Targeted genetic therapy for congenital melanocytic naevi

Abstract Severe cases of congenital melanocytic naevi (CMN) are associated with substantial cutaneous and neurological morbidity and an excess childhood mortality from melanoma. Effective treatments are lacking. This mosaic disorder is caused most commonly by a heterozygous oncogenic variant NRAS c.181C>A, p.(Q61K). As NRAS has multiple downstream effectors we have sought to treat this disease by precision genetic therapy rather than downstream pathway inhibition, hypothesising that silencing of the oncogenic allele may remove cellular protection from RAS induced apoptosis. Using a lead siRNA selected for allele specificity and fewest off-target effects on RNAseq (siNRASQ61K) we show here significant selective knockdown of variant mRNA expression and knockdown of NRAS protein levels. A single treatment to primary naevus cell cultures leads to normalisation of MAPK pathway activation, reduction in proliferation rate, and importantly, triggering of apoptosis. We go on to demonstrate that receptor-targeted lipid nanoparticles (RTNPs) successfully protect siRNA from degradation, deliver cargo to naevus cells in patient skin explants and improve naevus cell targeting. A single intradermal injection of siNRASQ61K-RTNPs into transgenic mice harbouring humanised NRASQ61K(Tyr::NRASQ61K) confirms significant selective knockdown of the variant NRAS allele in vivo.Successful targeting of variant NRAS and triggering of naevus cell apoptosis now paves the way for clinical trials of this genetic therapy for CMN.