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PubMed2026

Bayesian Integration of Tumor Mutational Signatures and Somatic Features Refines Pathogenicity Assessment of Germline Mismatch Repair Variants.

Variants of uncertain significance (VUS) in mismatch repair (MMR) genes represent a persistent bottleneck in germline interpretation for Lynch syndrome, creating a critical opportunity to leverage tumor biology to refine pathogenicity assessment. Although tumor features such as microsatellite instability (MSI) and immunohistochemistry (IHC) are routinely evaluated, they are typically interpreted separately from germline classification, and their quantitative contribution within ACMG/AMP frameworks remains poorly defined. We therefore analyzed paired germline and tumor sequencing data from 1110 tumors across 1073 patients with colorectal or endometrial cancer to determine whether mismatch repair-deficient (MMR-d) mutational signatures can be quantitatively integrated into Bayesian germline variant interpretation. Using COSMIC single-base substitution signatures, tumors were classified as MMR-d or MMR proficient, and an empirically derived likelihood ratio (LR) quantified the association between MMR-d signatures and pathogenic germline MMR variants. The presence of an MMR-d signature increased the likelihood of an underlying pathogenic germline MMR variant approximately eightfold (LR ≈ 8; log10 LR ≈ 0.90), whereas its absence provided moderate-to-strong benign evidence (LR ≈ 0.156; log10 LR ≈ -0.81). Applying this integrative framework to 45 germline MMR VUS, joint modeling of tumor mutational signatures with additional somatic and variant-level evidence resulted in clinically significant reclassification of 38 (84.4%) variants, including three reclassified as pathogenic or likely pathogenic and 35 as likely benign. A total of 16 downgraded variants were independently downgraded by Invitae. These findings demonstrate that tumor mutational signatures can be formally incorporated into Bayesian germline interpretation, transforming tumor data into quantitative pathogenicity evidence and offering a principled strategy to reduce VUS burden in hereditary cancer genetics.

Humans↗

PubMed2026

Mismatch repair protein MLH1 controls testis development by regulating the Hippo-YAP signaling pathway.

DNA mismatch repair (MMR) maintains genomic stability, and defects in MMR genes such as MLH1 and MSH2 predispose to cancer. Unlike other MMR components, MLH1 has unexplained roles in development, as Mlh1-deficient male mice exhibit severe testicular hypoplasia and sterility. Here, we uncover that MLH1 regulates testis development through the Hippo-Yes-associated protein (YAP) pathway. MLH1 directly binds YAP via its C-terminal domain and the WW domains of YAP, competitively inhibiting LATS1-mediated YAP phosphorylation. This interaction stabilizes YAP by suppressing ubiquitination and promotes its nuclear translocation dependent on MLH1's nuclear localization signal. Additionally, MLH1 facilitates YAP-TEAD complex formation, enabling expression of testicular development genes, including Wt1, Sox9, and Ctgf. These functions are independent of the MMR activity of MLH1. Mlh1-deficient mice show elevated YAP phosphorylation, reduced target gene expression, and impaired proliferation in developing testes. Pharmacological inhibition of the Hippo pathway kinases MST1/2 partially rescues testis hypoplasia in Mlh1-/- mice. These findings establish MLH1 as a Hippo pathway regulator and resolve its long-standing role in male gonad development.

Male↗

PubMed2026

Primary mismatch repair deficient glioma, IDH-wildtype and H3-wildtype: A giant cell tumor with potential for long-term survival occurring at all ages.

BACKGROUND: Replication repair deficiency is associated with increased risk of developing malignant gliomas. The aim of this study was to investigate primary mismatch repair deficient gliomas (PMMRDGs), a group of IDH-wildtype and H3-wildtype gliomas that is enriched among patients with CMMRD and Lynch syndrome. METHODS: We investigated how PMMRDGs differ from other gliomas with respect to DNA methylation profile, genomic alterations, histopathology, and clinical outcomes. RESULTS: PMMRDGs occur in pediatric, adolescents and the elderly, falling in two related methylation clusters and are characterized by a high frequency of replication repair deficiency. Histology showed multinucleated giant cells, and immunohistochemistry demonstrated loss of MMR protein expression. Survival analysis revealed long-term survival in patients with high mutational burden (>50 mut/Mb) and an intact chromosome 9p region, which was validated in an independent reference cohort. CONCLUSIONS: Overall, our findings indicate that PMMRDGs represent a distinct type of IDH-wildtype gliomas with potential for long-term survival likely driven by immune activation.

Humans↗

PubMed2026

Cooccurrence of Homologous Recombination Deficiency and Mismatch Repair Deficiency in Colorectal Cancer.

Homologous recombination deficiency (HRD) in colorectal cancer (CRC) remains largely unexplored. In contrast, mismatch repair deficiency (dMMR) occurs in ∼15% of patients with CRC. Although HRD and dMMR have historically been regarded as mutually exclusive, emerging evidence suggests that this mutual exclusivity may not be absolute. Here, we conducted a retrospective cohort study utilizing genomic and transcriptomic data to define HRD status in a Chinese dMMR CRC cohort (n = 99). Multiple machine learning approaches were employed to analyze the expression profiles of these tumors and to develop a classifier distinguishing HRD from homologous recombination proficiency (HRP) in dMMR CRCs. In the Chinese dMMR CRC cohort, 66% of tumors were classified as HRD. Compared with the HRP group, the HRD group had a significantly higher tumor mutational burden and better outcomes. The derived expression signature, comprising eight genes, successfully predicted HRD status in dMMR tumors with high accuracy in the training set (AUC = 0.88, Naïve Bayes) and the test set (AUC = 0.87). In this study, a subset of dMMR CRC tumors with co-occurring HRD was identified, which may have potential implications for patient stratification and the application of targeted therapies, such as PARP inhibitors, in this molecular subgroup.

colorectal cancer↗

PubMed2026

Uptake of germline testing for Lynch Syndrome in patients with deficient mismatch repair/ microsatellite-high colorectal cancer in the public hospital system in South Australia.

Lynch syndrome (LS) accounts for approximately 4% of colorectal cancer (CRC) cases and arises from pathogenic variants in mismatch repair (MMR) genes. Australian guidelines recommend universal MMR or microsatellite instability (MSI) screening in all CRC patients; however, real-world uptake remains variable. This study evaluated rates of MMR/MSI screening, germline testing, and genetics referrals across two major public hospitals in South Australia. A retrospective review of 1775 patients discussed at colorectal multidisciplinary team meetings in the Royal Adelaide and Queen Elizabeth hospitals between January 2021 and December 2023 was conducted to identify rates of MMR/MSI screening and subsequent referral of eligible patients to genetics. Of the 1129 colorectal cancer cases identified, MMR/MSI testing was performed in 93.2% (1052/1129), with deficiency detected in 12.5% (131/1052). Of these, 37% (49/131) were eligible for genetics referral after exclusion of somatic causes. Among eligible patients, 73.5% (36/49) were referred, and 43% (21/49) underwent germline testing. LS was confirmed in 12 patients (9% of deficient MMR CRC), while 9 patients (6.9%) were classified as having Lynch-like syndrome. Despite high screening rates, gaps remain in genetics referral and testing. Barriers included lack of reflex testing, loss to follow-up, and patient refusal. Targeted system-level interventions and improved genomic education are needed to enhance adherence to guidelines and optimise patient outcomes.

Humans↗

PubMed2026

Targeting DNA mismatch repair in Huntington's disease.

Somatic expansion of the HTT CAG repeat is a key feature of Huntington's disease (HD) pathogenesis. Mismatch repair (MMR) enzymes drive this process through erroneous DNA repair, with variants in MMR genes modifying the onset and progression of disease features. Cell-type-specific CAG repeat sizing recently confirmed that elevated somatic expansion underlies the selective vulnerability of HD medium spiny neurons, with expansion beyond certain CAG thresholds associated with distinct stages of cellular pathogenesis. In this review, we synthesise insights from post-mortem brain tissue, cell systems, and mouse models, detailing key CAG repeat-length-dependent changes. In addition, we critically evaluate the MMR proteins MSH3, MLH3, and PMS1 as therapeutic targets for slowing somatic expansion and outline key safety considerations for emerging MMR-modulating approaches.

Huntington Disease↗

PubMed2025

RNA/DNA Binding Protein TDP43 Regulates DNA Mismatch Repair Genes with Implications for Genome Stability.

TDP43 is an RNA/DNA binding protein increasingly recognized for its role in neurodegenerative conditions, including amyotrophic lateral sclerosis and frontotemporal dementia (FTD). As characterized by its aberrant nuclear export and cytoplasmic aggregation, TDP43 proteinopathy is a hallmark feature in over 95% of ALS/FTD cases, leading to the formation of detrimental cytosolic aggregates and a reduction in nuclear functionality within neurons. Building on our prior work linking TDP43 proteinopathy to the accumulation of DNA double-strand breaks (DSBs) in neurons, the present investigation uncovers a novel regulatory relationship between TDP43 and DNA mismatch repair (MMR) gene expressions. Here, we show that TDP43 depletion or overexpression directly affects the expression of key MMR genes. Alterations include MLH1, MSH2, MSH3, MSH6, and PMS2 levels across various primary cell lines, independent of their proliferative status. Our results specifically establish that TDP43 selectively influences the expression of MLH1 and MSH6 by influencing their alternative transcript splicing patterns and stability. We furthermore find aberrant MMR gene expression is linked to TDP43 proteinopathy in two distinct ALS mouse models and post-mortem brain and spinal cord tissues of ALS patients. Notably, MMR depletion resulted in the partial rescue of TDP43 proteinopathy-induced DNA damage and signaling. Moreover, bioinformatics analysis of the TCGA cancer database reveals significant associations between TDP43 expression, MMR gene expression, and mutational burden across multiple cancers. Collectively, our findings implicate TDP43 as a critical regulator of the MMR pathway and unveil its broad impact on the etiology of both neurodegenerative and neoplastic pathologies.

Amyotrophic lateral sclerosis↗

PubMed2024

Surviving without BRCA2: MLH1 gets R-looped in to curtail genomic instability.

While breast cancer 2 (BRCA2) loss of heterozygosity (LOH) promotes cancer initiation, it can also induce death in nontransformed cells. In contrast, mismatch repair gene mutL homolog 1 (MLH1) is a tumor-suppressor gene that protects cells from cancer development through repairing mismatched base pairs during DNA mismatch repair (MMR). Sengodan et al., in this issue of the JCI, reveal an interplay between the 2 genes: MLH1 promoted the survival of BRCA2-deficient cells independently of its MMR function. MLH1 protected replication forks from degradation, while also resolving R-loops, thereby reducing genomic instability. Moreover, MLH1 expression was regulated directly by estrogen, shedding light into the hormone-responsive nature of many BRCA2 mutant breast cancers. These results provide important insight into the genetics that drive the initiation of BRCA2-mutated breast cancers.

Humans↗

PubMed2026

Clinicopathologic Spectrum and Intraprostatic Heterogeneity of Primary Mismatch Repair-Deficient Prostate Cancer.

Mismatch repair deficiency (MMRd) is identified in a small subset of prostate cancers and has implications for therapy selection and germline testing. The clinicopathologic spectrum of primary MMRd prostate cancer remains incompletely characterized. We evaluated 32 primary treatment-naive MMRd prostatic adenocarcinomas identified at a single institution. Grade group distribution included GG5 (n=12, 38%), GG4 (n=4, 13%), GG3 (n=7, 22%), and GG2 (n=9, 28%). Intraductal and/or invasive cribriform carcinoma was identified in 78% of cases overall and in 78% of GG2 tumors. MMR protein loss predominantly involved MSH2/MSH6 (78%), with isolated MSH6 loss in 16% and MLH1/PMS2 loss in 6%. Concordant pathogenic MMR gene alterations were identified on targeted NGS in all cases, while concurrent Tier 1/2 HRR gene alterations were present in 11 cases (35%), including ATM, BRCA1, and BRCA2. Germline testing performed in 15 cases identified Lynch syndrome in 5 (33%). MMR immunohistochemistry performed on multiple tissue blocks in 14 cases revealed discordant MMR status between tumor foci in 8 cases (57%), with MMR deficiency consistently restricted to the highest-grade focus and retained expression in spatially separate lower-grade foci. One additional case demonstrated apparent intratumoral heterogeneity within a single biopsy core, with MMR deficiency restricted to a higher-grade component and retained expression in the adjacent lower-grade tumor. These findings demonstrate a broader clinicopathologic spectrum of primary MMRd prostate cancer than previously recognized. Frequent discordance of MMR status between tumor foci highlights the importance of evaluating the highest-grade tumor focus when assessing multifocal primary prostate cancer.

cribriform↗

PubMed2026

A PMS2-deficient pediatric high-grade glioma with PI3K-pathway mutations and adjacent developmental venous anomaly suggestive of CMMRD.

PURPOSE: Constitutional mismatch repair deficiency (CMMRD) is a rare hereditary cancer predisposition syndrome that frequently manifests with pediatric high-grade gliomas. However, recognition remains challenging, particularly in the absence of a clear family history. We report a pediatric high-grade glioma with PMS2 deficiency and complex molecular alterations to highlight key diagnostic clues and the importance of routine mismatch repair assessment. METHODS: Clinical, radiological, histopathological, immunohistochemical, and molecular findings of an 8-year-old girl presenting with a high-grade glioma were retrospectively evaluated. Immunohistochemistry included glial and mismatch repair markers. Targeted next-generation sequencing was performed to assess tumor mutational burden and pathogenic variants. RESULTS: Neuroimaging revealed a right frontoparietal mass associated with an adjacent developmental venous anomaly. Histopathology demonstrated a diffuse pediatric-type high-grade glioma with pseudopapillary architecture and marked mitotic activity. Immunohistochemistry showed diffuse p53 overexpression in tumor cells and complete loss of PMS2 expression in both tumor and non-neoplastic cells, supporting constitutional mismatch repair deficiency. Molecular analysis revealed an ultra-hypermutated profile with a tumor mutational burden of 117.4 mutations/Mb, a pathogenic PMS2 frameshift variant, and co-occurring alterations in TP53, PIK3CA, PIK3R1, and PTEN. The presence of PI3K-pathway mutations alongside a venous anomaly suggested a potential biological association. CONCLUSION: This case illustrates the characteristic clinicopathological and molecular features of CMMRD-associated pediatric high-grade glioma and underscores the critical role of routine mismatch repair immunohistochemistry. Integrated histological and genomic evaluation is essential for accurate diagnosis, appropriate genetic counseling, and potential therapeutic implications. Key Points • This case represents a pediatric high-grade glioma arising in the setting of PMS2-related constitutional mismatch repair deficiency (CMMRD). • The tumor exhibited an ultra-hypermutated profile with co-occurring TP53, PIK3CA, PIK3R1, and PTEN mutations. • Loss of PMS2 expression in both tumor and non-neoplastic cells was critical in establishing the diagnosis of CMMRD. • The presence of a developmental venous anomaly may relate to underlying PIK3R1 pathway alterations. • Routine mismatch repair immunohistochemistry is essential in pediatric high-grade gliomas, even in the absence of a family history.

Humans↗

PubMed2026

The Landmark Series: Mutation-Based Therapy of Pancreatic Cancer.

BACKGROUND: Pancreatic ductal adenocarcinoma (PDAC) remains a highly lethal malignancy with limited long-term survival despite advances in surgery and systemic therapy. PATIENTS: The population of interest comprises patients with PDAC characterized by targetable molecular alterations and biologically distinct transcriptomic subtypes. METHODS: We performed a narrative review of landmark and contemporary clinical trials, translational studies, and emerging molecular-classification platforms relevant to precision oncology in PDAC. RESULTS: Growing understanding of PDAC molecular biology has identified putative genetic mutations, including homologous recombination repair deficiency, mismatch repair deficiency, and mutated KRAS, enabling the development of targeted therapies and precision treatment strategies. Concurrently, transcriptomic profiling has revealed biologically distinct molecular subtypes associated with differences in prognosis and therapeutic response. Emerging tools such as molecular classifiers, deep learning models, and multiomic platforms may further refine patient selection and treatment personalization. CONCLUSIONS: This review highlights contemporary efforts of novel targeted therapies, ongoing advances in molecular subtyping, and the evolving role of precision oncology in improving outcomes for patients with PDAC.

Genomic alterations↗

PubMed2004

Lynch Syndrome

CLINICAL CHARACTERISTICS: Lynch syndrome is characterized by an increased risk for colorectal cancer (CRC) and cancers of the endometrium, ovary, stomach, small bowel, urinary tract, biliary tract, prostate, brain (usually glioblastoma), skin (sebaceous adenomas, sebaceous epithelioma, sebaceous carcinomas, and keratoacanthomas), and pancreas. Cancer risks and age of onset vary depending on the associated gene. Several other cancer types have been reported to occur in individuals with Lynch syndrome (e.g., sarcomas, adrenocortical carcinoma). However, the data are not sufficient to demonstrate that the risk of developing these cancers is increased in individuals with Lynch syndrome. DIAGNOSIS/TESTING: The diagnosis of Lynch syndrome is established in a proband with a germline heterozygous pathogenic variant in MLH1, MSH2, MSH6, or PMS2 or a 3' EPCAM deletion identified by molecular genetic testing, or, rarely, constitutional inactivation of the MLH1 promotor due to methylation identified by DNA methylation analysis. MANAGEMENT: Treatment of manifestations: Polypectomy at the time of colonoscopy; referral to advanced endoscopist for lesions requiring advanced resection techniques; surgical resection of polyps when needed; individualized surgical management for colon cancer based on tumor location, stage, Lynch syndrome-associated gene, age, comorbidity, bowel function, anticipated quality of life, and risk of metachronous CRC; mismatch repair (MMR) testing, microsatellite instability (MSI) testing, and multidisciplinary evaluation for rectal cancer prior to treatment; consider immune checkpoint inhibitor therapy for metastatic or unresectable MMR-deficient or MSI-high tumors; other tumors are managed as in the general population. Prevention of primary manifestations: Risk-reducing hysterectomy with bilateral salpingo-oophorectomy can be considered after childbearing is completed. Prophylactic colectomy prior to the development of colon cancer is generally not recommended for individuals known to have Lynch syndrome because screening colonoscopy with polypectomy is an effective preventive measure. Aspirin therapy has been shown to decrease the risk for CRC in individuals with Lynch syndrome. Surveillance: Colonoscopy with removal of precancerous polyps with frequency and initial screening based on gene involved and family history; annual education for females regarding the symptoms of endometrial and ovarian cancers; consider transvaginal ultrasound examination and endometrial biopsy every one to two years beginning at age 30 to 35 years; consider upper endoscopy examination particularly for individuals with a family history of gastric cancer and those of Asian ancestry with frequency and initial screening based on gene involved and family history; biopsies should be evaluated for H pylori infections so that appropriate treatment can be given as needed; consider capsule endoscopy and small bowel enterography for distal small bowel cancers in symptomatic persons; consider urinalysis with urine cytology annually beginning between ages 30 and 35 years; consider pancreatic cancer screening in individuals with a family history of pancreatic cancer; follow population screening guidelines and maintain awareness for signs and symptoms of other cancers. Agents/circumstances to avoid: Obesity, physical inactivity, cigarette smoking, alcohol consumption, and type 2 diabetes may increase CRC risk in individuals with Lynch syndrome. Evaluation of relatives at risk: Molecular genetic testing for the familial Lynch syndrome-related pathogenic variant is recommended for all first-degree relatives (parents, sibs, and offspring) of an affected individual in order to identify as early as possible those who would benefit from surveillance, risk-reducing interventions, and other preventive measures. Testing for constitutional MLH1 hypermethylation is recommended for all first-degree relatives of individuals with Lynch syndrome caused by constitutional MLH1 methylation. GENETIC COUNSELING: Lynch syndrome caused by a heterozygous germline Lynch syndrome-related pathogenic variant (i.e., a pathogenic variant in MLH1, MSH2, MSH6, or PMS2 or a 3' EPCAM deletion) is inherited in an autosomal dominant manner. Individuals with Lynch syndrome caused by constitutional inactivation of MLH1 by methylation typically represent simplex cases, although affected individuals from a few families have been reported with inherited MLH1 promoter methylation. The majority of individuals with a heterozygous germline Lynch syndrome-related pathogenic variant inherited the pathogenic variant from a parent who may or may not have had cancer. Each child of an individual with Lynch syndrome has a 50% chance of inheriting the Lynch syndrome-related pathogenic variant and the related cancer risks. If the reproductive partner of an individual with Lynch syndrome has a germline heterozygous pathogenic variant in the same Lynch syndrome-related gene, offspring are at risk of inheriting biallelic pathogenic variants and having constitutional mismatch repair deficiency. Once a germline Lynch syndrome-related pathogenic variant has been identified in an affected family member, predictive testing for at-risk asymptomatic family members and prenatal/preimplantation genetic testing are possible.

Hereditary Non-Polyposis Colorectal Cancer (HNPCC)↗

PubMed2015

Okazaki fragment maturation involves α-segment error editing by the mammalian FEN1/MutSα functional complex.

During nuclear DNA replication, proofreading-deficient DNA polymerase α (Pol α) initiates Okazaki fragment synthesis with lower fidelity than bulk replication by proofreading-proficient Pol δ or Pol ε. Here, we provide evidence that the exonuclease activity of mammalian flap endonuclease (FEN1) excises Pol α replication errors in a MutSα-dependent, MutLα-independent mismatch repair process we call Pol α-segment error editing (AEE). We show that MSH2 interacts with FEN1 and facilitates its nuclease activity to remove mismatches near the 5' ends of DNA substrates. Mouse cells and mice encoding FEN1 mutations display AEE deficiency, a strong mutator phenotype, enhanced cellular transformation, and increased cancer susceptibility. The results identify a novel role for FEN1 in a specialized mismatch repair pathway and a new cancer etiological mechanism.

Animals↗

PubMed2024

Adult-Onset Cancer Predisposition Syndromes in Children and Adolescents-To Test or not to Test?

With the increasing use of comprehensive germline genetic testing of children and adolescents with cancer, it has become evident that pathogenic variants (PV) in adult-onset cancer predisposition genes (aoCPG) underlying adult-onset cancer predisposition syndromes, such as Lynch syndrome or hereditary breast and ovarian cancer, are enriched and reported in 1% to 2% of children and adolescents with cancer. However, the causal relationship between PVs in aoCPGs and childhood cancer is still under investigation. The best-studied examples include heterozygous PVs in mismatch repair genes associated with Lynch syndrome in children with mismatch repair deficient high-grade glioma, heterozygous PVs in BARD1 in childhood neuroblastoma, and heterozygous PVs in BRCA2 in children with rhabdomyosarcoma. The low penetrance for pediatric cancers is considered to result from a combination of the low baseline risk of cancer in childhood and the report of only a modest relative risk of disease in childhood. Therefore, we do not advise that healthy children empirically be tested for PVs in an aoCPG before adulthood outside a research study. However, germline panel testing is increasingly being performed in children and adolescents with cancer, and exome and genome sequencing may be offered more commonly in this population in the future. The precise pediatric cancer risks and spectra associated with PVs in aoCPGs, underlying cellular mechanisms and somatic mutational signatures, as well as treatment response, second neoplasm risks, and psycho-oncological aspects require further research.

Adolescent↗

PubMed2026

Fluorescent reporter assay reveals ribonucleotides promote mismatch correction in vivo.

Ribonucleotides can serve as a strand discrimination signal in reconstituted in vitro biochemical mismatch repair (MMR) assays, but the influence of ribonucleotides on mismatch correction has not been measured directly in vivo. We have developed a fluorescence-based host cell reactivation assay that reports correction of a mismatch in proximity of a site-specifically incorporated ribonucleotide. A ribonucleotide leads to enhanced mismatch correction. While neither inactivation of a single allele nor knockdown of RNaseH2 is sufficient to suppress ribonucleotide directed MMR, a modest but statistically significant impairment for repair of mismatches in the presence of an embedded ribonucleotide is observed in RNaseH2 knockout cell lines. Reporter plasmids with ribonucleotides located in either the 3' or 5' orientation are robustly repaired in MMR-proficient cells but are weakly repaired in MMR-deficient cells, underscoring their utility as effective MMR reporters. Significant ribonucleotide-enhanced mismatch correction was consistently observed in MMR-deficient cells when the ribonucleotide is in the 3' orientation. The presence of a ribonucleotide led to enhanced MMR even in RNaseH2 knockout cells, suggesting that other enzymes may promote ribonucleotide-directed MMR. Loss of RNaseH2 was not sufficient to confer significant resistance to the alkylating agent, temozolomide, in support of a model in which ribonucleotide-directed repair events make minor contributions to the canonical MMR pathway in mammalian cells. We propose a model in which MMR-independent ribonucleotide enhanced correction of mismatches can proceed by ribonucleotide excision repair when the ribonucleotide is in the 5' direction, and proceeds by an unknown mechanism when the ribonucleotide is in the 3' direction.

DNA Mismatch Repair↗

PubMed2026

Human MutLα activates methylpurine DNA glycosylase to induce alkylation damage cytotoxicity.

Alkylation chemotherapy is commonly used against tumors such as glioblastoma, yet resistance often develops through downregulation of mismatch repair (MMR). Previous work has established that loss of MMR prevents the excision of the thymine-containing strand across O 6meG-T mismatches, thereby avoiding the futile repair cycle that ultimately leads to cell death. Here, we provide an alternative explanation to this prevailing mechanism of chemoresistance by MMR loss. We found that the MMR protein MutLα physically and functionally interacts with the base excision repair (BER) enzyme methylpurine DNA glycosylase (MPG), which processes common alkylation adducts, such as 7meG and 3meA. Biochemical reconstitution demonstrates that MutLα activates MPG glycosylase activity by promoting MPG substrate binding, and enhancing MPG release from the abasic site product, thereby facilitating enzyme turnover. This glycosylase stimulation requires ATP hydrolysis as well as the MLH1-interacting region on MPG. Both MutLα or its ability to interact with MPG promote the generation of alkylation-induced abasic sites in cells, which contribute to the cytotoxicity of methyl methanesulfonate (MMS), an SN2 alkylating agent that does not produce O 6meG. Our results provide new insight into the mechanism of alkylation chemoresistance and uncover an unappreciated cross-talk between MMR and BER.

DNA repair↗

PubMed2026

Heterozygous germline mutations in MSH3, and probably MLH3, act as classical tumour suppressors, leading to excess somatic deletion mutations, signature ID4 and increased colorectal cancer risk.

BACKGROUND: MSH3 and MLH3 are non-canonical DNA mismatch repair genes, involved in repairing insertion-deletion mutations. Colorectal cancer (CRC) and adenomas have been reported in patients with bi-allelic germline MSH3 mutations, and in a very few bi-allelic MLH3 mutation carriers. OBJECTIVES: We hypothesised that germline loss-of-function MSH3 and MLH3 mutations were akin to constitutional mismatch repair deficiency (cMMRd) and Lynch syndrome, such that CRC could result from either bi-allelic germline mutations or heterozygous germline mutations after second hits. DESIGN: About 12 000 CRC and multiple polyp cases and 460 000 controls were studied. 2023 patients underwent cancer genome sequencing. RESULTS: One CRC/multiple polyp case had bi-allelic MSH3 mutations and another, bi-allelic MLH3 mutations. MSH3 and MLH3 germline heterozygotes had an increased risk of CRC (2.2-fold, p=6.6×10-5 and 1.6-fold, p=0.028, respectively), owing to somatic 'second hits' that inactivated the wildtype allele. Single second hits sometimes inactivated both MSH3 and the nearby APC gene. All CRCs with MSH3 or MLH3 deficiency were microsatellite-stable but hypermutant. Deletions of ≥2 bp were particularly increased (~12-fold) and signature ID4 was usually present (p<0.0001). CRCs from heterozygotes without 'second hits' showed no hypermutation. CONCLUSION: The phenotypes of bi-allelic MSH3 and MLH3 mutation carriers resemble some patients with cMMRd. Heterozygous germline MSH3 and MLH3 alleles have incomplete penetrance, but increase CRC risk via hypermutation, phenotypically resembling PMS2-mutant Lynch syndrome. A causal association with specific mutations has not previously been reported for ID4 in human tumours. ID4 probably does not have a single aetiology, but can result from MSH3 or MLH3 deficiency.

COLONIC POLYPS↗

PubMed2026

Tumor-Infiltrating Basophils Are Associated With Improved Prognosis in Colorectal Cancer.

PURPOSE: Basophils are rare granulocytic cells known for their role in allergic reactions, but they may also be involved in other diseases such as cancer. However, the role of tumor-infiltrating basophils in colorectal cancer (CRC) remains unexplored. Here, we aimed to clarify the significance of basophils in CRC by analyzing tumor tissue from 2 CRC cohorts (n = 1830) and blood samples from 730 patients. MATERIALS AND METHODS: Tumor-infiltrating basophils were identified and quantified using double immunohistochemistry (proMBP1 for basophils and cytokeratin for tumor cells) combined with digital image analysis, and blood basophil counts were measured. The associations between basophils (in tumor tissue and blood) and clinicopathological features, prognosis, immune cell profiles, and systemic inflammation markers were examined. RESULTS: Higher densities of tumor-infiltrating basophils showed an inverse association with cancer-specific mortality, with stronger evidence in cohort 2 than in cohort 1. After adjusting for key prognostic factors (including disease stage and mismatch repair status), the hazard ratios for cancer-specific mortality comparing high versus low basophil density were 0.67 (95% CI, 0.43-1.03; Ptrend = .051) in cohort 1 (n = 749) and 0.54 (95% CI, 0.40-0.74; Ptrend < .001) in cohort 2 (n = 1039). Higher basophil densities were also associated with lower disease stage and less frequent lymphovascular invasion but not with mismatch repair deficiency. Blood basophil count correlated with tumor-infiltrating basophil density (r = 0.113; P = .003) but not with prognosis. CONCLUSIONS: Tumor-infiltrating basophils seem to be associated with favorable clinicopathological features and potentially improved CRC outcomes, suggesting that basophils may contribute to the tumor microenvironment.

Humans↗