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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.)

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.)