MAPK-driven MYC/p21 axis controls senescence bypass and BCL-XL dependence in vincristine-resistant rhabdomyosarcoma

Despite aggressive multimodal therapy, no new effective treatments have been introduced for relapsed RMS in more than three decades, underscoring the urgent need to elucidate mechanisms of therapy resistance. We generated vincristine-resistant fusion-negative (FN) RMS cell lines through long-term dose escalation and identified enhanced MAPK pathway activation, evidenced by increased ERK phosphorylation in vitro and a major subset of post-chemotherapy patient tumor specimens.

Pharmacologic MEK inhibition with trametinib or genetic suppression of MEK1 significantly impaired the growth of resistant cells and enhanced vincristine efficacy in a KRAS(G12D)-induced zebrafish FN RMS model, leading to reduced tumor burden and delayed relapse. Mechanistically, trametinib induced a senescence-associated cell-cycle arrest in resistant cells through a MYC–p21 regulatory axis, wherein MYC directly represses CDKN1A (p21) expression by binding to its promoter, and MYC depletion or trametinib-mediated MYC suppression result in p21 upregulation and senescence induction.