V. Bolduc

Recessive Mutations in the Putative Calcium-Activated Chloride Channel Anoctamin 5 Cause Proximal LGMD2L and Distal MMD3 Muscular Dystrophies

The recently described human anion channel Anoctamin (ANO) protein family comprises at least ten members, many of which have been shown to correspond to calcium-activated chloride channels. To date, the only reported human mutations in this family of genes are dominant mutations in ANO5 (TMEM16E, GDD1) in the rare skeletal disorder gnathodiaphyseal dysplasia. We have identified recessive mutations in ANO5 that result in a proximal limb-girdle muscular dystrophy (LGMD2L) in three French Canadian families and in a distal non-dysferlin Miyoshi myopathy (MMD3) in Dutch and Finnish families. These mutations consist of a splice site, one base pair duplication shared by French Canadian and Dutch cases, and two missense mutations. The splice site and the duplication mutations introduce premature-termination codons and consequently trigger nonsense-mediated mRNA decay, suggesting an underlining loss-of-function mechanism. The LGMD2L phenotype is characterized by proximal weakness, with prominent asymmetrical quadriceps femoris and biceps brachii atrophy. The MMD3 phenotype is associated with distal weakness, of calf muscles in particular. With the use of electron microscopy, multifocal sarcolemmal lesions were observed in both phenotypes. The phenotypic heterogeneity associated with ANO5 mutations is reminiscent of that observed with Dysferlin (DYSF) mutations that can cause both LGMD2B and Miyoshi myopathy (MMD1). In one MMD3-affected individual, defective membrane repair was documented on fibroblasts by membrane-resealing ability assays, as observed in dysferlinopathies. Though the function of the ANO5 protein is still unknown, its putative calcium-activated chloride channel function may lead to important insights into the role of deficient skeletal muscle membrane repair in muscular dystrophies.

No evidence that skewing of X chromosome inactivation patterns is transmitted to offspring in humans

Skewing of X chromosome inactivation (XCI) can occur in normal females and increases in tissues with age. The mechanisms underlying skewing in normal females, however, remain controversial. To better understand the phenomenon of XCI in nondisease states, we evaluated XCI patterns in epithelial and hematopoietic cells of over 500 healthy female mother-neonate pairs. The incidence of skewing observed in mothers was twice that observed in neonates, and in both cohorts, the incidence of XCI was lower in epithelial cells than hematopoietic cells. These results suggest that XCI incidence varies by tissue type and that age-dependent mechanisms can influence skewing in both epithelial and hematopoietic cells. In both cohorts, a correlation was identified in the direction of skewing in epithelial and hematopoietic cells, suggesting common underlying skewing mechanisms across tissues. However, there was no correlation between the XCI patterns of mothers and their respective neonates, and skewed mothers gave birth to skewed neonates at the same frequency as nonskewed mothers. Taken together, our data suggest that in humans, the XCI pattern observed at birth does not reflect a single heritable genetic locus, but rather corresponds to a complex trait determined, at least in part, by selection biases occurring after XCI.

G.P.126 “Strongman syndrome”: A new autosomal dominant herculean painful myopathy

Abstract The objective was to define clinical criteria for a novel autosomal dominant (AD) herculean painful myopathy. We reviewed the clinical assessment of 74 cases that presented with myalgia, muscle cramps, increased muscle bulk, above normal strength with a negative electrophysiological and genetic workup. We evaluated 42 members of the largest family and defined a clinical scale to establish affection status. Review of the 74 cases demonstrated the majority had a positive family history. The larger families support AD transmission with a variable phenotype. Cases presented with combinations of the following major symptoms and signs: above average strength, myalgia post-activity, deltoid fatigability after repeated contractions and percussion pseudomyotonia in more than two limbs. Many also presented with some of the following minor findings: poor quality sleep, excessive movements at night, weakness after repetitive movements, very large muscle bulk, calf hypertrophy, well defined muscles, clear superior strength by confrontation and proximal weakness affecting the iliopsoas. By doubling the weight given to the major findings we developed a 20-point affection scale. In the largest family this confirmed AD transmission. Few cases had elevated serum CPK levels. None had electrical myotonia, or decremental response on EMG. Muscle biopsies were unremarkable. Mutations in MYSN were excluded in 13 cases. No mutations were found in CAV3, CLCN1 or SCN4A. Though weakness after repeated contractions was common, none tested had electrophysiological evidence of myasthenia, nor DOK7 mutations. In some older cases a proximal weakness developed such that the condition evolved into a late-onset LGMD. We have identified a large number of Canadian and Australian cases affected by a variably painful herculean myopathy apparently transmitted in an AD fashion. Further genetic characterization of the larger families should uncover candidate loci and gene.