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Advances in the molecular understanding of facioscapulohumeral muscular dystrophy (FSHD) have revealed that FSHD results from epigenetic de-repression of the DUX4 gene in skeletal muscle, which encodes a transcription factor that is active in early embryonic development but is normally silenced in almost all somatic tissues. These advances also led to the identification of targets for disease-altering therapies for FSHD, as well as an improved understanding of the molecular mechanism of the disease and factors that influence its progression. Together, these developments led the FSHD research community to shift its focus towards the development of disease-modifying treatments for FSHD. This Review presents advances in the molecular and clinical understanding of FSHD, discusses the potential targeted therapies that are currently being explored, some of which are already in clinical trials, and describes progress in the development of FSHD-specific outcome measures and assessment tools for use in future clinical trials. Facioscapulohumeral muscular dystrophy is caused by aberrant expression of the transcription factor DUX4. Tihaya, Mul and colleagues describe advances in the development of targeted treatments for facioscapulohumeral muscular dystrophy and discuss potential clinical trial outcome measures as well as molecular and imaging biomarkers. Key points: Facioscapulohumeral muscular dystrophy (FSHD), a disorder for which there currently is no cure, is characterized by muscle weakness, predominantly affecting muscles in the face, shoulder girdle and upper arms. FSHD is associated with epigenetic de-repression of the DUX4 gene, which leads to aberrant expression of the transcription factor DUX4 and cytotoxicity in skeletal muscle cells. Clinical disease progression occurs in a nonlinear and muscle-by-muscle fashion with phases of muscle inflammation preceding rapid fatty replacement of muscle tissue and muscle wasting. Consensus has been reached on the pathogenetic mechanism of FSHD, and the field is entering a new era of targeted therapy development. Disease-altering therapies currently in development range from proof-of-principle gene-editing technologies focusing on reducing DUX4 expression to clinical trials of DUX4-blocking agents. Clinical trials in FSHD require the development of meaningful patient outcome measures, identification of reliable biomarkers and accurate methods of measuring disease progression, such as MRI and ultrasonography. [ABSTRACT FROM AUTHOR]

Structural Maintenance of Chromosomes Hinge Domain Containing 1 (SMCHD1) is a chromatin repressor, which is mutated in > 95% of Facioscapulohumeral dystrophy (FSHD) type 2 cases. In FSHD2, SMCHD1 mutations ultimately result in the presence of the cleavage stage transcription factor DUX4 in muscle cells due to a failure in epigenetic repression of the D4Z4 macrosatellite repeat on chromosome 4q, which contains the DUX4 locus. While binding of SMCHD1 to D4Z4 and its necessity to maintain a repressive D4Z4 chromatin structure in somatic cells are well documented, it is unclear how SMCHD1 is recruited to D4Z4, and how it exerts its repressive properties on chromatin. Here, we employ a quantitative proteomics approach to identify and characterize novel SMCHD1 interacting proteins, and assess their functionality in D4Z4 repression. We identify 28 robust SMCHD1 nuclear interactors, of which 12 are present in D4Z4 chromatin of myocytes. We demonstrate that loss of one of these SMCHD1 interacting proteins, RuvB-like 1 (RUVBL1), further derepresses DUX4 in FSHD myocytes. We also confirm the interaction of SMCHD1 with EZH inhibitory protein (EZHIP), a protein which prevents global H3K27me3 deposition by the Polycomb repressive complex PRC2, providing novel insights into the potential function of SMCHD1 in the repression of DUX4 in the early stages of embryogenesis. The SMCHD1 interactome outlined herein can thus provide further direction into research on the potential function of SMCHD1 at genomic loci where SMCHD1 is known to act, such as D4Z4 repeats, the inactive X chromosome, autosomal gene clusters, imprinted loci and telomeres. [ABSTRACT FROM AUTHOR]