Germline mutations of MLH1 are responsible for
tumor generation in nearly 50% of patients with
Lynch Syndrome, and around 15% of sporadic
colorectal cancers show MLH1-deficiency due to promotor hypermethylation. Although these
tumors are of lower aggressiveness the benefit for these patients from standard
chemotherapy is still under discussion. Recently, it was shown that the sensitivity to the
DNA-
PKcs inhibitor KU60648 is linked to loss of the MMR
protein MSH3. However, loss of MSH3 is rather secondary, as a consequence of
MMR-deficiency, and frequently detectable in MLH1-deficient
tumors. Therefore, we examined the expression of MLH1, MSH2, MSH6, and MSH3 in different MMR-deficient and proficient cell lines and determined their sensitivity to KU60648 by analyzing cell viability and survival. MLH1-dependent ability of double strand break (
DSB) repair was monitored after irradiation via γH2AX detection. A panel of 12
colon cancer cell lines, two pairs of cells, where MLH1 knock down was compared to controls with the same genetic background, and one MLH1-deficient cell line where MLH1 was overexpressed, were included. In summary, we found that MLH1 and/or MSH3-deficient cells exhibited a significantly higher sensitivity to KU60648 than MMR-proficient cells and that overexpression of MLH1 in MLH1-deficient cells resulted in a decrease of cell sensitivity. KU60648 efficiency seems to be associated with reduced
DSB repair capacity. Since the molecular testing of colon
tumors for MLH1 expression is a clinical standard we believe that MLH1 is a much better marker and a greater number of patients would benefit from KU60648 treatment.