The vast majority of the mammalian genome is transcribed giving rise to many different types of noncoding RNAs. Among them, long noncoding RNAs are the most numerous and functionally versatile class. Indeed, the lncRNA repertoire might be as rich as the proteome. LncRNAs have emerged as key regulators of gene expression at multiple levels. They play important roles in the regulation of development, differentiation and maintenance of cell identity and they also contribute to disease. In this review, we present recent advances in the biology of lncRNAs in muscle development and differentiation. We will also discuss the contribution of lncRNAs to muscle disease with a particular focus on Duchenne and facioscapulohumeral muscular dystrophies.

Alveolar rhabdomyosarcoma (ARMS) is an aggressive pediatric muscle cancer which arrested during the process of skeletal muscle differentiation. In muscle myoblast cells, ectopic expression of the histone H3 lysine 9 (H3K9) methytransferase KMT1A blocks differentiation by repressing a myogenic gene expression program. In this study, we tested the hypothesis that activation of a KMT1A-mediated program of transcriptional repression prevents ARMS cells from differentiating. We investigated whether KMT1A represses the expression of differentiation-associated genes in ARMS cells thereby blocking muscle differentiation. Our results demonstrate that expression of KMT1A is induced in human ARMS cancer cell lines when cultured under differentiation-permissible conditions. shRNA-mediated knock-down of KMT1A decreased anchorage dependent and independent cell proliferation and tumor xenograft growth, increased expression of differentiation-associated genes and promoted the appearance of a terminally differentiated-like phenotype. Lastly, shRNA-directed KMT1A knockdown restored the impaired transcriptional activity of the myogenic regulator MyoD. Together, our results suggested that high levels of KMT1A in ARMS cells under differentiation conditions impairs MyoD function thereby arresting myogenic differentiation in these tumor cells. Thus, targeting KMT1A may be a novel strategy for the treatment of this disease.

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Alveolar rhabdomyosarcoma (ARMS) comprises a rare highly malignant tumor presumed to be associated with skeletal muscle lineage in children. The hallmark of the majority of ARMS is a chromosomal translocation that generates the PAX3-FOXO1 fusion protein, which is an oncogenic transcription factor responsible for the development of the malignant phenotype of this tumor. ARMS cells are dependent to the oncogenic activity of PAX3-FOXO1 and its expression status in ARMS tumors correlates with worst patient outcome, suggesting that blocking this activity of PAX3-FOXO1 may be an attractive therapeutic strategy against this fusion-positive disease. In this study, we screened small-molecule chemical libraries for inhibitors of PAX3-FOXO1 transcriptional activity using a cell-based readout system. We identified the Sarco/Endoplasmic Reticulum Ca2+-ATPases (SERCA) inhibitor thapsigargin as an effective inhibitor of PAX3-FOXO1. Subsequent experiments in ARMS cells demonstrated that activation of AKT by thapsigargin inhibited PAX3-FOXO1 activity via phosphorylation. Moreover, this AKT activation appears to be associated with the effects of thapsigargin on intracellular calcium levels. Furthermore, thapsigargin inhibited the binding of PAX3-FOXO1 to target genes and subsequently promoted its proteosomal degradation. In addition, thapsigargin treatment decreases the growth and invasive capacity of ARMS cells while inducing apoptosis in vitro. Finally, thapsigargin can suppress the growth of an ARMS xenograft tumor in vivo. These data reveal that thapsigargin-induced activation of AKT is an effective mechanism to inhibit PAX3-FOXO1 and a potential agent for targeted therapy against ARMS.