Abnormal merosin in adults. A new form of late onset muscular dystrophy not linked to chromosome 6q2.

We have identified seven patients (including two sib pairs) with a predominantly late onset limb-girdle muscular dystrophy in whom an absence of merosin was noted on immunoblotting. Merosin immunocytochemistry was normal, and no abnormalities were detected on immunostaining for the various proteins known to be involved in the limb-girdle muscular dystrophies (alpha, beta, gamma, delta sarcoglycan and calpain 3). Apart from one patient, where muscle problems began in childhood, reported age at onset of muscle weakness involving initially the proximal muscles of the lower limbs ranged from 17 to 40 years. The pattern of muscle involvement was similar from patient to patient, with hypertrophy of at least the calf muscles, absence of scapular winging and predominant involvement of hip flexors and adductors and hamstrings more than quadriceps. Serum creatine kinase in all patients was at least 10 times normal, and muscle biopsies showed non-specific dystrophic features. We believe that the patients described here may represent a genetically distinct subset within the limb-girdle muscular dystrophy group.