HIGH-RESOLUTION FLUORESCENCE IN-SITU HYBRIDIZATION TO LINEARLY EXTENDED DNA VISUALLY MAPS A TANDEM REPEAT ASSOCIATED WITH FACIOSCAPULOHUMERAL MUSCULAR-DYSTROPHY IMMEDIATELY ADJACENT TO THE TELOMERE OF 4Q

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant neuromuscular disorder. The FSHD locus has been linked to the most di stal genetic markers on the long arm of chromosome 4. An EcoRI fragmen t length polymorphism segregates with the disease in most FSHD familie s. Within the EcoRI fragment lies a tandem array of 3.2 kb repeats. De letions of integral copies of this repeat have been associated with th e disease. The 3.2 kbp repeat has recently been shown to cross-hybridi ze to several regions of heterochromatin in the human genome and DNA s equence analysis reveals strong homology to a class of heterochromatin repeats, LSau. In this report, we demonstrate that the 3.2 kbp tandem repeat lies adjacent to a subtelomeric sequence, which is within 5-14 kb of the telomeric repeat (TTAGGG)(n). Direct visual fluorescence hy bridization to linearly extended strands of DNA enabled the visualizat ion of this subtelomeric sequence as a short string of signals at the end of a longer string of signals from the differentially labeled 3.2 kbp tandem repeat. Furthermore, in support of our data showing that th e 3.2 kbp repeat lies in close proximity to the telomere of 4q, we dem onstrated the lack of hybridization of total human DNA to this same re gion. Our results indicate that the tandem array of 3.2 kbp repeats, d isrupted in FSHD, lies immediately adjacent to the telomere of 4q and that the gene responsible for FSHD is likely located proximal to the t andem repeat.