By Gilang Cahya 
An international consortium of scientists, spearheaded by researchers at the University of California San Diego, has identified a potent microbial agent, the bacterial toxin colibactin, as a likely contributor to the concerning surge in early-onset colorectal cancer. This groundbreaking discovery, published in the prestigious journal Nature, offers a compelling explanation for a modern medical enigma that has baffled health professionals and public health organizations worldwide.
The intricate research meticulously analyzed 981 colorectal cancer genomes from individuals diagnosed with both early- and late-onset forms of the disease. These samples spanned 11 countries, encompassing regions with diverse incidences of colorectal cancer. The findings revealed a distinct genetic fingerprint left behind by colibactin, a toxin produced by specific strains of Escherichia coli that commonly inhabit the colon and rectum. This signature, characterized by a unique pattern of DNA mutations, was found to be an astonishing 3.3 times more prevalent in early-onset cases, particularly among adults under 40, when compared to those diagnosed after the age of 70. Furthermore, the study noted a particularly high concentration of these colibactin-associated mutation patterns in countries experiencing a greater burden of early-onset colorectal cancer.
"These mutation patterns serve as a historical record embedded within the genome, strongly suggesting that early-life exposure to colibactin is a primary driver behind the development of early-onset disease," stated Ludmil Alexandrov, a professor in the Department of Bioengineering and the Department of Cellular and Molecular Medicine at UC San Diego, and a senior author on the study. Alexandrov, also affiliated with the UC San Diego Moores Cancer Center and the Deputy Director of the Sanford Stem Cell Fitness and Space Medicine Center, emphasized the significance of this genetic imprint as a direct link to the disease’s origins.
While prior research, including earlier work from Alexandrov’s laboratory, had hinted at the involvement of colibactin by identifying related mutations in approximately 10-15% of all colorectal cancer cases, those studies either focused on later-onset disease or did not differentiate between age groups. This latest comprehensive analysis marks the first time a substantial enrichment of colibactin-related mutations has been definitively demonstrated in the context of early-onset colorectal cancer.
The Growing Tide of Early-Onset Colorectal Cancer
The implications of these findings are profound and underscore a disturbing trend. Colorectal cancer, once predominantly a disease affecting older adults, is now witnessing a dramatic increase in incidence among younger populations across at least 27 countries. The rate of new cases in adults under the age of 50 has roughly doubled every decade for the past 20 years. Projections indicate that if current trends persist, colorectal cancer is poised to become the leading cause of cancer-related mortality among young adults by the year 2030, a stark and concerning forecast.
The reasons behind this alarming surge have remained elusive until now. Young adults diagnosed with colorectal cancer frequently lack a family history of the disease and present with few of the traditionally recognized risk factors, such as obesity or hypertension. This lack of clear etiology has fueled speculation about the potential role of previously unidentified environmental or microbial exposures, a hypothesis that this new study directly addresses with compelling evidence.
A Serendipitous Discovery from Global Data
The genesis of this pivotal research was not initially focused on early-onset colorectal cancer. "When we embarked on this project, our intention was not to specifically investigate early-onset colorectal cancer," explained Marcos Díaz-Gay, a co-first author of the study and a former postdoctoral researcher in Alexandrov’s lab. "Our original objective was to explore global patterns of colorectal cancer to understand the disparities in incidence rates among different countries. However, as we delved deeper into the data, one of the most striking and unexpected findings was the remarkable frequency with which colibactin-related mutations appeared in the early-onset cases."
The team’s sophisticated molecular analysis revealed that the detrimental effects of colibactin commence at a very young age. By employing advanced techniques to chronologically map each identified mutational signature, the researchers demonstrated that colibactin-associated mutations emerge early in the tumor development process. This aligns with previous findings suggesting that such mutations can occur within the first decade of life. Furthermore, the study uncovered that colibactin-related mutations account for approximately 15% of what are known as "APC driver mutations" in colorectal cancer. APC driver mutations are among the earliest genetic alterations that directly contribute to the initiation and progression of cancer.
"If an individual acquires one of these critical driver mutations by the age of 10," Professor Alexandrov elaborated, "they could be decades ahead of the typical timeline for developing colorectal cancer, potentially experiencing a diagnosis at age 40 instead of the more common age of 60." This suggests that colibactin-producing bacteria may silently colonize the colons of children, initiating subtle yet significant molecular changes in their DNA and inadvertently laying the groundwork for colorectal cancer development long before any symptoms manifest.
While the evidence presented in the study offers robust support for this hypothesis, Professor Alexandrov cautioned that further rigorous research is essential to definitively establish causality and fully understand the intricate mechanisms involved.
Building on a Foundation of Mutational Signature Analysis
This significant contribution is part of the broader Cancer Grand Challenges initiative, specifically the Mutographs team, which is generously funded by Cancer Research UK. It represents a crucial advancement in the ongoing research led by Alexandrov, Díaz-Gay, and their colleagues over several years. Their collective expertise lies in deciphering the intricate patterns of DNA mutations induced by various environmental exposures, including ultraviolet radiation and bacterial toxins, as well as lifestyle factors such as smoking and alcohol consumption. Each of these agents leaves a distinctive genetic fingerprint, a unique mutational signature, within the genome, which can serve as a powerful tool in pinpointing the origins of specific cancers.
Through a sustained international collaboration involving UC San Diego, the International Agency for Research on Cancer (France), and the Wellcome Sanger Institute (UK), facilitated by Cancer Grand Challenges funding, the Mutographs team has already elucidated the mutational processes underlying esophageal, kidney, and head and neck cancers on a global scale. This latest research on colorectal cancer further expands the comprehensive understanding of cancer etiology by leveraging the power of mutational signature analysis.
By systematically cataloging these mutational patterns across thousands of cancer genomes, the researchers aim to identify novel causes of cancer that may have previously evaded detection. "Not every environmental factor or behavioral pattern we investigate leaves a discernible mark on our genome," Professor Alexandrov observed. "However, we have discovered that colibactin is indeed one of those agents that can leave such an imprint. In this specific instance, its genetic signature appears to be strongly associated with colorectal cancers occurring in young adults."
Unanswered Questions and Future Directions
The groundbreaking discovery by the research team has opened a Pandora’s Box of new questions that demand further investigation. Key among these are: How are children being exposed to colibactin-producing bacteria in the first place? What strategies can be implemented to prevent or mitigate this exposure? Are certain environmental conditions, dietary habits, or lifestyle choices more conducive to the proliferation of colibactin-producing bacteria? And crucially, how can individuals determine if they have already acquired these detrimental mutations?
The research team is actively pursuing several hypotheses to address these critical questions. They are continuing to meticulously examine the correlation between colibactin exposure and the risk of developing early-onset colorectal cancer. Furthermore, they are exploring the potential of probiotics as a safe and effective means to eliminate harmful bacterial strains from the gut. In parallel, efforts are underway to develop early detection tests that can analyze stool samples for the presence of colibactin-related mutations, offering a non-invasive screening method.
Meanwhile, the team remains committed to its global quest for identifying other cancer-linked mutational signatures. Their recent Nature study also identified an increase in specific mutational signatures in colorectal cancers originating from particular countries, including Argentina, Brazil, Colombia, Russia, and Thailand. This observation strongly suggests that local environmental exposures may also play a significant role in contributing to cancer risk within these regions.
"It is plausible that different countries harbor distinct, yet-to-be-identified causes of cancer," commented Dr. Díaz-Gay, who is now spearheading a new phase of the study from his recently established laboratory at the Spanish National Cancer Research Center (CNIO) in Madrid, Spain. "This could pave the way for the development of targeted, region-specific prevention strategies tailored to local environmental factors."
Professor Alexandrov highlighted a broader, paradigm-shifting implication of this research: the possibility that many cancers may originate from environmental or microbial exposures encountered in early life, long before any clinical diagnosis is made. "This fundamentally reshapes our understanding of cancer," he concluded. "It suggests that cancer development may not solely be a consequence of events occurring in adulthood, but could potentially be profoundly influenced by critical developmental periods in early life, perhaps even within the first few years of existence. Sustained investment in this type of fundamental research will be absolutely vital in our global endeavor to prevent and effectively treat cancer before it becomes too late."
