RARE-14. DEVELOPMENT OF ANAPLASTIC ASTROCYTOMA AS A THIRD MALIGNANCY IN A PEDIATRIC PATIENT WITH CONSTITUTIONAL MISMATCH REPAIR DEFICIENCY (CMMRD): A CASE REPORT AND EVALUATION OF TUMOR GENOMICS IDENTIFYING BIALLELIC MSH6 MUTATIONS

Congenital mismatch repair deficiency (CMMRD) is a pediatric cancer predisposition syndrome secondary to biallelic mutations in mismatch repair genes including MLH1, MSH2, MSH6, and PMS2. Due to the resulting lack of repair mechanisms, these patients develop a high intracellular mutational burden and have a high risk of development of multiple malignancies at a young age. Similar to patients with Lynch Syndrome (monoallelic mutations in MMR genes), these patients are at risk for development of central nervous system (CNS) tumors including high grade gliomas. Forty-eight percent of patients with CMMRD are diagnosed with a CNS malignancy. In this interesting case, a patient developed three metachronous malignancies prior to the age of 13, including Burkitt lymphoma, T-Cell lymphoma and anaplastic astrocytoma. Genomic analysis revealed a high mutational burden in his initial tumors, with multiple oncogenic mutations, as well as a previously unreported germline compound heterozygous MSH6 E744fs*12 and R248fs*8 alteration. He received a gross total resection of the tumor which in previous studies has been shown to have the highest impact on survival. Surgery was followed by radiation and ongoing treatment with an immune checkpoint inhibitor with stable disease at 6 months. The purpose of this case report is to describe the interesting presentation of CMMRD and discuss the previously unreported biallelic MSH6 mutations.