G.P.23 Phenotypic variability and survey in a series of Bethlem myopathy

Abstract

Mutations in COL6A1, COL6A2, and COL6A3 genes encoding collagen VI (ColVI) are responsible for a wide spectrum of muscle diseases, also termed collagen VI myopathies. At one end of the spectrum, Bethlem myopathy (BM) represent a mild proximal myopathy beginning within the first or second decade of life and characterized by joint contractures, a hallmark of this condition. We report the clinical presentation, the course and the genetic analysis of 35 BM patients followed at the Institute of Myology. Onset of the disease was noticed before the age of 10 (80%). Early onset was not correlated with a severe clinical course. The majority of patients presented a retractile phenotype. However, three patients showed atypical phenotype: a LGMD muscle weakness pattern without contractures was observed in two patients and a third one had a diagnosis of minicore myopathy. Finger hyperlaxity and scoliosis were observed in 23% and 26% of patients, respectively. The vast majority of patients had a slow disease progression, with the exception of eight patients who used clippers or rolling chair. On long-term follow-up, only four patients presented a vital capacity (VC) lower than 70% of the theoretical value (record age of 18–50years) which was not correlated with age disease onset. The pattern of involved muscles on muscle imaging was characteristic of ColVI-myopathies. Pathogenic mutations, including missense or exon skipping mutations affecting the triple helical domains, were identified in the three COL6A genes. The majority of the patients (91%) harbored autosomal dominant mutations (14% with de novo mutations), while three patients presented a recessive inheritance. In conclusion, we presented a series of 35 BM patients, three of them presenting an atypical phenotype namely without retractions misdiagnosed as LGMD or minicore myopathy. However, in these patients, muscle imaging was in accordance with the pattern of COLVI-related myopathies orienting towards molecular study of COL6 genes.