A MYOD1 Mutation Cooperates with PI3K Pathway Activation in ERMS

Alveolar rhabdomyosarcomas (ARMS) often are driven by FOXO1 gene fusions that block myogenic differentiation, but genetic events underlying impairment of terminal myogenic differentiation in embryonal rhabdomyosarcoma (ERMS) have yet to be defined. Through whole-exome sequencing, Kohsaka and colleagues uncovered a recurrent L122R substitution in myogenic differentiation 1 (MYOD1), which they went on to identify in 10 of 104 (10%) ERMS tumors. No MYOD1 mutations were found in ARMS tumor samples, nor were mutations in other related myogenic transcription factor genes identified in other ERMS samples. MYOD1 mutations were associated with an older age at diagnosis, tumors arising in the head and neck, frequent spindle cell morphology, and poor outcome, suggesting that MYOD1 mutations define a distinct subset of ERMS. Compared with overexpression of wild-type MYOD1, overexpression of the L122R mutant in mouse myoblast cells resulted in increased anchorage-independent growth and reduced myogenic differentiation. Of note, although PI3K pathway alterations are rare in ERMS, 5 of 10 MYOD1-mutant ERMS samples also contained PIK3CA mutations or PTEN deletions. PIK3CA^H1047R cooperated with MYOD1^L122R to inhibit myoblast differentiation and promote in vitro and in vivo growth more strongly than either mutant protein alone. Muscle cell differentiation genes were significantly downregulated as part of a MYOD1^L122R-induced gene signature that was also characterized by gene expression changes and chromatin histone marks more consistent with MYC expression than with wild-type MYOD1 expression, supporting an older mutagenesis study that found that the L122R mutant acquired the neomorphic ability to bind MYC consensus sites. Furthermore, even in the presence of wild-type MYOD1, the L122R mutant inhibited myogenic differentiation gene expression, suggesting that it can also act in a dominant-negative manner. The identification of MYOD1 mutations in a subset of ERMS with distinct clinical features and poor prognosis suggests that patients with MYOD1 mutations should be considered for high-risk protocols or therapies that target the PI3K pathway.

Kohsaka S, Shukla N, Ameur N, Ito T, Ng CK, Wang L, Lim D, et al. A recurrent neomorphic mutation in MYOD1 defines a clinically aggressive subset of embryonal rhabdomyosarcoma associated with PI3K–AKT pathway mutations. Nat Genet 2014 May 4 [Epub ahead of print].