ANALYSIS OF THE TANDEM REPEAT LOCUS D4Z4 ASSOCIATED WITH FACIOSCAPULOHUMERAL MUSCULAR-DYSTROPHY

The sequence of the tandem repeat sequence (D4Z4) associated with faci oscapulohumeral muscular dystrophy (FSHD) has been determined: each co py of the 3.3 kb repeat contains two homeoboxes and two previously des cribed repetitive sequences, LSau and a GC-rich low copy repeat design ated hhspm3. By Southern blotting, FISH and isolation of cDNA and geno mic clones we show that there are repeat sequences similar to D4Z4 at other locations in the human genome. Southern blot analysis of primate genomic DNA indicates that the copy number of D4Z4-like repeats has i ncreased markedly within the last 25 million years. Two cDNA clones we re isolated and found to contain stop codons and frameshifts within th e homeodomains. An STS was produced to the cDNAs and analysis of a som atic cell hybrid panel suggests they map to chromosome 14. No cDNA clo nes mapping to the chromosome 4q35 D4Z4 repeats that they encode a pro tein cannot be ruled out. Although D4Z4 may not encode a protein, ther e is an association between deletions within this locus and FSHD. The D4Z4 repeats contain LSau repeats and are adjacent to 68 bp Sau3A repe ats. Both of these sequences are associated with heterochromatic regio ns of DNA, regions known to be involved in the phenomenon of position effect variegation. We postulate that deletion of D4Z4 sequences could produce a position effect.