KRAS is one of the most frequently mutated oncogenes in human cancer, acting as a molecular switch that locks cells into a growth-promoting state. In most colorectal cancers, the same handful of canonical KRAS variants appear again and again, particularly substitutions at codons 12 and 13 that interfere with the protein’s ability to switch itself off. These canonical mutations are potent drivers, and their recurrence has traditionally been read as evidence of strong positive selection. Yet in hypermutant cancers, tumours burdened with extraordinarily high mutation loads, a strikingly different picture emerges. Atypical KRAS variants, mutations at unusual positions or of unusual types, appear far more often than their oncogenic strength would seem to justify.

This paradox has troubled cancer evolutionary biologists for years. If a weak driver confers less of a growth advantage than a canonical one, and if both are available to a tumour, conventional evolutionary logic predicts that the stronger driver should prevail and outcompete the weaker alternative. The persistence of weak drivers in hypermutant tumours therefore demanded an explanation. One intuitive hypothesis was that these tumours might somehow favour weak drivers through differential selection, perhaps because the cellular context of a hypermutant cancer changes what mutations are useful. Nicholson and Tomlinson set out to test whether such an appeal to altered selection was actually necessary.

Their approach was to build mathematical models of tumour evolution that track the appearance and expansion of driver mutations over the course of a cancer’s development. The models incorporate the basic currency of cancer genetics: cell divisions, mutation rates, the probability that a given division produces a driver mutation, and the selective growth advantage each driver confers. By varying these parameters, the researchers could ask under what conditions weak drivers would be expected to dominate the final tumour population. The answer they found was deceptively simple. When the mutation rate per cell division is elevated, further driver events occur so rapidly that the dynamics of tumour evolution are fundamentally altered, and weak drivers can rise to dominance even in direct competition with stronger alternatives.

The intuition behind this result is worth unpacking. In a tumour with a normal mutation rate, driver events are rare. A cell that acquires a strong driver mutation has time to expand into a large clone before the next driver arises, so the strongest available driver tends to sweep through the population. But in a hypermutant tumour, driver events arrive in quick succession. A cell carrying a weak driver may acquire additional driver mutations before any stronger competitor has had the chance to establish itself. The weak driver effectively gets a head start, and the sheer speed of subsequent mutation means that the tumour’s evolutionary trajectory is shaped more by the order and timing of events than by the fine ranking of selective advantages. Selection still matters, but the race is run so fast that the starting position can matter more than the finishing speed.

The researchers also found that this effect can be amplified by mutational biases, systematic tendencies of a mutational process to produce certain nucleotide changes in certain sequence contexts rather than others. Mutational processes are not random. Ultraviolet light, tobacco smoke, defective DNA repair enzymes and defective proofreading polymerases each leave characteristic fingerprints in the genome, preferentially mutating particular bases in particular neighbourhoods. If a hypermutational process happens to target the sequence contexts required for atypical KRAS variants, while rarely generating the contexts needed for canonical ones, then the supply of candidate weak drivers is inflated relative to the supply of strong ones. Combined with an elevated mutation rate, this bias can be sufficient to make atypical drivers the dominant species in the tumour.

To test whether this theoretical framework matched reality, the team focused on a specific and clinically important example: colorectal cancers with mutations in POLE, the gene encoding the proofreading domain of DNA polymerase epsilon. When this proofreading function fails, the enzyme makes copying errors at a greatly elevated rate, producing some of the highest mutation burdens seen in any human cancer. Nicholson and Tomlinson quantified the mutation bias of the POLE-mutant process for a range of KRAS driver variants, comparing the sequence contexts generated under POLE deficiency with those generated under non-hypermutant mutational processes. They found that the combination of the POLE-specific bias and the elevated mutation rate was sufficient to explain the observed enrichment of atypical KRAS drivers in these tumours. No special selective regime needed to be invoked.

The model’s predictive power extended beyond POLE-mutant cancers. Mismatch repair deficient colorectal cancers, another hypermutant class arising from a different DNA repair defect, also show an elevated prevalence of atypical KRAS drivers. When the researchers applied their model to this setting, its predictions were consistent with the observed variant frequencies. This is a notable result because POLE deficiency and mismatch repair deficiency generate mutations through different chemical mechanisms with different sequence biases, yet both produce the same qualitative outcome: weak, atypical drivers rising to prominence. The convergence suggests that the underlying principle, hypermutation reshaping the balance between mutation supply and selection, is general rather than specific to one repair pathway.

The broader implication of the study is a clarification of how mutation and selection interact during tumorigenesis. Textbook accounts of cancer evolution often treat driver mutations as the winners of a pure selection contest, with the fittest mutations sweeping to fixation. This work shows that in hypermutant tumours, the mutational process itself can act as a filter that determines which drivers are even available in meaningful numbers, and that the tempo of mutation can override fine-grained differences in selective advantage. Differential selection across cancer types need not be invoked to explain why different tumour classes carry different driver repertoires. The variation may simply be a consequence of how fast each tumour mutates and what kinds of mutations its particular defect produces.

There are also practical consequences for precision oncology. Atypical KRAS variants have historically been poorly characterised compared with their canonical counterparts, and their functional consequences are often uncertain. If many of these variants arise as passengers or weak drivers propelled by hypermutation rather than as potent oncogenic signals, this has implications for how they are interpreted in clinical sequencing reports and for which variants become targets of drug development. More broadly, the study adds to a growing appreciation that mutation rate is not merely a background parameter of cancer evolution but an active force that shapes which evolutionary paths tumours take. Understanding the interplay between mutational processes and selection, the authors conclude, is essential for explaining the diversity of driver mutations observed across human cancers, and mathematical modelling of the kind employed here offers a way to disentangle these forces that raw genomic data alone cannot provide.

Subject of Research: Evolutionary dynamics of weak KRAS driver mutations in hypermutant colorectal cancers

Article Title: Hyper-mutational processes provide a head-start for weak cancer drivers: Explaining atypical KRAS variants

Article References: Nicholson, M. D., & Tomlinson, I. (2026). Hyper-mutational processes provide a head-start for weak cancer drivers: Explaining atypical KRAS variants. PLOS Computational Biology, 22(10), e1014858. https://doi.org/10.1371/journal.pcbi.1014858

Image Credits: AI Generated

DOI: 10.1371/journal.pcbi.1014858

Keywords: KRAS, hypermutation, POLE, colorectal cancer, mathematical modelling, cancer evolution, driver mutations, mismatch repair deficiency, mutational bias, PLOS Computational Biology, tumorigenesis, selection

RIP works during the sexual cycle of certain filamentous fungi. When the fungus detects duplicated sequences in its genome, it floods them with C-to-T mutations, effectively scrambling and disabling repetitive elements such as transposons before they can spread. The process is famously efficient at silencing genome parasites, but it has a well-known drawback: mutations do not always stay confined to the repeats. Genes located near repetitive DNA, or genes that themselves arose through duplication, can be caught in the crossfire. What has remained unclear is just how much collateral damage natural populations actually tolerate, and whether genes can survive, and even function, under such heavy mutational bombardment.

To answer that question, the researchers took an unusually broad view. Rather than examining a single laboratory strain, they analyzed 134 strains of N. crassa collected across natural populations, and they sequenced and assembled a second, highly divergent reference genome from the strain FGSC2225 to complement the classic FGSC2489 reference. Having two distantly related references matters because RIP leaves a distinctive mutational signature: a strong excess of transition mutations, particularly changes at CpA dinucleotides, which produces an elevated ratio of transitions to transversions and a characteristic depletion of the dinucleotide CpA. Using these signatures, the team could scan protein-coding genes across the population and identify those that had been hit by RIP-like hypermutation with far greater accuracy than a single-reference approach would allow.

The scale of what they found was striking. On average, each strain carried roughly 259 to 293 genes classified as highly RIP-affected genes, or HRAGs, and across the entire population the team cataloged 3,188 non-redundant HRAGs. Most of these genes carried RIP-like mutations that were specific to individual lineages rather than shared across strains, a pattern pointing to independent, repeated origins of the mutational hits. The researchers confirmed this interpretation through parent-progeny sequencing: by examining the offspring of 117 tetrads produced during sexual reproduction, they showed that new RIP-like mutations arise independently in each sexual cycle rather than being inherited as a common ancestral burden.

Perhaps the most surprising aspect of the data concerns which genes get hit. Intuition might suggest that RIP would preferentially strike genes with duplicated ancestors or genes sitting close to transposable element nests, since proximity to repeats is what draws the mutation machinery in the first place. Instead, the analysis showed the opposite: RIP-like mutations significantly affected protein-coding genes, and the effect was strongest in single-copy genes that lack ancestral paralogs and are not located near repetitive sequences. In other words, even the most protected, evolutionarily solitary genes in the genome are routinely exposed to RIP-associated hypermutation, and they persist in the population despite carrying heavy mutational loads.

That persistence raises an obvious question: can genes riddled with RIP mutations still work? The team addressed this experimentally using gene knockouts, complementation assays, and fitness evaluations. The results demonstrated that highly RIP-affected genes can be essential for fitness or even for survival, meaning that N. crassa strains are living with functional genes that have been extensively rewritten by the defense system. In one particularly vivid example, a highly RIP-affected gene was linked to resistance to sorbic acid, a preservative widely used in food and industrial applications. A gene shaped by a genome-defense mutation system turns out to have consequences for how the fungus copes with a human-made chemical stress.

The study also used comparative genomics to place RIP in a broader evolutionary context. By comparing transition-to-transversion ratios and the proportion of highly RIP-affected genes across 27 species of Ascomycete fungi, the team found that fungi capable of RIP show elevated signatures of this mutational process relative to relatives that lack it. Simulation experiments, in which RIP-like mutations were artificially introduced into the genome of the related fungus Sordaria macrospora, helped validate the detection pipeline and rule out the possibility that the observed signals were artifacts of sequencing or annotation. Phylogenetic analyses of individual HRAGs further revealed inconsistencies with the species tree, consistent with lineage-specific mutational histories rather than shared ancestry.

For decades, RIP has been framed primarily as a gatekeeper against genome inflation, a system that suppresses gene duplication by destroying duplicated sequences before they can diverge into new functions. The new findings complicate that picture. If hundreds of single-copy genes in every natural strain carry RIP-like mutations, and if many of those genes remain functional and even essential, then RIP is not simply an evolutionary dead end for the genes it touches. It may instead act as an ongoing source of genetic variation, generating novel allelic diversity on which natural selection can act. The authors suggest that RIP, beyond suppressing gene duplication, may contribute directly to the evolution of functional genes, including those that exist in only a single copy per genome.

There is also a practical payoff. Because RIP leaves a recognizable and computable mutational signature, tracing RIP-associated marks across fungal genomes offers a rapid way to flag genes that matter functionally, without waiting for slow knockout screens. The authors highlight this as a tool for gene mining in fungal pathology, where identifying functionally important genes in pathogenic fungi is a persistent challenge. If a hypermutation system marks the genes under its influence, those marks can serve as a map, guiding researchers toward genes involved in stress tolerance, host interaction, or survival, including traits such as preservative resistance that have direct industrial relevance.

The work reframes one of biology’s classic genome-defense systems as a double-edged evolutionary instrument. N. crassa has long served as the model organism in which RIP was discovered, and this population-scale analysis shows that the mechanism’s reach extends deep into the functional core of the genome. A system evolved to silence selfish DNA appears, in practice, to be continuously rewriting the fungus’s own genes, sometimes with lethal consequences, sometimes with no effect, and occasionally in ways that change how the organism survives in the world. For evolutionary biologists, the message is that genome defense and genome innovation are not separate processes; in fungi at least, they may be two faces of the same mutational machinery.

Subject of Research: Repeat-induced point mutation and its evolutionary effects on single-copy functional genes in Neurospora crassa

Article Title: Pervasive influences of repeat-induced point mutation on single-copy functional genes reveal its evolutionary importance in Neurospora crassa

Article References: Tan, H., Chen, J., Ji, R., Sun, Y., He, Z., Xue, L., Zhang, X., Yang, S., Huang, J., & Wang, L. (2026). Pervasive influences of repeat-induced point mutation on single-copy functional genes reveal its evolutionary importance in Neurospora crassa. BMC Biology. https://doi.org/10.1186/s12915-026-02756-8

Image Credits: AI Generated

DOI: 10.1186/s12915-026-02756-8

Keywords: Neurospora crassa, repeat-induced point mutation, RIP, transposable elements, genome defense, hypermutation, single-copy genes, gene duplication, fungal genetics, population genomics, sorbic acid resistance, evolutionary biology

POLE and POLD1 encode the catalytic subunits of DNA polymerase epsilon and delta, the two enzymes responsible for replicating the vast majority of the human genome during cell division. Both enzymes carry exonuclease domains that act as molecular proofreaders, excising incorrectly inserted nucleotides before they can become permanent mutations. When the genes encoding these proofreading domains are themselves mutated, the editing function fails. Tumor cells then accumulate errors at an extraordinary pace, a phenomenon sometimes described as an ultramutated or hypermutated phenotype. The resulting flood of abnormal proteins is processed and displayed on the cell surface by human leukocyte antigen molecules, making the tumor look profoundly foreign to the immune system and rendering it unusually vulnerable to immune checkpoint inhibitors such as anti-PD-1 and anti-PD-L1 antibodies.

In the new study, a team led by Yiping Lang and corresponding author Tingting Zhu enrolled 450 consecutive colorectal cancer patients and systematically characterized the mutational profiles of POLE and POLD1, examining how the mutations related to clinicopathological features, immunotherapy efficacy, and survival. The overall mutation frequency was 5.78 percent, meaning roughly one in seventeen patients carried an alteration in at least one of the two genes. Notably, the mutations were heterogeneous in type, and the researchers found no significant association between mutation status and core clinical characteristics, TNM stage, or other tumor features. In other words, a clinician could not guess from a patient’s age, stage, or tumor appearance whether the proofreading machinery was broken; only sequencing could reveal it.

The immunotherapy results were striking. Among patients with POLE or POLD1 mutations, the objective response rate, the proportion whose tumors shrank measurably, reached 72.2 percent, compared with just 27.5 percent in wild-type patients. Disease control, which also includes patients whose tumors stopped growing, was achieved in 94.4 percent of the mutated group versus 71.0 percent of the wild-type group. Survival differences were even more dramatic: median progression-free survival was 16.0 months in patients with mutations versus 9.8 months in those without, and median overall survival was 77.5 months versus 22.7 months. For a disease in which metastatic colorectal cancer has historically been among the least immunotherapy-responsive tumor types, those numbers represent a potential reclassification of an entire molecular subset of patients.

The study also probed the biological mechanisms underlying this heightened sensitivity. Tumors with POLE or POLD1 mutations showed elevated tumor mutation burden, the total count of mutations carried by the cancer cells, which supplies the raw material for neoantigens that the immune system can recognize. They also displayed increased PD-L1 expression, the ligand through which tumors suppress attacking T cells and the direct target of checkpoint inhibitors, as well as enhanced infiltration of the tumor by lymphocytes, the immune cells that must be present and functional for checkpoint blockade to work. Together, these three features paint a coherent picture: proofreading-deficient tumors generate more targets, attract more immune cells, and depend more heavily on the PD-1/PD-L1 axis to escape destruction, making them exquisitely susceptible once that escape route is blocked pharmacologically.

Immune-related adverse events, the inflammatory side effects that arise when a reinvigorated immune system attacks healthy tissue, were also more common in the mutation carriers. However, the severe forms of these toxicities were actually less frequent in the mutated group than in wild-type patients. This asymmetry, more immune activation overall but less severe toxicity, is clinically meaningful, because fears of grade three or higher immune-related adverse events are one factor that can make clinicians hesitant to prescribe checkpoint inhibitors, particularly in older or frailer patients. The data suggest that in this population, the enhanced efficacy of immunotherapy in mutation carriers does not come at the price of a higher burden of dangerous side effects.

Beyond documenting the association, the researchers built and validated an integrated prognostic nomogram, a statistical tool that combines multiple variables into a single score to estimate an individual patient’s risk. The nomogram incorporated POLE and POLD1 mutation status alongside clinical factors and achieved a concordance index of 0.717 for predicting overall survival. A C-index of 0.5 indicates predictions no better than chance, while 1.0 indicates perfect discrimination, so 0.717 represents moderate predictive accuracy, useful for auxiliary risk stratification but not definitive on its own. The authors were careful to note that the tool has restricted clinically applicable threshold ranges, meaning its outputs should be interpreted within defined boundaries rather than treated as a universal oracle.

The significance of the work lies partly in how it complements existing biomarkers. Immunotherapy is currently standard in colorectal cancer primarily for tumors that are microsatellite instability-high or mismatch repair deficient, a subset accounting for only a small fraction of cases, most of them metastatic. POLE and POLD1 proofreading mutations represent a partially overlapping but distinct route to hypermutation. By demonstrating in a large consecutive cohort that these mutations independently predict favorable immunotherapy response and prognosis, the study strengthens the case for including POLE and POLD1 in routine molecular profiling panels for colorectal cancer, potentially extending the benefits of immunotherapy to patients whose tumors would not qualify under MSI testing alone.

There are important caveats. The study was retrospective, with informed consent waived by the ethics committee of Sir Run Run Shaw Hospital for that reason, and it was conducted at Chinese institutions, so the mutation spectrum and treatment patterns may not generalize perfectly to other populations. The mutation frequency of 5.78 percent confirms that POLE and POLD1 alterations are relatively uncommon, which means large multicenter collaborations will be needed to refine the findings and to determine whether specific mutation types, for example exonuclease domain mutations versus variants of uncertain significance, carry different prognostic weight. The moderate C-index of the nomogram likewise signals that mutation status alone cannot capture the full complexity of immunotherapy outcomes, which are shaped by tumor microenvironment factors, prior treatments, and host immunity.

Even so, the study lands at a moment when the field is actively searching for biomarkers that can extend immunotherapy beyond the microsatellite instability-high minority of colorectal cancers. The convergence of high tumor mutation burden, elevated PD-L1 expression, dense lymphocyte infiltration, and exceptional clinical responses in proofreading-deficient tumors offers a mechanistically grounded rationale for prospective trials that stratify patients by POLE and POLD1 status. If those trials confirm what this cohort suggests, a routine sequencing test already performed on most colorectal tumors could quietly become one of the most consequential decisions in the treatment pathway, directing a subset of patients toward therapies their tumors are primed to answer.

Subject of Research: POLE/POLD1 mutations as predictive biomarkers for immunotherapy response and prognosis in colorectal cancer

Article Title: Clinical features and prognostic impact of POLE/POLD1 mutations in colorectal cancer treated with immunotherapy

Article References: Clinical features and prognostic impact of POLE/POLD1 mutations in colorectal cancer treated with immunotherapy. (n.d.). https://doi.org/10.1007/s00432-026-06605-7

Image Credits: AI Generated

DOI: 10.1007/s00432-026-06605-7

Keywords: colorectal cancer, POLE, POLD1, immunotherapy, tumor mutation burden, PD-L1, biomarker, prognosis, immune checkpoint inhibitors, hypermutation, nomogram, tumor-infiltrating lymphocytes