Browsing by Subject "MLH1"

MLH1 is a gene that codes for one of the four mismatch repair (MMR) proteins alongside MSH2, MSH6, and PMS2. The main function of the MMR proteins is to recognize base mismatches and insertion-deletion loops formed during DNA replication and aid in their excision. Inherited heterozygous pathogenic variants in any of the four MMR genes lead to Lynch syndrome, an inherited cancer syndrome that predisposes to multiple different cancer types, most notably colorectal cancer. Loss of the expression of an MMR gene causes MMR-deficiency, which leads to microsatellite instability, the accumulation of mutations in microsatellite regions of the DNA. The higher mutational burden caused by MMR-deficiency is thought to be the main driving force of genomic instability and tumorigenesis in MMR-deficient cells. In addition to MMR, MLH1 and the MMR machinery have roles in other anticarcinogenic cellular processes, such as DNA damage signaling and DNA double-strand break repair. Recently, MLH1 has also been shown to have a significant role in regulating mitochondrial metabolism and oxidative stress responses. The identification of MMR-proficient tumors in Lynch syndrome patients begs the question whether the lower amount of functional MLH1 observed in MLH1 mutation carriers could cause problems with these functions and pose alternative routes to tumorigenesis. In line with this, it has been shown that the role of MLH1 in cell cycle regulation in DNA damage signaling is notably more sensitive to decreased amount of the protein compared to its role in MMR. The main goal of the thesis was to study the effects of decreased MLH1 expression on gene expression, cellular functions, and possible alternative tumorigenic pathways. In order to achieve this, the coding transcriptome of human fibroblast cell lines expressing MLH1 at different levels was sequenced and the resulting data analyzed. The study revealed that decreased MLH1 expression affects cellular functions associated with mitochondrial function and oxidative stress responses in cells with functional MMR. Particularly NRF2-controlled cytoprotective defence systems were observed to be downregulated. Decreased MLH1 expression was also observed to affect several cellular functions associated with reorganization of the cytoskeleton and interactions with the extracellular matrix. These results strengthen the recently made notions that MLH1 has a role in controlling the function of mitochondria and in mitigating oxidative stress, and that these two functions are connected. The study also brings to light new information on the possible role of MLH1 in controlling the organization of the cytoskeleton, which has previously received little attention. Dysfunction of mitochondria, increased oxidative stress, and reorganization of the cytoskeleton, as a result of decreased MLH1 expression, could pose events that facilitate malignant transformation of cells prior to the total loss of MMR function.