Progress in the molecular diagnosis of facioscapulohumeral muscular dystrophy and correlation between the number of KpnI repeats at the 4q35 locus and clinical phenotype

Genotype analysis by using the p13E-11 probe and other 4q35 polymorphic mar kers was performed in 122 Italian facioscapulohumeral muscular dystrophy fa milies and 230 normal controls. EcoRI-RI double digestion was routinely use d to avoid the interference of small EcoRI fragments of 10qter origin that were found in 15% of the controls. An EcoRI fragment ranging between 10 and 28 kb that was resistant to BlnI digestion was detected in 114 of 122 fami lies (93%) comprising 76 familial and 38 isolated cases. Among the unaffect ed individuals, 3 were somatic mosaics and 7, carrying an EcoRI fragment la rger than 20 kb, could be rated as nonpenetrant gene carriers. In a cohort of 165 patients with facioscapulohumeral muscular dystrophy we found an inv erse correlation between fragment size and clinical severity. A severe lowe r limb involvement was observed in 100% of patients with an EroRI fragment size of 10 to 13 kb (1-2 KpnI repeats left), in 53% of patients with a frag ment size of 16 to 20 kb (3-4 KpnI repeats left), and in 19% of patients wi th a fragment size larger than 21 kb (>4 KpnI repeats left). Our results co nfirm that the size of the fragment is a major factor in determining the fa cioscapulohumeral muscular dystrophy phenotype and that it has an impact on clinical prognosis and genetic counseling of the disease.