The impact of international care networks on the clinical management of constitutional mismatch repair deficiency (CMMRD): a review of recent developments

Constitutional mismatch repair deficiency (CMMRD) is a rare and likely the most penetrant cancer predisposition syndrome caused by biallelic germline variants in a mismatch repair gene. Patients typically develop a spectrum of malignancies, including brain tumors, gastrointestinal cancers, and hematological neoplasms within the first two decades of life. Since its initial description in 1999, two international consortia, the International Replication Repair Deficiency Consortium (IRRDC) and Care for CMMRD (C4CMMRD), have been established to better understand the syndrome, leading to the creation of diagnostic guidelines and surveillance protocols. This review summarizes recent data on the CMMRD tumor spectrum and discusses the consortia's updated diagnostic criteria and management guidelines, including novel blood-based assays for detecting constitutional microsatellite instability. Furthermore, we present the initial results and subsequent adjustments to international surveillance protocols. Finally, we discuss the demonstrated efficacy of immune checkpoint inhibitor (ICI) treatment, a key therapeutic advancement for CMMRD patients.