The role of MMR protein during DNA replication is to retain genomic stability by correcting the base mismatches [11]. We have been investigating the molecular alterations of the deficient MMR colon cancer and previously reported about the subset of poorly differentiated colon carcinoma with microsatellite instability harboring less expression of CDX2 [12]. Although CDX2 has a microsatellite (G)7 sequence tract in its coding region, our previous analysis showed that its marked reduction was a result of dominant-negative pathways regulating CDX2 transcription [13]. In contrast, in the present case, the reduced expression of MSH6 with a microsatellite (C)8 region is likely due to a somatic mutation caused by loss of an MMR component.
As shown in this case, this patient was preoperatively suspected of Lynch syndrome from his family history. Subsequent analysis of IHC for MMR proteins demonstrated MLH1/MSH6 deficiency in colon cancer and liver metastasis, and MLH1 deficiency in gastric cancer. On the other hand, colon cancer specimens of his father and brother showed only MLH1 deficiency. This family line was indicated to have MLH1 gene mutation and genetic testing revealed a frame shift mutation in MLH1 gene. Therefore, the reduced expression of MSH6 in colon cancer and liver metastasis was suspected as somatic mutation manner.
It has been shown that the coding region of MSH6 gene have repeat sequences which might be target gene for instability [14]. The coding region microsatellite (C)8 in exon 5 of MSH6 in the colon cancer and liver metastasis were analysed by DNA sequencing and showed mutation. The rational story is that microsatellite unstable colon cancer cell due to the MLH1 gene mutated condition metastasized to the liver and occurred somatic mutation into another type of allele in both tumor sites. The origin of metastatic liver tumor had already been diagnosed by morphological analysis, however, the results of DNA sequence also proved that the liver metastasis was not derived from stomach cancer, because the (C)8 region in exon 5 of MSH6 was wild type in the stomach cancer. We speculate that the reduced expression of MSH6 is a passenger mutation rather than a driver mutation in carcinogenesis in this case. However, it is possible that MSH6 deficiency due to the MLH1 gene mutation played a partial role in metastatic disease formation, which is consistent with the difference in MSH6 expression between the metastatic region and the primary site. Further investigations in similar cases comparing the primary tumor site and metastatic regions might provide a biological implication on whether secondary mutations in MSH6 contribute to carcinogenesis.
There must be a possible change of recognition potency of MSH6 antibody against MSH6 with the frameshift mutation in the repetitive mononucleotide tract. It has been reported that the truncated polypeptide due to 1 bp deletion or insertion at codon 1116 of MSH6 coding region was unstable and was not detected by the in vitro protein expression analysis [15]. We presume that the similar mechanism resulted in the loss of MSH6 immunoexpression. There have been some reports about the frequency of somatic frameshift mutations of the MSH6 gene induced by microsatellite instability. Duval A. et al. reported in their review article that somatic frameshift mutation of MSH6 was detected in more than 30% of colorectal cancers [14]. On the other hand, Shia J. et al. have shown the frequency of scanty staining of MSH6 was about 2% in colorectal carcinoma cases that meet the Revised Bethesda Guidelines [8], and Rondell et al. also showed the mutation ratio was less than 1% of colorectal cancer from the immunohistochemical analysis in a larger population [16].
Although the interpretation of IHC patterns require cooperation with pathologists, it has some advantages such as its inexpensiveness and convenience for direct detection of altered MMR genes, and its potency for visual evaluation by anyone [3]. However, there are some pitfalls of the interpretation of the results of IHC. As one of which was described in this case, the failure of MLH1 function associated with the somatic mutation of MSH6 coding region. Various staining patterns of MMR proteins, especially in MSH6, need to be recognized to prevent misinterpretation of IHC results, which do not necessarily represent the germline mutations.
This patient had been treated for anaplastic astrocytoma, therefore, this case was appeared to be related to Turcot syndrome type 1 which is characterized by glioblastoma and colorectal cancer or polyps associated with LS [1, 2]. However, the IHCs of the specimen of brain tumor showed intact staining of all MMR proteins (Supplementary Fig. 2). The staining pattern was discrepant with the previous report that the existence of germline mutation may influence the carcinogenesis of glioblastoma and colorectal neoplasm in Turcot’s syndrome patients [9]. It is also reported that chromosomal instability play an important role in the tumorigenesis of sporadic glioblastoma multiform [17], the brain tumor of this patient was considered to be a sporadic case or derived from another mechanism other than the deficiency of MMR system.
MMR status is meaningful in selecting pharmacological strategy, because deficient MMR cancers may be resistant to 5-FU based chemotherapy [18, 19]. Adjuvant chemotherapy using oxaliplatin has been commonly recognized as standard regimen for patients with node-positive colon cancer and advanced stage of gastric cancer regardless of the mismatch repair status. Some clinical trials showed more beneficial outcomes of the postoperative adjuvant FOLFOX treatment for patients with loss of MMR than those without MMR deficiency [20, 21]. Additionally, a cohort study proved the preference of oxaliplatin-using postoperative treatment for the prognosis of stage III colon cancer with MSI-high status [11, 22]. Furthermore, recent study has shown that MMR deficient carcinoma patients were shown to have better prognosis by immune checkpoint blockade [23].
This report indicates the underlying mechanism for the patchy expression pattern of MSH6 and it was proved by the frameshift mutation of the repetitive C8 region. This case supports a theoretical mechanism for the reduced expression of MSH6 due to a somatic mutation in the coding region that might arise in colon cancer of MLH1/PMS2-deficient type of LS.
