Molecular Analysis of Facioscapulohumeral Muscular Dystrophy (FSHD1)

Although facioscapulohumeral muscular dystrophy (FSHD) is genetically heterogeneous, in 97% of cases the disease is associated with a submicroscopic rearrangement on 4q35 (FSHD1). This rearrangement is the result of deletions of an integral number of tandemly arrayed 3.3-kb repeat units (1 ,2 ). The repeat number in the normal population varies between 12 and 100 copies, but FSHD1 patients consistently show one allele with 12 copies or less. Because the entire repeat array resides on a single Eco RI fragment, the rearrangement can be demonstrated by an Eco RI digestion, followed by hybridization with the probe, p13E-11, recognizing a sequence of this restriction fragment proximal to the repeat units themselves (Fig. 1 ; 1 ). Southern analysis has been complicated by the presence of a highly homologous repeat array of similar size on chromosome (chr) 10 (3 ). In most cases, chr-10-derived repeat units can be discriminated from chr-4-derived units by a chr-10-specific Bln I restriction site within each repeat unit (Fig. 1 ; 4 ). In 20% of the population, an exchange between chr-10- and chr-4 derived repeat units has been observed, complicating FSHD1 diagnosis in approx 1% of cases (5 ). Because of these complex rearrangements, and because of the size of the repeat array (Eco RI fragments ranging from 7 to 300 kb), the authors strongly encourage use of pulsed-field gel electrophoresis (PFGE) for the molecular analysis of FSHD1, which will enable visualization of all 4q35 and 10q26 alleles.