Flavones provide resistance to DUX4-induced toxicity via an mTor-independent mechanism.

Facioscapulohumeral muscular dystrophy (FSHD) is among the most common of the muscular dystrophies, affecting nearly 1 in 8000 individuals, and is a cause of profound disability. Genetically, FSHD is linked to the contraction and/or epigenetic de-repression of the D4Z4 repeat array on chromosome 4, thereby allowing expression of the DUX4 gene in skeletal muscle. If the DUX4 transcript incorporates a stabilizing polyadenylation site the myotoxic DUX4 protein will be synthesized, resulting in muscle wasting. The mechanism of toxicity remains unclear, as many DUX4-induced cytopathologies have been described, however cell death does primarily occur through caspase 3/7-dependent apoptosis. To date, most FSHD therapeutic development has focused on molecular methods targeting DUX4 expression or the DUX4 transcript, while therapies targeting processes downstream of DUX4 activity have received less attention. Several studies have demonstrated that inhibition of multiple signal transduction pathways can ameliorate DUX4-induced toxicity, and thus compounds targeting these pathways have the potential to be developed into FSHD therapeutics. To this end, we have screened a group of small molecules curated based on their reported activity in relevant pathways and/or structural relationships with known toxicity-modulating molecules. We have identified a panel of five compounds that function downstream of DUX4 activity to inhibit DUX4-induced toxicity. Unexpectedly, this effect was mediated through an mTor-independent mechanism that preserved expression of ULK1 and correlated with an increase in a marker of active cellular autophagy. This identifies these flavones as compounds of interest for therapeutic development, and potentially identifies the autophagy pathway as a target for therapeutics.

A human immune/muscle xenograft model of FSHD muscle pathology.

Background: Facioscapulohumeral muscular dystrophy (FSHD) disease progression is associated with muscle inflammation, although its role in FSHD muscle pathology is unknown. Methods: We have developed a novel humanized mouse strain, NSG-SGM3-W41, that supports the co- engraftment of human hematopoietic stem cells (HSCs) and muscle myoblasts as an experimental model to investigate the role of innate immunity in FSHD muscle pathology. Results: The NSG-SGM3-W41 mouse supports the selective expansion of human innate immune cell lineages following engraftment of human HSCs and the co-engraftment and differentiation of patient-derived FSHD or control muscle myoblasts. Immunohistological and NanoString RNA expression assays establish that muscle xenografts from three FSHD subjects were immunogenic compared to those from unaffected first-degree relatives. FSHD muscle xenografts preferentially accumulated human macrophages and B cells and expressed early complement genes of the classical and alternative pathways including complement factor C3 protein, which is a mediator of early complement function through opsonization to mark damaged cells for macrophage engulfment. FSHD muscle xenografts also underwent immune donor dependent muscle turnover as assayed by human spectrin ß1 immunostaining of muscle fibers and by NanoString RNA expression assays of muscle differentiation genes. Conclusions: The NSG-SGM3-W41 mouse provides an experimental model to investigate the role of innate immunity and complement in FSHD muscle pathology and to develop FSHD therapeutics targeting DUX4 and the innate immunity inflammatory responses.

Post-Translational Modifications of the DUX4 Protein Impact Toxic Function in FSHD Cell Models.

OBJECTIVE: Facioscapulohumeral muscular dystrophy (FSHD) is caused by abnormal de-repression of the myotoxic transcription factor DUX4. Although the transcriptional targets of DUX4 are known, the regulation of DUX4 protein and the molecular consequences of this regulation are unclear. Here, we used in vitro models of FSHD to identify and characterize DUX4 post-translational modifications (PTMs) and their impact on the toxic function of DUX4. METHODS: We immunoprecipitated DUX4 protein and performed mass spectrometry to identify PTMs. We then characterized DUX4 PTMs and potential enzyme modifiers using mutagenesis, proteomics, and biochemical assays in HEK293 and human myoblast cell lines. RESULTS: We identified 17 DUX4 amino acids with PTMs, and generated 55 DUX4 mutants designed to prevent or mimic PTMs. Five mutants protected cells against DUX4-mediated toxicity and reduced the ability of DUX4 to transactivate FSHD biomarkers. These mutagenesis results suggested that DUX4 toxicity could be counteracted by serine/threonine phosphorylation and/or inhibition of arginine methylation. We therefore sought to identify modifying enzymes that could play a role in regulating DUX4 PTMs. We found several enzymes capable of modifying DUX4 protein in vitro, and confirmed that protein kinase A (PKA) and protein arginine methyltransferase (PRMT1) interact with DUX4. INTERPRETATION: These results support that DUX4 is regulated by PTMs and set a foundation for developing FSHD drug screens based mechanistically on DUX4 PTMs and modifying enzymes. ANN NEUROL 2023;94:398-413.

Generation of iMyoblasts from Human Induced Pluripotent Stem Cells.

Skeletal muscle stem cells differentiated from human-induced pluripotent stem cells (hiPSCs) serve as a uniquely promising model system for investigating human myogenesis and disease pathogenesis, and for the development of gene editing and regenerative stem cell therapies. Here, we present an effective and reproducible transgene-free protocol for derivation of human skeletal muscle stem cells, iMyoblasts, from hiPSCs. Our two-step protocol consists of 1) small molecule-based differentiation of hiPSCs into myocytes, and 2) stimulation of differentiated myocytes with growth factor-rich medium to activate the proliferation of undifferentiated reserve cells, for expansion and cell line establishment. iMyoblasts are PAX3 + /MyoD1 + myogenic stem cells with dual potential to undergo muscle differentiation and to self-renew as a regenerative cell population for muscle regeneration both ex vivo and in vivo . The simplicity and robustness of iMyoblast generation and expansion have enabled their application to model the molecular pathogenesis of Facioscapulohumeral Muscular Dystrophy and Limb-Girdle Muscular Dystrophies, to both ex vivo and in vivo muscle xenografts, and to respond efficiently to gene editing, enabling the co-development of gene correction and stem cell regenerative therapeutic technologies for the treatment of muscular dystrophies and muscle injury. Graphical abstract.

iMyoblasts for ex vivo and in vivo investigations of human myogenesis and disease modeling.

Skeletal muscle myoblasts (iMyoblasts) were generated from human induced pluripotent stem cells (iPSCs) using an efficient and reliable transgene-free induction and stem cell selection protocol. Immunofluorescence, flow cytometry, qPCR, digital RNA expression profiling, and scRNA-Seq studies identify iMyoblasts as a PAX3+/MYOD1+ skeletal myogenic lineage with a fetal-like transcriptome signature, distinct from adult muscle biopsy myoblasts (bMyoblasts) and iPSC-induced muscle progenitors. iMyoblasts can be stably propagated for >12 passages or 30 population doublings while retaining their dual commitment for myotube differentiation and regeneration of reserve cells. iMyoblasts also efficiently xenoengrafted into irradiated and injured mouse muscle where they undergo differentiation and fetal-adult MYH isoform switching, demonstrating their regulatory plasticity for adult muscle maturation in response to signals in the host muscle. Xenograft muscle retains PAX3+ muscle progenitors and can regenerate human muscle in response to secondary injury. As models of disease, iMyoblasts from individuals with Facioscapulohumeral Muscular Dystrophy revealed a previously unknown epigenetic regulatory mechanism controlling developmental expression of the pathological DUX4 gene. iMyoblasts from Limb-Girdle Muscular Dystrophy R7 and R9 and Walker Warburg Syndrome patients modeled their molecular disease pathologies and were responsive to small molecule and gene editing therapeutics. These findings establish the utility of iMyoblasts for ex vivo and in vivo investigations of human myogenesis and disease pathogenesis and for the development of muscle stem cell therapeutics.

Muscular dystrophies are a group of inherited genetic diseases characterised by progressive muscle weakness. They lead to disability or even death, and no cure exists against these conditions. Advances in genome sequencing have identified many mutations that underly muscular dystrophies, opening the door to new therapies that could repair incorrect genes or rebuild damaged muscles. However, testing these ideas requires better ways to recreate human muscular dystrophy in the laboratory. One strategy for modelling muscular dystrophy involves coaxing skin or other cells from an individual into becoming 'induced pluripotent stem cells'; these can then mature to form almost any adult cell in the body, including muscles. However, this approach does not usually create myoblasts, the 'precursor' cells that specifically mature into muscle during development. This limits investigations into how disease-causing mutations impact muscle formation early on. As a response, Guo et al. developed a two-step protocol of muscle maturation followed by stem cell growth selection to isolate and grow 'induced myoblasts' from induced pluripotent stem cells taken from healthy volunteers and muscular dystrophy patients. These induced myoblasts can both make more of themselves and become muscle, allowing Guo et al. to model three different types of muscular dystrophy. These myoblasts also behave as stem cells when grafted inside adult mouse muscles: some formed human muscle tissue while others remained as precursor cells, which could then respond to muscle injury and start repair. The induced myoblasts developed by Guo et al. will enable scientists to investigate the impacts of different mutations on muscle tissue and to better test treatments. They could also be used as part of regenerative medicine therapies, to restore muscle cells in patients.

Precise therapeutic gene correction by a simple nuclease-induced double-stranded break.

Current programmable nuclease-based methods (for example, CRISPR-Cas9) for the precise correction of a disease-causing genetic mutation harness the homology-directed repair pathway. However, this repair process requires the co-delivery of an exogenous DNA donor to recode the sequence and can be inefficient in many cell types. Here we show that disease-causing frameshift mutations that result from microduplications can be efficiently reverted to the wild-type sequence simply by generating a DNA double-stranded break near the centre of the duplication. We demonstrate this in patient-derived cell lines for two diseases: limb-girdle muscular dystrophy type 2G (LGMD2G)1 and Hermansky-Pudlak syndrome type 1 (HPS1)2. Clonal analysis of inducible pluripotent stem (iPS) cells from the LGMD2G cell line, which contains a mutation in TCAP, treated with the Streptococcus pyogenes Cas9 (SpCas9) nuclease revealed that about 80% contained at least one wild-type TCAP allele; this correction also restored TCAP expression in LGMD2G iPS cell-derived myotubes. SpCas9 also efficiently corrected the genotype of an HPS1 patient-derived B-lymphoblastoid cell line. Inhibition of polyADP-ribose polymerase 1 (PARP-1) suppressed the nuclease-mediated collapse of the microduplication to the wild-type sequence, confirming that precise correction is mediated by the microhomology-mediated end joining (MMEJ) pathway. Analysis of editing by SpCas9 and Lachnospiraceae bacterium ND2006 Cas12a (LbCas12a) at non-pathogenic 4-36-base-pair microduplications within the genome indicates that the correction strategy is broadly applicable to a wide range of microduplication lengths and can be initiated by a variety of nucleases. The simplicity, reliability and efficacy of this MMEJ-based therapeutic strategy should permit the development of nuclease-based gene correction therapies for a variety of diseases that are associated with microduplications.