Abstract PO-36: Functional bypass of cell cycle entry checkpoints by MYC T58A mutation in germinal center-derived lymphomas

Somatic missense mutations targeting MYC9s coding sequence are found in >50% of Burkitt lymphomas and in a small fraction of diffuse large B-cell lymphomas. These mutations typically arise in translocated MYC alleles and cluster at specific protein residues, particularly Threonine 58 (T58). Previous studies in non-B cell models suggested that MYC mutations enhance oncogenic properties by escaping MYC-induced apoptosis. However, the functional impact of these mutations on the biology of germinal center (GC) B cells, where these mutations arise, has never been investigated. In GC B cells, MYC expression is induced by positive selection signals during cognate interactions with T follicular helper cells, concurrent with activation of PI3K and mTOR signaling. PI3K and mTOR are activated upon CD40 and B-cell receptor co-engagement and are critically required for positive selection and clonal expansion of GC B cells. Using newly generated MYC T58A knock-in mouse models, we found that MYC T58A mutation allows B cells to bypass a cell cycle entry checkpoint dependent on PI3K and mTOR activities. Exposure of wild-type (WT) B cells to rapamycin or, less markedly, to PI3K inhibitors, prevented the proliferation upon CD40 and IL4 stimulation ex vivo (a cytokine combination mimicking T-cell help) whereas MYC T58A mutant B cells divided multiple times and maintained adequate rates of active biosynthesis. Similarly, administration of rapamycin to immunized WT mice collapsed formed GCs while MYC T58A GCs maintained normal size and proliferation rates in vivo. These phenotypes were in part explained by the effects of PI3K/mTOR signaling on GSK3-beta, a kinase that normally phosphorylates MYC T58 to control protein turnover. Differences in metabolic and gene expression profiles between MYC WT and T58A cells offered additional insights into the mechanisms by which this mutation bypasses PI3K/mTOR checkpoint. Finally, we found that GC B cell-specific overexpression of MYC T58A, resulted in abnormally enlarged GCs upon immunization, a phenotype common to lymphoma models. Altogether, these results indicate that MYC somatic mutations confer competitive advantages to B cells by bypassing a strict requirement for upstream signals during cell cycle entry, which may support the clonal expansion of MYC mutant B cells during GC responses. These results have important translational implications, particularly given the increasing interest in the potential use of PI3K and/or mTOR inhibitors in the treatment of lymphomas. Citation Format: Jongkuen Lee, David Dominguez-Sola. Functional bypass of cell cycle entry checkpoints by MYC T58A mutation in germinal center-derived lymphomas [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-36.