Tibial muscular dystrophy--from clinical description to linkage on chromosome 2q31.

A genome scan with highly polymorphic markers has established linkage for tibial muscular dystrophy (TMD), a recently described late onset distal myopathy, to a novel myopathy locus on chromosome 2q31. The mode of inheritance in TMD is autosomal dominant and the typical symptom of ankle dorsiflexion weakness appears in the fourth to seventh decade. Weakness of lower leg muscles is slowly progressive eventually causing a moderate foot drop. Overall disability usually remains mild even in elderly patients and walking ability is preserved throughout the patient's lifetime. The main target of the disease, the tibial anterior muscle, shows progressive dystrophic changes with rimmed vacuoles at the early stages and complete replacement pathology at later stages of the disease. The linkage studies in four different TMD families revealed a common core haplotype with a set of markers on the chromosome 2q31 locus. This indicates one major ancient founder mutation for TMD in Finland. There is one superior candidate gene on the 2q31 locus, the gene encoding a giant protein titin, expressed in heart and skeletal muscle.