The frequency of microsatellite instability (MSI) is reportedly extremely low in breast cancer, despite widespread clinical expectations that many patients would be responsive to immune‐checkpoint inhibitors (ICI). Considering that some triple‐negative breast cancers (TNBC) responded well to ICI in a clinical trial and that a high density of tumor‐infiltrating lymphocytes (TILs) is frequently observed in other cancers with high levels of microsatellite instability (MSI‐H), we hypothesized that some TNBC with a high density of TILs would be MSI‐H. Medullary carcinoma (MedCa) of the breast, a rare histological type, is characterized by a high density of TILs. Considering that MedCa of the colon is often MSI‐H, we suspected that MedCa in breast cancer might also include MSI‐H tumors. Therefore, we conducted MSI tests on such breast cancers with a high density of TILs. The MSI status of 63 TIL‐high TNBC and 38 MedCa tumors, all from Asian women who had undergone curative surgery, were determined retrospectively. DNA mismatch repair (MMR) proteins and PD‐L1 expression were also investigated immunohistochemically. All samples were microsatellite stable, being negative for all microsatellite markers. TIL‐high TNBC with low MLH1 protein had higher levels of PD‐L1 in stromal immune cells ( P = .041). MedCa tumors showed significantly higher PD‐L1 expression in immune cells than in TIL‐high TNBC (<.001). We found that MSI‐H tumors were absent in TIL‐high breast cancers. Examination of MMR proteins, not a purpose of Lynch syndrome screening, may merit further studies to yield predictive information for identifying patients who are likely to benefit from ICI.

Aim Microsatellite instability (MSI), which reflects loss of DNA mismatch repair (MMR) activity, and immunohistochemistry (IHC) for MMR proteins are employed as screening examinations for Lynch syndrome (LS). Recent studies revealed that there is a population of MSI‐high tumors in sporadic endometrial cancer (EC). However, MSI data for Japanese EC patients are scarce. Furthermore, sporadic estrogen‐dependent EC (type I) is generally considered to arise from hyperplasia. Because LS is usually associated with type I EC, we hypothesized that MSI might be involved in the oncogenic process in some sporadic EC. We conducted MSI testing to reveal MSI status in sporadic Japanese EC. IHC for MMR proteins was also performed. Methods Ninety‐eight tissue samples of sporadic ECs from Japanese patients were used for IHC and MSI examinations. We also evaluated MMR protein expressions in the background normal endometrium. Results Microsatellite instability‐high was observed in 10.2% of 98 cases with sporadic EC, a lower percentage than that in Western studies. Loss of some MMR proteins was observed in 23 cases (23.5%) and there was a significant correlation with MSI‐high status (P < 0.001). Concerning the background endometrium, two cases showed partial loss of MLH1 and PMS2, corresponding to adjacent EC lesions, suggesting that MMR deficiency may already be present in the background endometrium. Conclusion The MSI‐high rate was low in our Japanese cohort. Our data confirmed the usefulness of MMR protein assessment for MSI screening in Japanese EC patients. Furthermore, IHC of the background endometrium might reveal the mechanism of MSI‐high tumorigenesis.

Background: Mesothelioma is histologically divided into three subgroups: epithelioid, sarcomatoid, and biphasic types. The epithelioid or sarcomatoid type is morphologically defined by polygonal or spindle-like forms of cells, respectively. The biphasic type consists of both components. It is not yet understood how histological differentiation of mesothelioma is regulated. ERC/mesothelin is expressed in most cases of the epithelioid type, but not in the sarcomatoid type of mesothelioma. Consequently, its expression is well correlated to the histological subtype. We hypothesized that ERC/mesothelin expression influences the histological differentiation of mesothelioma, and tested this hypothesis. Methods: We performed studies using the overexpression or knockdown of ERC/mesothelin in mesothelioma cells to examine its effect on cellular morphology, growth kinetics, or migration/invasion activity, in vitro. We then transplanted ERC/mesothelin-overexpressing and control cells into the intraperitoneal space of mice. We examined the effect of ERC/mesothelin overexpression on mouse survival and tumor phenotype. Results: In vitro cell culture manipulations of ERC/mesothelin expression did not affect cellular morphology or proliferation, although its overexpression enhanced cellular adhesion and the migration/invasion activity of mesothelioma cells. The survival rate of mice following intraperitoneal transplantation of ERC/mesothelinoverexpressing mesothelioma cells was significantly lower than that of mice with control cells. The histological evaluation of the tumors, however, did not show any morphological difference between two groups, and our hypothesis was not validated. Unexpectedly, both groups (ERC/mesothelin-overexpressing and control) of mesothelioma cells that were morphologically monophasic and spindle-like in vitro differentiated into a biphasic type consisting of polygonal and spindle-like components in the transplanted tumor, irrespective of ERC/mesothelin expression. Conclusions: These results suggested that the histological transition of mesothelioma between epithelioid and sarcomatoid types may be reversible and regulated not by ERC/mesothelin, but by other unknown mechanisms.

Overexpression of human epidermal growth factor receptor 2 (HER2) protein is well known to be more frequent in ductal carcinoma in situ (DCIS) than in invasive ductal carcinoma (IDC). However, the reasons for this difference are poorly understood. On the basis of the high frequency of estrogen receptor-positive (ER+) and HER2-positive (HER2+) DCIS, we hypothesized that this tumor type overexpresses HER2 protein without gene amplification and retrospectively investigated the HER2/neu gene status of 71 ER(+)HER2(+) DCIS, surgically removed during the 2007 to 2017 period, employing fluorescence in situ hybridization (FISH). To compare HER2 protein expressions between in situ and invasive components of individual tumors, 86 pT1mi/1a IDC with predominantly in situ disease were also examined. Furthermore, for comparison of FISH status between in situ and coexisting invasive components, another patient cohort, 78 FISH-positive IDC cases, were employed. To elucidate biological differences among DCIS with various combinations of ER and HER2 protein expressions, we also analyzed public microarray data of mRNA. HER2 gene amplification was observed in 35% of ER(+) and HER2 protein-overexpressing specimens, significantly lower than the 94% in ER-negative (ER−) and HER2 protein-overexpressing specimens (P<0.001). HER2 protein expression was decreased in the invasive component as compared with coexisting in situ portions in 40% of individual tumors, whereas the FISH status of these 2 components was well preserved. Moreover, ER(+) and HER2 protein-overexpressing DCIS showed significantly higher hypoxia-inducible factor-1α protein expression than the ER(+) and HER2 protein-nonoverexpressing tumors (P=0.016). We revealed that ER(+) and HER2 protein-overexpressing DCIS, especially ER-high tumors, frequently overexpress HER2 protein without gene amplification. Our data may provide novel insights for understanding the biology of DCIS.

Mesothelioma is a rare, aggressive malignancy with poor outcome, and has limited treatment options. The aim of this study was to perform a comprehensive analysis of programmed death ligand 1 (PD‐L1) and B7 homolog 3 (B7‐H3) expression in mesothelioma. We investigated the protein expression of PD‐L1 and B7‐H3 and their potential correlation with histological subtype, which might help to develop new therapies targeting these immune checkpoint molecules. Expression analysis of PD‐L1 and B7‐H3 was performed by immunohistochemistry using serial tissue sections of specimens obtained from 31 patients with mesothelioma. Tumors were classified into 22 epithelioid, 6 sarcomatoid, and 3 biphasic types. Of the 31 patients, 13 (41.9%) were positive for PD‐L1 and 28 (90.3%) were B7‐H3 positive. Twelve of the 13 PD‐L1 positive patients were positive for B7‐H3. PD‐L1 and B7‐H3 were widely co‐expressed in biphasic and sarcomatoid type tumor cells. These findings might provide a rationale for the use of combination therapy for mesothelioma by targeting PD‐L1 and B7‐H3, as well as the development of anti‐B7‐H3 or anti‐PD‐L1 single agents.