If the FBI turned their sights on prosecuting the molecular culprits behind cancer, they’d have their hands full. Researchers to date have implicated thousands of genes and pathways in hundreds of diverse cancers, which altogether deregulate our cells’ abilities to modulate their growth, differentiation, and survival in response to external cues. Nevertheless, among this sea of good actors turned bad, there are a few career criminals who would deserve top spots on any ‘Cancer’s Most Wanted’ list: genes that might sound familiar, like the tumor suppressors RB1 and TP53 (which produces the notorious p53 protein), members of the RAS gene family (the Sopranos of the cancer world), and an unassuming gene called MYC, which stars in today’s story. In its day job, MYC (which codes for a protein called c-MYC) is a transcription factor and master regulator of cell growth. After hours, however, MYC is also an oncogene—deregulation and over-activation of MYC drives aberrant cell growth in a stunningly large variety of cancers. Despite drawing researchers’ attention for many years, the complex regulation and cancer-driving mechanisms of MYC are still far from well understood. This is good news for Dr. Robert Eisenman, a professor in the Basic Sciences Division at Fred Hutch, who led some of the earliest studies on the function of MYC family genes and whose lab continues to study these enigmatic proteins. A recent study from the lab, published in Genes and Development, tackles the oncogenic mechanisms behind a specific MYC mutation with an innovative mouse model.
“While we often see cancer-associated mutations and large-scale genomic lesions that result in MYC upregulation, mutations to the coding regions of MYC are actually quite rare,” explains Dr. Brian Freie, a staff scientist in the Eisenman Lab and first author of the study. “One exception to this is a collection of specific mutations to MYC which affect threonine 58—a crucial amino acid whose phosphorylation leads to the degradation of MYC by the proteasome.” The logic makes sense—if cancers rely on high levels of MYC to thrive, mutating this threonine and reducing cell’s ability to tamp down MYC levels would do the trick. But do these mutations have the same effects on cells as simply overexpressing wild-type MYC? And are these mutations sufficient for tumorigenesis, or do cells need to pick up additional MYC alterations before they become cancerous? As Dr. Eisenman explains, “To date, attempts at studying the effects of these mutations use systems which require overexpressing mutant MYC, which makes it difficult to separate the specific effects of this mutation from the effects of simply having more MYC around (which we know also drives cancer).”
To circumvent these limitations, the team generated mice with a germline mutation to MYC which replaces threonine 58 with an alanine residue (Myc-T58A). The mice developed normally, and when Freie and colleagues started looking for evidence of cellular abnormalities, they were surprised to find…nothing, actually. “Even though we confirmed increased MYC protein levels in several organs, we found no evidence of cell over-proliferation or hyperplasia in these organs that is characteristic of other models featuring MYC overexpression,” notes Freie. The team didn’t give up that easily, however; they continued monitoring the mice and discovered that by one and a half years of age (roughly middle-aged, for a mouse), over 60% of Myc-T58A homozygotes developed hematopoietic malignancies—blood cancers. A barrage of follow-up experiments revealed a population of hematopoietic progenitor cells (cells which normally differentiate into mature blood cell types) as the likely culprit behind this effect: in Myc-T58A animals, these cells showed aberrant self-renewing capacity alongside an inability to differentiate into mature cell types—a classic oncogenic phenotype. Importantly, the team noticed these changes even before the mice had detectable tumors, bolstering the argument that it was this T58A mutation alone—and not other mutations picked up by tumors as they developed—that drove this phenotype.
To dig even further into the possible mechanism behind how Myc-T58A might be causing these malignancies, Freie and colleagues took a closer look at a subset of Myc-T58A mice which developed B-cell lymphomas. They isolated B-cells from mutant mice and wild-type littermates and subjected them to a bevy of high-throughput genomic methods which let them probe genome-wide mRNA levels, MYC binding site occupancy, and chromatin accessibility. Here again, they were surprised to find distinct differences between their model and traditional MYC-overexpression models. “While we did find many genes that were up- or downregulated in MYC T58A B-cells compared to controls, these represented only a subset of canonical MYC target genes,” explained Dr. Freie. “It’s also generally accepted that MYC increases the expression of its targets, so we expected to see greater MYC binding only in those genes which were upregulated in our mutant B-cells. When we looked, however, we were surprised to find that MYC binding was globally increased—upstream of genes which were upregulated in mutant B-cells, but also in the genes which were downregulated or even unaffected in the mutant cells.” Beyond these genomic effects, the team found yet more distinctions between their model and other MYC models. As Dr. Freie explained, “In cells which overexpress MYC, we generally see a shift in the metabolic pathways that cells used to generate energy and biosynthetic intermediates, as well as a transition into a more inflammatory state. In MYC T58A B-cells, however, we detect evidence of the metabolic effects of MYC without concurrent inflammatory effects.” Overall, these distinctions reveal important nuance with regard to how MYC functions in cancer—instead of simply increasing its abundance, mutations like T58A likely affect specific aspects of MYC’s function, which in turn contribute to specific phenotypic effects in the affected cells.
So, what insight are we left with? “On the highest level,” notes Dr. Eisenman, “this study really demonstrates that a single amino acid substitution in a single protein is enough to predispose the majority of mice to late-onset hematopoietic malignancies, which is both sobering and fascinating.” As Dr. Freie notes, “I think the fundamental advance of our model is the ability to mutate MYC in its native genetic context without concurrently overexpressing it—no matter where we looked, we found notable differences between our model and previous MYC overexpressing models, really highlighting that not all MYC alterations lead to the same oncogenic phenotypes and giving us a new tool to piece apart the intricate relationship between MYC function, expression, and carcinogenesis.” For sure, important questions remain to be addressed (why do mice with mutant MYC throughout their bodies preferentially develop blood cancers?), but equipped with insights from this new tool, the Eisenman Lab is poised to stay the course and come one step closer to bringing MYC to justice.
The spotlighted work was funded by the National Institutes of Health and supported by the Genomics and Bioinformatics and Comparative Medicine Shared Resources at Fred Hutch.
Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium members Drs. Robert Eisenman and Irwin Bernstein contributed to this study.
Freie, B., Carroll, P. A., Varnum-Finney, B. J., Ramsey, E. L., Ramani, V., Bernstein, I., & Eisenman, R. N. (2024). A germline point mutation in the MYC-FBW7 phosphodegron initiates hematopoietic malignancies. Genes & Development, 38(5–6), 253–272.