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Anoctamin-5 related muscle disease is caused by biallelic pathogenic variants in the anoctamin-5 gene (ANO5) and shows variable clinical phenotypes: limb-girdle muscular dystrophy type 12 (LGMD-R12), distal muscular dystrophy type 3 (MMD3), pseudometabolic myopathy or asymptomatic hyperCKaemia. In this retrospective, observational, multicentre study we gathered a large European cohort of patients with ANO5-related muscle disease to study the clinical and genetic spectrum and genotype–phenotype correlations. We included 234 patients from 212 different families, contributed by 15 centres from 11 European countries. The largest subgroup was LGMD-R12 (52.6%), followed by pseudometabolic myopathy (20.5%), asymptomatic hyperCKaemia (13.7%) and MMD3 (13.2%). In all subgroups, there was a male predominance, except for pseudometabolic myopathy. Median age at symptom onset of all patients was 33 years (range 23–45 years). The most frequent symptoms at onset were myalgia (35.3%) and exercise intolerance (34.1%), while at last clinical evaluation most frequent symptoms and signs were proximal lower limb weakness (56.9%) and atrophy (38.1%), myalgia (45.1%) and atrophy of the medial gastrocnemius muscle (38.4%). Most patients remained ambulatory (79.4%). At last evaluation, 45.9% of patients with LGMD-R12 additionally had distal weakness in the lower limbs and 48.4% of patients with MMD3 also showed proximal lower limb weakness. Age at symptom onset did not differ significantly between males and females. However, males had a higher risk of using walking aids earlier (P = 0.035). No significant association was identified between sportive versus non-sportive lifestyle before symptom onset and age at symptom onset nor any of the motor outcomes. Cardiac and respiratory involvement that would require treatment occurred very rarely. Ninety-nine different pathogenic variants were identified in ANO5 of which 25 were novel. The most frequent variants were c.191dupA (p.Asn64Lysfs*15) (57.7%) and c.2272C>T (p.Arg758Cys) (11.1%). Patients with two loss-of function variants used walking aids at a significantly earlier age (P = 0.037). Patients homozygous for the c.2272C>T variant showed a later use of walking aids compared to patients with other variants (P = 0.043). We conclude that there was no correlation of the clinical phenotype with the specific genetic variants, and that LGMD-R12 and MMD3 predominantly affect males who have a significantly worse motor outcome. Our study provides useful information for clinical follow up of the patients and for the design of clinical trials with novel therapeutic agents.

JAG2 encodes the Notch ligand Jagged2. The conserved Notch signaling pathway contributes to the development and homeostasis of multiple tissues, including skeletal muscle. We studied an international cohort of 23 individuals with genetically unsolved muscular dystrophy from 13 unrelated families. Whole-exome sequencing identified rare homozygous or compound heterozygous JAG2 variants in all 13 families. The identified bi-allelic variants include 10 missense variants that disrupt highly conserved amino acids, a nonsense variant, two frameshift variants, an in-frame deletion, and a microdeletion encompassing JAG2. Onset of muscle weakness occurred from infancy to young adulthood. Serum creatine kinase (CK) levels were normal or mildly elevated. Muscle histology was primarily dystrophic. MRI of the lower extremities revealed a distinct, slightly asymmetric pattern of muscle involvement with cores of preserved and affected muscles in quadriceps and tibialis anterior, in some cases resembling patterns seen in POGLUT1-associated muscular dystrophy. Transcriptome analysis of muscle tissue from two participants suggested misregulation of genes involved in myogenesis, including PAX7. In complementary studies, Jag2 downregulation in murine myoblasts led to downregulation of multiple components of the Notch pathway, including Megf10. Investigations in Drosophila suggested an interaction between Serrate and Drpr, the fly orthologs of JAG1/JAG2 and MEGF10, respectively. In silico analysis predicted that many Jagged2 missense variants are associated with structural changes and protein misfolding. In summary, we describe a muscular dystrophy associated with pathogenic variants in JAG2 and evidence suggests a disease mechanism related to Notch pathway dysfunction.

Neurotoksyna botulinowa typu A (BoNT-A, botulinum neurotoxin type A ) jest od 30 lat stosowana w leczeniu dystonii ogniskowych i segmentalnych. W tym czasie ewoluowaly poglądy na temat leczenia oraz przybylo wiele nowych publikacji. Szczegolna zmiana dotyczy dystonii szyjnej. Wprowadzenie dekade temu koncepcji collum-caput (Col-Cap) spowodowalo zmiane paradygmatu leczenia tej postaci dystonii ogniskowej, a takze rozwiniecie metod podawania leku pod kontrolą ultrasonografii. Obecnie w Polsce są dostepne trzy preparaty BoNT-A. Nazwy uznane przez amerykanską Agencje ds. Żywności i Lekow oraz nazwy handlowe preparatow to odpowiednio: onabotulinumtoxinA : Botox ® , abobotulinumtoxinA : Dysport ® , incobotulinum toxinA : Xeomin ® . Są to leki o zroznicowanych charakterystykach produktu leczniczego i zakresie wskazan klinicznych oraz dawkowania. Leczenie dystonii w Polsce odbywa sie w ramach programu lekowego i jest w pelni refundowane. Wiedza na temat rozpoznawania dystonii oraz mozliwości jej leczenia poglebila sie w ostatnim czasie, ale chorzy nadal dośc dlugo czekają na ustalenie diagnozy i wlączenie do leczenia. Celem niniejszych rekomendacji jest przegląd aktualnej wiedzy na temat leczenia dystonii ogniskowych oraz polowiczego kurczu twarzy za pomocą iniekcji BoNT-A.

OBJECTIVES We aimed to screen for late-onset Pompe disease using the dried blood spot (DBS) test in a cohort of patients with limb-girdle muscle weakness or persistent hyperCKemia. MATERIALS AND METHODS Patients with limb-girdle muscle weakness, persistently elevated CK, rigid spine syndrome, dyspnoea, myalgia or sibling of the patient diagnosed with LOPD were included in the study. Acid α-glucosidase (GAA) activity was measured on DBS by tandem mass spectrometry and followed by genetic testing when required. Study was conducted between June 2014 and May 2017. RESULTS A total of 337 patients aged 32.2 years (range 2-80) were included in the study. Late-onset Pompe disease was diagnosed in 10 patients (3.0% of tested cohort). All were compound heterozygotes with common c.32-13T>G mutation on one allele and missense or frameshift mutation on the other. Two of the mutations (c.1951delG and c.397T>G) were not reported previously. Seven of the patients started enzyme replacement therapy. CONCLUSIONS DBS test is a reliable method for screening for late-onset Pompe disease.

A review of literature on the dissection of internal carotid artery was presented with a presentation of a rare case of patient with transient left hypoglossal nerve palsy caused by mechanic compression from intramural hematoma in higher extracranial portion of dissected carotid artery confirmed in MRI and CT scans. The clinical presentation and management are discussed.

BACKGROUND: Late-onset Pompe disease is a rare genetic neuromuscular disorder caused by a primary deficiency of α-glucosidase and the associated accumulation of glycogen in lysosomal vacuoles. The deficiency of α-glucosidase can often be detected using an inexpensive and readily accessible dried blood spot test when Pompe disease is suspected. Like several neuromuscular disorders, Pompe disease typically presents with progressive weakness of limb-girdle muscles and respiratory insufficiency. Due to the phenotypic heterogeneity of these disorders, however, it is often difficult for clinicians to reach a diagnosis for patients with Pompe disease. Six hundred and six patients from a European population were recruited onto our study. Inclusion criteria stipulated that index cases must present with limb-girdle weakness or elevated serum creatine kinase activity. Whole exome sequencing with at least 250 ng DNA was completed using an Illumina exome capture and a 38 Mb baited target. A panel of 169 candidate genes for limb-girdle weakness was analysed for disease-causing variants. RESULTS: A total of 35 variants within GAA were detected. Ten distinct variants in eight unrelated index cases (and four siblings not sequenced in our study) were considered disease-causing, with the patients presenting with heterogeneous phenotypes. The eight unrelated individuals were compound heterozygotes for two variants. Six patients carried the intronic splice site c.-13 T > G transversion and two of the six patients also carried the exonic p.Glu176ArgfsTer45 frameshift. Four of the ten variants were novel in their association with Pompe disease. CONCLUSIONS: Here, we highlight the advantage of using whole exome sequencing as a tool for detecting, diagnosing and treating patients with rare, clinically variable genetic disorders. ispartof: Orphanet Journal of Rare Diseases vol:12 issue:1 pages:173-184 ispartof: location:England status: published

Introduction TRIM32-related myopathies represent a phenotypic spectrum of a rare autosomal recessive muscle disorder. The disease is described as a mild and progressive myopathy without characteristic clinical features. Originally classified as limb-girdle muscular dystrophy (LGMD) 2H (OMIM #254110), the disorder was first identified in the Hutterite population and the homozygous TRIM32 founder mutation, p.Asp487Asn, was identified as the cause of this disease.1 Only seven patients with definite non-Hutterite TRIM32-related myopathy have been reported in the literature. Apart from two missense mutations residing in the NHL repeats of TRIM32, only deletions, frameshift and nonsense mutations have been reported.2 Having applied next generation sequencing technologies to over 1000 patients with suspected genetic muscle disorders, we present nine patients with TRIM32-related myopathies and three patients with a homozygous TRIM32 variant of unknown significance (VUS). Subjects and methods DNA samples from 1000 patients with unexplained limb-girdle muscle weakness and/or elevated serum creatine kinase (CK) levels were gathered through the MYO-SEQ project. Samples were processed and whole exome sequencing (WES) performed as described previously.3 Additional patients with TRIM32-related myopathy were diagnosed by the Northern Molecular Genetics Service (NMGS) diagnostic laboratory through panel sequencing of 32 LGMD genes. Variants were classified according to ACMG guidelines.4 MRI imaging was performed for eight patients on a 1.5T MRI platform. Muscle biopsies for all patients were analysed following standard histological techniques. Results Genetic findings Of the 1000 MYO-SEQ patients, we identified 36 with rare coding variants in TRIM32 (minor allele frequency

Diseases related to DNA polymerase gamma dysfunction comprise of heterogeneous clinical presentations with variable severity and age of onset. Molecular screening for the common POLG variants: p.Ala467Thr, p.Trp748Ser, p.Gly848Ser, and p.Tre251Ile has been conducted in a large population cohort (n = 3123) and in a clinically heterogeneous group of 1289 patients. Recessive pathogenic variants, including six novel ones were revealed in 22/26 patients. Infantile Alpers-Huttenlocher syndrome and adulthood ataxia spectrum were the most common found in our group. Distinct molecular profile identified in the Polish patients with significant predominance of p.Trp748Ser variant (50% of mutant alleles) reflected strikingly low population frequency of the three remaining variants and slightly higher p.Trp748Ser allele frequency in the general Polish population as compared to the non-Finish European population.

LGMD2L is a subtype of limb-girdle muscular dystrophy (LGMD), caused by recessive mutations in ANO5, encoding anoctamin-5 (ANO5). We present the analysis of five patients with skeletal muscle weakness for whom heterozygous mutations within ANO5 were identified by whole exome sequencing (WES). Patients varied in the age of the disease onset (from 22 to 38 years) and severity of the morphological and clinical phenotypes. Out of the nine detected mutations one was novel (missense p.Lys132Met, accompanied by p.His841Asp) and one was not yet characterized in the literature (nonsense, p.Trp401Ter, accompanied by p.Asp81Gly). The p.Asp81Gly mutation was also identified in another patient carrying a p.Arg758Cys mutation as well. Also, a c.191dupA frameshift (p.Asn64LysfsTer15), the first described and common mutation was identified. Mutations were predicted by in silico tools to have damaging effects and are likely pathogenic according to criteria of the American College of Medical Genetics and Genomics (ACMG). Indeed, molecular modeling of mutations revealed substantial changes in ANO5 conformation that could affect the protein structure and function. In addition, variants in other genes associated with muscle pathology were identified, possibly affecting the disease progress. The presented data indicate that the identified ANO5 mutations contribute to the observed muscle pathology and broaden the genetic spectrum of LGMD myopathies.

BACKGROUND Patients with myotonic muscular dystrophy (dystrophia myotonica, DM) are at risk of sudden cardiac death due to diverse arrhythmias, especially progressive atrioventricular (AV) conduction abnormalities. However, there are limited data on supraventricular and potentially life-threatening ventricular arrhythmias, especially according to type 1 and type 2 DM. METHODS A group of 94 unselected consecutive patients with genetically confirmed DM and 45 healthy controls underwent electrocardiography, echocardiography, and 24-hour Holter monitoring. DM1 was diagnosed in 51, while DM2 in 43 patients (with mean age of 37.3 ± 12.5 and 48.3 ± 13.3 years, respectively). RESULTS DM1 subjects presented more frequently intraventricular conduction defects (29.4% vs 6.6%, P = 0.0003) and first-degree AV block (25.0% vs 4.6%, P = 0.008) than DM2 patients. Nonsustained supraventricular tachycardia (37.2% vs 3.8%, P = 0.001) and nonsustained ventricular tachycardia and/or R-on-T ventricular beats (23.2% vs 7.8%, P = 0.04) were more frequently observed in DM2 than in DM1. No relationship between disease duration and neurological status and occurrence of arrhythmias was observed. Multivariate analysis showed that independent predictor for bradyarrhythmias occurrence was DM1 only (odds ratio [OR] 6.4, 95% confidence interval [CI] 2.0-20.8, P = 0.002), while for supraventricular or ventricular arrhythmias occurrence it was DM2 (OR 4.1, 95% CI 1.5-11.4, P = 0.007) and increased age (OR 1.09, 95% CI 1.05-1.15, P < 0.0001). CONCLUSIONS In the relatively large groups of DM1 and DM2 patients, we observed frequent various arrhythmias, which warrant their close cardiac monitoring. DM1 subjects when compared to DM2 presented more frequently intraventricular and AV conduction defects. However, all types of tachyarrhythmias (except atrial fibrillation) were more frequently observed in DM2 patients.

Congenital myopathies and muscular dystrophies constitute a genetically and phenotypically heterogeneous group of rare inherited diseases characterized by muscle weakness and atrophy, motor delay and respiratory insufficiency. To date, curative care is not available for these diseases, which may severely affect both life-span and quality of life. We discuss prenatal diagnosis and genetic counseling for families at risk, as well as diagnostic possibilities in sporadic cases.

Background: Dystroglycanopathies are a clinically and genetically heterogeneous group of disorders that are typically characterised by limb-girdle muscle weakness. Mutations in 18 different genes have been associated with dystroglycanopathies, the encoded proteins of which typically modulate the binding of α-dystroglycan to extracellular matrix ligands by altering its glycosylation. This results in a disruption of the structural integrity of the myocyte, ultimately leading to muscle degeneration. Methods: Deep phenotypic information was gathered using the PhenoTips online software for 1001 patients with unexplained limb-girdle muscle weakness from 43 different centres across 21 European and Middle Eastern countries. Whole-exome sequencing with at least 250ng DNA was completed using an Illumina exome capture and a 38Mb baited target. Genes known to be associated with dystroglycanopathies were analysed for disease-causing variants. Results: Suspected pathogenic variants were detected in DPM3, ISPD, POMT1 and FKTN in one patient each, in POMK in two patients, in GMPPB in three patients, in FKRP in eight patients and in POMT2 in ten patients. This indicated a frequency of 2.7% for the disease group within the cohort of 1001 patients with unexplained limb-girdle muscle weakness. The phenotypes of the 27 patients were highly variable, yet with a fundamental presentation of proximal muscle weakness and elevated serum creatine kinase. Conclusions: Overall, we have identified 27 patients with suspected pathogenic variants in dystroglycanopathy-associated genes. We present evidence for the genetic and phenotypic diversity of the dystroglycanopathies as a disease group, while also highlighting the advantage of incorporating next-generation sequencing into the diagnostic pathway of rare diseases., SCOPUS: ar.j, info:eu-repo/semantics/published

Background In myotonia congenita provocative tests may induce transient or persistent changes in compound motor action potential (CMAP) parameters. These changes are variable and correspond to partial inexcitability of the myotonic muscle membrane. Specific patterns of response in electrophysiological exercise tests correlate with the underlying genotype, thus providing a guide to genetic analysis. The aim of this study was to estimate the utility of the short exercise test (SET) and short exercise test with cooling (SETC) in diagnosis of Becker disease (BMC). Material and Methods SET and SETC were performed in 30 patients with genetically confirmed BMC (19 men; 63.33%) with the mean age 29.4 ± 15.4 years; (range: 4–61). The mean age of disease onset was 8.1 ± 4.2 years (range: 0.5–19) and the mean disease duration 21.3 ± 16.8 years (range: 0–56.5). Results In the whole group we observed a significant decrease in CMAP amplitude immediately after effort in both tests. We noticed statistically significant sex differences in response to effort and cold. In men marked decline in CMAP amplitude was seen only in SETC whereas in SET no such changes were observed. In women evident CMAP decrement was seen in both tests. Conclusions In male patients with clinical suspicion of myotonia congenita atypical response in SET together with typical result in SETC does not exclude the diagnosis of BMC. Function of chloride channel in BMC might be modulated by sex specific factors, what needs further investigation in a larger group of patients.

Background Myotonia congenita (MC) is the most common nondystrophic myotonia caused by loss of function mutations in the CLCN-1 chloride channel. Aims: The purpose of this study was to describe detailed EMG characteristics of patients with MC as well as to test the hypothesis that needle EMG could help to distinguish between the recessive and dominant form of MC. Material and methods We evaluated 36 patients (23 men) aged 4–61 years with genetically proven MC. In 30 patients (19 men) autosomal recessive form of MC (BMC) and in 6 – autosomal dominant MC (TMC) was recognized. We analyzed EMG results for motor unit action potential (MUAP) morphology as well as the occurrence and distribution of myotonic discharges (MD). Results were recorded in 95.8% of examined muscles, in almost 100% muscles of patients with BMC and in about 80% muscles in TMC subgroup. The were described as common (in BMC) or widespread in TMC especially in distal limb muscles (first interosseous dorsalis and tibialis anterior, TA) where differences between groups reached statistical significance. For the whole MC group we observed a statistically significant positive correlation between MUAPs parameters in vastus lateralis and TA muscles and the duration of the disease. Similar correlation for biceps brachii muscle, was found in BMC. Conclusions This study supported the hypothesis that needle EMG could still be helpful in diagnosis of MC and together with short exercise and short exercise with cooling tests might be useful in differentiation between BMC and TMC.

The aim of this study was to evaluate cardiac autonomic nervous system function using Holter-derived and standard electrocardiographic parameters in patients with myotonic dystrophy (dystrophia myotonica, DM) and no clinically overt heart involvement. Eighty-four DM patients without conditions potentially influencing cardiac autonomic function were enrolled in the study: 44 with DM type 1 and 40 with DM type 2 (mean age 34.9 ± 11.5 and 47.8 ± 13.5 years, respectively). Two corresponding control groups of aged-matched healthy subjects were selected for DM1 (n = 35) and for DM2 (n = 30). Standard electrocardiography for QT interval dispersion and 24-h Holter monitoring with time-domain heart rate variability and heart rate turbulence were performed. No significant differences in time-domain heart rate variability parameters between DM1 or DM2 subjects and controls were observed. However, heart rate turbulence parameters were significantly impaired in DM1 patients as compared to their controls: turbulence onset (p = 0.025), and turbulence slope (p = 0.018). Moreover, turbulence slope was also impaired in DM2 patients (p = 0.042). As compared to controls, we observed an increased QT dispersion, both in DM1 (p = 0.003) and also in DM2 patients (p

Mutations in the superoxide dismutase-1 (SOD1) gene have been found in 12-23% of patients with a diagnosis of ALS. Here we describe a large ALS Polish family with a branch in France, carrying a G41S mutation in the SOD1, and characterized by an early onset of the disease and extremely short survival time. The mutation has been initially detected in Italian ALS families with common founder effect. However, in the Polish population the G41S mutation most probably originated from an independent mutation event, as indicated by haplotype analysis. Collected data support the hypothesis that a SOD1 mutation is not the sole factor determining the clinical ALS phenotype.

Myotonic dystrophy (DM), the most common dystrophy in adults, is an autosomal dominant disease characterized by a variety of multisystemic features. Two genetically distinct forms of DM are identified - type 1 (DM1), the classic form first described by Steinert, and type 2 (DM2), identified by Ricker. DM1 is caused by trinucleotide expansion of CTG in the myotonic dystrophy protein kinase gene, whereas in DM2 the expansion of tetranucleotide repeats (CCTG) in the zinc finger protein 9 gene was identified. Both mutations are dynamic and are located in non-coding parts of the genes. Phenotype variability of DM1 and DM2 is caused by a molecular mechanism due to mutated RNA toxicity. This paper reviews the clinical features of both types of myotonic dystrophies and summarizes current views on pathogenesis of myotonic dystrophy.

Introduction Multisystem manifestations of myotonic dystrophies type 1 (DM1) and 2 (DM2) are well known. Peripheral nerve involvement has been reported in DM1 but not in genetically confirmed DM2. The aim of our study was to assess peripheral nerve involvement in DM2 using nerve conduction studies and to compare these results with findings in DM1. Methods We prospectively studied patients with genetically confirmed DM2 ( n = 30) and DM1 ( n = 32). All patients underwent detailed neurological examination and nerve conduction studies. Results Abnormalities in electrophysiological studies were found in 26.67% of patients with DM2 and in 28.13% of patients with DM1 but the criteria of polyneuropathy were fulfilled in only 13.33% of patients with DM2 and 12.5% of patients with DM1. The polyneuropathy was subclinical, and no correlation was found between its presence and patient age or disease duration. Conclusions Peripheral nerves are quite frequently involved in DM2, but abnormalities meeting the criteria of polyneuropathy are rarely found. The incidence of peripheral nerve involvement is similar in both types of myotonic dystrophy.

Duchenne/Becker muscular dystrophy (DMD/BMD) is a recessive, X-linked disorder caused by a mutation in the dystrophin gene. Deletions account for approximately 60–65% of mutations, duplications for 5–10%. The remaining cases are mainly point mutations. According to Monaco theory clinical form of the disease depends on maintaining or disrupting the reading frame. The purpose of the study was to determine frequency and location of deletions and duplications in the dystrophin gene, to determine the compliance between maintaining/disrupting the reading frame and clinical form of the disease and to check the effectiveness of MLPA (multiplex ligation-dependent probe amplification) in the detection of these mutations in hemizygous patients and heterozygous female carriers. The material is composed of combined results of molecular diagnosis carried out in years 2009–2012 in 180 unrelated patients referred with the diagnosis of DMD/BMD tested by use of MLPA. We identified 110 deletions, 22 duplication (in one patient two different duplications were detected) and 2 point mutations. Deletions involved mainly exons 45–54 and 3–21, whereas most duplications involved exons 3–18. The compliance with Monaco theory was 95% for deletions and 76% for duplications. Most of mutations in the dystrophin gene were localized in the hot spots – different for deletions and duplications. MLPA enabled their quick identification, exact localization and determination whether or not they maintained or disrupted the reading frame. MLPA was also effective in detection of deletions and duplications in female carriers.

Rochester, MN, USA over 5 years (2009-2013). EMG was performed under conscious sedation in most of the children. Nerve conduction studies were performed in at least 2 nerves (one motor and sensory) and EMG performed in at least 2 muscles in ipsilateral upper and lower limb. 224 patients with the following referral diagnoses (myopathy, muscle weakness, neuromuscular disorders, myositis, myalgia, myoglobinuria, myasthenia, myotonia, cramps, periodic paralysis, hypotonia, and developmental delay) were reviewed. Myotonic discharges were defined as train of spontaneous myofiber action potentials, with waxing and waning amplitudes and frequencies. Myopathic EMG was defined as short duration, low amplitude motor unit potentials with rapid recruitment. Results: Eight children were identified with myotonic discharges during the study period (less than 0.5%). The age range was 3 to 16 years; 6 were males. Two children had isolated myotonic discharges without myopathic changes on EMG while the rest had myopathic EMG in addition to the myotonic discharges. The final diagnoses were: myotonia congenita in 2 (autosomal dominant inheritance in 1 and lack of definite family history in another), congenital myopathy in 3 (centronuclear myopathy-1, congenital fiber type disproportion-1 and congenital myopathy, indeterminate type -1), muscular dystrophy in 2 (facioscapulohumeral muscular dystrophy -1 and muscular dystrophy of indeterminate type-1), and inflammatory myopathy in 1. Two patients with myotonia congenita had clinical myotonia while the rest had only electrical myotonia. Myotonic discharges were diffuse in patients with myotonia congenita while patchy in other patients. Conclusions: Electrical myotonia is a rare electrophysiological finding encountered in our study similar to that reported in the published literature. Though rare, identification of myotonic discharges help to narrow the differential in children with suspected muscle disorders. Diffuse myotonic discharges without myopathic EMG are characteristic of non-dystrophic myotonic disorders.

Replication of the mitochondrial genome depends on the single DNA polymerase (pol gamma). Mutations in the POLG gene, encoding the catalytic subunit of the human polymerase gamma, have been linked to a wide variety of mitochondrial disorders that show remarkable heterogeneity, with more than 200 sequence variants, often very rare, found in patients. The pathogenicity and dominance status of many such mutations remain, however, unclear. Remarkable structural and functional conservation of human POLG and its S. cerevisiae ortholog (Mip1p) led to the development of many successful yeast models, enabling to study the phenotype of putative pathogenic mutations. In a group of patients with suspicion of mitochondrial pathology, we identified five novel POLG sequence variants, four of which (p.Arg869Ter, p.Gln968Glu, p.Thr1053Argfs*6, and p.Val1106Ala), together with one previously known but uncharacterised variant (p.Arg309Cys), were amenable to modelling in yeast. Familial analysis indicated causal relationship of these variants with disease, consistent with autosomal recessive inheritance. To investigate the effect of these sequence changes on mtDNA replication, we obtained the corresponding yeast mip1 alleles (Arg265Cys, Arg672Ter, Arg770Glu, Thr809Ter, and Val863Ala, respectively) and tested their effect on mitochondrial genome stability and replication fidelity. For three of them (Arg265Cys, Arg672Ter, and Thr809Ter), we observed a strong, partially dominant phenotype of a complete loss of functional mtDNA, whereas the remaining two led to partial mtDNA depletion and significant increase in point mutation frequencies. These results show good correlation with the severity of symptoms observed in patients and allow to establish these variants as pathogenic mutations. Electronic supplementary material The online version of this article (doi:10.1007/s00439-015-1578-x) contains supplementary material, which is available to authorized users.