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© 2020 The Authors. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/), The cause of autosomal-dominant retinitis pigmentosa (adRP), which leads to loss of vision and blindness, was investigated in families lacking a molecular diagnosis. A refined locus for adRP on Chr17q22 (RP17) was delineated through genotyping and genome sequencing, leading to the identification of structural variants (SVs) that segregate with disease. Eight different complex SVs were characterized in 22 adRP-affected families with >300 affected individuals. All RP17 SVs had breakpoints within a genomic region spanning YPEL2 to LINC01476. To investigate the mechanism of disease, we reprogrammed fibroblasts from affected individuals and controls into induced pluripotent stem cells (iPSCs) and differentiated them into photoreceptor precursor cells (PPCs) or retinal organoids (ROs). Hi-C was performed on ROs, and differential expression of regional genes and a retinal enhancer RNA at this locus was assessed by qPCR. The epigenetic landscape of the region, and Hi-C RO data, showed that YPEL2 sits within its own topologically associating domain (TAD), rich in enhancers with binding sites for retinal transcription factors. The Hi-C map of RP17 ROs revealed creation of a neo-TAD with ectopic contacts between GDPD1 and retinal enhancers, and modeling of all RP17 SVs was consistent with neo-TADs leading to ectopic retinal-specific enhancer-GDPD1 accessibility. qPCR confirmed increased expression of GDPD1 and increased expression of the retinal enhancer that enters the neo-TAD. Altered TAD structure resulting in increased retinal expression of GDPD1 is the likely convergent mechanism of disease, consistent with a dominant gain of function. Our study highlights the importance of SVs as a genomic mechanism in unsolved Mendelian diseases.

Background: More than one-third of genetic conditions involve the eye. Young adults with visual impairments experience psychosocial implications throughout their lives. The transition from adolescence to adulthood is a critical period during which young adults with hereditary visual impairment make choices regarding relationships, procreation and independence, setting the foundation for their future. Aim: This study explored the perceptions and lived experiences of visually impaired young adults. Setting: The study was conducted at three institutions for the blind in Cape Town. Methods: Using purposive sampling, in-depth qualitative interviews were conducted using a phenomenological approach with 15 young adults aged 18–23 years with genetic-based visual conditions. The data were analysed using thematic analysis. Results: Eleven participants had minimal knowledge and understanding of their condition, which also created uncertainty about their future health. Making decisions about procreation was the main implication of having a genetic-based visual impairment. Mixed feelings were reported regarding bearing offspring. Social alienation was experienced in some instances; many participants felt misunderstood by society, and intimate relationships were thought to be harder to obtain. Challenges with mobility, such as the inability to drive and the need to use public transportation, significantly impacted them, and various psychosocial aspects influenced how they perceived their lives and future. Conclusion: This research indicates that improved patient-centred healthcare services and support systems would be beneficial in aiding visually impaired individuals successfully transition into adulthood. The findings of this study serve as a useful guide for those who need to provide healthcare services, genetic services and support for visually impaired young adults.

Purpose: Missing heritability in human diseases represents a major challenge, and this is particularly true for ABCA4-associated Stargardt disease (STGD1). We aimed to elucidate the genomic and transcriptomic variation in 1054 unsolved STGD and STGD-like probands. Methods: Sequencing of the complete 128-kb ABCA4 gene was performed using single-molecule molecular inversion probes (smMIPs), based on a semiautomated and cost-effective method. Structural variants (SVs) were identified using relative read coverage analyses and putative splice defects were studied using in vitro assays. Results: In 448 biallelic probands 14 known and 13 novel deep-intronic variants were found, resulting in pseudoexon (PE) insertions or exon elongations in 105 alleles. Intriguingly, intron 13 variants c.1938-621G>A and c.1938-514G>A resulted in dual PE insertions consisting of the same upstream, but different downstream PEs. The intron 44 variant c.6148-84A>T resulted in two PE insertions and flanking exon deletions. Eleven distinct large deletions were found, two of which contained small inverted segments. Uniparental isodisomy of chromosome 1 was identified in one proband. Conclusion: Deep sequencing of ABCA4 and midigene-based splice assays allowed the identification of SVs and causal deep-intronic variants in 25% of biallelic STGD1 cases, which represents a model study that can be applied to other inherited diseases. Fil: Khan, Mubeen. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Cornelis, Stéphanie S.. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Del Pozo Valero, Marta. Hospital Universitario Fundación Jiménez Díaz; España. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Whelan, Laura. Trinity College; Estados Unidos Fil: Runhart, Esmee H.. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Mishra, Ketan. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Bults, Femke. Radboud University Nijmegen Medical Centre; Países Bajos Fil: AlSwaiti, Yahya. St John of Jerusalem Eye Hospital Group; Palestina (ANP) Fil: AlTalbishi, Alaa. St John of Jerusalem Eye Hospital Group; Palestina (ANP) Fil: De Baere, Elfride. University of Ghent; Bélgica Fil: Banfi, Sandro. Seconda Universita Degli Studi Di Napoli; Italia Fil: Banin, Eyal. The Hebrew University of Jerusalem; Israel Fil: Bauwens, Miriam. University of Ghent; Bélgica Fil: Ben Yosef, Tamar. The Ruth And Bruce Rappaport Faculty Of Medicine; Israel Fil: Boon, Camiel J. F.. Leiden University. Leiden University Medical Center; Países Bajos Fil: van den Born, L. Ingeborgh. Rotterdam Ophthalmic Institute; Países Bajos Fil: Defoort, Sabine. Universite Lille; Francia Fil: Devos, Aurore. Universite Lille; Francia Fil: Dockery, Adrian. Trinity College; Estados Unidos Fil: Dudakova, Lubica. Charles University and General University Hospital; República Checa Fil: Fakin, Ana. Charles University and General University Hospital; República Checa Fil: Farrar, G. Jane. Trinity College; Estados Unidos Fil: Ferraz Sallum, Juliana Maria. Universidade Federal de Sao Paulo; Brasil Fil: Fujinami, Kaoru. UCL Institute of Ophthalmology; Reino Unido Fil: Gilissen, Christian. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Glavac, Damjan. University of Ljubljana; Eslovenia Fil: Gorin, Michael B.. University of California at Los Angeles. School of Medicine; Estados Unidos Fil: Greenberg, Jacquie. University of Cape Town; Sudáfrica Fil: Hayashi, Takaaki. The Jikei University School of Medicine; Japón Fil: Hettinga, Ymkje M.. Bartiméus Diagnostic Center for Complex Visual Disorders; Países Bajos Fil: Hoischen, Alexander. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Hoyng, Carel B.. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Hufendiek, Karsten. University Eye Hospital Hannover Medical School; Alemania Fil: Jägle, Herbert. University Regensburg; Alemania Fil: Kamakari, Smaragda. OMMA Ophthalmological Institute of Athens; Grecia Fil: Karali, Marianthi. Seconda Universita Degli Studi Di Napoli; Italia Fil: Kellner, Ulrich. No especifíca; Fil: Klaver, Caroline C. W.. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Kousal, Bohdan. Charles University and General University Hospital; República Checa Fil: Lamey, Tina M.. University of Western Australia; Australia Fil: MacDonald, Ian M.. University of Alberta; Canadá Fil: Matynia, Anna. University of California at Los Angeles. School of Medicine; Estados Unidos Fil: McLaren, Terri L.. University of Western Australia; Australia Fil: Mena, Marcela D.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; Argentina Fil: Meunier, Isabelle. Université Montpellier II; Francia Fil: Miller, Rianne. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Newman, Hadas. Universitat Tel Aviv; Israel Fil: Ntozini, Buhle. University of Cape Town; Sudáfrica Fil: Oldak, Monika. No especifíca; Fil: Pieterse, Marc. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Podhajcer, Osvaldo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; Argentina Fil: Puech, Bernard. Universite Lille; Francia Fil: Ramesar, Raj. University of Cape Town; Sudáfrica Fil: Rüther, Klaus. No especifíca; Fil: Salameh, Manar. No especifíca; Fil: Salles, Mariana Vallim. Universidade de Sao Paulo; Brasil Fil: Sharon, Dror. The Hebrew University of Jerusalem; Israel Fil: Simonelli, Francesca. Seconda Universita Degli Studi Di Napoli; Italia Fil: Spital, Georg. No especifíca; Fil: Steehouwer, Marloes. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Szaflik, Jacek P.. No especifíca; Fil: Thompson, Jennifer A.. No especifíca; Fil: Thuillier, Caroline. Universite Lille; Francia Fil: Tracewska, Anna M.. No especifíca; Fil: van Zweeden, Martine. Radboud University Nijmegen Medical Centre; Países Bajos Fil: Vincent, Andrea L.. University of Auckland; Nueva Zelanda Fil: Zanlonghi, Xavier. No especifíca; Fil: Liskova, Petra. Charles University and General University Hospital; República Checa Fil: Stöhr, Heidi. Universitat Regensburg; Alemania Fil: De Roach, John N.. University of Western Australia; Australia Fil: Ayuso, Carmen. Hospital Universitario Fundación Jiménez Díaz; España Fil: Roberts, Lisa. University of Cape Town; Sudáfrica Fil: Weber, Bernhard H. F.. Universitat Regensburg; Alemania Fil: Dhaenens, Claire Marie. Universite Lille; Francia Fil: Cremers, Frans P. M.. Radboud University Nijmegen Medical Centre; Países Bajos

There is a glaring disparity in the populations included in genetic research; the majority of work involves European-derived cohorts, while other global populations - including Africans - are underrepresented. This is also true for the study of inherited retinal diseases. Being the most ancient of extant populations, African samples carry more variation than others, making them valuable for novel gene and variant discovery. The inclusion of diverse populations in research is essential to gain a more comprehensive understanding of genetic variation and molecular mechanisms of disease.

Huntington disease (HD) is the most common monogenic neurodegenerative disorder in populations of European ancestry, but occurs at lower prevalence in populations of East Asian or black African descent. New mutations for HD result from CAG repeat expansions of intermediate alleles (IAs), usually of paternal origin. The differing prevalence of HD may be related to the rate of new mutations in a population, but no comparative estimates of IA frequency or the HD new mutation rate are available. In this study, we characterize IA frequency and the CAG repeat distribution in fifteen populations of diverse ethnic origin. We estimate the HD new mutation rate in a series of populations using molecular IA expansion rates. The frequency of IAs was highest in Hispanic Americans and Northern Europeans, and lowest in black Africans and East Asians. The prevalence of HD correlated with the frequency of IAs by population and with the proportion of IAs found on the HD-associated A1 haplotype. The HD new mutation rate was estimated to be highest in populations with the highest frequency of IAs. In European ancestry populations, one in 5,372 individuals from the general population and 7.1% of individuals with an expanded CAG repeat in the HD range are estimated to have a molecular new mutation. Our data suggest that the new mutation rate for HD varies substantially between populations, and that IA frequency and haplotype are closely linked to observed epidemiological differences in the prevalence of HD across major ancestry groups in different countries.

Background To date, 43 types of Spinocerebellar Ataxias (SCAs) have been identified. A subset of the SCAs are caused by the pathogenic expansion of a CAG repeat tract within the corresponding gene. Ethnic and geographic differences are evident in the prevalence of the autosomal dominant SCAs. Few descriptions of the clinical phenotype and molecular genetics of the SCAs are available from the African continent. Established studies mostly concern the South African populations, where there is a high frequency of SCA1, SCA2 and SCA7. The SCA7 mutation in South Africa (SA) has been found almost exclusively in families of indigenous Black African ethnic origin. Objective To present the results of the first clinical description of seven Zambian families presenting with autosomal dominant SCA, as well as the downstream molecular genetic analysis of a subset of these families. Methods The study was undertaken at the University Teaching Hospital in Lusaka, Zambia. Ataxia was quantified with the Brief Ataxia Rating Scale derived from the modified international ataxia rating scale. Molecular genetic testing for 5 types of SCA (SCA1, SCA2, SCA3, SCA6 and SCA7) was performed at the National Health Laboratory Service at Groote Schuur Hospital and the Division of Human Genetics, University of Cape Town, SA. The clinical and radiological features were evaluated in seven families with autosomal dominant cerebellar ataxia. Molecular genetic analysis was completed on individuals representing three of the seven families. Results All affected families were ethnic Zambians from various tribes, originating from three different regions of the country (Eastern, Western and Central province). Thirty-four individuals from four families had phenotypic features of SCA7. SCA7 was confirmed by molecular testing in 10 individuals from 3 of these families. The age of onset of the disease varied from 12 to 59 years. The most prominent phenotypic features in these families were gait and limb ataxia, dysarthria, visual loss, ptosis, ophthalmoparesis/ophthalmoplegia, pyramidal tract signs, and dementia. Affected members of the SCA7 families had progressive macular degeneration and cerebellar atrophy. All families displayed marked anticipation of age at onset and rate of symptom progression. The pathogenic SCA7 CAG repeat ranges varied from 47 to 56 repeats. Three additional families were found to have clinical phenotypes associated with autosomal dominant SCA, however, DNA was not available for molecular confirmation. The age of onset of the disease in these families varied from 19 to 53 years. The most common clinical picture in these families included a combination of cerebellar symptoms with slow saccadic eye movements, peripheral neuropathy, dementia and tremor. Conclusion SCA is prevalent in ethnic Zambian families. The SCA7 families in this report had similar clinical presentations to families described in other African countries. In all families, the disease had an autosomal dominant pattern of inheritance across multiple generations. All families displayed anticipation of both age of onset and the rate of disease progression. Further clinical and molecular investigations of the inherited ataxias in a larger cohort of patients is important to understand the natural history and origin of SCAs in the Zambian population.

Missing heritability in human diseases represents a major challenge. Although whole-genome sequencing enables the analysis of coding and non-coding sequences, substantial costs and data storage requirements hamper its large-scale use to (re)sequence genes in genetically unsolved cases. The ABCA4 gene implicated in Stargardt disease (STGD1) has been studied extensively for 22 years, but thousands of cases remained unsolved. Therefore, single molecule molecular inversion probes were designed that enabled an automated and cost-effective sequence analysis of the complete 128-kb ABCA4 gene. Analysis of 1,054 unsolved STGD and STGD-like probands resulted in bi-allelic variations in 448 probands. Twenty-seven different causal deep-intronic variants were identified in 117 alleles. Based on in vitro splice assays, the 13 novel causal deep-intronic variants were found to result in pseudo-exon (PE) insertions (n=10) or exon elongations (n=3). Intriguingly, intron 13 variants c.1938-621G>A and c.1938-514G>A resulted in dual PE insertions consisting of the same upstream, but different downstream PEs. The intron 44 variant c.6148-84A>T resulted in two PE insertions that were accompanied by flanking exon deletions. Structural variant analysis revealed 11 distinct deletions, two of which contained small inverted segments. Uniparental isodisomy of chromosome 1 was identified in one proband. Integrated complete gene sequencing combined with transcript analysis, identified pathogenic deep-intronic and structural variants in 26% of bi-allelic cases not solved previously by sequencing of coding regions. This strategy serves as a model study that can be applied to other inherited diseases in which only one or a few genes are involved in the majority of cases.

Huntington disease (HD) is a fatal neurodegenerative disorder caused by a gain-of-function mutation in HTT. Suppression of mutant HTT has emerged as a leading therapeutic strategy for HD, with allele-selective approaches targeting HTT SNPs now in clinical trials. Haplotypes associated with the HD mutation (A1, A2, A3a) represent panels of allele-specific gene silencing targets for efficient treatment of individuals with HD of Northern European and indigenous South American ancestry. Here we extend comprehensive haplotype analysis of the HD mutation to key populations of Southern European, South Asian, Middle Eastern, and admixed African ancestry. In each of these populations, the HD mutation occurs predominantly on the A2 HTT haplotype. Analysis of HD haplotypes across all affected population groups enables rational selection of candidate target SNPs for development of allele-selective gene silencing therapeutics worldwide. Targeting SNPs on the A1 and A2 haplotypes in parallel is essential to achieve treatment of the most HD-affected subjects in populations where HD is most prevalent. Current allele-specific approaches will leave a majority of individuals with HD untreated in populations where the HD mutation occurs most frequently on the A2 haplotype. We further demonstrate preclinical development of potent and selective ASOs targeting SNPs on the A2 HTT haplotype, representing an allele-specific treatment strategy for these individuals. On the basis of comprehensive haplotype analysis, we show the maximum proportion of HD-affected subjects that may be treated with three or four allele targets in different populations worldwide, informing current allele-specific HTT silencing strategies.

There is a glaring disparity in the populations included in genetic research; the majority of work involves European-derived cohorts, while other global populations – including Africans – are underrepresented. This is also true for the study of inherited retinal diseases. Being the most ancient of extant populations, African samples carry more variation than others, making them valuable for novel gene and variant discovery. The inclusion of diverse populations in research is essential to gain a more comprehensive understanding of genetic variation and molecular mechanisms of disease.

Polyglutamine (polyQ) disorders are inherited neurodegenerative conditions defined by a common pathogenic CAG repeat expansion leading to a toxic gain-of-function of the mutant protein. Consequences of this toxicity include activation of heat-shock proteins (HSPs), impairment of the ubiquitin-proteasome pathway and transcriptional dysregulation. Several studies in animal models have shown that reducing levels of toxic protein using small RNAs would be an ideal therapeutic approach for such disorders, including spinocerebellar ataxia-7 (SCA7). However, testing such RNA interference (RNAi) effectors in genetically appropriate patient cell lines with a disease-relevant phenotype has yet to be explored. Here, we have used primary adult dermal fibroblasts from SCA7 patients and controls to assess the endogenous allele-specific silencing of ataxin-7 by two distinct siRNAs. We further identified altered expression of two disease-relevant transcripts in SCA7 patient cells: a twofold increase in levels of the HSP DNAJA1 and a twofold decrease in levels of the de-ubiquitinating enzyme, UCHL1. After siRNA treatment, the expression of both genes was restored towards normal levels. To our knowledge, this is the first time that allele-specific silencing of mutant ataxin-7, targeting a common SNP, has been demonstrated in patient cells. These findings highlight the advantage of an allele-specific RNAi-based therapeutic approach, and indicate the value of primary patient-derived cells as useful models for mechanistic studies and for measuring efficacy of RNAi effectors on a patient-to-patient basis in the polyQ diseases.

Spinocerebellar ataxia type 7 (SCA7) is an inherited dominant neurodegenerative disease caused by the expansion of a CAG repeat within the ATXN7 gene. Standard molecular diagnostic testing for SCA7 involves amplification of the region surrounding the CAG repeat via end-labeled PCR and subsequent capillary electrophoresis. In addition, multiplex methods exist that may be used to test for multiple polyglutamine spinocerebellar ataxias in a single assay. Herein, we used a SCA7 singleplex method to screen 111 individuals for whom the multiplex method detected a single normal allele. A total of six retested individuals (5.4%) were shown to have a pathogenic expansion at the ATXN7 locus. An additional triplet-primed PCR method was used to test the same cohort, and revealed no further disease-causing alleles. This study demonstrates the importance of using complementary methods to rule out apparent homoallelism during molecular testing for polyglutamine diseases.

Huntington disease (HD) is a neurodegenerative disorder resulting from the expansion of a CAG trinucleotide repeat in the huntingtin (HTT) gene. Worldwide prevalence varies geographically with the highest figures reported in populations of European ancestry. HD in South Africa has been reported in Caucasian, black and mixed subpopulations, with similar estimated prevalence in the Caucasian and mixed groups and a lower estimate in the black subpopulation. Recent studies have associated specific HTT haplotypes with HD in distinct populations. Expanded HD alleles in Europe occur predominantly on haplogroup A (specifically high-risk variants A1/A2), whereas in East Asian populations, HD alleles are associated with haplogroup C. Whether specific HTT haplotypes associate with HD in black Africans and how these compare with haplotypes found in European and East Asian populations remains unknown. The current study genotyped the HTT region in unaffected individuals and HD patients from each of the South African subpopulations, and haplotypes were constructed. CAG repeat sizes were determined and phased to haplotype. Results indicate that HD alleles from Caucasian and mixed patients are predominantly associated with haplogroup A, signifying a similar European origin for HD. However, in black patients, HD occurs predominantly on haplogroup B, suggesting several distinct origins of the mutation in South Africa. The absence of high-risk variants (A1/A2) in the black subpopulation may also explain the reported low prevalence of HD. Identification of haplotypes associated with HD-expanded alleles is particularly relevant to the development of population-specific therapeutic targets for selective suppression of the expanded HTT transcript.

Hemophilia A and B are X-linked recessive inherited bleeding disorders that have a profound impact on the family of affected individuals. Education is vital to enable women to appreciate the implications of being a carrier and the implications for a prospective child. Prior research has shown that cultural, socio-economic and linguistic issues in South Africa are major barriers to communication for first-language Xhosa-speakers. This exploratory study aimed to investigate the basic knowledge of genetic inheritance among this cultural group in order to promote culturally-sensitive, effective genetic counseling. Ten in-depth interviews were conducted with Xhosa-speaking mothers or caregivers of boys with hemophilia. Results suggest that the participants had a very limited understanding of the clinical management, genetic consequences and cause of hemophilia. While treatment and care by health care service providers was fully accepted, several participants believed that traditional methods would provide them with more satisfactory explanations. These findings suggest that there is a critical need for socio-culturally tailored, language-specific education for families with hemophilia.

Linkage analysis has been performed on a large South African family of British origin in which 39 persons in 6 generations had early onset Type I autosomal dominant retinitis pigmentosa (ADRP). Tight linkage was excluded between the disease and the D3S47 locus on chromosome 3. This finding is further evidence for genetic heterogeneity in ADRP.

Purpose Research over the past 25 years at the University of Cape Town has led to the identification of causative mutations in 17% of the 1416 families in the Retinal Degenerative Diseases (RDD) biorepository in South Africa. A low rate of mutation detection has been observed in patients of indigenous African origin, hinting at novel genes and mutations in this population. Recently, however, data from our translational research program showed two unrelated indigenous African families with Usher syndrome (USH), with the same homozygous MYO7A mutation. Therefore, the extent to which this mutation contributes toward the disease burden in South Africa was investigated. Methods Cohorts of unrelated indigenous South African probands with different RDD diagnoses were tested for the MYO7A c.6377delC mutation. Familial cosegregation analysis was performed for homozygous probands, clinical data were evaluated, and SNP haplotypes were analyzed. Results This homozygous MYO7A mutation underlies a remarkable 43% of indigenous African USH cases investigated in this study, the majority of which (60%) were diagnosed clinically with Type 2 USH. All homozygotes shared a common haplotype. This mutation does not appear to cause nonsyndromic vision loss. Conclusions Of interest is the origin of this common mutation relevant to the Bantu population migration into southern Africa. Further investigation of the phenotype may elucidate the disease biology, and perhaps reveal a larger cohort with the same mutation, with which to assess the impact of environmental and genetic modifiers and evaluate therapeutic trials.

BackgroundAlthough there is a high incidence of tendon injury as a result of participation in physical activity, the mechanisms responsible for such injuries are poorly understood. Investigators have suggested that some people may have a genetic predisposition to develop tendon injuries; in particular, genes on the tip of the long arm of chromosome 9 might, at least in part, be associated with this condition. The tenascin-C gene, which has been mapped to chromosome 9q32-q34, encodes for a structural component of tendons.HypothesisThe tenascin-C gene is associated with Achilles tendon injury.Study DesignCase control study; Level of evidence, 3.MethodsA total of 114 physically active white subjects with symptoms of Achilles tendon injury and 127 asymptomatic, physically active white control subjects were genotyped for the guanine-thymine dinucleotide repeat polymorphism within the tenascin-C gene.ResultsA significant difference in the allele frequencies of this polymorphism existed between the 2 groups of subjects (χ2 = 51.0, P =. 001). The frequencies of the alleles containing 12 repeats (symptomatic group, 18.9% vs control group, 10.2%) and 14 repeats (symptomatic group, 9.2% vs control group, 0.8%) were significantly higher in the symptomatic group, while the frequencies of the alleles containing 13 repeats (symptomatic group, 8.8% vs control group, 24.0%) and 17 repeats (symptomatic group, 7.5% vs control group, 20.1%) were significantly lower in this same group. Subjects who were homozygous or heterozygous for the underrepresented alleles (13 and 17 repeats) but who did not possess an overrepresented allele (12 and 14 repeats) may have a lower risk of developing Achilles tendon injuries (odds ratio, 6.2; 95% confidence interval, 3.5-11.0; P ConclusionsThe guanine-thymine dinucleotide repeat polymorphism within the tenascin-C gene is associated with Achilles tendon injury. Alleles containing 12 and 14 guanine-thymine repeats were overrepresented in subjects with tendon injuries, while the alleles containing 13 and 17 repeats were underrepresented.Clinical RelevancePersons who have variants of the tenascin-C gene with 12 and 14 guanine-thymine repeats appear to have a 6-fold risk of developing Achilles tendon injuries.

There is little data on the spectrum and frequencies of the autosomal dominant spinocerebellar ataxias (SCAs) from the African continent. We undertook a large prospective population-based study over a 10-year period in South Africa (SA). Affected persons were clinically evaluated, and the molecular analysis for the SCA1, 2, 3, 6 and 7 expansions was undertaken. Of the 54 SA families with dominant ataxia, SCA1 accounted for 40.7%, SCA2 for 13%, SCA3 for 3.7%, SCA6 for 1.9%, SCA7 for 22.2% and 18.5% were negative for all these mutations. The frequency of the SCA1 and SCA7 expansions in SA represents one of the highest frequencies for these expansions reported in any country. In this study, the SCA7 mutations have only been found in SA families of Black ethnic origin.

Purpose: To understand which clinical presentations suggest a diagnosis of choroideremia (CHM). Methods: Retrospective chart review. Included were patients for whom a clinical diagnosis of CHM was suggested, but either protein analysis or direct sequencing of the CHM gene could not confirm the diagnosis. Clinical presentation, family history and fundus photographs were reviewed. Results: We analyzed protein and DNA samples from members of more than 100 families in which at least I member had a clinical diagnosis of CHM. For 26 of these families, the clinical diagnosis of CHM could not be confirmed by laboratory analysis. Relevant clinical information was requested from the referring ophthalmologists so that alternative diagnoses could be considered. Sufficient information was provided for 13 of the 26 families. Four patients were reclassified as having retinitis pigmentosa (RP) from the clinical phenotype; only two clearly had X-linked inheritance. One patient had a syndrome including macular dystrophy, hearing loss, developmental delay and cerebral palsy. One patient was reclassified as having congenital stationary night blindness on the basis of an electronegative electroretinogram and a normal fundus. One patient had hearing loss suggesting Usher syndrome. One patient had signs consistent with cone-rod dystrophy (CRD). Five patients could not be reclassified on the basis of the clinical presentation. Conclusion: RP, Usher syndrome and CRD are clinical phenotypes that may overlap with CHM. Clinical features that suggest CHM include severe chorioretinal atrophy with preservation of the macula, X-linked inheritance and retinal changes in a related female.

Analysis of exon ORF15 of the RPGR gene has revealed a novel mutation in a South African family with X-linked retinitis pigmentosa (XLRP), which has implications for the rest of the family in terms of pre-symptomatic testing. The ability to test for this mutation will be beneficial for the accurate determination of carrier status in female relatives who may have been unaware of their risk before this study was performed. This work also highlights the need to be aware of the ramifications of mutation testing in what may appear to be small families. This is the first report of an RPGR ORF15 mutation in a South African family of mixed ancestry.

The RP17 locus for autosomal dominant retinitis pigmentosa has previously been mapped to chromosome 17q by linkage analysis. Two unrelated South African families are linked to this locus and the identification of key recombination events assigned the RP17 locus to a 10 cM interval on 17q22. The work reported here refines the mapping of the locus from a 10 cM to a 1 cM interval between the microsatellite markers D17S1604 and D17S948. A physical map of this interval was constructed using information from the Whitehead/MIT YAC contig WC 17.8. Sequence-tagged site (STS) content mapping of seven overlapping YACs from this contig was employed in order to build the map. A BAC library was screened to cover a gap in the YAC contig and two positive BACs were identified. Intragenic polymorphisms in the retinal fascin gene provided evidence for the exclusion of this candidate as the RP17 disease gene.

Waardenburg syndrome (WS) type 1 is an autosomal dominant disorder characterized by sensorineural hearing loss, pigmentary abnormalities of the eye, hair, and skin, and dystopia canthorum. The phenotype is variable and affected individuals may exhibit only one or a combination of several of the associated features. To assess the relationship between phenotype and gene defect, clinical and genotype data on 48 families (271 WS individuals) collected by members of the Waardenburg Consortium were pooled. Forty-two unique mutations in the PAX3 gene, previously identified in these families, were grouped in five mutation categories: amino acid (AA) substitution in the paired domain, AA substitution in the homeodomain, deletion of the Ser-Thr-Pro-rich region, deletion of the homeodomain and the Ser-Thr-Pro-rich region, and deletion of the entire gene. These mutation classes are based on the structure of the PAX3 gene and were chosen to group mutations predicted to have similar defects in the gene product. Association between mutation class and the presence of hearing loss, eye pigment abnormality, skin hypopigmentation, or white forelock was evaluated using generalized estimating equations, which allowed for incorporation of a correlation structure that accounts for potential similarity among members of the same family. Odds for the presence of eye pigment abnormality, white forelock, and skin hypopigmentation were 2, 8, and 5 times greater, respectively, for individuals with deletions of the homeodomain and the Pro-Ser-Thr-rich region compared to individuals with an AA substitution in the homeodomain. Odds ratios that differ significantly from 1.0 for these traits may indicate that the gene products resulting from different classes of mutations act differently in the expression of WS. Although a suggestive association was detected for hearing loss with an odds ratio of 2.6 for AA substitution in the paired domain compared with AA substitution in the homeodomain, this odds ratio did not differ significantly from 1.0.

In direct-to-consumer (DTC) genetic testing, laboratory-based genetic services are offered directly to the public without an independent healthcare professional being involved. The committee of the Southern African Society for Human Genetics (SASHG) appeals to the public and clinicians to be cautious when considering and interpreting such testing. It is important to stress that currently, the clinical validity and utility of genetic tests for complex multifactorial disorders such as type 2 diabetes mellitus and cardiovascular diseases is questionable. The majority of such tests are not scientifically validated and are based on a few preliminary studies. Potential consumers should be aware of the implications of genetic testing that could lead to stigmatisation and discrimination by insurance companies or potential employers of themselves and their family members. Guidelines and recommendations for DTC genetic testing in South Africa (SA) are currently lacking. We provide recommendations that seek to protect consumers and healthcare providers in SA from possible exploitation.

Spinocerebellar ataxia type 7 (SCA7) is an inherited dominant neurodegenerative disease caused by the expansion of a CAG repeat within the ATXN7 gene. Standard molecular diagnostic testing for SCA7 involves amplification of the region surrounding the CAG repeat via end-labeled PCR and subsequent capillary electrophoresis. In addition, multiplex methods exist that may be used to test for multiple polyglutamine spinocerebellar ataxias in a single assay. Herein, we used a SCA7 singleplex method to screen 111 individuals for whom the multiplex method detected a single normal allele. A total of six retested individuals (5.4%) were shown to have a pathogenic expansion at the ATXN7 locus. An additional triplet-primed PCR method was used to test the same cohort, and revealed no further disease-causing alleles. This study demonstrates the importance of using complementary methods to rule out apparent homoallelism during molecular testing for polyglutamine diseases.

Polyglutamine diseases are inherited neurodegenerative conditions arising from expanded trinucleotide CAG repeats in the disease-causing gene, which are translated into polyglutamine tracts in the resultant protein. Although these diseases share a common type of mutation, emerging evidence suggests that pathogenesis is complex, involving disruption of key cellular pathways, and varying with the disease context. An understanding of polyglutamine disease mechanisms is critical for development of novel therapeutics. Here we summarise theories of molecular pathogenesis, and examine ways in which this knowledge is being harnessed for therapy, with reference to work under way at the University of Cape Town. Despite a plethora of preclinical data, clinical trials of therapies for polyglutamine diseases have had only limited success. However, recently initiated trials, including those using gene silencing approaches, should provide valuable insights into the safety and efficacy of therapies directly targeting polyglutamine pathogenesis. This is particularly relevant in the South African context, where the frequencies of two polyglutamine diseases, spinocerebellar ataxia types 1 and 7, are among the highest globally.

This study of allelic association using three intra- and two extragenic markers within 150 kb of the Huntington disease (HD) mutation has provided evidence for linkage disequilibrium for four of five markers. Haplotype analysis of 67 HD families using markers in strong linkage disequilibrium with HD identified two haplotypes underlying 77.6% of HD chromosomes. Normal chromosomes with these two haplotypes had a mean number of CAG repeats significantly larger than and an altered distribution of CAG repeats compared with other normal chromosomes. Furthermore, haplotype analysis of five new mutation families reveals that HD has arisen on these same two chromosomal haplotypes. These findings suggest that HD arises more frequently on chromosomes with specific DNA haplotypes and higher CAG repeat lengths. We then studied CAG and CCG repeat lengths in the HD gene on 896 control chromosomes from different ancestries to determine whether the markedly reduced frequency of HD in Finland, Japan, China and African Blacks is associated with an altered frequency of DNA haplotypes and subsequently lower CAG lengths on control chromosomes compared to populations of Western European descent. The results show a highly significant inverse relationship between CAG and CCG repeat lengths. In populations with lowered prevalence rates of HD, CAG repeat lengths are smaller and the distribution of CCG alleles is markedly different from Western European populations. These findings suggest that, in addition to European emigration, new mutations make a contribution to geographical variation of prevalence rates and is consistent with a multistep model of HD developing from normal chromosomes with higher CAG repeat lengths.

Retinitis pigmentosa (RP) is a group of genetically and clinically heterogeneous retinopathies, some of which have been shown to result from mutations in two different known retinal genes, rhodopsin (3q) and peripherin-rds (6p). Three additional anonymous loci at 7p, 7q and pericentric 8 have been implicated by linkage studies. There are still, however, a few families in which all known loci have been excluded. In this report we present data indicating a location, on the short arm of chromosome 17, for the autosomal dominant RP (ADRP) locus in a large South African (SA) family of British ancestry. Positive two-point lod scores have been obtained for nine markers (D17S938, Z = 5.43; D17S796, Z = 4.82; D17S849, Z = 3.6; D17S786, Z = 3.55; TP53, Z = 3.55; D17S578, Z = 3.29; D17S960, Z = 3.16; D17S926, Z = 1.51; D17S804, Z = 0.47 all at theta = 0.10 except D17S804 and D17S926, theta = 0.20). These data provide definitive evidence for the localization of an ADRP gene on chromosome 17p. The human recoverin gene has been localized to 17p13.1 and was consequently a prime candidate for ADRP in the family studied. However, mutation screening of the three exons of this gene failed to produce any evidence of recoverin being the gene involved in the pathogenesis of ADRP in this SA family.

Purpose Technology for quantitative measurement of fundus autofluorescence (AF). Methods Scanning laser ophthalmoscopes (SLO’s, HRA2 and Spectralis, Heidelberg Eng.) were modified by insertion of an internal AF reference to account for variable laser power and detector sensitivity. 30° images (mean of 9 without histogram stretching) were acquired from normal test subjects (10-60 y/o) after a 20 s bleaching period,.. Quantified AF (qAF) was calculated from the reference gray level (GL), the zero GL, the media and the magnification (refractive error). The linearity of the system, field uniformity, effect of refractive error, and reproducibility were tested. qAF measurements were then done on 22 patients with Stargardt disease (STGD ) and 11 patients with retinitis pigmentosa (RP). Results The linear detection range extended to 175 GL. Field uniformity was better than 5% in a central 20°-diameter circle. The recorded AF and the square of the image magnification were inversely related. Different day reproducibility was 4.1 % at the fovea. Median inter-instrument reproducibility (Spectralis and the HRA2) was 7.4 %. Normals. qAF highly correlated with age (central qAF = 8.8+3.5*Age, p

Usher syndrome is the most commonly recognized cause of combined visual and hearing loss in technologically developed countries. There are several different types and all are inherited in an autosomal recessive manner. There may be as many as five different genes responsible for at least two closely related phenotypes. The nature of the gene defects is unknown, and positional cloning strategies are being employed to identify the genes. This is a report of the localization of one gene for Usher syndrome type I to chromosome 11q, probably distal to marker D11S527. Another USH1 gene had been previously localized to chromosome 14q, and this second localization establishes the existence of a new and independent locus for Usher syndrome.

Spinocerebellar ataxia type 7 is a polyglutamine disorder caused by an expanded CAG repeat mutation that results in neurodegeneration. Since no treatment exists for this chronic disease, novel therapies such post-transcriptional RNA interference-based gene silencing are under investigation, in particular those that might enable constitutive and tissue-specific silencing, such as expressed hairpins. Given that this method of silencing can be abolished by the presence of nucleotide mismatches against the target RNA, we sought to identify expressed RNA hairpins selective for silencing the mutant ataxin-7 transcript using a linked SNP. By targeting both short and full-length tagged ataxin-7 sequences, we show that mutation-specific selectivity can be obtained with single nucleotide mismatches to the wild-type RNA target incorporated 3′ to the centre of the active strand of short hairpin RNAs. The activity of the most effective short hairpin RNA incorporating the nucleotide mismatch at position 16 was further studied in a heterozygous ataxin-7 disease model, demonstrating significantly reduced levels of toxic mutant ataxin-7 protein with decreased mutant protein aggregation and retention of normal wild-type protein in a non-aggregated diffuse cellular distribution. Allele-specific mutant ataxin7 silencing was also obtained with the use of primary microRNA mimics, the most highly effective construct also harbouring the single nucleotide mismatch at position 16, corroborating our earlier findings. Our data provide understanding of RNA interference guide strand anatomy optimised for the allele-specific silencing of a polyglutamine mutation linked SNP and give a basis for the use of allele-specific RNA interference as a viable therapeutic approach for spinocerebellar ataxia 7.

Mutation analysis of retinal candidate genes is performed as part of an ongoing research to identify the causative genetic defect in South African families with retinal degenerative disorders (RDDs). A translational research protocol has been established whereby probands are counseled and given their molecular genetic results to take back to other family members, who can then request individual diagnostic testing. A Thr17Met mutation of the rhodopsin gene was identified in a Caucasian South African family with autosomal dominant retinitis pigmentosa. Patients with this mutation appear to benefit from treatment using oral vitamin A supplementation. This family has been informed that a molecular diagnosis is available; however, one individual has refused testing and none of the younger generation has shown interest in receiving molecular results or genetic counseling. Adapting the established protocol for the translation of RDD research results and contacting mutation positive individuals may be justifiable in light of the potential benefit of therapy.

The article by Ricki Lewis, Nature, Volume 449,October 18, 2007, correctly points out that the genetic counseling profession is on the "verge of being discovered by the rest of the world". The rapid recognition of genes associated with single-gene disorders and complex conditions has deepened our understanding of the role of genetics in health and illness. The impact of genetic conditions on individuals and families, particularly in ethical, legal and psychosocial arenas, requires specially trained professionals to work in this unique and growing dimension of healthcare. The Transnational Alliance for Genetic Counseling (TAGC) represents fifteen countries currently providing genetic counselor education across five continents.

In considering inherited disease, one important distinction is between diseases with a clear, unequivocal genetic cause and other diseases whose occurrence is influenced by genetic factors, but whose causes are multifactorial. The first type, simple or Mendelian inherited diseases, typically have a distinct mode of inheritance-autosomal dominant, autosomal recessive or X-linked-and are the result of rare, pathogenic mutations with high penetrance (presence of the mutation or mutations has a high likelihood of causing disease). Examples of Mendelian diseases that include neovascularization are X-linked Norrie’s disease (1) and autosomal dominant glaucoma caused by mutations in myocilin (2). Even “simple” inherited diseases are genetically complicated, as mutations in different genes may cause the same disease, and different mutations in the same gene may cause different diseases. However, it is reasonable to assume that the cause of a Mendelian disease in a given individual and family is one mutation only (or two, if recessive) in a specific gene.

Of the 14 loci so far associated with autosomal dominant retinitis pigmentosa (RP), the most recently cloned gene is the one responsible for RP17 which occurs in six South African families comprising 187 individuals. Of these, 60 individuals are currently affected with RP and 16 are at risk by virtue of their relatedness to a known mutation carrier. The gene responsible for RP17 is carbonic anhydrase IV (CA4), one of a family of carbonic anhydrases which are involved in the interconversion of carbon dioxide and carbonic acid (11). The proposed mechanism of disease suggests that a drug-based treatment may be possible.

The success of genetic research studies depend on patients' willingness to participate. It is thus important to explore the attitudes of individuals that participate in such studies. This study used qualitative methods to explore how individuals with inherited retinal degenerative disorders (RDD) perceived participating in genetic research and subsequently receiving mutation results. Individual interviews were conducted with all the individuals in the Cape Town Metropolitan area who had received mutation results after participating in a genetic research program (4 individuals). Although experiences differed significantly, the study revealed that the participants had positive attitudes towards participating in the RDD research program. This study illustrates the importance of using qualitative methods in ophthalmic populations to explore important issues.

This study involved the detailed investigation of the region surrounding the huntingtin gene in families with a history of Huntington Disease (HD) in South Africa. The primary aim was to investigate the origins of the HD mutation in South Africa by constructing a single-nucleotide polymorphism (SNP) haplotype around the HD gene and to determine how many haplotypes there are in two different South African populations. Haplotypes were created by genotyping six SNPs in a total of 13 HD families – seven Caucasian and six Mixed Ancestry. Of the six Mixed Ancestry families, four shared a common SNP haplotype, which was observed in two Afrikaans-speaking Caucasian HD families thus indicating that a founder effect was present in the South African population. The genotyping of a recently identified highly polymorphic marker close to the HD disease-causing mutation further corroborated the SNP haplotype results. Computational analysis was used to analyze the extent of the common haplotype identified in the study cohort in additional South African HD individuals. The results strongly suggest that the common haplotype extends further into the South African Mixed Ancestry HD population and is predominant in the Mixed Ancestry HD families.

Stem cell science is advancing at an unprecedented rate, with thousands of research papers being published every year and many clinical trials for a wide range of conditions underway as registered on ClinicalTrials.gov. This rapidly expanding and alluring field has brought with it ever more complex and multifaceted ethical issues, many of which require new guidelines, consent protocols and even change in legislation, since they do not fit comfortably in the existing bioethical regulations and protocols. Keeping up with the ethical implications of stem cell research is daunting to the expert and non-expert. We review the various types of stem cells and then focus on multipotent and pluripotent cell types, since it is these cell types that bring with them the greatest research and therapeutic potential, while concurrently delivering novel ethical conundrums. Certain key considerations are currently lacking and what is needed is how to obtain permission from individuals who donate their biological material for both scientific inquiry and eventually, for their potential therapeutic utility.

Mutations in the hotspot of RP1 are reportedly responsible for 4-7% of autosomal dominant retinitis pigmentosa (ADRP) in the United States, Canada, and Europe. South Africa (SA) has unique subpopulations and a comparatively low observed frequency of rhodopsin mutations, which lead to this investigation of the contribution of RP1 mutations to the ADRP disease burden in SA.Fifty-seven affected, unrelated South African individuals with ADRP were selected for mutation screening of the RP1 hotspot, using denaturing high performance liquid chromatography (HPLC). Variants were identified by direct sequencing, after which cosegregation analysis and population frequency studies were performed using restriction fragment length polymorphism analysis, nondenaturing HPLC, or denaturing HPLC.Three mutations were identified, including two novel sequence variations and the common Arg677X mutation. A wide spectrum of disease severity was observed in the families with these RP1 gene mutations. Two nondisease-associated polymorphisms were also detected, with the frequency of one of these variants being significantly low in Black African individuals.Mutations were only found in Caucasian families with origins in the British Isles. The observed RP1 mutation frequency of 5.3% in SA ADRP patients is comparable to the frequency reported in other populations.

PURPOSE. To assess the mutation spectrum of ABCA4 underlying Stargardt disease (STGD) in South Africa (SA) and to determine whether there is a single or a few founder chromosomes in SA STGD families. METHODS. Sixty-four probands exhibiting the STGD phenotype were screened for mutations in the 50 exons of ABCA4 by single-strand conformational polymorphism- heteroduplex analysis sequencing and restriction fragment length polymorphism analysis. Microsatellite marker haplotyping was used to determine the ancestry in 10 families. RESULTS. Fifty-seven ABCA4 disease-associated alleles were identified that comprised 16 different sequence variants, of which two were novel, in 40 individuals of the cohort of 64 subjects. The most common variants identified included the C1490Y, L2027F, R602W, V256splice, R152X, and 2588G→C mutations. The C1490Y variant was the most common disease-associated variant identified (19/64 subjects) and was absent in 392 control chromosomes. At least 10 ABCA4 disease-associated haplotypes were identified. Two of these haplotypes, which carried the C1490Y mutation, were identified in three unrelated families. CONCLUSIONS. Results suggest that ABCA4 is the major gene underlying STGD in the cohort investigated. Five of the six common sequence variants identified were at a higher frequency in the SA cohort than reported in published data on individuals of similar ancestry. The mutation and haplotype data suggests that there are several ancestral haplotypes underlying STGD in SA. There seems to be at least two different origins for the common C1490Y mutation, as well as two for the R602W mutation, thereby suggesting several founder effects for STGD in SA.

Genetic and physical mapping of the RP17 locus on 17q identified a 3.6-megabase candidate region that includes the gene encoding carbonic anhydrase IV ( CA4 ), a glycosylphosphatidylinositol-anchored protein that is highly expressed in the choriocapillaris of the human eye. By sequencing candidate genes in this region, we identified a mutation that causes replacement of an arginine with a tryptophan (R14W) in the signal sequence of the CA4 gene at position -5 relative to the signal sequence cleavage site. This mutation was found to cosegregate with the disease phenotype in two large families and was not found in 36 unaffected family members or 100 controls. Expression of the mutant cDNA in COS-7 cells produced several findings, suggesting a mechanism by which the mutation can explain the autosomal dominant disease. In transfected COS-7 cells, the R14W mutation ( i ) reduced the steady-state level of carbonic anhydrase IV activity expressed by 28% due to a combination of decreased synthesis and accelerated turnover; ( ii ) led to up-regulation of immunoglobulin-binding protein, double-stranded RNA-regulated protein kinase-like ER kinase, and CCAAT/enhancer-binding protein homologous protein, markers of the unfolded protein response and endoplasmic reticulum stress; and ( iii ) induced apoptosis, as evidenced by annexin V binding and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling staining, in most cells expressing the mutant, but not the WT, protein. We suggest that a high level of expression of the mutant allele in the endothelial cells of the choriocapillaris leads to apoptosis, leading in turn to ischemia in the overlying retina and producing autosomal dominant retinitis pigmentosa.

Having identified a disease-associated rhodopsin mutation in a patient with retinitis pigmentosa (RP), the issue is to address the question of whether to offer genetic testing to at-risk family members. Two members of a South African (SA) family, one of whom suffers from RP, as well as 54 unrelated SA RP patients from the same population group were investigated using single-stranded conformational polymorphism analysis followed by DNA sequencing. A rare homozygous mutation at the intron-exon boundary of exon 4 in the rhodopsin gene was identified in the proband. One of his siblings was found to be heterozygous for the same mutation. The mutation was not detected in the 54 unrelated SA RP patients examined, 11 of whom were sporadic cases. A low incidence of RP amongst heterozygous carriers of this mutation has been reported; however, in the past it has been unclear whether the mutation has an effect in single copy or dual copy. To the best of our knowledge, this is the first time that this mutation has been reported as homozygous in an affected individual, thereby resolving the issue and confirming that it is a recessive disease-associated mutation. This is also the first autosomal recessive RP disease-causing rhodopsin mutation that has been identified in Southern Africa. Analysis of the extended pedigree indicated obligate heterozygous carriers of the mutation, without obvious signs of visual impairment in early adulthood. The extent to which potential heterozygous carriers should be pursued and clinically examined is discussed and the question is addressed as to whether to inform the family of these molecular findings.

Retinitis pigmentosa (RP) is a genetically heterogeneous disorder characterized by progressive degeneration of the peripheral retina leading to night blindness and loss of visual fields. With an incidence of approximately 1 in 4000, RP can be inherited in X-linked, autosomal dominant or autosomal recessive modes. The RP13 locus for autosomal dominant RP (adRP) was placed on chromosome 17p13.3 by linkage mapping in a large South African adRP family. Using a positional cloning and candidate gene strategy, we have identified seven different missense mutations in the splicing factor gene PRPC8 in adRP families. Three of the mutations cosegregate within three RP13 linked families including the original large South African pedigree, and four additional mutations have been identified in other unrelated adRP families. The seven mutations are clustered within a 14 codon stretch within the last exon of this large 7 kb transcript. The altered amino acid residues at the C-terminus exhibit a high degree of conservation across species as diverse as humans, Arabidopsis and trypanosome, suggesting that some functional significance is associated with this part of the protein. These mutations in this ubiquitous and highly conserved splicing factor offer compelling evidence for a novel pathway to retinal degeneration.

The average adult eye contains approximately 92 000 000 rods and 5 000 000 cones distributed throughout the retina. Cone cells function in colour perception, while rod cells are required for vision in low light. Degeneration of primarily the rod photoreceptor cells is the cause of Retinitis Pigmentosa (RP). This is a group of hereditary disorders characterised by night-blindness and gradual constriction of the visual fields, resulting in partial or total blindness. RP can be inherited in dominant, recessive, X-linked, mitochondrial or digenic modes. It may also present as part of a syndrome.1

South Africa represents an anthropological and geographical cul de sac to which several ancient African populations have migrated from Central, East and West Africa. This subcontinent may thus represent access to some of the most ancient homo sapiens genetic material that is still in the process of being passed from one generation to the next. The indigenous African subpopulations may also carry some of the most ancient (non-lethal) disease-causing mutations. Apart from this interesting niche for the study of inherited disorders, this subcontinent has also attracted waves of immigrations over the past 500 years. The first major European settlement was by Dutch seamen in the region of the Cape of Good Hope. This was followed by colonists from Germany and France and other parts of Europe, including a large contingent from Britain. From the East, immigrants also hailed from India and Malaysia. With regard to the potential of genetic investigations, there are significant numbers of communities that have remained as true as is possible to their ancestral lineages from their lands of origin. This has been reinforced by the sociopolitical practise of apartheid. The high degree of clinical and genetic heterogeneity in our cohort of research subjects with retinal degenerative disorders, coupled with the observation of certain disorders segregating in a manner which fits a ’land of origin’ scenario has led us to use the information of ancestral lineages to assist in deterring the strategy to defining the underlying genetic causes in RDDs.