Recurrent MSC E116K mutations in ALK-negative anaplastic large cell lymphoma

Anaplastic large cell lymphomas (ALCLs) represent a relatively common group of T-cell non-Hodgkin lymphomas (T-NHLs) that are unified by similar pathologic features but demonstrate marked genetic heterogeneity. ALCLs are broadly classified as being anaplastic lymphoma kinase (ALK) or ALK , based on the presence or absence of rearrangements. Exome sequencing of 62 T-NHLs identified a previously unreported recurrent mutation in the musculin gene, , exclusively in ALK ALCLs. Additional sequencing for a total of 238 T-NHLs confirmed the specificity of for ALK ALCL and further demonstrated that 14 of 15 mutated cases (93%) had coexisting rearrangements. Musculin is a basic helix-loop-helix (bHLH) transcription factor that heterodimerizes with other bHLH proteins to regulate lymphocyte development. The E116K mutation localized to the DNA binding domain of musculin and permitted formation of musculin-bHLH heterodimers but prevented their binding to authentic target sequence. Functional analysis showed MSC acted in a dominant-negative fashion, reversing wild-type musculin-induced repression of and cell cycle inhibition. Chromatin immunoprecipitation-sequencing and transcriptome analysis identified the cell cycle regulatory gene as a direct transcriptional target of musculin. MSC reversed E2F2-induced cell cycle arrest and promoted expression of the CD30-IRF4-MYC axis, whereas its expression was reciprocally induced by binding of IRF4 to the promoter. Finally, ALCL cells expressing were preferentially targeted by the BET inhibitor JQ1. These findings identify a novel recurrent mutation as a key driver of the CD30-IRF4-MYC axis and cell cycle progression in a unique subset of ALCLs.