[Fukuyama congenital muscular dystrophy and related alpha-dystroglycanopathies]

Alpha-dystroglycan (alpha-DG) is a glycoprotein that binds to laminin in the basal lamina and helps provide mechanical support. A group of muscular dystrophies are caused by glycosylation defects of alpha-DG and are hence collectively called alpha-dystroglycanopathy (alpha-DGP). Alpha-DGP is clinically characterized by a combination of muscular dystrophies, structural brain anomalies, and ocular involvement. So far, 6 causative genes have been identified: LARGE, POMGNT1, POMT1, POMT2, FKRP, and FKTN. Initially, alpha-DGP was classified under congenital muscular dystrophies; however, the clinical phenotype is now expanded to include a markedly wide spectrum ranging from the most severe, lethal congenital muscular dystrophy with severe brain deformity to the mildest limb girdle muscular dystrophy with minimal muscle weakness. This is exemplified by Fukuyama congenital muscular dystrophy (FCMD), which is the most prevalent alpha-DGP in Japan, and is caused by mutations in FKTN. FCMD is clinically characterized by a triad of mental retardation, brain deformities, and congenital muscular dystrophy, and a majority of FCMD patients have a homozygous 3-kb retrotransposal insertion in the 3'non-coding region. Typically, they are able to sit but never attain independent ambulation in their lives. Recently, a patient from Turkey harboring homozygous 1-bp insertion reportedly showed a severe brain deformity with hydrocephalus and died 10 days after birth. In contrast, the mildest FKTN phenotype, LGMD2L, was identified in 6 cases from 4 families in Japan. These patients harbored compound heterozygous mutation with 3-kb retrotransposal insertion in the 3'non-coding region and a novel missense mutation in the coding region. Clinically, these patients presented with minimal muscle weakness and dilated cardiomyopathy and had normal intelligence. These data clearly indicate that FKTN mutations can cause a broad spectrum of muscular dystrophies. Therefore, clinicians should always bear in mind the possibility of alpha-DGP when they have a patient suspected to have muscular dystrophy.