*2.3.1 MSH2-MSH6 (MutS*α*)*

MMR pathway is responsible for repairing base mismatches, insertions and deletions arise from DNA replication, genomic recombination, and other error-prone DNA repair systems [69]. MSH2-MSH6 (MutSα) complex plays an important role in this pathway by recognizing the mismatched bases and starting the MMR process [69]. Different polymorphisms and expression levels of MMR components have shown to be associated with prognosis and survival in cancer patients [70–72].

A study conducted by Li et al., emphasized the importance of MMR pathway in Ewing sarcoma and showed that the expression levels of MSH2 and MSH6 is correlated with an increased chance of metastasis and poor prognosis in these patients [73]. They used the GEO database to investigate the correlation of the key dysregulated genes and pathways with prognosis information and metastasis status of the Ewing sarcoma patients [73]. The findings highlighted the MMR pathway as the most significantly enriched KEGG pathway in EWS patients [73]. The expression levels of key MMR components including MSH2 and MSH6 are found to be significantly associated with metastasis, shorter EFS, and overall poor prognosis in Ewing sarcoma patients [73]. Several studies have investigated the role of MMR pathway in osteosarcoma. Liu et al., investigated the growth-suppressive effects of MSH6 gene silencing in combination with cisplatin in osteosarcoma [74]. Microarray-based gene expression analysis of samples obtained from 67 osteosarcoma patients along with 24 normal patients demonstrated that MSH6 is significantly up-regulated in osteosarcoma patients [74]. Then, they evaluated cell proliferation, cell cycle distribution, gene and protein expression, and apoptosis of osteosarcoma cell line MG63 after co-treatment with cisplatin and siRNA targeting MSH6 [74]. The data showed that silencing MSH6 in combination with cisplatin reduced expression levels c-Myc, cyclin D1, Bcl-2, Stathmin, and PCNA and increased BAX expression in osteosarcoma cells [74]. This combination treatment also induced significant anti-proliferative effects, indicating that MSH6 could be considered as a potential therapeutic target for treatment of osteosarcoma patients [74]. In another study, proteomic analysis for identification of proteins that are differentially expressed between osteosarcoma and normal osteoblastic cells revealed that chromosome segregation 1-like (CSE1L) protein is significantly associated with the growth of osteosarcoma cells [75]. Co-immunoprecipitation and RNA-seq analysis in this study showed that CSE1L acts as a positive regulator of MSH6 in osteosarcoma cells [75]. In addition, they knocked down CSE1L protein in osteosarcoma cells and found significant growth suppression [75]. Furthermore, to investigate the role of MSH6, they overexpressed MSH6 in

CSE1L-knockdown osteosarcoma cells [75]. The results showed that overexpression of MSH6 significantly increased cell proliferation rate and reversed the anti-tumor effects observed in CSE1L-knockdown cells, indicating that CSE1L activity is dependent on MSH6 expression [75]. Moreover, down-regulation of MSH6 resulted in suppression of cell growth in both *in vitro* and *in vivo* experiments [75]. The prognostic potential of MSH6 and CSE1L was also explored by evaluation of the MSH6 and CSE1L expression levels in tumor samples [75]. They found a significant correlation between the expression of these two proteins and overall poor prognosis in osteosarcoma patients [75]. Similarly, another study on osteosarcoma patients showed that overexpression of MSH2 and MSH6 is significantly associated with shorter survival time, lower sensitivity to chemotherapy, and higher chances of metastasis [76]. These studies underscored the significance of MMR proteins as both prognostic biomarkers and possible therapeutic targets in sarcoma.
