Pulse Field Gel Electrophoresis for the Detection of Facioscapulohumeral Muscular Dystrophy Gene Rearrangements

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant myopathy with a frequency of 1 in 20,000 and a penetrance of 95 % by the age of 20. affects specific muscle groups (facial, upper girdle, upper arm, pelvic girdle, and foot extensor) and displays a variety of phenotypic expression, ranging from almost asymp tomatic forms to more severe wheelchair-bound cases. Linkage and physical mappin strategies have identified a polymorphic EcoRI locus on human chromosome 4q35 (1 ) that is composed of a variable number of 3.3 kb KpnI tandem repeat units (D4Z4) and appears to be tightly linked to FSHD. In normal subjects the p13E-1 1 EcoRI fragment containing the KpnI repeats varies in size between 50 and 300 kb, while in sporadic and familial cases of FSHD the disease cosegregates with a fragment widely below this range, i.e., between 10 and 34 kb (2 –7 ). It has been demonstrated that the 4q35 rearrangement involves the deletion of an integral number of 3.3 kb repeat units (8 ). In th human genome the 3.3 kb repeats are present on several chromosomes other than 4q, specifically the short arms of acrocentric chromosomes, 1q12 and 10qter, as shown by in situ hybridization experiments (9 ,10 ). The spreading of KpnI repeat sequences o human chromosomes generates artifacts in the interpretation of DNA analysis in normal subjects and FSHD patients, because multiple EcoRI fragments are observed afte hybridization with p13E-11 probe and time-consumimg linkage analysis with 4q3 and 10qter markers is required to assign the chromosomal origin of the alleles (5 ,11 ). We found that the 10qter locus shows a high degree of homology with the 4q35 locuas shown by restriction mapping and in situ hybridization experiments (12 ).