A new cell-by-cell atlas of 189 colorectal tumors has revealed biological differences that may help explain why the disease strikes younger adults with particular force. Researchers using single-nucleus RNA sequencing found that tumors in younger patients contain a higher proportion of connective tissue cells, which may shield cancer from both the immune system and chemotherapy.
The Rising Tide in Younger Adults
Colorectal cancer is now the leading cause of cancer deaths for people under 50. Cases in this age group have risen nearly 60 percent since the early 2000s, with diagnoses before age 50 increasing 1 to 3 percent per year since the mid-1990s. Dr. Jason Hall, surgeon-in-chief at Tufts Medical Center, has called the trend an epidemic, noting that such rapid change across a single generation is unusual for any disease.
A separate report published in NEJM Evidence shows the increase is no longer confined to younger adults. Overall colorectal cancer rates rose for the first time in 20 years, climbing to 38.3 cases per 100,000 Americans in 2023 from 36.7 in 2022. The steepest climb appeared in the 45-to-49 age group, where rates jumped about 13 percent annually from 2019 to 2023. Increases also emerged in people aged 65 to 69, while rates plateaued for those 70 to 74.
Biological Differences Revealed in Single-Nucleus Study
The study, published in Nature Medicine, analyzed frozen tumor tissue from patients aged 29 to over 90. The cohort included 54 people diagnosed before 50 and 135 diagnosed later. By reading gene activity in individual cells across this lifespan range, researchers could track gradual changes rather than simply comparing two age groups.
Early findings suggest the tumor microenvironment differs markedly by age. In older patients, the pattern reversed. Dr. Ana Anderson, a co-author and professor of neurology at Harvard Medical School, noted that clinical data indicates the disease in younger patients is more aggressive overall, and that these patients, despite being otherwise healthy, do not do well with standard treatment.
How Connective Tissue May Shield Tumors
The encouraging part of this discovery is that it points to a specific mechanism. Stromal cells are known to release compounds that suppress immune cells, potentially helping tumors evade immune attack. Dense connective tissue can also act as a physical barrier, limiting the reach of chemotherapy drugs. Either effect could help explain why younger patients often respond poorly to standard regimens.
Researchers also found heightened gene activity linked to epithelial-to-mesenchymal transition, or EMT, signaling in younger patients’ tumors. This process makes cancer cells more mobile and capable of invading other organs, which may contribute to faster spread. The tumors also contained many long noncoding RNAs, genetic molecules whose role here remains unknown.
Updated Screening Guidelines and Recognizing Warning Signs
Doctors and the U.S. Preventive Services Task Force recommend that average-risk adults begin colorectal cancer screening at age 45. Options include stool-based tests that check for hidden blood or abnormal DNA, and colonoscopy, which allows a doctor to examine the colon and rectum for polyps and cancer.
Warning signs include blood in the stool, constipation, and diarrhea. Dr. Yixing Jiang, an oncologist at Penn State Cancer Institute, emphasizes that symptoms can be subtle. Many patients have such minor bleeding they do not notice it, and changes in bowel habits are often dismissed as stress. Largely, colorectal cancers are found during routine colonoscopy in people with no symptoms at all.
Broader Epidemiological Trends
The NEJM Evidence analysis, based on National Cancer Institute SEER data from 2000 to 2023, suggests a birth cohort effect: people born during and after the 1960s appear to carry higher risk as they age. Lead author Caitlin Murphy of the University of Chicago said the findings mean rising colorectal cancer rates can no longer be viewed as a problem limited to younger adults.
The Nature Medicine study describes differences in tumor makeup but does not prove those differences cause worse outcomes, nor does it explain why incidence is rising. The sample, while large for this type of analysis, came from banked tissue and may not represent the broader population. Anderson said the work opens a new avenue for research into a disease we previously had no idea about. Patients should talk to their doctor about screening timing and any persistent changes in bowel habits.
Sources
- The Medical Minute: Why are GI cancers rising in young adults?
- New clues in the hunt to explain why aggressive colorectal cancer is rising in younger people
- A Cell-by-Cell Atlas of 189 Tumors Offers New Clues to Why Colorectal Cancer Hits Younger Adults So Hard
- The Rise in Colorectal Cancer Is No Longer Only Limited to Young Adults, New Study Finds
- New clues in the hunt to explain why aggressive colorectal cancer is rising in younger people
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