Evolutionary history of linked D4Z4 and Beta satellite clusters at the FSHD locus (4q35)

A distal auxiliary element facilitates cleavage and polyadenylation of Dux4 mRNA in the pathogenic haplotype of FSHD

The degenerative muscle disorder facioscapulohumeral dystrophy (FSHD) is thought to be caused by the inappropriate expression of the Double Homeobox 4 (Dux4) protein in muscle cells leading to apoptosis. Expression of Dux4 in the major form of FSHD is a function of two contributing molecular changes: contractions in the D4Z4 microsatellite repeat region where Dux4 is located and an SNP present within a region downstream of the D4Z4. This SNP provides a functional, yet non-consensus polyadenylation signal (PAS) is used for the Dux4 mRNA 3' end processing. Surprisingly, the sequences flanking the Dux4 PAS do not resemble a typical cleavage and polyadenylation landscape with no recognizable downstream sequence element and a suboptimal cleavage site. Here, we conducted a systematic analysis of the cis-acting elements that govern Dux4 cleavage and polyadenylation. Using a transcriptional read-through reporter, we determined that sequences downstream of the SNP located within the β-satellite region are critical for Dux4 cleavage and polyadenylation. We also demonstrate the feasibility of using antisense oligonucleotides to target these sequences as a means to reduce Dux4 expression. Our results underscore the complexity of the region immediately downstream of the D4Z4 and uncover a previously unknown function for the β-satellite region in Dux4 cleavage and polyadenylation.

DUX4 induces a transcriptome more characteristic of a less-differentiated cell state and inhibits myogenesis

Skeletal muscle wasting in facioscapulohumeral muscular dystrophy (FSHD) results in substantial morbidity. On a disease-permissive chromosome 4qA haplotype, genomic and/or epigenetic changes at the D4Z4 macrosatellite repeat allows transcription of the DUX4 retrogene. Analysing transgenic mice carrying a human D4Z4 genomic locus from an FSHD-affected individual showed that DUX4 was transiently induced in myoblasts during skeletal muscle regeneration. Centromeric to the D4Z4 repeats is an inverted D4Z4 unit encoding DUX4c. Expression of DUX4, DUX4c and DUX4 constructs, including constitutively active, dominant-negative and truncated versions, revealed that DUX4 activates target genes to inhibit proliferation and differentiation of satellite cells, but that it also downregulates target genes to suppress myogenic differentiation. These transcriptional changes elicited by DUX4 in mouse have significant overlap with genes regulated by DUX4 in man. Comparison of DUX4 and DUX4c transcriptional perturbations revealed that DUX4 regulates genes involved in cell proliferation, whereas DUX4c regulates genes engaged in angiogenesis and muscle development, with both DUX4 and DUX4c modifing genes involved in urogenital development. Transcriptomic analysis showed that DUX4 operates through both target gene activation and repression to orchestrate a transcriptome characteristic of a less-differentiated cell state.

ZNF555 protein binds to transcriptional activator site of 4qA allele and ANT1: potential implication in Facioscapulohumeral dystrophy

Facioscapulohumeral dystrophy (FSHD) is an epi/genetic satellite disease associated with at least two satellite sequences in 4q35: (i) D4Z4 macrosatellite and (ii) β-satellite repeats (BSR), a prevalent part of the 4qA allele. Most of the recent FSHD studies have been focused on a DUX4 transcript inside D4Z4 and its tandem contraction in FSHD patients. However, the D4Z4-contraction alone is not pathological, which would also require the 4qA allele. Since little is known about BSR, we investigated the 4qA BSR functional role in the transcriptional control of the FSHD region 4q35. We have shown that an individual BSR possesses enhancer activity leading to activation of the Adenine Nucleotide Translocator 1 gene (ANT1), a major FSHD candidate gene. We have identified ZNF555, a previously uncharacterized protein, as a putative transcriptional factor highly expressed in human primary myoblasts that interacts with the BSR enhancer site and impacts the ANT1 promoter activity in FSHD myoblasts. The discovery of the functional role of the 4qA allele and ZNF555 in the transcriptional control of ANT1 advances our understanding of FSHD pathogenesis and provides potential therapeutic targets.