DNA repair process — Role in health and disease
In Escherichia coli, during the repair process, the protein MutS gets recruited to the mismatch region. Through a sequence of events it attracts a DNA exonuclease to remove the wrong sequence. Following this, a polymerase synthesises the missing nucleotides and finally a DNA ligase joins the DNA fragment to the remaining strand. In hereditary non-polyposis colon cancer (HNPCC) families (called Lynch syndrome), germline mutations in any MutS homologue alleles increases predisposition to organ cancers. The EU-funded MISMATCH2MODEL proposal employed a systems biology approach to understand the MMR process. Combining state-of-the art structural, biophysical and biochemical methods, scientists found that the MutS protein acts as a molecular switch to initiate DNA mismatch recognition. Upon mismatch binding and recognition by MutS, MutL protein assembles onto MutS and then recruits the nuclease MutH to carry out incision of the newly synthesised strand. Association of MutS and MutL with the DNA was found to be a rapid process ranging from seconds to minutes. However, these two protein components were retained on DNA for long periods (tens of minutes). The consortium also demonstrated that the DNA remodelling process was temperature-driven rather than adenosine triphosphate (ATP)-dependent. An important line of work of the MISMATCH2MODEL project entailed the reconstruction of the human MMR process in bacteria. Researchers successfully identified the rate-limiting step of MMR to be the mismatch detection and degradation of the error-containing strand. The innovative nanomanipulation and visualisation technologies developed represent a powerful new way of analysing protein–DNA and protein–protein interactions. The quantitative data on prokaryotic and human MMR processes were integrated into a mathematical model that could be useful in further dissection of the pathway. Given the importance of the MMR process in genomic integrity, the MISMATCH2MODEL project accomplishments may provide new insight into cancer development.