Mechanisms involved in genetic differences in response to raltitrexed-induced thymineless stress

Current concepts that colorectal cancer develops from normal epithelial cells through gene mutations indicate that genetic differences between cancer cells and normal tissues may lead to differences in the response to chemotherapy. In this study, murine embryo fibroblasts (MEFs) with loss of MLH1 (MLH1 -/-), a key component of DNA mismatch repair, showed a lower sensitivity to treatment with the thymidylate synthase inhibitor, raltitrexed (RTX), than their MLH1+/+ counterparts. Interestingly, the cytotoxicity of RTX, which was completely prevented by thymidine supplementation in MLH1 +/+ MEFs, was only reduced by 20% in MLH1-/- MEFs. In MLH1-/- MEFs, the sensitivity to RTX treatment increased as the duration of treatment increased. Further studies showed that RTX treatment led to changes in gene expression profile that are consistent with continually increased uptake of thymidine in MLH1+/+ MEFs, but not in MLH1-/- MEFs. These studies may open new avenues for enhancing anticancer effect in cancer cells with the MLH1-/- genotype, while protecting normal tissue with functional MLH1, through prolonging RTX treatment and co-administration of thymidine.;APMIN/+ mice are heterozygous for a truncating mutation in the APC gene and are a model of human intestinal cancer. RTX treatment ameliorated the disease-related anemia in APCMIN/+ mice, while it caused bone marrow suppression in their wild-type (WT) littermates. We examined the hypothesis that the APCMIN/+ genotype compromised erythropoiesis in this study, because APCMIN/+ genotype may lead to an increase in Wnt erythropoiesis in this study, because APC MIN/+ genotype may lead to an increase in Wnt signaling, a critical pathway for maintaining self-renewal of hematopoietic stem cells and suppressing their differentiation. Without RTX treatment, red blood cell (RBC) counts and hemoglobin content in 6- and 9-week APCMIN/+ mice were lower than those in age-matched WT littermates, while tumor bleeding and changes in RBC turnover were not observed. Expression levels of Notch1 and HoxB4 genes increased in the spleens of 6-week APCMIN/+ mice. While severe anemia in 12-week-old APCMIN/+ mice was correlated with tumor bleeding, a blockade in erythropoiesis was suggested by premature bone marrow. The premature bone marrow and altered gene expression may underlie compromised erythropoiesis in APCMIN/+ mice, which leads to insufficient compensation for blood loss caused by tumor hemorrhage.