Tumor Molecular Testing Guides Anti-PD-1 Therapy and Provides Evidence for Pathogenicity of Mismatch Repair Variants

Lynch syndrome is characterized by germline abnormalities in mismatch repair (MMR) genes, leading to predisposition to multiple cancers. A second hit to the unaffected allele is required for tumorigenesis. MMR proteins repair incorrectly paired nucleotides and prevent generation of insertions and deletions at microsatellites. Aberrancies in these MMR proteins can be a result of germline mutations or somatic alterations. Defective MMR results in microsatellite instability (MSI) and a high mutational burden. The clinical implications of MSI are becoming readily apparent, as presence of MSI leads to the generation of neoantigens, stimulating tumor-associated lymphocytes. This has led to the use of programmed cell death protein 1 blockade for MMR-deficient tumors. The U.S. Food and Drug Administration recently approved pembrolizumab for any advanced solid tumor demonstrating MSI and nivolumab for metastatic MSI colorectal cancer. However, the clinical significance of numerous MMR gene variants remains uncertain. The International Society for Gastrointestinal Hereditary Tumors classification system categorizes 2,360 MMR variants, which can be used to gauge pathogenicity. There are many variants of uncertain significance (VUS; or class 3) for which clinicians are unable to provide recommendations. In this study, we employed the combination of germline testing and tumor mutational assessment to help discern the clinical relevance of VUS and guide immunotherapeutic decisions. Key Points: A clinical dilemma arises when genomic testing yields variants of uncertain significance (VUS). Germline VUS were identified in two patients with gastrointestinal malignancies, but only one patient had a second-hit mutation in a mismatch repair gene leading to mismatch repair deficiency that conferred response to immunotherapy. The combination of germline testing along with tumor mutational assessment can help discern the clinical relevance of VUS and can help guide therapeutic decision-making toward individualized patient care.