Bethlem myopathy: long-term follow-up identifies COL6 mutations predicting severe clinical evolution

OBJECTIVE

Mutations in one of the 3 genes encoding collagen VI (COLVI) are responsible for a group of heterogeneous phenotypes of which Bethlem myopathy (BM) represents the milder end of the spectrum. Genotype-phenotype correlations and long-term follow-up description in BM remain scarce.

METHODS

We retrospectively evaluated the long-term clinical evolution, and genotype-phenotype correlations in 35 genetically identified BM patients (23 index cases).

RESULTS

Nineteen patients showed a typical clinical picture with contractures, proximal weakness and slow disease progression while 11 presented a more severe evolution. Five patients showed an atypical presentation, namely a limb girdle muscle weakness in 2 and a congenital myopathy pattern with either no contractures, or only limited to ankles, in 3 of them. Pathogenic COL6A1-3 mutations were mostly missense or in frame exon-skipping resulting in substitutions or deletions. Twenty one different mutations were identified including 12 novel ones. The mode of inheritance was, autosomal dominant in 83% of the index patients (including 17% (N=4) with a de novo mutation), recessive in 13%, and undetermined in one patient. Skipping of exon 14 of COL6A1 was found in 35% of index cases and was mostly associated with a severe clinical evolution. Missense mutations were detected in 39% of index cases and associated with milder forms of the disease.

CONCLUSIONS

Long-term follow-up identified important phenotypic variability in this cohort of 35 BM patients. However, worsening of the functional disability appeared typically after the age of 40 in 47% of our patients, and was frequently associated with COL6A1 exon 14 skipping.

Early onset collagen VI myopathies: Genetic and clinical correlations

OBJECTIVE

Mutations in the genes encoding the extracellular matrix protein collagen VI (ColVI) cause a spectrum of disorders with variable inheritance including Ullrich congenital muscular dystrophy, Bethlem myopathy, and intermediate phenotypes. We extensively characterized, at the clinical, cellular, and molecular levels, 49 patients with onset in the first 2 years of life to investigate genotype-phenotype correlations.

METHODS

Patients were classified into 3 groups: early-severe (18%), moderate-progressive (53%), and mild (29%). ColVI secretion was analyzed in patient-derived skin fibroblasts. Chain-specific transcript levels were quantified by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), and mutation identification was performed by sequencing of complementary DNA.

RESULTS

ColVI secretion was altered in all fibroblast cultures studied. We identified 56 mutations, mostly novel and private. Dominant de novo mutations were detected in 61% of the cases. Importantly, mutations causing premature termination codons (PTCs) or in-frame insertions strikingly destabilized the corresponding transcripts. Homozygous PTC-causing mutations in the triple helix domains led to the most severe phenotypes (ambulation never achieved), whereas dominant de novo in-frame exon skipping and glycine missense mutations were identified in patients of the moderate-progressive group (loss of ambulation).

INTERPRETATION

This work emphasizes that the diagnosis of early onset ColVI myopathies is arduous and time-consuming, and demonstrates that quantitative RT-PCR is a helpful tool for the identification of some mutation-bearing genes. Moreover, the clinical classification proposed allowed genotype-phenotype relationships to be explored, and may be useful in the design of future clinical trials.

A single homozygous point mutation in a 3'untranslated region motif of selenoprotein N mRNA causes SEPN1-related myopathy

Mutations in the SEPN1 gene encoding the selenoprotein N (SelN) have been described in different congenital myopathies. Here, we report the first mutation in the selenocysteine insertion sequence (SECIS) of SelN messenger RNA, a hairpin structure located in the 3' untranslated region, in a patient presenting a classical although mild form of rigid spine muscular dystrophy. We detected a significant reduction in both mRNA and protein levels in the patient's skin fibroblasts. The SECIS element is crucial for the insertion of selenocysteine at the reprogrammed UGA codon by recruiting the SECIS-binding protein 2 (SBP2), and we demonstrated that this mutation abolishes SBP2 binding to SECIS in vitro, thereby preventing co-translational incorporation of selenocysteine and SelN synthesis. The identification of this mutation affecting a conserved base in the SECIS functional motif thereby reveals the structural basis for a novel pathological mechanism leading to SEPN1-related myopathy.

Prenatal diagnosis in laminin α2 chain (merosin)-deficient congenital muscular dystrophy: A collective experience of five international centers

The congenital muscular dystrophies (CMD) are clinically and genetically heterogeneous. The merosin (laminin alpha2 chain) deficient form (MDC1A), is characterized clinically by neonatal hypotonia, delayed motor milestones and associated contractures. It is caused by deficiency in the basal lamina of muscle fibers of the alpha2 chain of laminins 2 and 4 (LAMA2 gene at 6q22-23). Laminin alpha2 chain is also expressed in fetal trophoblast, which provides a suitable tissue for prenatal diagnosis in families where the index case has total deficiency of the protein. This article reports the collective experience of five centers over the past 10 years in 114 prenatal diagnostic studies using either protein analysis of the chorionic villus (CV) of the trophoblast plus DNA molecular studies with markers flanking the 6q22-23 region and intragenic polymorphisms (n=58), or using only DNA (n=44) or only protein (n=12) approaches. Of the 102 fetuses studied by molecular genetics, 27 (26%) were predicted to be affected while 75 (74%) were considered as unaffected, with 52 (51%) being heterozygous, thus conforming closely to an autosomal recessive inheritance. In 18 of the 27 affected fetuses, the trophoblast was studied by immunocytochemistry and there was a total or only traces deficiency of the protein in CV basement membrane in all. In 10 cases material from the presumably affected fetus was available for analysis after termination of the pregnancy and immunohistochemical study confirmed the diagnosis in all of them. Prenatal studies of 'at risk' pregnancies in the five centers produced neither false negative (merosin-deficiency in CVs in a normal fetus), nor false positive (normal merosin expression in CVs and affected child), indicating the reliability of the technique, when all the necessary controls are done. Our experience suggests that protein and DNA analysis can be used either independently or combined, according to the facilities of each center, to provide accurate prenatal diagnosis of the MDC1A, and have an essential role in genetic counseling.

Hypertrophic cardiomyopathy: distribution of disease genes, spectrum of mutations, and implications for a molecular diagnosis strategy

BACKGROUND

Hypertrophic cardiomyopathy is an autosomal-dominant disorder in which 10 genes and numerous mutations have been reported. The aim of the present study was to perform a systematic screening of these genes in a large population, to evaluate the distribution of the disease genes, and to determine the best molecular strategy in clinical practice.

METHODS AND RESULTS

The entire coding sequences of 9 genes (MYH7, MYBPC3, TNNI3, TNNT2, MYL2, MYL3, TPM1, ACTC, andTNNC1) were analyzed in 197 unrelated index cases with familial or sporadic hypertrophic cardiomyopathy. Disease-causing mutations were identified in 124 index patients ( approximately 63%), and 97 different mutations, including 60 novel ones, were identified. The cardiac myosin-binding protein C (MYBPC3) and beta-myosin heavy chain (MYH7) genes accounted for 82% of families with identified mutations (42% and 40%, respectively). Distribution of the genes varied according to the prognosis (P=0.036). Moreover, a mutation was found in 15 of 25 index cases with "sporadic" hypertrophic cardiomyopathy (60%). Finally, 6 families had patients with more than one mutation, and phenotype analyses suggested a gene dose effect in these compound-heterozygous, double-heterozygous, or homozygous patients.

CONCLUSIONS

These results might have implications for genetic diagnosis strategy and, subsequently, for genetic counseling. First, on the basis of this experience, the screening of already known mutations is not helpful. The analysis should start by testing MYBPC3 and MYH7 and then focus on TNNI3, TNNT2, and MYL2. Second, in particularly severe phenotypes, several mutations should be searched. Finally, sporadic cases can be successfully screened.

Homozygotes for a R869G mutation in the beta -myosin heavy chain gene have a severe form of familial hypertrophic cardiomyopathy

Familial Hypertrophic Cardiomyopathy (FHC) is an autosomal dominant disease characterised by ventricular hypertrophy, with predominant involvement of the interventricular septum. It is a monogenic disease with a high level of genetic heterogeneity (nine genes and more than 110 mutations reported so far). We describe a family with a new R869G mutation in the beta -myosin heavy chain gene (MYH7). This mutation was found in the heterozygous status in both parents and in the homozygous status in the two children. A haplotype analysis on the MYH7 locus with microsatellite markers showed that the same haplotype is transmitted within the family, suggesting a founder effect. Clinically, the father was asymptomatic with mild left ventricular hypertrophy on echocardiography. The mother had a mild form of hypertrophic cardiomyopathy and remained asymptomatic until 60 years old when an atrial fibrillation occurred. For the two children, clinical diagnosis was performed at 12 and 8 years and atrial fibrillation occurred at 17 years. For both children, the evolution was characterized by left ventricle (LV) systolic dysfunction and a severe dilatation of the left atrium before 40 years of age.

CONCLUSIONS

In this family, a new R869G mutation in the MYH7 gene was found. Interestingly, a mutation was found at the homozygous status for the first time in FHC. This finding suggests that this particular mutation is compatible with life, but for homozygous subjects, age at onset of symptoms was earlier and the disease much more severe than in the heterozygous subjects, suggesting a gene-dose effect.

First description of germline mosaicism in familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy is a genetically and phenotypically heterogeneous disease caused by mutations in seven sarcomeric protein genes. It is known to be transmitted as an autosomal dominant trait with rare de novo mutations.A French family in which two members are affected by hypertrophic cardiomyopathy was clinically screened with electrocardiography and echocardiography. Genetic analyses were performed on leucocyte DNA by haplotype analysis with microsatellite markers at the MYH7 locus and mutation screening by single strand conformation polymorphism analysis. Two subjects exhibited severe hypertrophic cardiomyopathy. A mutation in the MYH7 gene was found in exon 14 (Arg453Cys). The two affected patients were carriers of the mutation, which was not found in the circulating lymphocytes of their parents. Haplotype analysis at the MYH7 locus with two intragenic microsatellite markers (MYOI and MYOII) and the absence of the mutation in the father's sperm DNA suggested that the mutation had been inherited from the mother. However, it was not found in either her fibroblasts or hair. This is the first description of germline mosaicism shown by molecular genetic analysis in an autosomal dominant disorder and more especially in hypertrophic cardiomyopathy. This mosaicism had been inherited from the mother but did not affect her somatic cells. Such a phenomenon might account for some de novo mutations in familial hypertrophic cardiomyopathy.