Rearrangements within the facioscapulohumeral muscular dystrophy locus: mechanism, timing and consequences.

Abstract

The autosomal dominant myopathy facioscapulohumeral muscular dystrophy (FSHD) is the third most common inherited neuromuscular disorder and is characterized by progressive muscle weakness and atrophy, initially involving muscles in the face, upper arm and shoulder girdle. FSHD is caused by contractions of the D4Z4 repeat on chromosome 4q35. An almost identical D4Z4 repeat is present on chromosome 10q26 but contractions of this repeat have never been associated with FSHD. Because transcriptional activity from D4Z4 has never been demonstrated, other disease models have been proposed in which the D4Z4 contraction causes inappropriate transcription of cis and/or trans genes. This thesis describes the elucidation of the mechanism and timing of mitotic D4Z4 contractions. We show that D4Z4 contractions often occur mitotically, likely during the first zygotic divisions and that the resulting mosaicism is often not detected in FSHD patients. Furthermore, we show a significant reduction of the CpG-methylation in D4Z4 repeats of FSHD-alleles, which is suggestive of a local chromatin alteration. In addition, we study a telomeric variation distal to D4Z4, 4qA and 4qB, and show that this variation is equally common in the control population, but FSHD is exclusively associated with the 4qA allele. We demonstrate that the subtelomeric FSHD locus has been subjected to multiple rearrangements, including duplications, deletions and translocations. Duplications have been identified between the subtelomeres of chromosomes 4q and 10q, and 4p and 4q, and might have initiated more recent translocations of D4Z4 between chromosomes 4 and 10. Together, these rearrangements are hampering the molecular diagnosis of FSHD. To address these problems, new methods were developed for the identification and characterization of the complicated FSHD genotypes, which are caused by these rearrangements. In addition, the characterization of these unusual FSHD genotypes contributes to the refinement of the definition of the FSHD allele and possibly to the elucidation of the disease mechanism of FSHD.