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Asymptomatic female carriers of the FMR1 premutation at childbearing age have been mostly identified through prenatal genetic testing, which is routinely proposed in Israel. During the last few years, a premutation phenotype in males and females has been defined-FXAND, including neuropsychiatric disorder, learning difficulties, endocrine dysfunction, and premature ovarian failure. So when a family at risk is identified, should individuals be tested for premutation even if minors? In order to understand what professionals' views are with regard to testing FMR1 premutation in minors, we performed a questionnaire testing both ethical attitudes and knowledge. Eighty-two percent of professionals would positively consider fragile X testing in minors, and an additional 15.4% would consider it in boys only. The specific phenotype of full mutation is recognized well by health professionals, while the premutation phenotype is not well known. There is a need to expand awareness on the fragile X premutation phenotype through better information. Testing of fragile X premutation status in minors should be considered, when at risk due to family history.

Psychopathology is prevalent in Williams (WS), fragile X (FXS) and Prader–Willi (PWS) syndromes. However, little is known about the potential correlates of psychopathology in these groups. A questionnaire study was completed by 111 caregivers of individuals with WS (n = 35); FXS (n = 50) and PWS (n = 26). Mean age was 26 years (range 12–57 years); 74 (67%) were male. Multiple regression analyses indicated that higher rates of health problems and sensory impairments predicted higher psychopathology in WS (p

Fragile X syndrome (FXS) is a devastating developmental disability that has profound effects on cognition, behavior and seizure susceptibility. There are currently no treatments that target the underlying cause of the disorder and recent clinical trials have been unsuccessful. In 2007, seminal work demonstrated that amyloid-beta protein precursor (APP) is dysregulated in Fmr1(KO) mice through a metabotropic glutamate receptor 5 (mGluR(5))-dependent pathway. These findings raise the hypotheses that: (1) APP and/or APP metabolites are potential therapeutic targets as well as biomarkers for FXS, and (2) mGluR(5) inhibitors may be beneficial in the treatment of Alzheimer’s disease. Herein, advances in the field over the past decade that have reproduced and greatly expanded upon these original findings are reviewed, and required experimentation to validate APP metabolites as potential disease biomarkers as well as therapeutic targets for FXS are discussed.

Screening for fragile X syndrome (FXS) is essential in children with developmental delay or intellectual disability (ID). In addition, using clinical screening checklists remains of high interest in resource-limited settings. We aimed to gain insight into the prevalence of FXS and the distribution of CGG alleles and to evaluate the usefulness of three checklists in specialized institutions in Kinshasa, DR Congo. We recruited 80 males and 25 females from six specialized institutions in Kinshasa and administered a questionnaire comprising items from the following FXS checklists: Hagerman, Maes, and Guruju. FMR1 CGG repeats were assessed for every patient. About 37% of patients were referable for FX testing based on Hagerman's checklist, 35% for Maes', and 43.80% for Guruju's, but none of them was molecularly confirmed to have FXS. Thus, specificities were 62.86, 64.76, and 56.5%, respectively, for Hagerman, Maes, and Guruju, respectively. The mean CGG allele size was 28.55 ± 2.83 (ranges, 17-48). The 29 CGG was the most frequent allele (24.61%). Thus, existing checklists should not be automatically applied to Congolese patients without adjustments. The distribution of CGG repeats and the number of CGG alleles are similar to other African studies. ispartof: JOURNAL OF COMMUNITY GENETICS vol:10 issue:1 pages:153-159 ispartof: location:Germany status: published

Background Fragile X syndrome (FXS) is characterized by a range of developmental, neuropsychiatric, and behavioral symptoms that cause significant impairment in those with the disorder. Cannabidiol (CBD) holds promise as a potential treatment for FXS symptoms due to its safety profile and positive effects on a number of emotional and behavioral symptoms associated with FXS. The aim of the current study was to evaluate the safety, tolerability, and initial efficacy of ZYN002, a transdermal CBD gel, in a pediatric population with FXS. Methods Twenty children and adolescents (aged 6–17 years) with a diagnosis of FXS (confirmed through molecular documentation of FMR1 full mutation) were enrolled in an open-label, multi-site, trial of ZYN002. Transdermal CBD gel was administered twice daily for 12 weeks, titrated from 50 mg to a maximum daily dose of 250 mg. The primary efficacy endpoint was change from screening to week 12 on the Anxiety, Depression, and Mood Scale (ADAMS). Secondary endpoint measures included the Aberrant Behavior Checklist—Community for FXS (ABC-CFXS), Pediatric Anxiety Rating Scale (PARS-R), Pediatric Quality of Life Inventory (PedsQL™), three Visual Analogue Scales (VAS), and the Clinical Global Impression Scale—Severity (CGI-S) and Improvement (CGI-I). Results The majority of treatment-emergent AEs (reported by 85% of participants) were mild in severity (70%), and no serious adverse events were reported. There was a statistically significant reduction in ADAMS total score from screening to week 12 and significant reductions on nearly all other secondary endpoints, including all ADAMS subscales (except depressed mood), all ABC-CFXS subscale scores (e.g., social avoidance, irritability), PARS-R total severity score, and PedsQL total score. Conclusions ZYN002 was well tolerated and produced clinically meaningful reductions in anxiety and behavioral symptoms in children and adolescents with FXS. These findings support further study of ZYN002 in a randomized, well-controlled trial for the treatment of behavioral symptoms of FXS. Trial registration ANZCTR, ACTRN12617000150347 Registered 27 January 2017

Background Individuals with premutation alleles of the fragile X mental retardation 1 (FMR1) gene are at risk of developing fragile X-associated tremor/ataxia syndrome (FXTAS) during aging. Characterization of motor issues associated with aging in FMR1 premutation carriers is needed to determine neurodegenerative processes and establish new biobehavioral indicators to help identify individuals at greatest risk of developing FXTAS. Methods We examined postural stability in 18 premutation carriers ages 46–77 years and 14 age-matched healthy controls. Participants completed a test of static stance and two tests of dynamic postural sway on a force platform to quantify postural variability and complexity. CGG repeat length was measured for each premutation carrier, and MRI and neurological evaluations were conducted to identify carriers who currently met criteria for FXTAS. Of the 18 premutation carriers, seven met criteria for definite/probable FXTAS (FXTAS+), seven showed no MRI or neurological signs of FXTAS (FXTAS−), and four were inconclusive due to insufficient data. Results Compared to controls, premutation carriers showed increased center of pressure (COP) variability in the mediolateral (COPML) direction during static stance and reduced COP variability in the anterior-posterior (COPAP) direction during dynamic AP sway. They also showed reductions in COPML complexity during each postural condition. FXTAS+ individuals showed reduced COPAP variability compared to FXTAS− carriers and healthy controls during dynamic AP sway. Across all carriers, increased sway variability during static stance and decreased sway variability in target directions during dynamic sways were associated with greater CGG repeat length and more severe neurologically rated posture and gait abnormalities. Conclusion Our findings indicate that aging FMR1 premutation carriers show static and dynamic postural control deficits relative to healthy controls implicating degenerative processes of spinocerebellar and cerebellar-brainstem circuits that may be independent of or precede the onset of FXTAS. Our finding that FXTAS+ and FXTAS− premutation carriers differed on their level of intentional AP sway suggests that neural mechanisms of dynamic postural control may be differentially impacted in patients with FXTAS, and its measurement may be useful for rapidly and precisely identifying disease presence and onset. Electronic supplementary material The online version of this article (10.1186/s11689-018-9261-x) contains supplementary material, which is available to authorized users.

Hyperintensities in the middle cerebellar peduncles (MCP), known as the MCP sign, and progressive late-onset ataxia constitute major characteristics of the fragile X tremor/ataxia syndrome (FXTAS). Here, we describe a 60-year-old male affected by ataxia due to biallelic mutations in the mitochondrial polymerase gamma (POLG) gene in which hyperintensities of the middle cerebellar peduncles (MCP) were found. The initial suspicion of FXTAS was however ruled out by a normal CGG expansion size in the FMR1 gene. We discuss the features of late-onset POLG-A as a phenocopy of FXTAS.

Extensive research on fragile X mental retardation gene knockout mice and mutant Drosophila models has largely expanded our knowledge on mechanism-based treatment of fragile X syndrome (FXS). In light of these findings, several clinical trials are now underway for therapeutic translation to humans.Electronic literature searches were conducted using the PubMed database and ClinicalTrials.gov. The search terms included "fragile X syndrome", "FXS and medication", "FXS and therapeutics" and "FXS and treatment". Based on the publications identified in this search, we reviewed the neuroanatomical abnormalities in FXS patients and the potential pathogenic mechanisms to monitor the progress of FXS research, from basic studies to clinical trials.The pathological mechanisms of FXS were categorized on the basis of neuroanatomy, synaptic structure, synaptic transmission and fragile X mental retardation protein (FMRP) loss of function. The neuroanatomical abnormalities in FXS were described to motivate extensive research into the region-specific pathologies in the brain responsible for FXS behavioural manifestations. Mechanism-directed molecular medicines were classified according to their target pathological mechanisms, and the most recent progress in clinical trials was discussed.Current mechanism-based studies and clinical trials have greatly contributed to the development of FXS pharmacological therapeutics. Research examining the extent to which these treatments provided a rescue effect or FMRP compensation for the developmental impairments in FXS patients may help to improve the efficacy of treatments.

The aim of the present study was to investigate the association between FMR1 premutation and premature ovarian failure (POF) patients in Indian population, and a meta-analysis of published results was undertaken to clarify whether FMR1 premutation consistently contributed to the susceptibility.A total of 289 POF samples and 360 control samples were included in the study. Repeat variation was checked using GeneScan technique. Results were analyzed with GeneMapper software. Meta-analysis was performed using the Open Meta-Analyst and STATA 12.0 software. The crude odds ratio with 95 % confidence interval (CI) was computed to assess the strength of the associations.The assayed case and control population showed 29 different CGG repeat sizes (alleles), ranging from 7 to 40. Within this population, we found that the CGG repeat length polymorphisms were within the normal range of 6-55 in both patients as well as control samples. Eleven case-control studies were included in the meta-analysis with a total of 1,313 POF cases and 3,132 control subjects. Our meta-analysis revealed that there was a significant difference in the incidence of FMR1 premutation between POF cases and control subjects with p value0.001 (OR 5.41; 95 % CI 2.53, 11.61).We found no significant association between FMR1 CGG repeat premutation and POF in Indian population. However, the meta-analysis showed an increased risk of POF associated with a premutation, especially among populations from European descent. Further functional research should be performed to explain the inconsistent results in different ethnicities and POF susceptibility.

Background Epigenetic modifications of the fragile X mental retardation 1 (FMR1) gene locus may impact the risk for reproductive and neurological disorders associated with expanded trinucleotide repeats and methylation status in the 5′ untranslated region. FMR1 methylation is commonly assessed by Southern blot (SB) analysis, yet this method suffers a cumbersome workflow and relatively poor sizing resolution especially for premutation allele characteristic for fragile X-associated disorders. In this study, a methylation PCR (mPCR) assay was used to evaluate correlations among genotype, epitype, and phenotype in fragile X premutation (PM) carrier women in order to advance the understanding of the association between molecular determinants and the presence of fragile X-associated tremor and ataxia (FXTAS). Results Activation ratios (ARs) in 39 PM women were determined by mPCR and compared with SB analysis. ARs were distributed across a range of values including five samples with 80% AR. The two methods were correlated (R 2 of 0.87 and F test of

The mechanism underlying CCG-repeat expansions in patients with fragile X premutation is not well understood. Using a new experimental system in mammalian cells, a study in this issue reports that break-induced replication has a role in CGG-repeat instability.

The clinical failure of two large mGluR5-antagonist programmes in Fragile X syndrome has forced drug developers to rethink trial design and target selection for neurodevelopmental indications.

Premutation fragile X carriers have a CGG repeat expansion (55 to 200 repeats) in the promoter region of the fragile X mental retardation 1 (FMR1) gene. Amygdala dysfunction has been observed in premutation symptomatology, and recent research has suggested the amygdala as an area susceptible to the molecular effects of the premutation. The current study utilizes structural magnetic resonance imaging (MRI) to examine the relationship between amygdala volume, CGG expansion size, FMR1 mRNA, and psychological symptoms in male premutation carriers without FXTAS compared with age and IQ matched controls. No significant between group differences in amygdala volume were found. However, a significant negative correlation between amygdala volume and CGG was found in the lower range of CGG repeat expansions, but not in the higher range of CGG repeat expansions.

Bei Patienten mit Klinefelter-Syndrom konnen eine begleitende hyperkinetische Storung des Sozialverhaltens eine Medikation mit Methylphenidat oder Risperidon notwendig machen. Seit diesem Jahr steht als alternative Behandlung der Noradrenalinwiederaufnahmehemmer Atomoxetin zur Verfugung. Wir berichten von 2 Patienten mit Klinefelter-Syndrom und hyperkinetischer Storung des Sozialverhaltens, bei denen Atomoxetin nach unzureichenden Behandlungsergebnissen mit Methylphenidat und Risperidon eine gute Wirksamkeit zeigte.

Fragile X syndrome is caused by the loss of an RNA-binding protein called FMRP (for fragile X mental retardation protein). FMRP seems to influence synaptic plasticity through its role in mRNA transport and translational regulation. Recent advances include the identification of mRNA ligands, FMRP-mediated mRNA transport and the neuronal consequence of FMRP deficiency. FMRP was also recently linked to the microRNA pathway. These advances provide mechanistic insight into this disorder, and into learning and memory in general.

Orchestration of gene expression enables coordination between the nucleus and synapses. A report now uncovers a function for the fragile X mental retardation protein in RNA editing necessary for synaptic development.

In humans, absence of Fragile X mental retardation protein (FMRP), an RNA-binding protein, results in Fragile X syndrome (FXS), the most common inherited form of intellectual disability. Here we report through biochemical and electrophysiological studies that FMRP binds the C-terminus of the Slack sodium-activated potassium channel to activate the channel. The findings suggest that Slack activity may provide a link between patterns of neuronal firing and changes in protein translation.

Fragile X syndrome is the second leading cause of mental retardation after Down syndrome. Most women carriers of the fragile X mutation are unaware of their condition. We critically evaluated whether screening pregnant women at low risk for FMR1 mutation would be feasible as a routine part of antenatal care in general practice. We also studied acceptance and attitudes to gene testing. From July 1995 until December 1996, a carrier test was offered at the Kuopio City Health Centre free of charge to all pregnant women in the first trimester following counselling given by midwives on fragile X syndrome. All women found to be carriers of FMR1 gene mutations underwent detailed genetic counselling and were offered prenatal testing. Attitudes towards the gene test were elicited by questionnaire. Most pregnant women (85%) elected to undertake the gene test. Six women were found to be carriers (a rate of 1 in 246), and all subsequently accepted prenatal testing. Three foetuses had a normal FMR1 gene, one had a large premutation, one a 'size mosaic' mutation pattern, and another a full mutation. This observational and interventional study demonstrates that antenatal screening provides an effective way of identifying carriers and incorporating prenatal testing into this process.

We used Drosophila olfactory memory in order to understand in vivo the molecular basis of cognitive defect in Fragile X syndrome. We observed that Fragile X protein (FMRP) was required acutely and interacted with argonaute1 and staufen in long-term memory (LTM). Occlusion of long-term memory formation in Fragile X mutants could be rescued by protein synthesis inhibitors, suggesting that excess baseline protein synthesis could impact negatively on cognition.

In order to look for linkage disequilibrium between the fragile X locus and its flanking markers, we analysed the FRAXAC1 and DXS548 microsatellites in normal and fragile X individuals of Portuguese origin. We observed differences in allele and haplotype frequencies between these two samples. Four haplotypes (A-2, C-2, C-5 and D-6) accounted for 76% of all fragile X chromosomes, whereas a single haplotype (C-7) accounted for 70% of the normal population and less than 3% of the fragile X chromosomes. Among the four observed high-risk haplotypes, A-2 and D-6 had been previously reported in other studies, but C-2 and C-5 seem characteristic of Portuguese patients, as suggested by the high frequency (38%) in fragile X chromosomes and virtual absence in controls. In accordance with previous studies, a greater heterozygosity of the fragile X sample was noted when compared to that of controls. The high frequency of C-7 haplotype in the normal population and its virtual absence in the fragile X sample may reflect the existence of linkage disequilibrium between the two loci and/or selective advantage (protector effect) of this haplotype.

The relationship between the fragile X syndrome (FXS) and autism is reviewed. Shortly after the FXS was first described, it was noted that certain behaviors commonly found in afflicted individuals resemble certain features of autism. Research concerning a possible relationship between these conditions is summarized. The outcome of this research indicates that FXS is not a common cause of autism, although the number of individuals with FXS who meet diagnostic criteria for autism is higher than can be accounted for by chance. The major focus of this paper highlights that FXS is a well-defined neurogenetic disease that includes a cognitive behavioral phenotype, and has both a known biological cause and an increasing well-delineated pathogenesis. Autism is a behaviorally defined syndrome whose syndromic boundaries and biological causes are not known. These profound differences complicate comparisons and causal discussions. However, the behavioral neurogenetic information available about FXS suggests certain pathways for future research directed at elucidating the syndrome of autism.

Background A previous study reported enhanced psychomotor speed, and subtle but significant cognitive impairments, modulated by age and by mutations in the fragile X mental retardation 1 (FMR1) gene in adult female fragile X premutation carriers (fXPCs). Because male carriers, unlike females, do not have a second, unaffected FMR1 allele, male fXPCs should exhibit similar, if not worse, impairments. Understanding male fXPCs is of particular significance because of their increased risk of developing fragile X-associated tremor/ataxia syndrome (FXTAS). Methods Male fXPCs (n = 18) and healthy control (HC) adults (n = 26) aged less than 45 years performed two psychomotor speed tasks (manual and oral) and two visuospatial tasks (magnitude comparison and enumeration). In the magnitude comparison task, participants were asked to compare and judge which of two bars was larger. In the enumeration task, participants were shown between one and eight green bars in the center of the screen, and asked to state the total number displayed. Enumeration typically proceeds in one of two modes: subitizing, a fast and accurate process that works only with a small set of items, and counting, which requires accurate serial-object detection and individuation during visual search. We examined the associations between the performance on all tasks and the age, full-scale intelligent quotient, and CGG repeat length of participants. Results We found that in the magnitude comparison and enumeration tasks, male fXPCs exhibited slower reaction times relative to HCs, even after controlling for simple reaction time. Conclusions Our results indicate that male fXPCs as a group show impairments (slower reaction times) in numerical visuospatial tasks, which are consistent with previous findings. This adds to a growing body of literature characterizing the phenotype in fXPCs who are asymptomatic for FXTAS. Future longitudinal studies are needed to determine how these impairments relate to risk of developing FXTAS.

BackgroundFragile X syndrome is the most common inherited form of mental impairment characterized by cognitive impairment, attention deficit and autistic behaviours. The mouse model of Fragile X is used to study the underlying neurobiology associated with behavioral deficiencies. The effect of Fragile X glial cells on the development of neurons has not been studied. We used a co-culture technique in combination with morphometrics on immunostained neurons to investigate the role of astrocytes in the development delays associated with hippocampal neuron development.ResultsWe found that hippocampal neurons grown on Fragile X astrocytes exhibited a significant difference from the neurons grown with normal astrocytes after 7 days in vitro for many parameters including increases in dendritic branching and in area of the cell body. However, after 21 days in culture, the neurons grown on Fragile X astrocytes exhibited morphological characteristics that did not differ significantly from the neurons grown on normal astrocytes. With antibodies to the pre-synaptic protein, synapsin, and to the excitatory post-synaptic protein, PSD-95, we quantified the number of developing excitatory synapses on the dendrites. In addition to the delays in dendritic patterning, the development of excitatory synapses was also delayed in the hippocampal neurons.ConclusionsThese experiments are the first to establish a role for astrocytes in the delayed growth characteristics and abnormal morphological features in dendrites and synapses that characterize the Fragile X syndrome.

The patient was a 9-year-old boy. He began to walk at the age 1 year and 8 months and began to speak at the age of 2 years, suggesting retarded mental and motor development. A diagnosis of DMD was made when he was 7 years old. On admission, the patient exhibited a peculiar thin and long face, large auricles, narrow palate, malalignment of the teeth, epicanthus, saddle nose, and simian lines in addition to symptoms consistent with DMD. Chromosome analysis showed fragile X at Xq27 at a frequency of 20%. His mother also showed fragile X at the same position. Since the atypical features of this DMD patient are all explained as fragile X syndrome, this case was considered to be a very rare instance of DMD whose clinical pictures were modified by fragile X syndrome.

In fragile X syndrome, the absence of fragile X mental retardation protein (FMRP) allows the production of certain proteins to go unchecked in the brain. A recent study in mice suggests that bringing excessive translation into balance may provide a key to treating this syndrome (pages 1473–1477).

A recent theory about the basis for fragile X syndrome is now validated in a mouse model. The findings point the way to treatment options targeting group 1 metabotropic glutamate receptors.

MIT researchers have shown that the widely used cholesterol drug lovastatin can correct fragile X syndrome in mice. The findings add to a growing list of unconventional strategies for this common form of mental retardation.

Fragile X syndrome, the most common genetic cause of autism, arises from the loss of fragile X mental retardation protein (FMRP). Jung et al. found that FMRP-null mice are deficient in a form of metabotropic glutamate receptor 5 (mGluR5)-dependent long-term depression that occurs in the ventral striatum and prefrontal cortex. Loss of FMRP in these regions disrupted a protein complex that couples the production of the endocannabinoid 2-arachidonoylglycerol (2-AG) to mGluR5 activation. Enhancing 2-AG signalling reversed certain behavioural deficits in FMRP-null mice, indicating that this complex may be a candidate drug target.

This article discusses the research on drugs to be used in the treatment of autism being conducted by Massachusetts-based biotechnology firm Seaside Therapeutics under chief executive officer (CEO) Randall L. Carpenter. Topics include the belief that the fragile X single-gene glitch could account for idiopathic autism, the development of the drug arbaclofen, which targets the chemical messenger glutamate, and the ages that drug therapy would ideally begin for autism patients.

Clinical utility gene card for: fragile X mental retardation syndrome, fragile X-associated tremor/ataxia syndrome and fragile X-associated primary ovarian insufficiency

We would like to comment on the paper "The role of recombination in the evolvement of the fragile X mutation" by Tamar Schaap recently published in this journal (Schaap 1989). In this paper, Dr. Schaap purported to show that recombination around the fragile X (flaX) locus was more common in the daughters of transmitting males than in phenotypically normal women with a maternally derived fraX chromosome. We consider this study to be flawed in its design and execution, and - in

Ten years after the identification of the gene responsible for fragile X syndrome, recent studies have revealed a list of mRNAs bound by the fragile X gene product and have identified specific sequences required for the interaction between the fragile X protein and its targets. These results are a breakthrough in understanding why absence of the fragile X protein leads to mental retardation.

X Fra is the second cause of mental retardation (M.R.) after Down's Syndrome, representing 50% of the X-linked cases. The X-Fra phenotype has been described and common features are being defined in multicentric programs. We studied 46 children presenting 4 or more of the signs in a score: 1)Family history suggestive of X-linked pathology; 2) Height and cephalic circunference on P50 or more; 3)Large ears; 4)Prominent ears; 5)Long face; 6)High palate; 7)Calluses on the dorsal aspect of hands; 8)Joint hiperlaxity; 9) Depressed sternum; 10)Mitral prolapse; 11)Macroorchidism; 12)Poor visual contact; 13)Hiperactivity; 14)Stereotypes behavior; 15)Self-agression; 16)Disorders language development; 17)Variable degrees of M.R. We assign one point for each item. Twelve children, aged 3 to 14 years resulted X-Fra+ in lymphocyte cultures with added Fudr (50 cells analyzed). Negative X-patients showed less than seven signs of the score, whereas twelve or more signs were found in X-Fra+ children. Each patient was evaluated by geneticist, a neurologist and a neuropsychologist. The future aims of this program are: A) To identify X-Fra+ cases in a population of M.R. children without known etiology. B) To select through the score those patients to be evaluated cytogenetically. C) To provide genetic counselling to mothers and female relatives at risk.

The clinical and cytogenetic features of 15 families with mental retardation linked to the fragile site on the X chromosome are presented. The 15 propositi were all prepubertal, and one was a girl. Although the clinical picture varied in severity, it was sufficiently constant to suggest the diagnosis from the facial features and the encephalopathy with language retardation and disturbed behavior. Macroorchidism was not seen before puberty. The fragile X chromosome was found in seven of the nine mothers studied and in two mildly retarded sisters and has also been demonstrated in fibroblasts in eleven subjects with the abnormality.

The coagulation factor IX gene and two other polymorphic loci corresponding to DNA probes 52 A and St 14 have been previously localized in the q27 to qter region of the human X chromosome. In order to study their localization with respect to the fragile site at Xq27-28, we have hybridized the three DNA probes to metaphase chromosomes of a boy with fragile X mental retardation. We show that probe 52 A is located in the proximal part of the Xq27 band, while the coagulation factor IX gene is on the distal part of this band, but proximal to the fragile site. The very polymorphic St 14 probe is located in the distal part of the Xq28 band, on the other side of the fragile site.

Linkage data, using the polymorphic markers 52A (DXS51), F9, 4D-8(DXS98), and St14(DXS52), are presented from 14 fragile X pedigrees and from 7 normal pedigrees derived from the collection of the Centre d'Etude du Polymorphisme Humaine. A multipoint linkage analysis indicates that the most probable order of these four loci in normal families is DXS51-F9-DXS98-DXS52. Recombination frequencies (\(\hat \theta \)) corresponding to maximum LOD scores (\(\hat Z\)) were obtained by two-point linkage analysis for a nuber of linkage groups, including: DXS51-F9 (\(\hat Z\)=5.94, \(\hat \theta \)=0.03), F9-DXS98 (\(\hat Z\)=0.51, \(\hat \theta \)=0.26), F9-DXS52 (\(\hat Z\)=0.84, \(\hat \theta \)=0.27), and DXS98-DXS52 (\(\hat Z\)=0.32, \(\hat \theta \)=0.20). A multipoint linkage analysis of these loci, including the fragile X locus, was also performed for the fragile X population and the data support the relative order (DSX51, F9, DXS98)-FRAXA-DXS52. Recombination frequencies and maximum LOD scores, which again were derived from two-point linkage analyses, were obtained for the linkage groups DXS51-F9 (\(\hat Z\)=9.96, \(\hat \theta \)=0) and F9-DXS52 (\(\hat Z\)=0.07, \(\hat \theta \)=0.45), as well as for the groups DXS51-FRAXA (\(\hat Z\)=2.42, \(\hat \theta \)=0.15), F9-FRAXA (\(\hat Z\)=1.30, \(\hat \theta \)=0.18), DXS98-FRAXA (\(\hat Z\)=0.05 \(\hat \theta \)=0.36), and DXS52-FRAXA (\(\hat Z\)=2.42 \(\hat \theta \)=0.15). The linkage data was further tested for the presence of genetic heterogeneity both within and between the fragile X and normal families for the intervals DXS51-F9, F9-DXS52, F9-FRAXA, and DXS52-FRAXA using a modification of the A test. Except for the interval F9-FRAXA (P

We report an extended family in which two brothers with a fragile X chromosome are mentally retarded while a third brother with the fragile site is both phenotypically and mentally normal. The study of six probes detecting restriction fragment length polymorphisms on either sides of the fragile site Xq27 confirmed that the fragile X regions inherited by these three brothers were identical from DXS 102 to the telomere. These data highlight the heterogeneity of the fragile X syndrome, which is discussed in the framework of the different hypotheses previously proposed.