Expression of laminin subunits in congenital muscular dystrophy

The expression of laminin subunits M, A, B1 and B2 was studied immunocytochemically in 25 cases of classical congenital muscular dystrophy (CMD), 11 hypotonic infants, 20 cases of a variety of inherited and acquired neuromuscular disorders, and 11 controls. Merosin, as indicated by labelling for the M chain, was deficient in 12 (48%) of the cases of classical CMD. Seven cases had no detectable labelling for the M chain whereas five showed traces, including three cousins from the same family. This suggests that very low expression may relate to a possible difference in the molecular defect, compared with cases completely devoid of the M chain. The A chain was abundant in regenerating fibres and in immature fibres expressing fetal myosin. In all merosin-deficient cases the A chain was over-expressed but this was not due to immaturity. A secondary reduction in sarcolemmal expression of the B1 chain occurred in five merosin-deficient cases, whilst expression in vascular tissue was normal. B1 was also reduced in one merosin-positive case of CMD, suggesting that other subunits may be involved in other forms of CMD. No differences in the expression of the B2 chain were observed in any of the cases studied. No abnormality in laminin subunits was found in controls or other neuromuscular disorders.