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Background: Gliomas are a major cause of cancer-related death among children, adolescents, and young adults (age 0-40 years). Primary mismatch repair deficiency (MMRD) is a pan-cancer mechanism with unique biology and therapeutic opportunities. We aimed to determine the extent and impact of primary MMRD in gliomas among children, adolescents, and young adults., Methods: Clinical and molecular data were collected from a population-based cohort of children, adolescents, and young adults with gliomas from Toronto (TOR-Ped, age 0-18 years, collected Jan 1, 2000, to Dec 31, 2021; and TOR-AYA, age 18-40 years, collected Jan 1, 2000, to June 30, 2019). Additional validation paediatric cohorts from St Jude Children's Research Hospital (0-18 years, 2015-21) and the Children's Brain Tumor Network (0-18 years, 1981-2021) were used. Functional genomic tools were applied with the primary aim of assessing primary MMRD prevalence among glioma subgroups and germline impact. To evaluate the effect of primary MMRD on therapy and overall survival, Kaplan-Meier estimates were used on an additional cohort of patients with primary MMRD gliomas treated with immunotherapy., Findings: 1389 gliomas were included in the study. The prevalence of primary MMRD ranged between 3·7% and 12·4% in high-grade gliomas (overall 30 of 483; 6·2%, 95% CI 4·2-8·7) and less than 1% in low-grade gliomas (four of 899; 0·4%, 0·1-1·1; p<0·0001 by χ 2 test). Specific molecular analysis for all gliomas showed that primary MMRD was absent among oligodendrogliomas (none of 67) and uncommon in BRAF V600E gliomas (one of 110) and histone mutant-driven gliomas (one of 150). In the paediatric age group (<18 years), primary MMRD was common in IDH WT and H3 WT gliomas harbouring pathogenic TP53 variants (21 of 61; 34·4%, 22·7-47·7) and in malignant IDH mut gliomas (five of eight; 62·5%, 24·5-91·5). Germline aetiology accounted for 33 (94·3%) of 35 primary MMRD gliomas, including children, adolescents, and young adults with previously unrecognised Lynch syndrome. Survival was poor for patients with primary MMRD gliomas. Particularly poor survival was observed for those with IDH mut astrocytomas with primary MMRD when compared with those with mismatch repair-proficient gliomas (HR 12·6, 95% CI 2·8-57·5; p=0·0011 by multivariable Cox regression). Immune checkpoint blockade was associated with improved survival for patients with primary MMRD gliomas compared with conventional chemoradiotherapy regimens (HR 0·4, 0·3-0·7; p=0·0017 by multivariable Cox regression), regardless of age or germline status., Interpretation: Primary MMRD is more common than previously reported in gliomas in children, adolescents, and young adults, is enriched in specific molecular subgroups, and is associated with poor outcomes. Accurate detection, genetic testing, early diagnosis through surveillance, and implementation of immunotherapy might improve survival for these patients., Funding: The Canadian Institutes for Health Research, Stand Up to Cancer-Bristol Myers Squibb Catalyst, US National Institutes of Health, Canadian Cancer Society, Brain Canada, The V Foundation for Cancer Research, BioCanRx, Canada's Immunotherapy Network, Harry and Agnieszka Hall, Meagan's Hug, BRAINchild Canada, and the LivWise Foundation., Competing Interests: Declaration of interests AFG reports consulting fees and speaker honoraria from Servier Canada. DST reports consulting fees for Need (getneed.com). GG receives research funds from IBM, Pharmacyclics/AbbVie, Bayer, Genentech, Calico, and Ultima Genomics. GG is an inventor on patent applications related to MSMuTect, MSMutSig, POLYSOLVER, SignatureAnalyzer-GPU, MSIDetect/MSEye, LOGIC, and MinimuMM-seq; is a founder, consultant, and has privately held equity in Scorpion Therapeutics; is a founder of and has privately held equity in PreDICTA Biosciences; and has consulted for Merck. JK reports funding from Canadian Institutes of Health Research and Brain Canada; consulting as a neuropathologist for the Ontario Forensic Pathology Services; provides expert witness testimony, support to attend Canadian and American Association of Neuropathologists annual meetings, and support from the Department of Laboratory Medicine and Molecular Biology at Sunnybrook Health Science Centre to undertake a forensic neuropathology sabbatical; is the Vice Chair of the Royal College of Physicians and Surgeons of Canada Specialty Committee in Neuropathology; and reports leadership positions in the Canadian Association of Neuropathologists. SDa reports grants from Ontario Institute for Cancer Research, New Frontiers in Research Fund, Canadian Institutes of Health Research, Canadian Cancer Society, Breast Cancer Canada, American Brain Tumor Association, Synaptive Medical, VPIX Medical, Gratitude 10, and Macbeth Family Trust; is the Keenan Chair in Surgery at the Temerty Faculty of Medicine at the University of Toronto; reports royalties or licenses from Oxford University Press; and holds leaderships roles at Ontario Health (Cancer Care Ontario) and Subortical Surgery Group. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.)

DNA replication-repair deficiency (RRD) arises from pathogenic variants in the mismatch repair and/or polymerase-proofreading genes. Multiple germline cancer predisposition syndromes in children and young adults, including constitutional mismatch repair deficiency (CMMRD), Lynch, polymerase-proofreading deficiency, and rare digenic syndromes can lead to RRD cancers. The most frequent brain tumors in these children are high-grade gliomas. Embryonal tumors like medulloblastoma have also been described. Lower-grade tumors are reported from cancer surveillance initiatives. The latter has an extremely high rate of malignant transformation. Novel functional assays quantifying the genomic microsatellite indel load have been demonstrated to be highly sensitive and specific for the diagnosis of RRD cancers and children with germline CMMRD. Importantly, RRD brain tumors uniformly harbor high mutation and microsatellite burden. High T-cell infiltration makes these aggressive cancers amenable to immune checkpoint inhibition, irrespective of their germline genetic background. Synergistic combinations are reported to be successful in patients failing checkpoint inhibitor monotherapy. Future directions include the development of innovative approaches to improve immune surveillance for RRD brain cancers. Additionally, the use of novel tools including circulating tumor DNA and quantifying microsatellite indel load over time can be useful to monitor disease burden and treatment responses in patients., Competing Interests: None., (© The Author(s) 2024. Published by Oxford University Press, the Society for Neuro-Oncology and the European Association of Neuro-Oncology.)

Background: Constitutional mismatch repair deficiency (CMMRD) syndrome is a rare and aggressive cancer predisposition syndrome. Because a scarcity of data on this condition contributes to management challenges and poor outcomes, we aimed to describe the clinical spectrum, cancer biology, and impact of genetics on patient survival in CMMRD., Methods: In this cohort study, we collected cross-sectional and longitudinal data on all patients with CMMRD, with no age limits, registered with the International Replication Repair Deficiency Consortium (IRRDC) across more than 50 countries. Clinical data were extracted from the IRRDC database, medical records, and physician-completed case record forms. The primary objective was to describe the clinical features, cancer spectrum, and biology of the condition. Secondary objectives included estimations of cancer incidence and of the impact of the specific mismatch-repair gene and genotype on cancer onset and survival, including after cancer surveillance and immunotherapy interventions., Findings: We analysed data from 201 patients (103 males, 98 females) enrolled between June 5, 2007 and Sept 9, 2022. Median age at diagnosis of CMMRD or a related cancer was 8·9 years (IQR 5·9-12·6), and median follow-up from diagnosis was 7·2 years (3·6-14·8). Endogamy among minorities and closed communities contributed to high homozygosity within countries with low consanguinity. Frequent dermatological manifestations (117 [93%] of 126 patients with complete data) led to a clinical overlap with neurofibromatosis type 1 (35 [28%] of 126). 339 cancers were reported in 194 (97%) of 201 patients. The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99). Median time between cancer diagnoses for patients with more than one cancer was 1·9 years (IQR 0·8-3·9). Neoplasms developed in 15 organs and included early-onset adult cancers. CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%]). Patients with CNS tumours had the poorest overall survival rates (39% [95% CI 30-52] at 10 years from diagnosis; log-rank p<0·0001 across four cancer types), followed by those with haematological cancers (67% [55-82]), gastrointestinal cancers (89% [81-97]), and other solid tumours (96% [88-100]). All cancers showed high mutation and microsatellite indel burdens, and pathognomonic mutational signatures. MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival (overall survival at age 15 years 63% [95% CI 55-73] for PMS2, 49% [35-68] for MSH6, 19% [6-66] for MLH1, and 0% for MSH2; p<0·0001). Frameshift or truncating variants within the same gene caused earlier cancers and inferior outcomes compared with missense variants (p<0·0001). The greater deleterious effects of MLH1 and MSH2 variants as compared with PMS2 and MSH6 variants persisted despite overall improvements in survival after surveillance or immune checkpoint inhibitor interventions., Interpretation: The very high cancer burden and unique genomic landscape of CMMRD highlight the benefit of comprehensive assays in timely diagnosis and precision approaches toward surveillance and immunotherapy. These data will guide the clinical management of children and patients who survive into adulthood with CMMRD., Funding: The Canadian Institutes for Health Research, Stand Up to Cancer, Children's Oncology Group National Cancer Institute Community Oncology Research Program, Canadian Cancer Society, Brain Canada, The V Foundation for Cancer Research, BioCanRx, Harry and Agnieszka Hall, Meagan's Walk, BRAINchild Canada, The LivWise Foundation, St Baldrick Foundation, Hold'em for Life, and Garron Family Cancer Center., Competing Interests: Declaration of interests ALa reports payment from Alexion, support from Servier and stock from Gilead, outside of the submitted work. AV is co-lead role of the Consortium for Childhood Cancer Predisposition, outside of the submitted work. BH reports payment and stock from Invitae, outside of the submitted work. BC reports participation as data safety and monitoring board member in ReRad Study, participation in the chapter advisory board for Make a Wish Canada Nova Scotia, and participation in the medical advisory committee for Make a Wish Canada, outside of the submitted work. CCP reports grants from St Baldrick's Foundation, outside of the submitted work. DSZ reports consulting fees for Bayer, AstraZeneca, Accendatech, Novartis, Day One, FivePhusion, Alexion, Amgen, and Norgine, outside of the submitted work. DTB reports grants from MSD and Novocure, consulting fees from Nanocarry Therapeutics and Servier, and payment from Servier, outside of the submitted work. EO reports payment and support from Alexion for educational event, outside of the submitted work. EB reports grants from Roche and board participation for Novartis, Alexion and Gilead, outside of the submitted work. FC reports grants from Hall Hunter Foundation (UK), outside of the submitted work. HBF reports payments from Illumina and Sanofi Genzyme, support from Illumina, participation as scientific advisory committee for Sanofi Genzyme, International Gaucher Alliance and Igentify, stock from Igentify, and receipt of materials from Illumina, outside of the submitted work. IW reports grants from Chimerix and payment from COG Partners, outside of the submitted work. IS reports grants from Fondation Saint-Luc and FNRS-CDR, outside of the submitted work. JK reports other financial interests at Servier and PRA Health Sciences, outside of the submitted work. JRG-S reports participation on the board of the Philippine Society of Pediatric Oncology and Philippine Board of Pediatric Oncology, and stock in Macrogenics, Moderna, Mirati Therapeutics, CRISPR Therapeutics, Repligen, Quidelortho, and Shockwave Medical, outside of the submitted work. KJB reports consulting fees from US WorldMeds, Springworks Therapeutics, Alexion, and YmAbs, and payment from Alexion, outside of the submitted work. MS reports grants and support from the Swedish Childhood Cancer Fund, participation as a data safety and monitoring board member for clinical trial NCT05230758, and participation in the Swedish Pediatric CNS tumour group, outside of the submitted work. MAC reports financial support from SUNY Upstate Department of Pediatrics and board participation for Paige's Butterfly Run, outside of the submitted work. MO reports payment from Aptitude Health and participation on a data safety monitoring board or advisory board for Ultragenyx and Abeona, outside of the submitted works. MZ reports payment and support from and board participation for AstraZeneca. NW reports grants from CANSEARCH Foundation, Childhood Cancer Research Switzerland, and the Foundation for Children and Adolescents with Cancer; payment, support, and advisory board participation for Swedish Orphan Biovitrum; and board participation for Childhood Cancer Switzerland, outside of the submitted work. NH reports grants from the National Institutes of Health (NIH) and board participation for Incyte and Pfizer, outside of the submitted work. PCN reports grants from the Canadian Institutes for Health Research (CIHR) Foundation, US Department of Defense, and Garron Family Cancer Centre, outside of the submitted work. RP reports participation in the Collaborative Group of the Americas on Inherited Gastrointestinal Cancer, outside of the submitted work. RT reports consulting fees from Fennec Pharmaceuticals and Day One Biopharmaceuticals and payment from Fennec Pharmaceuticals, outside of the submitted work. SS reports payments from Sanofi Pharmaceuticals and Mylan Pharmaceutical, and board participation for Sanofi Pharmaceuticals, outside of the submitted work. SB reports consulting fees from Hoffmann-La Roche, YmAbs, MAP Biopharma and SERB SAS, and payment from Zschimmer & Schwarz Mohsdorf, outside of the submitted work. UI reports board participation in Pakistan Society of Pediatric Oncology, outside of the submitted work., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

We report a case of Mismatch Repair Deficiency (MMRD) caused by germline homozygous EPCAM deletion leading to tissue-specific loss of MSH2. Through the use of patient-derived cells and organoid technologies, we performed stepwise in vitro differentiation of colonic and brain organoids from reprogrammed EPCAM del iPSC derived from patient fibroblasts. Differentiation of iPSC to epithelial-colonic organoids exhibited continuous increased EPCAM expression and hypermethylation of the MSH2 promoter. This was associated with loss of MSH2 expression, increased mutational burden, MMRD signatures and MS-indel accumulation, the hallmarks of MMRD. In contrast, maturation into brain organoids and examination of blood and fibroblasts failed to show similar processes, preserving MMR proficiency. The combined use of iPSC, organoid technologies and functional genomics analyses highlights the potential of cutting-edge cellular and molecular analysis techniques to define processes controlling tumorigenesis and uncovers a new paradigm of tissue-specific MMRD, which affects the clinical management of these patients., (© 2024. The Author(s).)

Immune checkpoint inhibition (ICI) is effective for replication-repair-deficient, high-grade gliomas (RRD-HGG). The clinical/biological impact of immune-directed approaches after failing ICI monotherapy is unknown. We performed an international study on 75 patients treated with anti-PD-1; 20 are progression free (median follow-up, 3.7 years). After second progression/recurrence (n = 55), continuing ICI-based salvage prolonged survival to 11.6 months (n = 38; P < 0.001), particularly for those with extreme mutation burden (P = 0.03). Delayed, sustained responses were observed, associated with changes in mutational spectra and the immune microenvironment. Response to reirradiation was explained by an absence of deleterious postradiation indel signatures (ID8). CTLA4 expression increased over time, and subsequent CTLA4 inhibition resulted in response/stable disease in 75%. RAS-MAPK-pathway inhibition led to the reinvigoration of peripheral immune and radiologic responses. Local (flare) and systemic immune adverse events were frequent (biallelic mismatch-repair deficiency > Lynch syndrome). We provide a mechanistic rationale for the sustained benefit in RRD-HGG from immune-directed/synergistic salvage therapies. Future approaches need to be tailored to patient and tumor biology., Significance: Hypermutant RRD-HGG are susceptible to checkpoint inhibitors beyond initial progression, leading to improved survival when reirradiation and synergistic immune/targeted agents are added. This is driven by their unique biological and immune properties, which evolve over time. Future research should focus on combinatorial regimens that increase patient survival while limiting immune toxicity. This article is featured in Selected Articles from This Issue, p. 201., (©2023 American Association for Cancer Research.)

Purpose: Checkpoint inhibitors have limited efficacy for children with unselected solid and brain tumors. We report the first prospective pediatric trial (NCT02992964) using nivolumab exclusively for refractory nonhematologic cancers harboring tumor mutation burden (TMB) ≥5 mutations/megabase (mut/Mb) and/or mismatch repair deficiency (MMRD)., Patients and Methods: Twenty patients were screened, and 10 were ultimately included in the response cohort of whom nine had TMB >10 mut/Mb (three initially eligible based on MMRD) and one patient had TMB between 5 and 10 mut/Mb., Results: Delayed immune responses contributed to best overall response of 50%, improving on initial objective responses (20%) and leading to 2-year overall survival (OS) of 50% [95% confidence interval (CI), 27-93]. Four children, including three with refractory malignant gliomas are in complete remission at a median follow-up of 37 months (range, 32.4-60), culminating in 2-year OS of 43% (95% CI, 18.2-100). Biomarker analyses confirmed benefit in children with germline MMRD, microsatellite instability, higher activated and lower regulatory circulating T cells. Stochastic mutation accumulation driven by underlying germline MMRD impacted the tumor microenvironment, contributing to delayed responses. No benefit was observed in the single patient with an MMR-proficient tumor and TMB 7.4 mut/Mb., Conclusions: Nivolumab resulted in durable responses and prolonged survival for the first time in a pediatric trial of refractory hypermutated cancers including malignant gliomas. Novel biomarkers identified here need to be translated rapidly to clinical care to identify children who can benefit from checkpoint inhibitors, including upfront management of cancer. See related commentary by Mardis, p. 4701., (©2023 The Authors; Published by the American Association for Cancer Research.)

Cancers arising from germline DNA mismatch repair deficiency or polymerase proofreading deficiency (MMRD and PPD) in children harbour the highest mutational and microsatellite insertion-deletion (MS-indel) burden in humans. MMRD and PPD cancers are commonly lethal due to the inherent resistance to chemo-irradiation. Although immune checkpoint inhibitors (ICIs) have failed to benefit children in previous studies, we hypothesized that hypermutation caused by MMRD and PPD will improve outcomes following ICI treatment in these patients. Using an international consortium registry study, we report on the ICI treatment of 45 progressive or recurrent tumors from 38 patients. Durable objective responses were observed in most patients, culminating in a 3 year survival of 41.4%. High mutation burden predicted response for ultra-hypermutant cancers (>100 mutations per Mb) enriched for combined MMRD + PPD, while MS-indels predicted response in MMRD tumors with lower mutation burden (10-100 mutations per Mb). Furthermore, both mechanisms were associated with increased immune infiltration even in 'immunologically cold' tumors such as gliomas, contributing to the favorable response. Pseudo-progression (flare) was common and was associated with immune activation in the tumor microenvironment and systemically. Furthermore, patients with flare who continued ICI treatment achieved durable responses. This study demonstrates improved survival for patients with tumors not previously known to respond to ICI treatment, including central nervous system and synchronous cancers, and identifies the dual roles of mutation burden and MS-indels in predicting sustained response to immunotherapy., (© 2022. The Author(s).)

Purpose: Constitutional mismatch repair deficiency syndrome (CMMRD) is a lethal cancer predisposition syndrome characterized by early-onset synchronous and metachronous multiorgan tumors. We designed a surveillance protocol for early tumor detection in these individuals., Patients and Methods: Data were collected from patients with confirmed CMMRD who were registered in the International Replication Repair Deficiency Consortium. Tumor spectrum, efficacy of the surveillance protocol, and malignant transformation of low-grade lesions were examined for the entire cohort. Survival outcomes were analyzed for patients followed prospectively from the time of surveillance implementation., Results: A total of 193 malignant tumors in 110 patients were identified. Median age of first cancer diagnosis was 9.2 years (range: 1.7-39.5 years). For patients undergoing surveillance, all GI and other solid tumors, and 75% of brain cancers were detected asymptomatically. By contrast, only 16% of hematologic malignancies were detected asymptomatically ( P < .001). Eighty-nine patients were followed prospectively and used for survival analysis. Five-year overall survival (OS) was 90% (95% CI, 78.6 to 100) and 50% (95% CI, 39.2 to 63.7) when cancer was detected asymptomatically and symptomatically, respectively ( P = .001). Patient outcome measured by adherence to the surveillance protocol revealed 4-year OS of 79% (95% CI, 54.8 to 90.9) for patients undergoing full surveillance, 55% (95% CI, 28.5 to 74.5) for partial surveillance, and 15% (95% CI, 5.2 to 28.8) for those not under surveillance ( P < .0001). Of the 64 low-grade tumors detected, the cumulative likelihood of transformation from low-to high-grade was 81% for GI cancers within 8 years and 100% for gliomas in 6 years., Conclusion: Surveillance and early cancer detection are associated with improved OS for individuals with CMMRD., Competing Interests: Hagit N. BarisConsulting or Advisory Role: Sanofi, Igentify LtdSpeakers' Bureau: Sanofi, Takeda, Pfizer Donald BaselHonoraria: BioMarinConsulting or Advisory Role: iQvia Deborah T. BlumenthalConsulting or Advisory Role: AstraZeneca, Novocure, Takeda Miriam BornhorstConsulting or Advisory Role: AstraZeneca/MedImmune Sara RhodeHonoraria: Myriad GeneticsSpeakers' Bureau: Myriad GeneticsTravel, Accommodations, Expenses: Myriad Genetics Roula FarahHonoraria: Novo NordiskConsulting or Advisory Role: Novo Nordisk William D. FoulkesResearch Funding: AstraZeneca David GassEmployment: X4 PharmaceuticalsHonoraria: X4 PharmaceuticalsSpeakers' Bureau: Precisionscientia Heather HampelStock and Other Ownership Interests: Genome MedicalConsulting or Advisory Role: InVitae, Genome Medical, Promega, 23andMe Jordan R. HansfordConsulting or Advisory Role: Bayer Craig HarlosTravel, Accommodations, Expenses: GlaxoSmithKline Nobuko HijiyaHonoraria: NovartisConsulting or Advisory Role: Novartis, IncyteResearch Funding: Pfizer Junne KamiharaStock and Other Ownership Interests: PanTher Therapeutics, ROME Therapeutics, TellBioHonoraria: Pfizer, NanoString Technologies, Third Rock Ventures, Foundation MedicineConsulting or Advisory Role: ROME Therapeutics, Third Rock VenturesResearch Funding: PureTech, Ribon Therapeutics, ACD BiotechnePatents, Royalties, Other Intellectual Property: Patent on drug delivery device licensed to PanTher Therapeutics, Patents on Repeat RNA biomarkers and therapeutics licensed to Rome Therapeutics, Patents on Circulating Tumor Cell Biomarkers Licensed to TellBio Inc Jeffrey KnipsteinEmployment: PRA Health SciencesConsulting or Advisory Role: Atheneum Alvaro LassalettaStock and Other Ownership Interests: Gilead SciencesConsulting or Advisory Role: Shire, Jazz Pharmaceuticals, Roche, ServierTravel, Accommodations, Expenses: Shire, Gilead Sciences Simon C. LingHonoraria: AbbvieResearch Funding: Abbvie, Gilead Sciences Michael P. LinkConsulting or Advisory Role: Incyte, ADC Therapeutics, Lilly, Steba Biotech, Mesoblast, GlaxoSmithKline, SyndaxResearch Funding: Seattle Genetics, Janssen Oncology Rebecca Loret de MolaEmployment: Huron Consulting Maura MassiminoConsulting or Advisory Role: Oncoscience, Novartis Gary MasonEmployment: Janssen Research & Development, MerckStock and Other Ownership Interests: Johnson & Johnson, MerckTravel, Accommodations, Expenses: Janssen Research & Development Kim E. NicholsStock and Other Ownership Interests: IncyteResearch Funding: Incyte/Novartis, Alpine Immune Sciences Enrico OpocherConsulting or Advisory Role: AstraZenecaTravel, Accommodations, Expenses: AstraZeneca Michael OsbornTravel, Accommodations, Expenses: Amgen, Pfizer Benjamin OshrineHonoraria: Mesoblast Alyssa ReddyConsulting or Advisory Role: Novartis, AstraZeneca Lara ReichmanResearch Funding: Illumina Marc RemkeStock and Other Ownership Interests: Bayer, BB Biotech Ventures, BioNTech AG, InVitae, IDEXX Laboratories Kami Wolfe SchneiderOther Relationship: Journal of Genetic Counseling Duncan StearnsConsulting or Advisory Role: AstraZenecaOpen Payments Link: https://openpaymentsdata.cms.gov/physician/792397 Patrick TombocHonoraria: Unicare Health PlanConsulting or Advisory Role: UniCare Health Plan An Van DammeConsulting or Advisory Role: Octapharm, Pfizer, BayerResearch Funding: Johnson & JohnsonTravel, Accommodations, Expenses: Pfizer, Sobi, Shire, Roche Ira WinerResearch Funding: Oncoceutics David S. ZieglerConsulting or Advisory Role: Bayer, AmgenTravel, Accommodations, Expenses: Bayer Cynthia HawkinsConsulting or Advisory Role: BayerPatents, Royalties, Other Intellectual Property: IP for low-grade glioma and sarcoma fusion panels as well as medulloblastoma subgrouping panel David MalkinConsulting or Advisory Role: Bayer Eric BouffetConsulting or Advisory Role: NovartisResearch Funding: Roche, Bristol Myers SquibbNo other potential conflicts of interest were reported.

POLE mutations are a major cause of hypermutant cancers, yet questions remain regarding mechanisms of tumorigenesis, genotype-phenotype correlation, and therapeutic considerations. In this study, we establish mouse models harboring cancer-associated POLE mutations P286R and S459F, which cause rapid albeit distinct time to cancer initiation in vivo , independent of their exonuclease activity. Mouse and human correlates enabled novel stratification of POLE mutations into three groups based on clinical phenotype and mutagenicity. Cancers driven by these mutations displayed striking resemblance to the human ultrahypermutation and specific signatures. Furthermore, Pole -driven cancers exhibited a continuous and stochastic mutagenesis mechanism, resulting in intertumoral and intratumoral heterogeneity. Checkpoint blockade did not prevent Pole lymphomas, but rather likely promoted lymphomagenesis as observed in humans. These observations provide insights into the carcinogenesis of POLE -driven tumors and valuable information for genetic counseling, surveillance, and immunotherapy for patients. SIGNIFICANCE: Two mouse models of polymerase exonuclease deficiency shed light on mechanisms of mutation accumulation and considerations for immunotherapy. See related commentary by Wisdom and Kirsch p. 5459 ., (©2020 American Association for Cancer Research.)