Fascioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant neuromuscular disorder linked to partial deletion of integral numbers of a 3.3 kb polymorphic repeat, D4Z4, within the subtelomeric region of chromosome 4q. Although the relationship between deletions of D4Z4 and FSHD is well established, how this triggers the disease remains unclear. We have mapped the DNA loop domain containing the D4Z4 repeat cluster in human primary myoblasts and in murine-human hybrids. A nuclear matrix attachment site was found located in the vicinity of the repeat. Prominent in normal human myoblasts and nonmuscular human cells, this site is much weaker in muscle cells derived from FSHD patients, suggesting that the D4Z4 repeat array and upstream genes reside in two loops in nonmuscular cells and normal human myoblasts but in only one loop in FSHD myoblasts. We propose a model whereby the nuclear scaffold͞matrix attached region regulates chromatin accessibility and expression of genes implicated in the genesis of FSHD. The disorder is related to a short repeat array that remains after deletion of an integral number of tandemly arrayed 3.3-kb repeat units on chromosome 4. The size of this polymorphic locus (D4Z4) varies in normal individuals from 35 to 300 kb, whereas in FSHD patients it is consistently shorter than 35 kb (3). Partial deletion of the D4Z4 array on chromosome 4 ultimately leads to FSHD and is currently used as a diagnostic tool in genetic counseling to predict the probability of the disease (1,(3)(4)(5). A correlation exists between the extent of the deletion and its clinical expression: Indeed, patients with one to three repeats develop an early FSHD, whereas individuals with nine to 10 repeats exhibit a weaker form of the disease (5).Extensive efforts to identify gene transcripts associated with the 4q35-specific D4Z4 repeat, as potential FSHD candidate genes, have been largely unsuccessful (6). The 3.3-kb D4Z4 elements contain a cryptic DUX4 gene potentially coding for a double homeodomain protein (7), and an overall perturbation of mRNA expression profiles can be observed in FSHD patients (8-10), but the disease appears to result from an as yet unexplained mechanism with a genetic alteration not residing within a causative gene for the disease.The 4q35 genomic region ( Fig. 1) displays heterochromatic features and might exert repressive effects on neighboring genes with a mechanism similar to position effect variegation. A decreased D4Z4 repeat number consistently results in inappropriate up-regulation of adjacent FRG2, FRG1, and Ant1 in FSHD muscle (11-13). It has also been shown that a transcriptional repressor complex binds D4Z4, whose deletion would trigger overexpression by lack of repression (11). Indeed, overexpression of FRG1 in transgenic mice provokes a phenotype similar to that of FSHD (14). However, such a model of position effect has been recently challenged in two reports of an apparent lack of up-regulation of any 4q35 gene and because of the histone H4 acetylation state in FSHD lymph...

Facio-scapulo-humeral dystrophy (FSHD), a muscular hereditary disease with a prevalence of 1 in 20,000, is caused by a partial deletion of a subtelomeric repeat array on chromosome 4q. Earlier, we demonstrated the existence in the vicinity of the D4Z4 repeat of a nuclear matrix attachment site, FR-MAR, efficient in normal human myoblasts and nonmuscular human cells but much weaker in muscle cells from FSHD patients. We now report that the D4Z4 repeat contains an exceptionally strong transcriptional enhancer at its 5′-end. This enhancer up-regulates transcription from the promoter of the neighboring FRG1 gene. However, an enhancer blocking activity was found present in FR-MAR that in vitro could protect transcription from the enhancer activity of the D4Z4 array. In vivo, transcription from the FRG1 and FRG2 genes could be down- or up-regulated depending on whether or not FR-MAR is associated with the nuclear matrix. We propose a model for an etiological role of the delocalization of FR-MAR in the genesis of FSHD.

The number of D4Z4 repeats in the subtelomeric region of chromosome 4q is strongly reduced in patients with Facio-Scapulo-Humeral Dystrophy (FSHD). We performed chromosome conformation capture (3C) analysis to document the interactions taking place among different 4q35 markers. We found that the reduced number of D4Z4 repeats in FSHD myoblasts was associated with a global alteration of the three-dimensional structure of the 4q35 region. Indeed, differently from normal myoblasts, the 4qA/B marker interacted directly with the promoters of the FRG1 and ANT1 genes in FSHD cells. Along with the presence of a newly identified transcriptional enhancer within the 4qA allele, our demonstration of an interaction occurring between chromosomal segments located megabases away on the same chromosome 4q allows to revisit the possible mechanisms leading to FSHD.