A pan-cancer analysis of microsatellite instability as a predictor of Lynch syndrome in Chinese population.

Abstract

1574 Background: Microsatellite instability (MSI) and mismatch repair deficiency (dMMR) are primarily tested in colorectal (CRC) and endometrial cancer (EC) to aid Lynch Syndrome (LS) diagnosis. A pan-cancer study presented at 2018 ASCO, however, revealed that up to 50% of LS patients had tumors not typically associated with LS, suggesting that patients with an MSI-high (MSI-H) phenotype should proceed to germline testing regardless of tumor type or family history. We thus set out to examine this potentially practice changing notion in Chinese population. Methods: MSI status and germline mutations in MLH1, MSH2, MSH6 and PMS2 genes were determined using a targeted next generation sequencing panel covering 100 MSI loci as well as MMR genes. Tumor mutation burden (TMB) levels were calculated for LS patients with MSI-H and MSS tumors, and intergroup differences were assessed using Mann Whitney U test or Fisher’s exact test. Results: Of 6,288 advanced tumors spanning > 27 cancers, 0.8% (48/6,288) were EC, 21.6% (1,362/6,288) were CRC, and 77.6% (4,878/6,288) were other malignancies. 3.6% (224/6,288) of the samples were found to be MSI-H and 3.5% (217/6,288) harbored MMR mutations (somatic and germline). Germline mutations indicative of LS were identified in 0.1% (8/6,064) of the MSS group and 17% (38/224) of the MSI-H group (p < 0.001). In contrast with the 2018 ASCO report, up to 63.8% of the 224 MSI-H tumors were CRC/EC, and only 8.9% (3/38) of the LS patients had MSI-H non-CRC/EC tumors (1 ovarian clear cell, 1 small bowel, and 1 gastric cancer). LS patients with non-CRC/EC tumors were more likely to be MSS compared to those with CRC/EC (70.0% vs 2.7%, p < 0.001). Alterations in MLH1/ MSH2 were present in 78.3% (36/46) of the LS patients, and they demonstrated significantly better correlation with a MSI-H phenotype than MSH6/PMS2 alterations (94.4% vs. 40.0%, p = 0.0005). Additionally, in line with previous reports showing co-ocurrence of MSI-H and high TMB in gastrointestinal cancers, the LS patients with MSS tumors had a significantly lower median TMB compared with the MSI-H population (4.6 muts/Mb vs. 91.8 muts/Mb, p < 0.001). Conclusions: Our study showed that in Chinese population, CRC/EC still predominated among LS-associated cancers, while non-CRC/EC LS patients were more likely to present with an MSS phenotype. The value of MSI-H/dMMR as a predictor of LS across different tumor types warrant further investigation.