# Colorectal cancer in a 13‐year‐old with constitutional mismatch repair deficiency and MUTYH heterozygosity

## TL;DR

A 13-year-old boy with a rare genetic condition developed colorectal cancer, highlighting the challenges in diagnosing overlapping genetic disorders.

## Contribution

The case highlights the diagnostic complexity when CMMRD overlaps with other conditions like NF1.

## Key findings

- A 13-year-old boy was diagnosed with CMMRD due to MSH6 and MUTYH mutations.
- The patient was initially misdiagnosed with neurofibromatosis type 1.
- Accurate diagnosis is critical for managing cancer risk and treatment.

## Abstract

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.