Differential expression of HRK regulates proliferation of acquired melanocytic naevi

BACKGROUND: Naevi are considered benign analogues for melanoma, and they share many clinical and molecular features including oncogenic activation, and the transient adoption of senescence-like phenotypes. We previously determined dynamic mechanisms for maintenance of benign naevi (showing conventional features under histopathology) via DNA methylation which was not shared by dysplastic naevi. OBJECTIVES AND METHODS: Using whole-transcriptome sequencing, we evaluated 32 naevi (plus matching adjacent perilesional skin) characterised by histopathology and dermoscopic patterns, to reveal a novel gene-set enriched in actively growing naevi. RESULTS: We focussed on an apoptosis-related gene, HRK (Harakiri/DP5), a Bcl2 family member, which has been found to be downregulated in therapy-induced senescent melanoma cells. Whole transcriptome profiling confirmed that increased HRK expression was significantly correlated (R2=0.86; P=1.55 x 10-17) with dysplastic naevi and in naevi featuring a peripheral rim of globules (R2=0.67; P=1.54 x 10-8). Immunohistochemistry confirmed that HRK was lowly expressed in benign (conventional) naevi compared to dysplastic naevi, with a marked increase in thin invasive T1a melanoma tissues. Next, siRNA knockdown of HRK in melanoma cell lines resulted in up to twofold BAX (pro-apoptotic) expression and significant increases in sub G0/G1, indicative of apoptotic cells. Decreased HRK expression led to significantly reduced proliferative capacity, wound healing, colony formation, and an increase in senescent cells; indicating HRK activity promotes escape from senescence. CONCLUSIONS: In summary, we show HRK expression is distinctly regulated between different subtypes of naevi and melanoma, with supporting evidence this differential expression contributes to regulation of melanocytic proliferation.