Mismatch Repair in Eukaryotes
Explore and order pathway-specific siRNAs, real-time PCR assays, and expression vectors. View pathway information and literature references for your pathway.
  • Click on your proteins of interest in the pathway image or review below
  • Select your genes of interest and click "add selection"
  • When you have finished your gene selection, click "Find Products" to find assays, arrays, or create custom products
Download Image Terms of Use Download PPT
Pathway Navigator
Mismatch Repair in Eukaryotes
The major DNA Repair mechanisms take advantage of the facts that DNA is double-stranded and the same information is present in both strands. Consequently, in cases where damage is present in just one strand, the damage can be accurately repaired by cutting it out (excision) and replacing it with new DNA synthesized using the complementary strand as template. All organisms, prokaryotic and eukaryotic, employ at least three excision mechanisms: Mismatch Repair, Base Excision Repair, And Nucleotide Excision Repair. Mismatch repair successfully meets a number of challenges. First, it recognizes a wide spectrum of rare mismatches, embedded in millions of correctly base-pairing nucleotides. Second, mismatch repair unequivocally discriminates between a correct nucleotide in the template strand and a perfectly normal, but incorrect, nucleotide in the newly replicated DNA so as to prevent rather than fix mutations. Next, mismatch repair removes a patch of nascent DNA, including the misincorporated nucleotide, terminating excision just beyond the mismatch. Finally, mismatch repair fills the excision gap by high-fidelity DNA synthesis; ligation subsequently restores strand continuity (Ref. 1).

DNA polymerases delta is required for repair of base-base mismatches and small insertion/deletion heterologies, and that the requirement for this activity is evident when the single strand break that directs the reaction is located either 5 or 3 to the mispair. The residual level of pol eta in repair-deficient is insufficient to support repair. Thus, either the human mismatch repair reaction requires both DNA polymerases delta and DNA polymerases alpha and eta (Ref. 2 & 3). DNA mismatch repair in eukaryotes is a complex process involving multiple steps. First, the mismatch is bound by a heterodimer of MutS-Alpha homologues, either Msh2/Msh6 (for base–base mismatches and one or two base loops) or MutS-Beta heterodimer (Msh2/Msh3) (for loops of 2–14 bp). This binding triggers an ATP-dependent conformational change that results in the recruitment of a MutL-Alpha heterodimer (Mlh1/Pms2) (Ref. 4). The mismatch-bound Msh2/ Msh6 heterodimer undergoes an ATP-dependent conformational change, which converts it to a sliding clamp capable of translocating along the DNA backbone. The Msh2/ Msh6 •ATP•DNA complex is bound by a second heterodimer, composed of Mlh1 and Pms2 in a second ATP-dependent step. This complex can translocate in either direction, in search of a strand discontinuity. Mismatch Repair must be directed to the newly synthesised strand, which in eukaryotes can be distinguished from the template strand solely by the presence of gaps between Okazaki fragments on the lagging strand, or by the free 3-terminus on the leading strand (Ref. 5 & 6). In addition to its ability to complex with MSH6Msh2  also forms a heterodimer with a second MutS homolog called Msh3. This MutS-Beta complex binds to ID mispairs but displays little if any affinity for base-base mismatches. Further, cell lines genetically defective in the Msh6 subunit of MutS-Alpha are deficient in base-base mismatch correction but retain partial activity on ID mispairs, and this residual activity has been shown to be due to the MutS-Beta complex. MutS-Alpha thus functions in base-base and ID mismatch repair, but MutS-Beta is apparently restricted to processing of the latter class of heterology. Hence MutS-Alpha and MutS-Beta has a role in the recognition and processing of ID mispairs. In strand excision the strand break that directs correction can be located either 3 or 5 to the mispair, with mismatch-provoked excision removing that portion of the incised strand spanning the two DNA sites. This mechanism is largely based on study of the fate of nicked heteroduplex DNA in nuclear extracts under conditions of repair DNA synthesis inhibition by aphidicolin or by omission of exogenous dNTPs (Ref. 7 & 8). Fen1(Flap endonuclease 1), and Exo1  is involved in DNA mismatch repair by interaction with Msh2  and Mlh1, genes that cause HNPCC  (Hereditary Nonpolyposis Colorectal Cancer) (Ref. 9).

In strand resynthesis the Msh2/Msh6 sliding clamp stimulates the activity of Exo1 (Exonuclease1), a 5_ → 3_ exonuclease that subsequently degrade a stretch of several hundred nucleotides, starting from a nick situated 5_ from the mispair and traveling towards the mispair. Exo1 is a candidate gene for colorectal tumor susceptibility because it is believed to play a role in mismatch repair. The region of single-stranded DNA was stabilized by RPA (Replication Protein A); this made the gapped substrate refractory to further degradation by Exo1, until this protein was stimulated by further molecules of ATP-bound Msh2/Msh6 heterodimer arriving from the direction of the mispair (Ref. 10 , 11 & 12). MutS, MutL, Exo1, RPA, PCNA, RFC(Replication Factor C), and DNA polymerase-delta supports bidirectional mismatch repair directed by a strand break located either 3 or 5 to the mispair. While repair directed by a 5-strand break can occur in the absence of MutL, this activity is required for mismatch correction directed by a 3- strand break. Repair DNA synthesis by polymerase-delta depends on presence of RPA, PCNA, and RFC for optimal activity. Polymerase-delta and its cofactors PCNA and RFC will then fill in the resulting single-stranded gap, and DNA ligase I will seal the remaining nick. The key aspects of this repair are (i) the binding of a mismatch or a modified DNA base(s) by the Msh2/Msh6 heterodimer, or by the Msh2/Msh6/Mlh1/PMS2 complex, will not bring about DNA incision at this site, nor will it lead to damage excision; (ii) the exonucleolytic degradation of DNA, triggered by the activated Msh2/Msh6 heterodimer, will take place only if a pre-existing strand discontinuity is present in the vicinity; (iii) the modified DNA bases will not be removed during the DNA degradation step unless they are present in the strand containing the discontinuity; (iv) different DNA modifications bound by the Msh2/Msh6 heterodimer will not necessarily be processed in the same way by the MMR system, or give rise to similar intermediates (Ref. 13 & 14).

Cadmium (Cd2+) is a known carcinogen that inactivates the DNA mismatch repair pathway. The inhibition of mismatch repair by Cd2+ is through the inactivation of the ATPase activity of the MSH2MSH6 heterodimer, resulting in a dominant negative effect and causing a mutator phenotype. Cd2+ is highly inhibitory to the ATP binding and hydrolysis activities of Msh2Msh6 and less inhibitory to its DNA mismatch binding activity (Ref.15 & 16).