Detecting Collagen VI in Bethlem Myopathy

Abstract

Zamurs et al. (1) investigated the functional consequences of a homozygous COL6A2 p.D871N mutation in muscle biopsy and fibroblast cultures of a recessive Bethlem myopathy patient. The authors reported the absence of collagen VI (col6) in muscle biopsy, a pattern considered distinctive for Ullrich congenital muscular dystrophy (UCMD). This finding appears inconsistent with the data provided for the patient's skin fibroblasts, which clearly show that col6 was secreted in the extracellular matrix (1). Moreover, other authors showed that recessive mutations in the α2(VI) C2 domain, with consequences similar to those reported for this patient, cause a partial col6 deficiency in muscle biopsies (2, 3). The study of col6 in muscle was performed with a monoclonal antibody (VI-26) raised against an uncharacterized triple helical epitope (1). It is noteworthy that col6 was not detected with this antibody in muscle and skin biopsies of a UCMD patient, but it was instead revealed by 3C4 antibody (see Fig. 2, patient P3, in Ref. 4). These observations highlight the importance of using different and well characterized col6 antibodies, including those raised against single col6 chains, especially in the absence of a correlation with biochemical data. In addition, alkaline treatment on frozen sections to retrieve antigen is unusual and may affect the detection of extracellular matrix proteins (5). Indeed, if ever a lack of col6 is ascertained in Bethlem myopathy muscle, it will indicate that other factors are responsible for the different clinical outcomes, questioning the criteria of diagnosis, prognosis, and potential therapeutic approaches. Given the relevance of this issue, we recommend caution in data interpretation.