Clinical outcomes of BRAF-mutant melanoma patients have significantly improved with combination BRAF and MEK inhibitors (BRAFi/MEKi), which target components of the MAPK/ERK signalling pathway. However, the success of these targeted therapies is hampered by drug resistance; mainly contributed by overactivation of the MAPK/ERK pathway. Recently published data from our lab have demonstrated that co-inhibiting CDK4/6 and PRMT5, both downstream of the MAPK/ERK pathway, was an effective therapeutic strategy in BRAF-mutant melanoma. Hence, we hypothesised that BRAFi-resistant melanoma cells would also respond to the dual inhibition of CDK4/6 and PRMT5, considering CDK4/6 and PRMT5 lie downstream of activating events that lead to BRAFi/MEKi-resistance.
Through dose-response assays, we found that melanoma cells that have increased resistance to BRAFi remain sensitive to CDK4/6 inhibitors (CDK4/6i) and PRMT5 inhibitors (PRMT5i). More importantly, co-inhibiting CDK4/6 and PRMT5 suppressed cell proliferation in resistant cells. Only in p53 wild-type (p53WT) cells did shRNA knockdown of p53 restore their ability to proliferate in the presence of both a CDK4/6 and PRMT5 inhibitor, suggesting p53 plays a role in the robust response of p53WT cells to this novel combination. These findings were further supported by our RNA sequencing and whole genome CRISPR/CAS9 knockout screen data on p53WT melanoma cells which showed an enrichment in the p53 pathway following PRMT5 drug pressure. This reactivation of p53 is contributed by alternative splicing of MDM4, a negative regulator of p53, resulting in a loss of full length MDM4 in line with literature findings.
Overall, evidence from our studies support the functional cooperativity of CDK4/6i and PRMT5i as a combination therapy which exert their effects in part through reactivation of the p53 pathway, achieved by PRMT5’s role in regulating splicing. Thus, co-inhibiting CDK4/6 and PRMT5 serve as a potential treatment strategy for BRAF-mutant melanoma patients who are resistant to BRAFi/MEKi. Nonetheless, further studies are required to functionally validate our preliminary findings and to better characterise the response of melanoma cells to PRMT5 inhibitors to maximise PRMT5’s potential as an anti-cancer therapy.