The Role of the DNA Mismatch Repair Pathway in Adult Stem Cells In Vivo

Lynch syndrome (LS) leads to an increased risk of early-onset colorectal and other types of cancer and is caused by germline mutations in DNA mismatch repair (MMR) genes. Loss of MMR function results in a mutator phenotype that likely underlies its role in tumorigenesis. However, loss of MMR also results in the elimination of a DNA damage-induced checkpoint/apoptosis activation barrier that may allow damaged cells to grow unchecked. A fundamental question is whether loss of MMR function provides cells an immediate selective advantage during tumorigenesis. We predict that the loss of the damage response role of the MMR pathway can provide adult stem cells a survival advantage in a mutagenic environment. To test this hypothesis in an in vivo system, we utilized the freshwater planarian Schmidtea mediterranea which contains a significant population of identifiable adult stem cells. We identified planarian homologs of human MSH2, MSH6, and MLH1—MMR genes which are mutated in LS. The planarian Smed-msh2 is expressed in stem cells and some progeny. We depleted Smed-msh2 mRNA levels by RNA-interference and found a striking survival advantage in these animals treated with a cytotoxic DNA alkylating agent compared to control animals. We have demonstrated that this tolerance to DNA damage is due to the survival of mitotically active, MMR-deficient stem cells. Finally, we found that long-term depletion of Smed-msh2 by RNAi resulted in a shrinking phenotype. Our results suggest that loss of MMR provides an in vivo survival advantage to the stem cell population in the presence of DNA damage that may have implications for tumorigenesis.