(PA-297) Functional Evaluation of RHEB as a Lineage-Selective Dependency in Multiple Myeloma Using Integrated CRISPR, Epigenetic, and Expression Analyses

Introduction:  Multiple myeloma (MM) remains incurable and is driven by lineage-specific dependencies rather than consistent genomic mutations. RHEB, a GTPase activating mTORC1, was identified among 116 genes with MM-preferential essentiality in a genome-scale CRISPR screen by de Matos Simoes et al. (2023). Given its role in supporting proteostasis and managing ER stress, RHEB represents a plausible MM-selective therapeutic target.

Methods:   Candidate surface and plasma cell–specific genes were first curated via the Human Protein Atlas and cross-validated using RNA-seq data from the MMRF CoMMpass study. Functional prioritization was guided by CERES-based CRISPR screening data from DepMap. To independently assess RHEB dependency, we merged CRISPR gene-effect (Chronos and CERES), dependency probability, and OncoTree lineage data across 700+ cell lines. Mann–Whitney U tests were applied to compare MM versus non-MM dependency. Chromatin accessibility (ATAC-seq) and tissue expression (GTEx) were used to assess transcriptional
activity and specificity.

Results:   RHEB was initially identified as an MM-preferential gene in CERES datasets, with elevated chromatin accessibility in MM models and low expression in most normal tissues. However, our independent validation using Chronos data revealed no statistically significant
enrichment of RHEB dependency in MM versus non-MM lines (gene effect p = 0.228; dependency p = 0.111). Despite this, functional literature supports RHEB's critical role in managing MM cell survival via mTORC1 signaling, proteostasis, and ER stress buffering.

Conclusions:   Although our Chronos-based analysis did not confirm statistical enrichment of RHEB dependency in MM, converging evidence from CRISPR studies, epigenomic activation, and transcriptomic specificity suggests a lineage-selective vulnerability. Future work includes
CRISPR knockout studies in MM vs. normal plasma cells and preclinical evaluation of RHEB- targeted inhibitors or combination regimens with proteasome inhibitors. Keywords: RHEB, multiple myeloma, CRISPR screening, DepMap, mTORC1, plasma cell, proteostasis, therapeutic target