Cellular and animal models for facioscapulohumeral muscular dystrophy

Abstract

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most common forms of muscular dystrophy and presents with weakness of the facial, scapular and humeral muscles, which frequently progresses to the lower limbs and truncal areas, causing profound disability. Myopathy results from epigenetic de-repression of the D4Z4 microsatellite repeat array on chromosome 4, which allows misexpression of the developmentally regulated DUX4 gene. DUX4 is toxic when misexpressed in skeletal muscle and disrupts several cellular pathways, including myogenic differentiation and fusion, which likely underpins pathology. DUX4 and the D4Z4 array are strongly conserved only in primates, making FSHD modeling in non-primate animals difficult. Additionally, its cytotoxicity and unusual mosaic expression pattern further complicate the generation of in vitro and in vivo models of FSHD. However, the pressing need to develop systems to test therapeutic approaches has led to the creation of multiple engineered FSHD models. Owing to the complex genetic, epigenetic and molecular factors underlying FSHD, it is difficult to engineer a system that accurately recapitulates every aspect of the human disease. Nevertheless, the past several years have seen the development of many new disease models, each with their own associated strengths that emphasize different aspects of the disease. Here, we review the wide range of FSHD models, including several in vitro cellular models, and an array of transgenic and xenograft in vivo models, with particular attention to newly developed systems and how they are being used to deepen our understanding of FSHD pathology and to test the efficacy of drug candidates.

Pathomechanisms and biomarkers in facioscapulohumeral muscular dystrophy: roles of DUX4 and PAX7

“…DUX4 can inhibit the functions of myogenic regulatory factors MYOD and MYOGENIN that are required for myogenic differentiation and disrupt the enhancer of MYF5 (Bosnakovski et al , 2008 ; Bosnakovski et al , 2018 ), as well as indirectly activate HEY1, a myogenic repressor (Young et al , 2013 ). Apoptosis is induced by DUX4 in many cell types and species (Kowaljow et al , 2007 ; DeSimone et al , 2020 ) but exactly how remains unclear. While DUX4 induces p53‐dependent apoptosis (Wallace et al , 2010 ), it drives apoptosis in TP53 ‐null mice too, possibly via upregulation of p21 (Bosnakovski et al , 2017a ).…”

“…Although DUX4 is restricted to apes and Old World monkeys, animal models have been developed to examine the role of DUX4 in vivo (DeSimone et al , 2020 ). In particular, mouse models with inducible/controllable DUX4 expression demonstrate hallmarks of FSHD including reduced muscle strength and histopathological features such as muscle atrophy, degeneration, inflammation and fibrosis.…”

“…Discovery studies identify both direct and indirect DUX4 target genes to inform about affected pathways that can then be validated in FSHD. DUX4 discovery studies have been performed in human myoblasts, rhabdomyosarcoma cells, murine myoblasts, murine embryonic stem cells and embryos of Danio rerio , Xenopus laevis and Drosophila melanogaster (DeSimone et al , 2020 ). Comparing transcriptional changes induced by DUX4 in mouse and human show significant correlations in global gene expression changes, particularly in genes associated with cell death and myogenesis (Sharma et al , 2013 ; Knopp et al , 2016 ).…”

“…In vivo models of FSHD are necessary to both understand pathomechanisms and test potential therapeutics (DeSimone et al , 2020 ). Muscle‐specific DUX4 expression in mouse can model muscle pathology, including inflammatory changes, e.g.…”

Genetic Approaches for the Treatment of Facioscapulohumeral Muscular Dystrophy

“…Needless to say, developing therapies that would address the unique characteristics of FSHD would require the use of animal models that resemble the disease fairly well. As previously mentioned, recent advancements in the conditional control of DUX4 expression have helped us get closer to generating such models, resulting in mice with dystrophic histopathology, impaired muscle strength, and asymmetric muscle degeneration similar to those seen in patients (DeSimone et al, 2020). Due to the inducible nature of DUX4 expression in these models, the severity of resulting phenotypes is also tunable, allowing for therapeutic testing in a variety of disease states.…”