Altered Expression of the Dna Mismatch Repair Proteins Hmlh1 and Hmsh2 in Cutaneous Dysplastic Nevi and Malignant Melanoma

Deficient mismatch repair is detected in large-to-giant congenital melanocytic naevi: providing new insight into aetiology and diagnosis

BACKGROUND

The aetiologies of large-to-giant congenital melanocytic naevi (LGCMN) remain ambiguous. A previous study discovered signatures associated with deficient mismatch repair (dMMR) in patients with LGCMN. However, a screening diagnostic immunohistochemistry (IHC) panel of dMMR in patients with LGCMN has not been performed to date.

OBJECTIVES

To identify the MMR status and aetiologies of LGCMN.

METHODS

A total of 110 patients with CMN, including 30 giant CMN, 30 large CMN, 30 medium CMN and 20 small CMN, underwent diagnostic IHC (for MSH6, MSH2, PMS2 and MLH1) screening of dMMR. The control group comprised normal skin samples from 20 healthy people. MMR proteins with little effect (MSH3 and PMS1) on the MMR system were stained in all samples. The surgical procedures conducted on each patient were noted because they might alter the behaviour of CMN and confound the results. Binary logistic regression analyses were performed between the phenotypic data and MMR status to identify associations. Whole-exome sequencing was performed on the main naevi, satellite naevi and normal skin tissues of four patients to detect variants. Mutational signature analyses were conducted to explore the aetiologies of LGCMN.

RESULTS

dMMR was detected in 37% (11 of 30) of giant, 23% (7 of 30) of large and 7% (2 of 30) of medium CMNs, but were not identified in small CMNs or normal skin tissues. Moreover, multiple LGCMNs had a much higher dMMR rate than did single LGCMNs. The regression analyses showed that MMR status was significantly associated with CMN size and the presence of satellites, but was not correlated with age, sex, location, satellite diversity or tissue expansion. Notably, the pattern of protein loss in LGCMN mainly consisted of PMS2 loss. Mutational signature analyses detected dMMR-related signatures in patients with LGCMN. Additionally, rare deleterious germline mutations in DNA repair genes were detected in LGCMN, mainly in MSH6, ATM, RAD50, BRCA1 and ERCC8. These germline mutations were single-patient variants with unknown significance.

CONCLUSIONS

dMMR is one of the aetiologies underlying LGCMN, particularly in patients with giant main lesions and multiple satellite lesions. Further studies are necessary to investigate the role of the DNA repair system, particularly MMR, in LGCMN.

[Dysplastic melanocytic nevus]

Dysplastic nevus is still a controversial entity both clinically and histologically. The occurrence of dysplastic nevus especially in the context of dysplastic nevus cell syndrome is associated with an increased risk for melanoma. The following minimal histological criteria should be fulfilled: nests of melanocytes varying in size and shape, bridging and confluent, proliferation of single melanocytes basal and suprabasal, cytoplasmic and nuclear atypia of melanocytes and subepidermal fibroplasia. The biological behavior (common nevus variant or precursor of melanoma?) is difficult to evaluate by presently available methods. The further development of new molecular biology techniques may allow a better prognosis of dysplastic nevi in an objective and reproducible manner. Against this background complete excision followed by clinical surveillance has to be recommended for the routine practice.

High expression of the mismatch repair protein MSH6 is associated with poor patient survival in melanoma

OBJECTIVES

The outcome of patients with primary melanoma (PM) cannot be completely explained based on currently adopted clinical-histopathologic criteria. In this study, we evaluated the potential prognostic value of mismatch repair protein expression in PMs.

METHODS

We examined the immunohistochemical staining of mismatch repair proteins in 18 benign nevi and 101 stage I to III PMs and investigated their association with tumor clinicopathologic variables and melanoma mortality.

RESULTS

Expression of MSH2, MLH1, and PMS2 was high in benign nevi and reduced in a subset of PMs. Conversely, MSH6 expression was absent or extremely low in benign nevi and increased in a subset of PMs. In the multivariate analysis, including sex, age, Breslow thickness, and ulceration, high MSH6 expression in PMs (ie, immunostaining in >20% of tumor cells) was significantly associated with an increased risk of melanoma mortality (relative risk, 3.76; 95% confidence interval, 1.12-12.70).

CONCLUSIONS

MSH6 protein expression can be a valuable marker to improve prognosis assessment in PMs.

Widespread mismatch repair protein expression in canine cutaneous mast cell tumors

Cutaneous mast cell tumors (MCTs) are common canine neoplasms. Some dog breeds more frequently develop MCTs, suggesting a genetically mediated predisposition. In humans, the most common inherited cancer predisposition is caused by germline defects in the mismatch repair (MMR) genes. To investigate whether inherited defects in the MMR genes predispose some dogs to MCT development, MMR expression in 22 MCTs from young and predisposed breed dogs was compared with MMR expression in 22 MCTs from old dogs of non-MCT-predisposed breeds. MMR expression was investigated immunohistochemically using antibodies against MLH1, MSH2, and MSH6. Mast cells within all MCTs expressed MLH1, MSH2, and MSH6. There were no significant differences in the intensity of immunoreactivity or the percentage of cells expressing MMR proteins between MCTs from the 2 groups of dogs. There were no significant differences in MMR protein expression between grade II and grade III MCTs. These results do not support the hypothesis that inherited MMR defects predispose some dogs to MCT development.

Temozolomide- and fotemustine-induced apoptosis in human malignant melanoma cells: response related to MGMT, MMR, DSBs, and p53

Malignant melanomas are highly resistant to chemotherapy. First-line chemotherapeutics used in melanoma therapy are the methylating agents dacarbazine (DTIC) and temozolomide (TMZ) and the chloroethylating agents BCNU and fotemustine. Here, we determined the mode of cell death in 11 melanoma cell lines upon exposure to TMZ and fotemustine. We show for the first time that TMZ induces apoptosis in melanoma cells, using therapeutic doses. For both TMZ and fotemustine apoptosis is the dominant mode of cell death. The contribution of necrosis to total cell death varied between 10 and 40%. The O⁶-methylguanine-DNA methyltransferase (MGMT) activity in the cell lines was between 0 and 1100 fmol mg⁻¹ protein, and there was a correlation between MGMT activity and the level of resistance to TMZ and fotemustine. MGMT inactivation by O⁶-benzylguanine sensitized all melanoma cell lines expressing MGMT to TMZ and fotemustine-induced apoptosis, and MGMT transfection attenuated the apoptotic response. This supports that O⁶-alkylguanines are critical lesions involved in the initiation of programmed melanoma cell death. One of the cell lines (MZ7), derived from a patient subjected to DTIC therapy, exhibited a high level of resistance to TMZ without expressing MGMT. This was related to an impaired expression of MSH2 and MSH6. The cells were not cross-resistant to fotemustine. Although these data indicate that methylating drug resistance of melanoma cells can be acquired by down-regulation of mismatch repair, a correlation between MSH2 and MSH6 expression in the different lines and TMZ sensitivity was not found. Apoptosis in melanoma cells induced by TMZ and fotemustine was accompanied by double-strand break (DSB) formation (as determined by H2AX phosphorylation) and caspase-3 and -7 activation as well as PARP cleavage. For TMZ, DSBs correlated significantly with the apoptotic response, whereas for fotemustine a correlation was not found. Melanoma lines expressing p53 wild-type were more resistant to TMZ and fotemustine than p53 mutant melanoma lines, which is in marked contrast to previous data reported for glioma cells treated with TMZ. Overall, the findings are in line with the model that in melanoma cells TMZ-induced O⁶-methylguanine triggers the apoptotic (and necrotic) pathway through DSBs, whereas for chloroethylating agents apoptosis is triggered in a more complex manner.