Brian Freie1, Patrick A. Carroll1, Barbara J. Varnum-Finney2, Erin L. Ramsey1, Vijay Ramani3, Irwin Bernstein2 and Robert N. Eisenman1 1Basic Sciences Division, Fred Hutchinson Cancer Center, Seattle, Washington 98109, USA; 2Clinical Research Division, Fred Hutchinson Cancer Center, Seattle, Washington 98109, USA; 3Gladstone Institute for Data Science and Biotechnology, University of California, San Francisco, San Francisco, California 94158, USA Corresponding authors: eisenmanfredhutch.org, bfreiefredhutch.org Abstract

Oncogenic activation of MYC in cancers predominantly involves increased transcription rather than coding region mutations. However, MYC-dependent lymphomas frequently acquire point mutations in the MYC phosphodegron, including at threonine 58 (T58), where phosphorylation permits binding via the FBW7 ubiquitin ligase triggering MYC degradation. To understand how T58 phosphorylation functions in normal cell physiology, we introduced an alanine mutation at T58 (T58A) into the endogenous c-Myc locus in the mouse germline. While MYC-T58A mice develop normally, lymphomas and myeloid leukemias emerge in ∼60% of adult homozygous T58A mice. We found that primitive hematopoietic progenitor cells from MYC-T58A mice exhibit aberrant self-renewal normally associated with hematopoietic stem cells (HSCs) and up-regulate a subset of MYC target genes important in maintaining stem/progenitor cell balance. In lymphocytes, genomic occupancy by MYC-T58A was increased at all promoters compared with WT MYC, while genes differentially expressed in a T58A-dependent manner were significantly more proximal to MYC-bound enhancers. MYC-T58A lymphocyte progenitors exhibited metabolic alterations and decreased activation of inflammatory and apoptotic pathways. Our data demonstrate that a single point mutation stabilizing MYC is sufficient to skew target gene expression, producing a profound gain of function in multipotential hematopoietic progenitors associated with self-renewal and initiation of lymphomas and leukemias.

Received October 23, 2023. Accepted March 19, 2024.