By Gilang Cahya 
In a groundbreaking development that sheds light on a perplexing modern medical enigma, an international consortium of researchers, spearheaded by the University of California San Diego (UC San Diego), has pinpointed a likely microbial instigator behind the escalating incidence of early-onset colorectal cancer: a potent bacterial toxin known as colibactin. This revelation, published on April 23 in the prestigious journal Nature, offers a critical new perspective on the factors contributing to a disease that is increasingly striking individuals at younger ages.
The investigation, a monumental undertaking involving the analysis of 981 colorectal cancer genomes from patients spanning 11 countries with diverse colorectal cancer risk profiles, has uncovered a distinct genetic signature left by colibactin. This signature, characterized by specific patterns of DNA mutations, was found to be a staggering 3.3 times more prevalent in early-onset cases – particularly in adults under the age of 40 – when compared to individuals diagnosed with the disease after the age of 70. Furthermore, these colibactin-associated mutation patterns showed a pronounced correlation with countries that exhibit a higher incidence of early-onset colorectal cancer.
A Microbial Culprit in the Making
Colibactin is produced by specific strains of Escherichia coli (E. coli) bacteria that commonly inhabit the human colon and rectum. While these bacteria are often benign residents, certain strains possess the capability to produce colibactin, a toxin with a remarkable ability to directly alter DNA. The UC San Diego-led study provides compelling evidence that exposure to colibactin, potentially commencing in early childhood, imprints a unique and enduring mark on the DNA of colon cells. This genetic imprinting, researchers posit, significantly elevates the risk of developing colorectal cancer before the age of 50.
“These mutation patterns are a kind of historical record in the genome, and they point to early-life exposure to colibactin as a driving force behind early-onset disease,” stated senior author Ludmil Alexandrov, a professor in the Shu Chien-Gene Lay Department of Bioengineering and the Department of Cellular and Molecular Medicine at UC San Diego. Professor Alexandrov, who also holds affiliations with the UC San Diego Moores Cancer Center and serves as Deputy Director of the Sanford Stem Cell Fitness and Space Medicine Center, emphasized the significance of these genetic fingerprints as diagnostic tools.
Previous research, including earlier investigations by Professor Alexandrov’s laboratory, had identified colibactin-related mutations in approximately 10 to 15 percent of all colorectal cancer cases. However, these earlier studies either focused exclusively on late-onset disease or did not differentiate between early- and late-onset diagnoses. This latest comprehensive study represents a pivotal advancement by demonstrating, for the first time, a substantial and statistically significant enrichment of colibactin-related mutations specifically within the early-onset cohort.
The Escalating Crisis of Early-Onset Colorectal Cancer
The implications of these findings are profound and underscore a growing public health crisis. Colorectal cancer, once predominantly considered a disease affecting older adults, has seen a dramatic and alarming surge in incidence among younger populations across at least 27 countries worldwide. Over the past two decades, the incidence of colorectal cancer in adults under the age of 50 has roughly doubled every decade. Projections indicate that if current trends persist, colorectal cancer is poised to become the leading cause of cancer-related death among young adults by the year 2030, a stark statistic that highlights the urgency of understanding its underlying causes.
The reasons behind this disturbing rise have, until now, remained largely elusive. Young adults diagnosed with colorectal cancer often present without a family history of the disease and typically have few of the traditionally recognized risk factors, such as obesity or hypertension. This absence of clear explanations has fueled intense speculation about the role of potential, yet undiscovered, environmental or microbial exposures, a hypothesis that this new study directly and robustly investigates.
“When we started this project, we weren’t planning to focus on early-onset colorectal cancer,” confessed study co-first author Marcos Díaz-Gay, who was a postdoctoral researcher in Professor Alexandrov’s lab at the time of the study. “Our original goal was to examine global patterns of colorectal cancer to understand why some countries have much higher rates than others. But as we dug into the data, one of the most interesting and striking findings was how frequently colibactin-related mutations appeared in the early-onset cases.”
The Molecular Timeline of Colibactin’s Damage
The research team’s meticulous analysis indicates that colibactin’s damaging effects begin remarkably early in life. By employing molecular techniques to precisely date each identified mutational signature within the tumor genomes, the scientists have demonstrated that colibactin-associated mutations emerge early in the process of tumor development. This aligns with prior research suggesting that such mutations can occur within the first decade of life. Furthermore, the study revealed that colibactin-related mutations account for approximately 15 percent of what are known as APC driver mutations. These APC mutations are among the earliest genetic alterations that directly promote the development of cancer in the colon.
Professor Alexandrov elaborated on the long-term consequences of these early-life events: “If someone acquires one of these driver mutations by the time they’re 10 years old, they could be decades ahead of schedule for developing colorectal cancer, getting it at age 40 instead of 60.” This suggests a scenario where colibactin-producing bacteria may silently colonize the colons of children, initiating subtle yet critical molecular changes in their DNA and potentially laying the groundwork for colorectal cancer to develop long before any outward symptoms manifest.
While the findings provide powerful evidence supporting this hypothesis, Professor Alexandrov cautioned that further rigorous research is essential to definitively establish causality.
Building on a Foundation of Mutational Signature Analysis
This latest breakthrough is a significant milestone within the broader context of the Cancer Grand Challenges team, Mutographs, an initiative funded by Cancer Research UK. Professor Alexandrov, Dr. Díaz-Gay, and their international colleagues have been at the forefront of advancing research in decoding the patterns of DNA mutations caused by various environmental exposures, including ultraviolet radiation and bacterial toxins, as well as lifestyle behaviors such as smoking and alcohol consumption. Their work is predicated on the understanding that each of these factors leaves a distinct genetic fingerprint – a unique mutational signature – within the genome, which can serve as a crucial clue in pinpointing the origins of specific cancers.
As part of a sustained, long-term collaboration involving UC San Diego, the International Agency for Research on Cancer (IARC) in France, and the Wellcome Sanger Institute in the UK, facilitated by Cancer Grand Challenges funding, the Mutographs team has previously elucidated the mutational processes underlying esophageal, kidney, and head and neck cancers globally. The current findings on colorectal cancer further expand this comprehensive understanding of cancer etiology through the sophisticated analysis of mutational signatures.
By systematically cataloging these intricate mutational patterns across thousands of cancer genomes, the researchers aim to identify novel causes of cancer that have previously evaded detection. “Not every environmental factor or behavior we study leaves a mark on our genome,” Professor Alexandrov remarked. “But we’ve found that colibactin is one of those that can. In this case, its genetic imprint appears to be strongly associated with colorectal cancers in young adults.”
Addressing the Unknowns and Future Directions
The team’s groundbreaking discovery naturally gives rise to a multitude of pressing questions. How are children being exposed to colibactin-producing bacteria? What preventative measures can be implemented to mitigate this exposure? Are there specific environmental conditions, dietary habits, or lifestyle choices that may foster colibactin production? And crucially, how can individuals ascertain if they have already acquired these colibactin-induced mutations?
In response to these critical questions, the research team is actively pursuing several avenues of investigation. They are delving deeper into the correlation between colibactin and the risk of early-onset colorectal cancer. Concurrently, they are exploring the potential therapeutic role of probiotics in safely eradicating harmful bacterial strains from the gut microbiome. Furthermore, the development of early detection tests that analyze stool samples for the presence of colibactin-related mutations is a key priority.
Beyond the immediate focus on colibactin, the team continues its global endeavor to identify other cancer-linked mutational signatures. The recent Nature study also highlighted an increase in specific mutational signatures within colorectal cancers originating from certain countries, including Argentina, Brazil, Colombia, Russia, and Thailand. This observation suggests that localized environmental exposures may also play a significant role in shaping cancer risk within particular regions.
“It’s possible that different countries have different unknown causes,” Dr. Díaz-Gay, who is now leading a new phase of the study from his recently established laboratory at the Spanish National Cancer Research Center (CNIO) in Madrid, Spain, commented. “That could open up the potential for targeted, region-specific prevention strategies.”
Professor Alexandrov articulated a broader implication of this line of research: the growing possibility that many cancers may originate from environmental or microbial exposures encountered in early life, long before any clinical diagnosis becomes apparent. “This reshapes how we think about cancer,” he concluded. “It might not be just about what happens in adulthood – cancer could potentially be influenced by events in early life, perhaps even the first few years. Sustained investment in this type of research will be critical in the global effort to prevent and treat cancer before it’s too late.” This paradigm shift in understanding cancer’s origins holds immense promise for developing more effective, and potentially earlier, preventative and therapeutic interventions.
