JCO Precision Oncology · 7 authors, 6 centres
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This is a case report of a 6-year-old boy with stage IV rectal adenocarcinoma and mismatch repair deficiency, caused by somatic MLH1 promoter hypermethylation, who was treated with combination nivolumab and ipilimumab immunotherapy. The patient achieved a sustained partial response after 17 months of treatment.
Tumor analysis revealed mismatch repair deficiency with loss of MLH1 and PMS2 proteins, high microsatellite instability, high tumor mutational burden, and somatic MLH1 promoter hypermethylation. Germline testing for Lynch syndrome was negative. Due to the tumor's molecular profile, the patient was treated with combination immunotherapy (nivolumab and ipilimumab), followed by nivolumab maintenance. Follow-up imaging at 2 months and 6 months showed a decrease in tumor size and resolution of the liver lesion. The patient maintained stable disease for 17 months with manageable toxicity. The report highlights the efficacy of immunotherapy for mismatch repair-deficient tumors in a pediatric patient and the importance of molecular profiling, even in the absence of a hereditary cancer syndrome.
