A Haworth

C9orf72 expansions are the most common genetic cause of Huntington disease phenocopies

Objective: In many cases where Huntington disease (HD) is suspected, the genetic test for HD is negative: these are known as HD phenocopies. A repeat expansion in the C9orf72 gene has recently been identified as a major cause of familial and sporadic frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Our objective was to determine whether this mutation causes HD phenocopies. Methods: A cohort of 514 HD phenocopy patients were analyzed for the C9orf72 expansion using repeat primed PCR. In cases where the expansion was found, Southern hybridization was performed to determine expansion size. Clinical case notes were reviewed to determine the phenotype of expansion-positive cases. Results: Ten subjects (1.95%) had the expansion, making it the most common identified genetic cause of HD phenocopy presentations. The size of expansion was not significantly different from that associated with other clinical presentations of C9orf72 expanded cases. The C9orf72 expansion-positive subjects were characterized by the presence of movement disorders, including dystonia, chorea, myoclonus, tremor, and rigidity. Furthermore, the age at onset in this cohort was lower than previously reported for subjects with the C9orf72 expansion and included one case with pediatric onset. Discussion: This study extends the known phenotype of the C9orf72 expansion in both age at onset and movement disorder symptoms. We propose a revised clinico-genetic algorithm for the investigation of HD phenocopy patients based on these data.

The inherited ataxias: Genetic heterogeneity, mutation databases, and future directions in research and clinical diagnostics

The inherited cerebellar ataxias are a diverse group of clinically and genetically heterogeneous neurodegenerative disorders. Inheritance patterns of these disorders can be complex with autosomal dominant, autosomal recessive, X‐linked, and mitochondrial inheritance demonstrated by one or more ataxic syndromes. The broad range of mutation types found in inherited ataxia contributes to the complex genetic etiology of these disorders. The majority of inherited ataxias are caused by repeat expansions; however, conventional mutations are important causes of the rarer dominant and recessive ataxias. Advances in sequencing technology have allowed for much broader testing of these rare ataxia genes. This is relevant to the aims of the Human Variome Project, which aims to collate and store gene variation data through mutation databases. Variant data is currently located in a range of public and commercial resources. Few locus‐specific databases have been created to catalogue variation in the dominant ataxia genes although there are several databases for some recessive genes. Developing these resources will facilitate a better understanding of the complex genotype–phenotype relationships in these disorders and assist interpretation of gene variants as testing for rarer ataxia genes becomes commonplace. Hum Mutat 33:1324–1332, 2012.

Prevalence study of genetically defined skeletal muscle channelopathies in England.

Objectives: To obtain minimum point prevalence rates for the skeletal muscle channelopathies and to evaluate the frequency distribution of mutations associated with these disorders. Methods: Analysis of demographic, clinical, electrophysiologic, and genetic data of all patients assessed at our national specialist channelopathy service. Only patients living in the United Kingdom with a genetically defined diagnosis of nondystrophic myotonia or periodic paralysis were eligible for the study. Prevalence rates were estimated for England, December 2011. Results: A total of 665 patients fulfilled the inclusion criteria, of which 593 were living in England, giving a minimum point prevalence of 1.12/100,000 (95% confidence interval [CI] 1.03–1.21). Disease-specific prevalence figures were as follows: myotonia congenita 0.52/100,000 (95% CI 0.46–0.59), paramyotonia congenita 0.17/100,000 (95% CI 0.13–0.20), sodium channel myotonias 0.06/100,000 (95% CI 0.04–0.08), hyperkalemic periodic paralysis 0.17/100,000 (95% CI 0.13–0.20), hypokalemic periodic paralysis 0.13/100,000 (95% CI 0.10–0.17), and Andersen-Tawil syndrome (ATS) 0.08/100,000 (95% CI 0.05–0.10). In the whole sample (665 patients), 15 out of 104 different CLCN1 mutations accounted for 60% of all patients with myotonia congenita, 11 out of 22 SCN4A mutations for 86% of paramyotonia congenita/sodium channel myotonia pedigrees, and 3 out of 17 KCNJ2 mutations for 42% of ATS pedigrees. Conclusion: We describe for the first time the overall prevalence of genetically defined skeletal muscle channelopathies in England. Despite the large variety of mutations observed in patients with nondystrophic myotonia and ATS, a limited number accounted for a large proportion of cases.

A new explanation for recessive myotonia congenita Exon deletions and duplications in CLCN1

Objective: To assess whether exon deletions or duplications in CLCN1 are associated with recessive myotonia congenita (MC). Methods: We performed detailed clinical and electrophysiologic characterization in 60 patients with phenotypes consistent with MC. DNA sequencing of CLCN1 followed by multiplex ligation-dependent probe amplification to screen for exon copy number variation was undertaken in all patients. Results: Exon deletions or duplications in CLCN1 were identified in 6% of patients with MC. Half had heterozygous exonic rearrangements. The other 2 patients (50%), with severe disabling infantile onset myotonia, were identified with both a homozygous mutation, Pro744Thr, which functional electrophysiology studies suggested was nonpathogenic, and a triplication/homozygous duplication involving exons 8–14, suggesting an explanation for the severe phenotype. Conclusions: These data indicate that copy number variation in CLCN1 may be an important cause of recessive MC. Our observations suggest that it is important to check for exon deletions and duplications as part of the genetic analysis of patients with recessive MC, especially in patients in whom sequencing identifies no mutations or only a single recessive mutation. These results also indicate that additional, as yet unidentified, genetic mechanisms account for cases not currently explained by either CLCN1 point mutations or exonic deletions or duplications.

Call for participation in the neurogenetics consortium within the Human Variome Project

The rate of DNA variation discovery has accelerated the need to collate, store and interpret the data in a standardised coherent way and is becoming a critical step in maximising the impact of discovery on the understanding and treatment of human disease. This particularly applies to the field of neurology as neurological function is impaired in many human disorders. Furthermore, the field of neurogenetics has been proven to show remarkably complex genotype-to-phenotype relationships. To facilitate the collection of DNA sequence variation pertaining to neurogenetic disorders, we have initiated the “Neurogenetics Consortium” under the umbrella of the Human Variome Project. The Consortium’s founding group consisted of basic researchers, clinicians, informaticians and database creators. This report outlines the strategic aims established at the preliminary meetings of the Neurogenetics Consortium and calls for the involvement of the wider neurogenetic community in enabling the development of this important resource.

In vivo impact of presynaptic calcium channel dysfunction on motor axons in episodic ataxia type 2.

Episodic ataxia type 2 is caused by mutations in CACNA1A, which encodes the pore-forming subunit of the voltage-gated calcium channel Cav2.1. Tomlinson et al. reveal altered peripheral axonal excitability in patients compared to controls. The changes may reflect malformation of the nodes of Ranvier, as demonstrated in animal models.

P39 Prevalence study of skeletal muscle channelopathies in England

Introduction The non-dystrophic myotonias (NDM) and periodic paralyses (PP) are a group of skeletal muscle disorders caused by mutations in genes encoding ion channels. To date, very few studies have systematically evaluated the prevalence of these disorders, never in England, and most of them predate genetic diagnosis. Objective To obtain prevalence data on skeletal muscle channelopathies and to evaluate the relative frequency of common mutations. Methods The study covered all patients with NDM or PP living in the UK that were referred to the UK national reference centre for assessment. Inclusion criteria were clinical and electrophysiological features of NDM or PP, and confirmed mutations in genes encoding ion channels (96% of cases). England was selected as the geographical area for prevalence analysis. Results From a total of 582 patients identified, 286 had myotonia congenita, 70 paramyotonia congenita, 23 sodium-channel myotonias, 97 hypokalemic PP, 66 hyperkalemic PP, four normokalemic PP, and 36 Andersen-Tawil syndrome (ATS). 530 patients were from England, giving a point prevalence of 1/100 000. Significant allelic heterogeneity was associated with NDM and ATS. However, a limited number of mutations were responsible for most cases. Conclusion We have analysed the largest series of patients with skeletal muscle channelopathies reported so far, and documented for the first time their overall prevalence. The spectrum of mutations was similar to that previously reported.

015 Identifying the cause of phenotypic variability in a family with non-dystrophic myotonia

Non-dystrophic Myotonia (NDM) is the commonest group of skeletal muscle channelopathies often causing severe myotonia in patients. It is either caused by mutations in the skeletal muscle chloride channel gene, CLCN1, or the sodium channel gene, SCN4A resulting in increased membrane excitability. An important unexplained issue in NDM is the significant phenotypic variability seen between patients with the same mutation and even within the same pedigree. We investigated the cause of this phenotypic variability in a pedigree where one branch of the family had several severely affected individuals, whereas those in another branch were more mildly affected. We tested the genes known to be associated with myotonia in both the severely and mildly affected individuals in this pedigree. Sequencing of the sodium channel gene, SCN4A, revealed that all affected individuals carried the known mutation c.3917G>T; p.Gly1306Val, consistent with a diagnosis of Paramyotonia Congenita. Sequencing of CLCN1 in all affected individuals also revealed the presence of a mutation known to cause Myotonia Congenita in exon 8, c.938C>T; p.Ala313Val in the severely affected individuals. This mutation was absent in the mildly affected individuals. In this pedigree the phenotypic variability is therefore most likely due to a double hit of two myotonia-causing mutations in different genes. This suggests that it is important to sequence all myotonia genes in pedigrees with marked phenotypic variability.

1154 Skeletal muscle chloride channel gene (CLCN1) copy number variation can cause myotonia congenita

Myotonia Congenita (MC) is the commonest skeletal muscle channelopathy. It associates with recessive or dominant mutations in the chloride channel gene CLCN1. A significant number of patients from recessive pedigrees (14% in our series of 439 cases) only harbour a single recessive mutation despite full sequencing of CLCN1. The genetic basis of such cases remains unexplained; introducing uncertainties for diagnosis and genetic counselling. We hypothesised that CLCN1 copy number variations (CNV) may be a new genetic mechanism in such MC patients. We performed detailed clinical and neurophysiological characterisation in 60 patients with a MC phenotype. DNA sequencing of CLCN1 was followed by multiplex ligation dependent probe amplification (MLPA) to screen for CNV in CLCN1. We identified the first reported group of MC patients with CNV in CLCN1. Eight per cent of cases in our cohort carried a CNV in CLCN1. In two families, rearrangements affecting exons 8 to 14 had a very severe infantile onset myotonia. This indicates that CLCN1 CNV are an important previously unreported cause of recessive MC suggesting that MLPA should be done in the genetic work-up of patients with MC; especially in cases where direct DNA sequencing identifies no mutations or only a single recessive mutation. These data also indicate that additional unidentified genetic mechanisms exist to account for cases not currently explained by either exonic CLCN1 point mutations or copy number variation.