A Constitutional Mismatch Repair Deficiency Syndrome Presented With an Advanced Rectal Cancer in a Juvenile Female: A Case Report and Literature Review.

The constitutional mismatch repair deficiency (CMMRD) syndrome is a rare and challenging condition with a poor prognosis. It results from biallelic mismatch repair gene mutations and leads to multiorgan cancers. Therefore, we report the first case of advanced juvenile rectal cancer related to CMMRD syndrome in the Gulf region. She is a 13-year-old female, born to non-consanguineous parents with a positive family history of malignancy, presented with an eight-month history of a retractable bulging anal mass associated with diarrhea mixed with blood and constitutional symptoms. She was cachectic with café au lait spots all over her body. Upon investigation, she was found to have invasive rectal adenocarcinoma. The case was started on neoadjuvant chemoradiotherapy in addition to genetic testing which showed a homozygous pathogenic variant in PMS2, indicating CMMRD syndrome. The patient underwent pre-operative post-neoadjuvant reassessment followed by laparoscopic total proctocolectomy with ileal J-pouch creation and ileoanal anastomosis with temporary diverting loop ileostomy which later on was reversed with no complications or recurrence. The family declined to continue the adjuvant therapy but accepted the surveillance programs and genetic testing. Unusual or late presentation secondary to a very rare syndrome like CMMRD is a major challenge to clinicians, hence a high index of suspicion and proper utilization of genetics programs might be the best available solutions.