Variable penetrance of COL6A1 null mutations: implications for prenatal diagnosis and genetic counselling in Ullrich congenital muscular dystrophy families.

Collagen VI mutations cause mild Bethlem myopathy and severe, progressive Ullrich congenital muscular dystrophy (UCMD). We identified a novel homozygous COL6A1 premature termination mutation in a UCMD patient that causes nonsense-mediated mRNA decay. Collagen VI microfibrils cannot be detected in muscle or fibroblasts. The parents are heterozygous carriers of the mutation and their fibroblasts produce reduced amounts of collagen VI. The molecular findings in the parents are analogous to those reported for a heterozygous COL6A1 premature termination mutation that causes Bethlem myopathy. However, the parents of our UCMD proband are clinically normal. The proband's brother, also a carrier, has clinical features consistent with a mild collagen VI phenotype. Following a request for prenatal diagnosis in a subsequent pregnancy we found the fetus was a heterozygous carrier indicating that it would not be affected with severe UCMD. COL6A1 premature termination mutations exhibit variable penetrance necessitating a cautious approach to genetic counselling.