Abstract LB293: Distinct regulation of Cyclin D mediates heterogenous response to RAS inhibition in colorectal cancer models

Abstract KRAS inhibitors have demonstrated limited efficacy as monotherapy in KRAS-mutant colorectal cancer (CRC). Here we tested a RAS(ON) multi-selective inhibitor (RMC-7977) in KRAS-mutant CRC models in vitro and find a heterogeneous response to treatment. We analyzed variables that correlate with sensitivity and find that the degree of Cyclin D1 inhibition upon RMC-7977 treatment correlates strongly with overall sensitivity to the inhibitor. We find the level of inhibition of Cyclin D1 by RMC-7977 reflects the extent of ERK dependency of its mRNA expression levels. On one end of the spectrum are cell lines in which Cyclin D1 mRNA (CCND1) is strongly ERK-dependent. In these cells, either RMC-7977 or the MEK inhibitor trametinib are sufficient to fully inhibit Cyclin D1 and cause G1 cell cycle arrest. In these tumors, resistance to RAS or MEK inhibition arises through ERK-pathway re-activation and a rebound in CCND1 mRNA. A vertical inhibition strategy with RAS + MEK inhibition to fully and/or durably suppress ERK pathway activation leads to decreased cell proliferation and increased apoptosis in these cells, exemplifying the need for a combinatorial approach to achieve complete RAS pathway inhibition in this context. On the other end of the spectrum are cell lines in which Cyclin D1 mRNA levels are relatively ERK-independent. In these lines, CCND1 mRNA and protein expression persist despite trametinib or RMC-7977 treatment. We observe that these tumor cell lines are often associated with PI3K pathway or HER2 alterations. We find that in these cells, RAS + PI3K combination inhibition can more effectively block cap-dependent Cyclin D1 protein translation through its negative effect on mTORC1 activity. In addition, PI3K inhibition, but not mTOR inhibition, can also further synergize with RAS inhibition by downregulating CCND1 mRNA. Thus, we find a potential mechanism of resistance to RAS inhibition in CRC is persistent Cyclin D1 expression, mediated either through ERK-pathway re-activation or a shift in CCND1 mRNA regulation away from the ERK pathway. Depending on the tumor’s distinct regulation of Cyclin D1, its full suppression can be achieved through increased inhibition of RAS-MAPK pathway signaling or a parallel inhibition strategy with RAS + PI3K combination inhibition. Citation Format: Philip Choi, Julia Meredith, Mallika Singh, Neal Rosen. Distinct regulation of Cyclin D mediates heterogenous response to RAS inhibition in colorectal cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB293.