DNA Repair

The DNA mismatch repair (MMR) pathway has evolved to correct errors made by DNA polymerase during DNA replication. Such errors fall into two broad categories: base substitutions and insertions/deletions. A base substitution error occurs when DNA polymerase inserts an incorrect (noncomplementary) nucleotide opposite the template base, like a T opposite G instead of C, or A opposite C instead of G. These incorrect base pairs are referred to as mispairs or mismatches. Often, DNA polymerase will make a base substitution error when copying a base that has been damaged by alkylation. For example, DNA polymerase will frequently insert a T opposite O⁶-methylguanine on the other strand.

An insertion error occurs when DNA polymerase adds one or more extra nucleotides (+1, +2, +3, and so on) to a sequence; a deletion error is made when one or more nucleotides (−1, −2, −3, and so on) are omitted from a sequence. Sequences that contain repeats of the same nucleotide (mononucleotide repeat), such as AAAAAAAA, are particularly vulnerable to +1 or −1 insertion/deletion errors when copied by DNA polymerase. Such sequences might be called "slippery," in that DNA polymerase can "slide" on the DNA and lose its place. Other repetitive sequences, like the dinucleotide repeat CACACACA and the trinucleotide repeat CTGCTGCTG, are prone to +2 and +3, or −2 and −3 insertion/deletion errors, respectively. These repetitive DNA sequences are called microsatellites.

Defects in DNA mismatch repair have been found in several types of cancer, notably colon cancer, and microsatellite sequences that are either shorter or longer than normal are a hallmark of defective MMR. Expansion of trinucleotide repeat sequences is associated with a number of hereditary neurological disorders, such as fragile X syndrome, myotonic dystrophy, and Huntington's disease.

The process of MMR, like the BER and NER pathways, comprises damage recognition, damage excision, DNA repair synthesis, and DNA ligation. First, a mismatch or insertion/deletion error must be recognized by a complex of proteins specialized for the particular type of damage (mismatch, or small or large insertion/deletion). Just how the mismatch recognition protein complex "knows" which DNA strand contains the "right" nucleotide and/or which DNA strand contains the "wrong" one has not yet been determined.

Next, a phosphodiester bond in the DNA strand containing the mismatched nucleotide is cleaved by an endonuclease, the strand is displaced Figure 5. The nucleotide excision repair pathway is used to repair damage that distorts the double helix or prevents replication. After recognition, helicase unwinds the two strands, which are stabilized by single-strand binding proteins. Multiple nucleotides are removed from either side of the damage. DNA polymerase fills the gap, and ligase relinks the strand. by DNA helicase, and a portion of the strand is removed by a combination of DNA exonuclease and DNA polymerase. Lastly, DNA polymerase carries out DNA repair synthesis, and DNA ligase restores the continuity of the sugar-phosphate-DNA backbone. The patch of DNA newly synthesized by the MMR DNA polymerase is relatively large, approximately 1,000 nucleotides long, compared to the DNA repair synthesis that takes place during BER, which typically replaces 1 nucleotide, or NER, which replaces approximately 30 nucleotides. MMR is especially important in tissues that are constantly regenerating, like the intestinal lining and the endometrium (the lining of the uterus), since growth requires DNA replication, which sometimes makes mistakes.