The vast majority of skeletal muscle progenitors during vertebrate embryogenesis are derived from somites, epithelial condensations

INTRODUCTION Rhabdomyosarcoma (RMS) is a pediatric malignancy thought to arise from the aberrant regulation of developmental programs during myogenesis. It is the most common soft-tissue sarcoma of childhood (Wexler and Helman, 1994; Arndt and Crist, 1999). Treatment approaches are often aggressive and multimodal, involving a combination of chemotherapeutic agents and local surgical resection and/or irradiation. Patients with locally unresectable or metastatic disease are particularly difficult to treat, and current treatment approaches have a considerable number of severe sequelae, including secondary malignancies (Wexler and Helman, 1994; Arndt and Crist, 1999). There is a need for novel therapies that specifically target the unique molecular underpinnings of this disease. There are two major pediatric subtypes of RMS, embryonal (ERMS) and alveolar (ARMS), distinguished by histological appearance (Wexler and Helman, 1994; Arndt and Crist, 1999). ARMS is characterized in most cases by chromosomal translocations between the PAX3 or PAX7 and the FKHR (FOXO1 – Human Gene Nomenclature Database) loci (Barr, 2001). By contrast, ERMS is often associated with loss of heterozygosity at chromosome 11p15 and a number of whole chromosomal gains (Wexler and Helman, 1994; Barr, 1997; Arndt and Crist, 1999). Activating mutations in RAS family members have been described in a minority of ERMS cases (Stratton et al., 1989; Chen et al., 2006; Martinelli et al., 2009), and gene expression analysis on a number of tumors suggests that RAS pathway activation might be a common event in ERMS formation, even in the absence of RAS mutations (Langenau et al., 2007). The vast majority of skeletal muscle progenitors during vertebrate embryogenesis are derived from somites, epithelial condensations of the paraxial mesoderm that transiently form during anteriorposterior patterning of the embryo (Pownall et al., 2002; Parker et al., 2003). The myogenic regulatory factors (MRFs) MyoD (Myod1), Myf5, myogenin (also known as Myf4) and Myf6 (also known as Mrf4) are basic helix-loop-helix (bHLH) transcription factors that are important regulators of muscle development and differentiation. MyoD and Myf5 are expressed in early muscle progenitors within somites and together are required for and regulate muscle progenitor specification (Pownall et al., 2002). Myf6 and myogenin are expressed in more differentiated muscle cells and are the key regulators of muscle differentiation (Pownall et al., 2002). Postnatal muscle progenitors are thought to arise from satellite cells, the resident stem cells of adult skeletal muscle (Parker et al., 2003). The MRFs function similarly to their embryonic roles in directing the differentiation of satellite cell-derived progenitors into terminally differentiated muscle (Parker et al., 2003). The myogenic program is disrupted in human RMS. RMS tumors consistently overexpress MYOD1, and variably express the other MRFs (Anderson et al., 1999; Blandford et al., 2006). They also express other myogenic factors that are characteristic of multiple stages of myogenesis and often express various structural proteins that are typically found in terminally differentiated muscle (Anderson et al., 1999; Blandford et al., 2006). Yet RMS tumors do not readily form myotubes and myofibers, suggesting an inability to activate myogenic programs appropriately (Anderson et al., 1999). Interestingly, expression of myogenin is correlated with disease progression and poor clinical outcome (Blandford et al., 2006; Heerema-McKenney et al., 2008), yet it is unclear whether it directly modulates disease phenotype or represents a marker of disease state in RMS. 1Stem Cell Program and Division of Hematology/Oncology, Boston Children’s Hospital and Dana-Farber Cancer Institute, Boston, MA 02115, USA. 2Harvard Medical School, Boston, MA 02115, USA. 3Memorial Sloan-Kettering Cancer Center, New York, NY 10065, USA. 4Massachusetts General Hospital, Boston, MA 02114, USA.