FSHD: A Repeat Contraction Disease Finally Ready to Expand

PLoS Genetics: FSHD: A Repeat Contraction Disease Finally Ready to Expand (Our Understanding of Its Pathogenesis)

Christopher E. Pearson^1,2*

1 Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario, Canada, 2 Department of Molecular Genetics, University of Toronto, Toronto, Ontario, Canada

Facioscapulohumeral muscular dystrophy (FSHD), was one of the first diseases shown to be caused by an unstable repeat in the early 1990s along with spinal and bulbar muscular atrophy (SBMA), myotonic dystrophy (DM1), and fragile X mental retardation (FRAXA), where the latter three are caused by genetically expanding trinucleotide repeats [1]. However, FSHD differs considerably from the trinuclotide repeat diseases, as it is caused by a contraction of a macrosatellite (D4Z4 repeat, 3.3 kb/unit). Moreover, far less is understood about the pathogenic mechanism for FSHD, relative to SBMA, DM1, and FRAXA. This is not due to a shortage of experimental efforts, plausible hypotheses, or collaborative efforts towards understanding FSHD [2], [3]. The elucidation of FSHD is hampered by the size of the unstable repeat, its sequence complexity, the number of repeat units, and the presence of the repeat on Chromosomes 4 and 10, making analysis technically difficult. The difficulty is compounded further by the absence of an obvious gene, transcript, or protein in the unstable or proximal region; in fact, the D4Z4 repeats have been referred to as “junk” DNA or are thought to be a pseudogene, at best. As a result, FSHD has proved to be one of the most complex and challenging genetic diseases to even a glimpse an underlying pathogenic cause for FSHD. Several recent papers, including one in this issue of PLoS Genetics [4], have made significant advances that now permit us to expand our understanding of FSHD pathogenesis, a repeat contraction disease.

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