Facioscapulohumeral muscular dystrophy: Dux gene evolution and molecular diagnostics

Facioscapulohumeral muscular dystrophy (FSHD) is a progressive muscle-wasting disease that affects about 7/100,000 people. The first symptoms usually appear in the muscles of the shoulder, the face and the foot. The molecular cause of FSHD has been traced to the macrosatellite D4Z4, located on the chromosome 4 subtelomere. D4Z4 is usually composed of 11- 120 copies of 3.3 kb repeats with identical DNA sequences. The repeats are oriented head-to-tail in a tandem array, and there is a copy of the intronless gene DUX4 embedded within each. Healthy individuals have large D4Z4 macrosatellites with more than 11 repeats; the high repeat number is linked to the epigenetic silencing of D4Z4. Most FSHD patients, however, only have 1- 10 D4Z4 repeats (,FSHD 1 '). This causes the D4Z4 chromatin to become 'de-repressed ' and epigenetically active, resulting in the aberrant production of DUX4 transcripts. These transcripts are thought to cause the muscle wasting symptoms in FSHD. Interestingly, a small number of 'FSHD2' patients share the chromatin de-repression and DUX4 expression with FSHDl patients, without having a reduction in D4Z4 repeat number. In this thesis, I discuss two different aspects of this unusual disease. The function of DUX genes is still unknown, and a better understanding of their evolutionary history would aid the interpretation of functional studies of related homologues. Using a combination of bioinformatics and laboratory experiments, I dissect the evolutionary origins of DUX4 and other DUX genes in mammals. I also show that the unusual high-copy tandem array arrangement of DUX4 is shared by the related gene DUXC. My data suggest a model for the origin of DUX4, its intron-loss and its high copy number. It is unknown what triggers the epigenetic D4Z4 changes in FSHD2. Here, I report the molecular characterisation of patients with FSHD2, who may participate in a future exome sequencing mutation screen. In a proof-of-principle study, I demonstrate how exome sequencing can be used to correct a clinical misdiagnosis. iEThOS - Electronic Theses Online ServiceGBUnited Kingdo