In this study, we identified MMR-D TCs and investigated their mutation profiles, clinicopathological features, and prognostic significance. Previous studies have shown that the MMR-D phenotype occurs not only in hereditary non-polyposis CRC but also in endometrial cancer and breast cancer [21–23]. Mutations in MMR genes have been reported in approximately 14% of ATC cases . We examined the rate of MMR-D tumors in patients with PTC (6.7%), PDTC (4.7%), and ATC (22.2%), and our findings were in line with the results of two prior studies that evaluated the genetic landscape of TCs [12, 24].
Our cohort contained some cases wherein two MMR proteins were deficient (MLH1 and PMS2, MLH1 and MSH2, MSH2 and MSH6, MSH6 and PMS2), and our results showed a higher proportion of MSH6 and PMS2 loss. Latham et al. noted that 20% (2/10) of small bowel adenocarcinoma (SBA) patients harbored PMS2 germline mutations, and suggesting that these mutations drove the malignancy . Sugano highlighted that homozygous mutations in the PMS2 gene were related to early-onset gastrointestinal adenocarcinoma (< 20 years), multiple adenomatous polyps, childhood brain lymphoma/leukemia, and neurofibromatosis type 1 [26, 27].
In this study, we observed MSI in 3 cases. It is worth noting that 1 of the included tumors was assessed for MSI-H/MMR-D and 2 were assessed for MSI-L/MMR-D status. Studies on SBA, endometrial cancer, and CRC have reported differences between MSI testing and IHC [21, 22, 25]. The other 11 cases showed microsatellite stability (MSS). MLH1/PMS2 and MSH6 expression losses due to germline or somatic DNA variants in MLH1 or due to MLH1 promoter hypermethylation. MSH2/MSH6 expression losses due to germline or somatic DNA variants. Solitary MSH6 losses due to germline or somatic DNA variants. In these patients, there was no history of preoperative radiotherapy, although MSH6 protein expression has been reported to be lost after neoadjuvant radiation. Moreover, it is known that MMR protein loss typically corresponds to MLH1 promoter hypermethylation and MSI within small portions of tumors. The advantage of IHC is the ability to detect MMR-D cases that can potentially be missed by MSI testing, specifically for MSH6 mutations, which tend to cause weak or no MSI in endometrial carcinomas.
Our data also demonstrated that MMR-D may be associated with better disease prognosis, especially in stage IVB ATC  and stage III CRC . Related studies have also demonstrated that the improvement in survival rate may be due to the enhanced anti-tumor immune response caused by an increased neoantigen load resulting from the hypermutation phenotype of MMR-D tumors . Konishi et al pointed out that MMR CRC with a better prognosis may have fewer mutations in p53 [12, 33] and more frequent mutations in the β-catenin, transforming growth factor βreceptor type II genes , and the intense lymphocytic infiltrates observed in tumors . The penetrance of MSH6 and PMS2 mutations in CRC has been controversial because these mutations do not significantly increase the risk of extracolonic malignancies.
We found that 22% of MMR-D patients were PD-L1-positive, and OS analysis shows that a PD-L1-positive MMR-D phenotype was linked to a poor prognosis. Previous research has shown a positive rate of 14.4% among MTC patients. The five-year structural recurrence-free survival rate of patients with PD-L1-positive tumors has been reported to be nearly 28% lower than that of those with PD-L1-negative tumors. MMR status is correlated with an increased neoantigen load, causing stronger immune responses and the upregulation of immune checkpoints, including PD-1/PD-L1. Hence, positive patients are generally more likely to obtain a survival benefit from ICIs [20, 37]. Although nivolumab has been approved by the FDA for the treatment of adult and pediatric patients (age≥12 years), a multicenter study that investigated the combination of nivolumab and ipilimumab in MMR-D/MSI-H CRC patients reported an overall response rate of 55%. A more comprehensive assessment of MMR/MSI, PD-1/PD-L1 expression, TMB, and T-cell-inflamed gene expression profile (GEP) could help in improved identification of patients with higher odds of responding to ICIs in future clinical trials for advanced TCs.
There are some limitations to the present study. First, although the sample size was large in terms of TC cases, considering the retrospective nature of the study, further research is still warranted. Second, MMR associated gene mutations may need to be further involved in the present study, including mutations in the GTPase (KRAS), KRAS proto-oncogene, and serine/threonine kinase (BRAF). Finally, it is necessary to further performed next-generation sequencing along with targeted gene sequencing or whole exome/genome sequencing for cases with MMR IHC results to infer the MMR genotype status.
Overall, we have depicted the DNA MMR and PD-L1 expression patterns in TC patients in the present study. A comprehensive analysis of multiple markers could help in the development of optimal strategies for identifying patients with LA, PDTC, and ATC sensitive to immunotherapy in the near future.