RhoA wild-type allele loss unleashes the cancer driver function of RhoA E40Q

Cancer hotspot mutations of unknown function often obscure the functional understanding of the molecular pathways underlying cancer formation and limit precision medicine progress. Here, we investigated unresolved driver functions of RhoA in Head and neck squamous cell carcinoma (HNSCC). Our investigation reveals that RhoA E40Q is a partial loss-of-function allele which paradoxically promotes tumorigenesis only in the absence of wild-type RhoA. Therefore, mice expressing RhoA E40Q specifically in keratinocytes lacking wild-type RhoA spontaneously developed squamous cell carcinoma and showed defective hair shafts formation. Mechanistically, RhoA E40Q expression in RhoA-deficient cells impaired genome maintenance as shown by increased nuclear foci of gh2AX, a marker for DNA double-strand breaks. This genomic instability was mirrored by reduced cellular fitness upon RhoA E40Q knock-in in MCF10A cells, a phenotype also observed in BRCA2 loss. These convergent data establish RhoA E40Q as an unusual, context-dependent oncogenic driver: a seemingly inactive variant that unleashes its tumor-promoting potential only when the wild-type allele is absent.