Differential expression of HRK regulates proliferation of acquired melanocytic naevi

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.

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.

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.

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.