Background: Facioscapulohumeral dystrophy (FSHD) is a degenerative muscle disease whose pathophysiology is attributable to epigenetic de-repression of the double homeobox 4 (DUX4) transcription factor in skeletal muscle cells. Activation of DUX4 drives a pro-apoptotic transcriptional program, leading to muscle atrophy and loss of function. Bromo- and extraterminal domain (BET) family proteins have been shown to enable DUX4 transcription in FSHD muscle cells. Apabetalone, a BET inhibitor (BETi) in late-stage clinical development for cardiovascular disease, has selectivity for the second bromodomain (BD2) within BET proteins and a well-established clinical safety record.
Objectives: Using primary human skeletal muscle cells (pHSMC) from affected FSHD patients, we evaluated apabetalone’s ability to epigenetically repress DUX4 gene expression and counter its downstream effects on mediators of FSHD pathology.
Methods: We applied RNA-sequencing (RNA-seq) and bioinformatic analysis to categorize treatment-associated impacts on the transcriptome. We compared the effects of apabetalone’s BD2 selective BET inhibition with the pan BET inhibitor JQ1, and losmapimod – a p38 mitogen-activated protein kinase (MAPK) inhibitor and DUX4 transcriptional repressor.
Results: BETi downregulated expression of DUX4 downstream markers, including ZSCAN4, MBD3L2, and TRIM43. Pan BET inhibition by JQ1 detrimentally suppressed muscle cell differentiation and significantly induced apoptosis. In contrast, BD2-selective BET inhibition by apabetalone preserved differentiation and viability. Pathway analysis showed that BET inhibitors reversed DUX4-mediated gene expression in pathways of metabolism and cell death. Treatment with losmapimod altered different FSHD-associated pathways (including some linked to transcription and intracellular adhesion).
Conclusions: Apabetalone dose-dependently inhibits expression of DUX4 target genes and reverses transcriptional activity that contributes to FSHD pathology in pHSMC. No effects on myotube differentiation or apoptosis illustrated the preferential safety profile of BD2-selective apabetalone versus pan BETi. This, coupled with its established clinical safety record, makes apabetalone a promising candidate therapeutic for FSHD.