Targeting SHOC2 and GAB2 in acral melanomas driven by RAS-GTP

Acral and cutaneous melanomas both rely on sustained mitogen-activated protein (MAP) kinase pathway activation for growth, however, unlike cutaneous melanoma, acral melanomas harbor distinct genomic drivers. These include frequent amplifications of GAB2 , loss of function mutations in NF1 , activating mutations in the KIT receptor tyrosine kinase (RTK), and mutations in canonical RAS (often codons 12 or 13 of NRAS). These mutations may occur separately or co-occur, and converge to increase RAS-GTP, resulting in oncogenic MAPK signaling.RAS-driven cutaneous melanomas have recently been found to be particularly dependent on SHOC2 for sustained MAPK signaling, however the extent to which RAS-driven acral melanomas rely on SHOC2 and/or GAB2 for sustained MAPK signaling has not extensively studied to date. To determine the extent to which SHOC2, GAB, and RAS-GTP contribute to MAPK signaling in acral melanoma, we used targeted si-RNA to SHOC2, GAB1, GAB2, or a combination of these, to determine the necessary RAS/MAPK pathway components critical for pathway activation. In parallel, we also utilized the RAS-ON inhibitor RMC-6236 (Daraxonrasib), the MEK inhibitor Trametinib, or a combination of both agents in RAS-driven acral melanoma cell lines. We found that the NRAS G12-mutant acral melanoma cell lines HS852T and Ma-Mel-27 were markedly sensitive to SHOC2 knockdown, with concurrent SHOC2/GAB1/GAB2 knockdown conferring the greatest inhibition of MAPK activity. Similarly, both cell lines were sensitive to RAS-GTP inhibition with RMC-6236, with the addition of Trametinib resulting in the greatest decrease in MAPK activity.These results demonstrate that RAS-driven acral melanomas rely on components upstream of RAS that activate MAPK signaling, such as GAB, as well as components downstream of RAS such as SHOC2 for maximal MAPK pathway activation. In keeping with this, pharmacologic inhibition of RAS-GTP with RMC-6236 combined with the MEK kinase inhibitor Trametinib resulted in maximal pathway inhibition. Therefore, targeting RAS/MAPK signaling using a vertical inhibition approach warrants further exploration in RAS-driven acral melanomas.