DNA mismatch repair protein Msh2 also known as MutS homolog 2 or MSH2 is a protein that in humans is encoded by the MSH2 gene, which is located on chromosome 2. MSH2 is a tumor suppressor gene and more specifically a caretaker gene that codes for a DNA mismatch repair (MMR) protein, MSH2, which forms a heterodimer with MSH6 to make the human MutSα mismatch repair complex. It also dimerizes with MSH3 to form the MutSβ DNA repair complex. MSH2 is involved in many different forms of DNA repair, including transcription-coupled repair, [5] homologous recombination, [6] and base excision repair. [7]
Mutations in the MSH2 gene are associated with microsatellite instability and some cancers, especially with hereditary nonpolyposis colorectal cancer (HNPCC). At least 114 disease-causing mutations in this gene have been discovered. [8]
Hereditary nonpolyposis colorectal cancer (HNPCC), sometimes referred to as Lynch syndrome, is inherited in an autosomal dominant fashion, where inheritance of only one copy of a mutated mismatch repair gene is enough to cause disease phenotype. Mutations in the MSH2 gene account for 40% of genetic alterations associated with this disease and is the leading cause, together with MLH1 mutations. [9] Mutations associated with HNPCC are broadly distributed in all domains of MSH2, and hypothetical functions of these mutations based on the crystal structure of the MutSα include protein–protein interactions, stability, allosteric regulation, MSH2-MSH6 interface, and DNA binding. [10] Mutations in MSH2 and other mismatch repair genes cause DNA damage to go unrepaired, resulting in an increase in mutation frequency. These mutations build up over a person's life that otherwise would not have occurred had the DNA been repaired properly.
The viability of MMR genes including MSH2 can be tracked via microsatellite instability, a biomarker test that analyzes short sequence repeats which are very difficult for cells to replicate without a functioning mismatch repair system. Because these sequences vary in the population, the actual number of copies of short sequence repeats does not matter, just that the number the patient does have is consistent from tissue to tissue and over time. This phenomenon occurs because these sequences are prone to mistakes by the DNA replication complex, which then need to be fixed by the mismatch repair genes. If these are not working, over time either duplications or deletions of these sequences will occur, leading to different numbers of repeats in the same patient.
71% of HNPCC patients show microsatellite instability. [11] Detection methods for microsatellite instability include polymerase chain reaction (PCR) and immunohistochemical (IHC) methods, polymerase chain checking the DNA and immunohistochemical surveying mismatch repair protein levels. "Currently, there are evidences that universal testing for MSI starting with either IHC or PCR-based MSI testing is cost effective, sensitive, specific and is generally widely accepted." [12]
In eukaryotes from yeast to humans, MSH2 dimerizes with MSH6 to form the MutSα complex, [13] which is involved in base mismatch repair and short insertion/deletion loops. [14] MSH2 heterodimerization stabilizes MSH6, which is not stable because of its N-terminal disordered domain. Conversely, MSH2 does not have a nuclear localization sequence (NLS), so it is believed that MSH2 and MSH6 dimerize in the cytoplasm and then are imported into the nucleus together. [15] In the MutSα dimer, MSH6 interacts with the DNA for mismatch recognition while MSH2 provides the stability that MSH6 requires. MSH2 can be imported into the nucleus without dimerizing to MSH6, in this case, MSH2 is probably dimerized to MSH3 to form MutSβ. [16] MSH2 has two interacting domains with MSH6 in the MutSα heterodimer, a DNA interacting domain, and an ATPase domain. [17]
The MutSα dimer scans double stranded DNA in the nucleus, looking for mismatched bases. When the complex finds one, it repairs the mutation in an ATP dependent manner. The MSH2 domain of MutSα prefers ADP to ATP, with the MSH6 domain preferring the opposite. Studies have indicated that MutSα only scans DNA with the MSH2 domain harboring ADP, while the MSH6 domain can contain either ADP or ATP. [18] MutSα then associates with MLH1 to repair the damaged DNA.
MutSβ is formed when MSH2 complexes with MSH3 instead of MSH6. This dimer repairs longer insertion/deletion loops than MutSα. [19] Because of the nature of the mutations that this complex repairs, this is probably the state of MSH2 that causes the microsatellite instability phenotype. Large DNA insertions and deletions intrinsically bend the DNA double helix. The MSH2/MSH3 dimer can recognize this topology and initiate repair. The mechanism by which it recognizes mutations is different as well, because it separates the two DNA strands, which MutSα does not. [20]
Msh2 modulates accurate homologous recombination, a prominent DNA double-strand break repair pathway in mammalian chromosomes. [21] Repair of DNA double-strand breaks by accurate homologous recombination predominates over the inaccurate double-strand break repair pathway of “non-homologous end joining” in hamster, mouse and human somatic cells. [21]
MSH2 has been shown to interact with:
DNA damage appears to be the primary underlying cause of cancer, [34] and deficiencies in expression of DNA repair genes appear to underlie many forms of cancer. [35] [36] If DNA repair is deficient, DNA damage tends to accumulate. Such excess DNA damage may increase mutations due to error-prone translesion synthesis and error prone repair (see e.g. microhomology-mediated end joining). Elevated DNA damage may also increase epigenetic alterations due to errors during DNA repair. [37] [38] Such mutations and epigenetic alterations may give rise to cancer.
Reductions in expression of DNA repair genes (usually caused by epigenetic alterations) are very common in cancers, and are ordinarily much more frequent than mutational defects in DNA repair genes in cancers.[ citation needed ] (See Frequencies of epimutations in DNA repair genes.) In a study of MSH2 in non-small cell lung cancer (NSCLC), no mutations were found while 29% of NSCLC had epigenetic reduction of MSH2 expression. [39] In acute lymphoblastoid leukemia (ALL), no MSH2 mutations were found [40] while 43% of ALL patients showed MSH2 promoter methylation and 86% of relapsed ALL patients had MSH2 promoter methylation. [41] There were, however, mutations in four other genes in ALL patients that destabilized the MSH2 protein, and these were defective in 11% of children with ALL and 16% of adults with this cancer. [40]
Methylation of the promoter region of the MSH2 gene is correlated with the lack of expression of the MSH2 protein in esophageal cancer, [42] in non-small-cell lung cancer, [39] [43] and in colorectal cancer. [44] These correlations suggest that methylation of the promoter region of the MSH2 gene reduces expression of the MSH2 protein. Such promoter methylation would reduce DNA repair in the four pathways in which MSH2 participates: DNA mismatch repair, transcription-coupled repair [5] homologous recombination, [6] [45] [46] and base excision repair. [7] Such reductions in repair likely allow excess DNA damage to accumulate and contribute to carcinogenesis.
The frequencies of MSH2 promoter methylation in several different cancers are indicated in the Table.
Cancer | Frequency of MSH2 promoter methylation | Ref. |
---|---|---|
Acute lymphoblastic leukemia | 43% | [41] |
Relapsed Acute lymphoblastic leukemia | 86% | [41] |
Renal cell carcinoma | 51–55% | [47] [48] |
Esophageal squamous cell carcinoma | 29–48% | [42] [49] |
Head and neck squamous-cell carcinoma | 27–36% | [50] [51] [52] |
Non-small cell lung cancer | 29–34% | [39] [43] |
Hepatocellular carcinoma | 10–29% | [53] |
Colorectal cancer | 3–24% | [44] [54] [55] [56] |
Soft-tissue sarcoma | 8% | [57] |
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur. This can eventually lead to malignant tumors, or cancer as per the two-hit hypothesis.
Hereditary nonpolyposis colorectal cancer (HNPCC) is a hereditary predisposition to colon cancer.
A neoplasm is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists in growing abnormally, even if the original trigger is removed. This abnormal growth usually forms a mass, which may be called a tumour or tumor.
DNA glycosylases are a family of enzymes involved in base excision repair, classified under EC number EC 3.2.2. Base excision repair is the mechanism by which damaged bases in DNA are removed and replaced. DNA glycosylases catalyze the first step of this process. They remove the damaged nitrogenous base while leaving the sugar-phosphate backbone intact, creating an apurinic/apyrimidinic site, commonly referred to as an AP site. This is accomplished by flipping the damaged base out of the double helix followed by cleavage of the N-glycosidic bond.
DNA mismatch repair (MMR) is a system for recognizing and repairing erroneous insertion, deletion, and mis-incorporation of bases that can arise during DNA replication and recombination, as well as repairing some forms of DNA damage.
Microsatellite instability (MSI) is the condition of genetic hypermutability that results from impaired DNA mismatch repair (MMR). The presence of MSI represents phenotypic evidence that MMR is not functioning normally.
Muir–Torre syndrome is a rare hereditary, autosomal dominant cancer syndrome that is thought to be a subtype of HNPCC. Individuals are prone to develop cancers of the colon, genitourinary tract, and skin lesions, such as keratoacanthomas and sebaceous tumors. The genes affected are MLH1, MSH2, and more recently, MSH6, and are involved in DNA mismatch repair.
DNA mismatch repair protein Mlh1 or MutL protein homolog 1 is a protein that in humans is encoded by the MLH1 gene located on chromosome 3. The gene is commonly associated with hereditary nonpolyposis colorectal cancer. Orthologs of human MLH1 have also been studied in other organisms including mouse and the budding yeast Saccharomyces cerevisiae.
MSH6 or mutS homolog 6 is a gene that codes for DNA mismatch repair protein Msh6 in the budding yeast Saccharomyces cerevisiae. It is the homologue of the human "G/T binding protein," (GTBP) also called p160 or hMSH6. The MSH6 protein is a member of the Mutator S (MutS) family of proteins that are involved in DNA damage repair.
Mismatch repair endonuclease PMS2 is an enzyme that in humans is encoded by the PMS2 gene.
Methylated-DNA--protein-cysteine methyltransferase(MGMT), also known as O6-alkylguanine DNA alkyltransferaseAGT, is a protein that in humans is encoded by the MGMT gene. MGMT is crucial for genome stability. It repairs the naturally occurring mutagenic DNA lesion O6-methylguanine back to guanine and prevents mismatch and errors during DNA replication and transcription. Accordingly, loss of MGMT increases the carcinogenic risk in mice after exposure to alkylating agents. The two bacterial isozymes are Ada and Ogt.
Exonuclease 1 is an enzyme that in humans is encoded by the EXO1 gene.
DNA mismatch repair protein, MutS Homolog 3 (MSH3) is a human homologue of the bacterial mismatch repair protein MutS that participates in the mismatch repair (MMR) system. MSH3 typically forms the heterodimer MutSβ with MSH2 in order to correct long insertion/deletion loops and base-base mispairs in microsatellites during DNA synthesis. Deficient capacity for MMR is found in approximately 15% of colorectal cancers, and somatic mutations in the MSH3 gene can be found in nearly 50% of MMR-deficient colorectal cancers.
Methyl-CpG-binding domain protein 4 is a protein that in humans is encoded by the MBD4 gene.
PMS1 protein homolog 1 is a protein that in humans is encoded by the PMS1 gene.
DNA mismatch repair protein Mlh3 is a protein that in humans is encoded by the MLH3 gene.
Mouse models of colorectal cancer and intestinal cancer are experimental systems in which mice are genetically manipulated, fed a modified diet, or challenged with chemicals to develop malignancies in the gastrointestinal tract. These models enable researchers to study the onset, progression of the disease, and understand in depth the molecular events that contribute to the development and spread of colorectal cancer. They also provide a valuable biological system, to simulate human physiological conditions, suitable for testing therapeutics.
Genome instability refers to a high frequency of mutations within the genome of a cellular lineage. These mutations can include changes in nucleic acid sequences, chromosomal rearrangements or aneuploidy. Genome instability does occur in bacteria. In multicellular organisms genome instability is central to carcinogenesis, and in humans it is also a factor in some neurodegenerative diseases such as amyotrophic lateral sclerosis or the neuromuscular disease myotonic dystrophy.
Cancer epigenetics is the study of epigenetic modifications to the DNA of cancer cells that do not involve a change in the nucleotide sequence, but instead involve a change in the way the genetic code is expressed. Epigenetic mechanisms are necessary to maintain normal sequences of tissue specific gene expression and are crucial for normal development. They may be just as important, if not even more important, than genetic mutations in a cell's transformation to cancer. The disturbance of epigenetic processes in cancers, can lead to a loss of expression of genes that occurs about 10 times more frequently by transcription silencing than by mutations. As Vogelstein et al. points out, in a colorectal cancer there are usually about 3 to 6 driver mutations and 33 to 66 hitchhiker or passenger mutations. However, in colon tumors compared to adjacent normal-appearing colonic mucosa, there are about 600 to 800 heavily methylated CpG islands in the promoters of genes in the tumors while these CpG islands are not methylated in the adjacent mucosa. Manipulation of epigenetic alterations holds great promise for cancer prevention, detection, and therapy. In different types of cancer, a variety of epigenetic mechanisms can be perturbed, such as the silencing of tumor suppressor genes and activation of oncogenes by altered CpG island methylation patterns, histone modifications, and dysregulation of DNA binding proteins. There are several medications which have epigenetic impact, that are now used in a number of these diseases.
DNA methylation in cancer plays a variety of roles, helping to change the healthy cells by regulation of gene expression to a cancer cells or a diseased cells disease pattern. One of the most widely studied DNA methylation dysregulation is the promoter hypermethylation where the CPGs islands in the promoter regions are methylated contributing or causing genes to be silenced.