Introduction The MMR system keeps the

Introduction The MMR system keeps the fidelity of replication by repairing replication errors such as base-base mispairs and insertions/deletions. It plays a substantial role in restoring the fidelity of DNA and contributing to genome integrity [1]. The components of MMR are conserved in a broad range of species including E.coli, yeast and human [2]. The main components of MMR in human p-Chlorophenylalanine are MutS homologs (MSH2, MSH3, MSH6), MutLhomologs (MLH1, MLH3, PMS1, PMS2), and the PCNA sliding clamp. RPA. EXO1, HMGB1, RFC, DNA polymerase δ and Ligase 1 also play a crucial role in MMR [3,4]. The MutS homologs form the heterodimers MSH2-MSH6 (known as MutSα) and MSH2-MSH3 (MutSβ), which play a critical role in mismatch recognition and initiation of repair. MLH1 heterodimerizes with PMS2, PMS1, or MLH3 to form MutLα (MLH1-PMS2), MutLβ (MLH1-PMS1), or MutLγ (MLH1-MLH3), respectively [5]. During MMR, DNA mismatches are recognized by MutS which recruits MutL to the mismatch sites. EXO1 assists in removing the daughter-strand DNA containing mismatches, while the parental strand DNA is stabilized by RPA. RPA and HMGB1 stimulate MutSα-activated, EXO1-catalyzed excision. The gap is then filled by Pol δ and sealed by DNA ligase I [4]. Mutations in MSH2, MSH6, MLH1 or PMS2 that cause constitutional MMR defects lead to the development of Lynch Syndrome (also known as hereditary non-polyposis colorectal cancer), which is associated with high risk of colorectal cancer and other cancers [6,7]. Cumulative evidence supports that MMR proteins are also involved in MMR-independent pathways including homologous recombination, DSB repair, and DNA damage response [8,9]. Deficiency in MMR genes has been associated with increased resistance to apoptosis after DNA damage and the elevated frequency of drug resistance [[10], [11], [12]]. It has been reported that MSH2 and MSH6 are recruited to DNA damage sites in the G1 phase in XPA cells after UV irradiation [13], and the MutLα-defective 293T cells are deficient in UVA-induced DNA damage response, and expressing recombinant MutLα rescues such deficiency [14]. The human MLH1 is a 756 amino-acids protein that can be roughly divided into two domains: the N-terminal domain (NTD) and the C-terminal domain (CTD) [15]. The N-terminus harbors an ATPase domain and the MSH2-binding motif, whereas the C-terminal domain enables MLH1 interaction with its binding partners and also possesses the binding site for PMS2 that is important for the endonucleolytic function of PMS2. Numerous studies have demonstrated the importance of the ATPase activity in MLH1 function. Mutations in the ATPase domain of hMutLα abolish the ATP hydrolytic activity of hMutLα [16]. ATP hydrolysis is indispensable for hMutLα activity in MMR, and mutant hMutLα lacks the MMR activity, indicating the important function of hMutLα ATP hydrolysis in MMR, which is consistent with studies of MutL in E.coli [[17], [18], [19]]. In Lynch syndrome, missense mutations in or near the ATP binding site confer increased mutagenesis, perhaps due to diminished ATP hydrolysis [17]. Previous studies also report that the ATPase-deficient MLH1 knockin mice exhibit a dramatic reduction in class-switch recombination but not somatic hypermutation, as well as decreased accumulation of 53BP1, suggesting that MLH1 is involved in DNA end processing through its ATPase activity [20]. Finally, loss of MLH1 N-terminus also abrogates its recruitment to DNA damage sites induced by laser-irradiated or etoposide [21,22]. Studies have also shown that MMR proteins also participant in signaling dysfunctional telomeres [23,24]. Telomeres are repetitive TTAGGG sequences present at chromosomal ends that are pivotal for the maintenance of genome stability [25]. However, in some vertebrate species, short stretches of (TTAGGG)n tandem repeats can also be found at intra-chromosome loci, the so-called interstitial telomeric sequences (ITSs). ITSs contribute to chromosome breakage, recombination and rearrangement. Therefore, ITSs may be an important source of genome instability [22,[26], [27], [28], [29], [30], [31]]. We have previously identified that the N-terminus of MLH1 (residues 1–389) is required for recruiting MLH1 to DNA damage sites and also suppresses aberrant formation of ITSs in telomerase-positive cells. The aberrant formation of ITSs appears to be associated with DSB formation, and such formation may induce chromosome instability such as chromosome loss and micronuclei formation [22].