Transactivating mutation of the MYOD1 gene is a frequent event in adult spindle cell rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma in children and adolescents, being characterized by expression of genes and morphological and ultrastructural features of sarcomeric differentiation. The spindle cell variant of rhabdomyosarcoma (spindle cell RMS) in adults has been defined as an entity, separated from embryonal rhabdomyosarcoma (ERMS), with unfavourable clinical outcome. So far, no recurrent genetic alteration has been identified in the adult form of spindle cell RMS. We studied a case of adult spindle cell RMS using next-generation sequencing (NGS) after exome capture. Using this approach, we identified 31 tumour-specific somatic alterations and selected four genes with predicted functional relevance to muscle differentiation and growth. MYOD1, KIF18A, NOTCH1, and EML5 were further tested for mutations using Sanger sequencing on DNA from FFPE samples from 16 additional, adult spindle cell RMS samples. The highly conserved sequence homology of MYOD1 with other myogenic transcription factors prompted us to screen the basic DNA-binding domains of MYF5, MYF6 and MYOG for mutations. From the investigated 17 samples, seven (41%) showed homozygous mutation of MYOD1, indicating a critical role in this rare subtype of adult spindle cell RMS, while no mutations were found in any of the other genes involved in myogenic differentiation. The p.L122R mutation occurs in the conserved DNA binding domain in MYOD1 and leads to transactivation and MYC-like functions. MYOD1 homozygous mutations are frequent, recurrent and pathognomonic events in adult-type spindle cell RMS.