In this episode of JCO PO Article Insights, host Dr. Jiasen He summarizes the article, "Somatic Mutation Profiles of Colorectal Cancer by Birth Cohort" by Gilad, et al published October 11, 2025.

TRANSCRIPT

Jiasen He: Hello, and welcome to the JCO Precision Oncology Article Insights. I am your host, Jiasen He, and today, we will be discussing the JCO Precision Oncology article, "Somatic Mutation Profiles of Colorectal Cancer by Birth Cohort," by Dr. Gilad and colleagues.

Early-onset colorectal cancer is defined as colorectal cancer diagnosed before the age of 50. Several reports have suggested that early-onset colorectal cancer has unique characteristics. Compared with late-onset colorectal cancer, early-onset colorectal cancer cases are more commonly found in the distal colon or rectum, tend to be diagnosed at more advanced stages, and may display unfavorable histologic features.

Although the overall incidence of colorectal cancer has declined in recent decades, the incidence of early-onset colorectal cancer continues to rise. This increase appears to be driven by birth cohort effects. The reasons behind this rise remain unclear but are likely multifactorial, involving changes in demographics, diet, lifestyle, environmental exposures, and genetic predisposition. At the same time, studies have shown conflicting results regarding whether there are differences in the mutation profiles between early-onset and late-onset colorectal cancer. Therefore, it is crucial to explore whether colorectal cancer somatic mutational landscape differs across birth cohorts, as this could provide important insight into generational shifts in colorectal cancer incidence.

To address this question, the authors conducted a retrospective study to characterize the mutation spectrum of colorectal cancer across different birth cohorts. Consecutive colorectal cancer patients who underwent somatic next-generation sequencing at the University of Chicago pathology laboratory between 2015 and 2022 were retrospectively identified. Tumors were tested for 154 to 168 genes and categorized as either microsatellite stable or high according to established thresholds. Patients with hereditary cancer syndromes or inflammatory bowel disease were excluded. Participants were then grouped into birth cohorts by decades, as well as into two major groups: those born before 1960 and after 1960. Genes that were identified in at least 5% of the sample were selected and grouped into 10 canonical cancer signaling pathways. These genes and pathways were then included in the analysis to explore their association with colorectal cancer across different birth cohorts and age groups.

A total of 369 patients were included in the study, with a median birth year of 1955 and a median age at colorectal cancer diagnosis of 62.9 years. 5.4% were identified as having microsatellite-high tumors. The median tumor mutational burden was 5 mutations per megabase for microsatellite-stable tumors and 57.7 mutations per megabase for microsatellite-high tumors. Patients with microsatellite-high tumors tended to have earlier birth years and were diagnosed at an older age. However, after adjusting for potential confounders, neither birth year nor age remained statistically significant. Similarly, after controlling for confounders, no significant associations were observed between birth year or age and mutation burden.

In this cohort, APC, TP53, and KRAS were the most frequently mutated genes. No statistically significant differences in the prevalence of gene mutations were observed across birth cohorts. Correspondingly, the most affected signaling pathways were the Wnt, TP53, and (RTK)/RAS pathways. Similar to the gene-level finding, no significant differences in the prevalence of these pathways were identified among birth cohorts.

When examining patients born before and after 1960, the authors found that the older birth cohorts were diagnosed at an older age and had higher tumor mutational burden. However, no significant differences were observed in any of the genes or pathways analyzed. Among microsatellite-stable tumors, 18.3% were classified as early-onset colorectal cancer, while 81.1% were late-onset colorectal cancer. Consistent with previous reports, early-onset colorectal cancers in this cohort were more likely to be left-sided and more common among more recent birth cohorts. However, no significant differences were identified in any of the examined genes or pathways when comparing early-onset to late-onset colorectal cancer.

In this cohort, a higher prevalence of early-onset colorectal cancer was observed among more recent birth cohorts, consistent with previous reports. Still, no distinct mutational signature was identified between the early and late birth cohorts. The authors proposed that the lack of distinct mutational profile by age or birth cohort may be due to the limited number of key molecular pathways driving colorectal cancer. Although environmental exposures likely differ across generations, the downstream effects may have converged on similar biological mechanisms, leading to comparable somatic mutations across cohorts. Alternately, they proposed that the observed birth cohort differences in colorectal incidence may be driven by distinct mutation signatures, epigenetic alterations, or changes in the immune microenvironment rather than variations in canonical gene mutations.

As the authors noted, given the retrospective nature of this study, its modest sample size, and the predominance of advanced-stage tumors, larger prospective studies are needed to validate these findings.

In summary, this study found no significant differences in the mutational landscape of colorectal cancer across birth cohorts or age groups. The authors proposed that the generational shift in colorectal cancer incidence is unlikely to be driven by changes in the underlying tumor genomics. However, larger prospective studies are needed to validate these findings.

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Somatic Mutations of Colorectal Cancer by Birth Cohort

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