Durbeej, M. | bioRxiv Science

bioRxiv2022

Skeletal muscle regeneration is altered in the R6/2 mouse model of Huntington's disease

Huntingtons disease (HD) is caused by CAG repeat expansion in the huntingtin (HTT) gene. Skeletal muscle wasting alongside central pathology is a well-recognized phenomenon seen in patients with HD and HD mouse models. HD muscle atrophy progresses with disease and affects prognosis and quality of life. Satellite cells, progenitors of mature skeletal muscle fibers, are essential for proliferation, differentiation, and repair of muscle tissue in response to muscle injury or exercise. In this study, we aim to investigate the effect of mutant HTT on the differentiation and regeneration capacity of HD muscle by employing in vitro mononuclear skeletal muscle cell isolation and in vivo acute muscle damage model in R6/2 mice. We found that, similar to R6/2 adult mice, neonatal R6/2 mice also exhibit a significant reduction in myofiber width and morphological changes in gastrocnemius and soleus muscles compared to WT mice. Cardiotoxin (CTX)-induced acute muscle damage in R6/2 and WT mice showed that the Pax7+ satellite cell pool was dampened in R6/2 mice at 4 weeks post-injection, and R6/2 mice exhibited an altered inflammatory profile in response to acute damage. Our results suggest that, in addition to the mutant HTT degenerative effects in mature muscle fibers, expression of mutant HTT in satellite cells might alter developmental and regenerative processes to contribute to the progressive muscle mass loss in HD. Taken together, the results presented here encourage further studies evaluating the underlying mechanisms of satellite cell dysfunction in HD mouse models.

pathology↗

bioRxiv2018

A mutation-independent approach via transcriptional upregulation of a disease modifier gene rescues muscular dystrophy in vivo.

Introductory paragraphIdentification of protective and/or pathogenic genetic modifiers provides important insight into the heterogeneity of disease presentations in individuals affected by neuromuscular disorders (NMDs), despite having well-defined pathogenic variants. Targeting modifier genes to improve disease phenotypes could be especially beneficial in cases where the causative genes are large, structurally complex and the mutations are heterogeneous. Here, we report a mutation-independent strategy to upregulate expression of a compensatory disease-modifying gene in Congenital Muscular Dystrophy type 1A (MDC1A) using a CRISPR/dCas9-based transcriptional activation system. MDC1A is caused by nonfunctional Laminin 2, which compromises muscle fibers stability and axon myelination in peripheral nerves 1. Transgenic overexpression of Lama1, encoding a structurally similar protein Laminin 1, ameliorates muscle wasting and paralysis in the MDC1A mouse models, demonstrating its role as a protective disease modifier 2. Yet, upregulation of Lama1 as a postnatal gene therapy is hampered by its large size, which exceeds the current genome packaging capacity of clinically relevant delivery vehicles such as adeno-associated viral vectors (AAVs). In this study, we sought to upregulate Lama1 using CRISPR/dCas9-based transcriptional activation system, comprised of catalytically inactive S. aureus Cas9 (dCas9) fused to VP64 transactivation domains and sgRNAs targeting the Lama1 promoter. We first demonstrated robust upregulation of Lama1 in myoblasts, and following AAV9-mediated intramuscular delivery, in skeletal muscles of dy2j/dy2j mouse model of MDC1A.\n\nWe therefore assessed whether systemic upregulation of Lama1 would yield therapeutic benefits in dy2j/dy2j mice. When the intervention was started early in pre-symptomatic dy2j/dy2j mice, Lama1 upregulation prevented muscle fibrosis and hindlimb paralysis. An important question for future therapeutic approaches for a variety of disorders concerns the therapeutic window and phenotypic reversibility. This is particularly true for muscular dystrophies as it has long been hypothesized that fibrotic changes in skeletal muscle represent an irreversible disease state that would impair any therapeutic intervention at advanced stages of the disease. Here, we demonstrate that dystrophic features and disease progression were significantly improved and partially reversed when the treatment was initiated in symptomatic 3-week old dy2j/dy2j mice with already-apparent hind limb paralysis and significant muscle fibrosis.\n\nCollectively, our data demonstrate the feasibility and therapeutic benefit of CRISPR/dCas9-mediated modulation of a disease modifier gene, which opens up an entirely new and mutation-independent treatment approach for all MDC1A patients. Moreover, this treatment strategy provides evidence that muscle fibrosis can be reversible, thus extending the therapeutic window for this disorder. Our data provide a proof-of-concept strategy that can be applied to a variety of disease modifier genes and a powerful therapeutic approach for various inherited and acquired diseases.

genetics↗