Early onset collagen VI myopathies: Genetic and clinical correlations

Abstract

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. ANN NEUROL 2010;68:511–520

A Mouse Model for Dominant Collagen VI Disorders

“…Skipping of COL6A3 exon 16, due to splice site mutations or small deletion, is the most common molecular defect in dominant UCMD patients (11,13,23,25). In this study we deleted exon 16 and its flanking sequence in the mouse Col6a3 gene by gene targeting.…”

“…We have previously shown that the Col6a3 hm/hm mice display reduced muscle contractile forces after twitch and tetanic stimulation at age 4 months (22). In UCMD patients, whereas recessive mutations causing an absence of collagen VI almost invariantly display an "early-severe (never acquired ambulation)" phenotype, dominant mutations causing skipping of COL6A3 exon 16 result in both early-severe and "moderate-progressive (loss of ambulation at a mean age of 10 years)" manifestations (13). These observations suggest that dominant exon skipping mutation may result in a somewhat less severe phenotype compared with recessive mutations that lead to mRNA decay and that modifier genes likely contribute to the phenotypic variations of dominant exon skipping mutations.…”

“…In this study we generated a mutant mouse bearing the most common molecular defect found in dominant UCMD patients, i.e. skipping of exon 16 in the COL6A3 mRNA (11,13,23,25). We show that mutant mice heterozygous for the Col6a3 exon 16 deletion, named…”

“…velvety skin, hyperkeratosis, and keloid formation, are common. Severely affected patients either never achieve independent ambulation or can walk initially, but the ability is lost by the teen years (8,12,13). Respiratory insufficiency develops progressively, and ultimately patients almost invariably need ventilation support.…”

Inherited Connective Tissue Disorders of Collagens: Lessons from Targeted Mutagenesis

“…Autophagy has a key role in the clearance of damaged organelles and in the turnover of cell components and is thus essential for tissue homeostasis. Recently, 56 novel mutations have been described, allowing a clinical classification and revealing the complexity of genotype-phenotype relationships (Briñas et al, 2010).…”