Concomitant IDH wild-type glioblastoma and IDH1-mutant anaplastic astrocytoma in a patient with constitutional mismatch repair deficiency syndrome

Constitutional mismatch repair deficiency (CMMRD) is a rare and often under‐recognized tumour predisposition syndrome, presenting with both extracranial and malignant brain tumours that occur in children and/or young adults 1. The genetic defects underlying this disease are biallelic germline mutations in one of the DNA MMR genes leading to a constitutional DNA repair defect that causes a cancer predisposition syndrome with early onset 2. This mechanism is different from Lynch syndrome (LS) where a heterozygous germline loss‐of‐function mutation is observed and the patients are more prone to develop colon and genitourinary cancers as adults