Colorectal cancer in a 13-year-old with constitutional mismatch repair deficiency and MUTYH heterozygosity.

Constitutional mismatch repair deficiency (CMMRD) is a rare hereditary cancer syndrome resulting from biallelic mutations in DNA mismatch repair (MMR) genes that lead to early-onset cancers in children, including lymphoma and colorectal cancer (CRC). This case report presents a 13-year-old boy diagnosed with CMMRD due to a homozygous MSH6 mutation and a heterozygous MUTYH mutation. The patient's initial misdiagnosis as neurofibromatosis type 1 (NF1) highlights the overlap between CMMRD and NF1, as their overlapping genetic pathologies can yield similar clinical manifestations. This case emphasizes the complexity of genetic diagnoses, particularly when multiple predispositions like MMR and MUTYH mutations coexist. Accurate identification of CMMRD and associated mutations is crucial for timely management and genetic counseling, given its significant implications for cancer risk and treatment strategies.