The Shizosaccharomyces pombe homolog (SpMYH) of the Escherichia coli MutY is required for removal of guanine from 8-oxoguanine/guanine mispairs to prevent G:C to C:G transversions

DNA Repair in Haploid Context

DNA repair is a well-covered topic as alteration of genetic integrity underlies many pathological conditions and important transgenerational consequences. Surprisingly, the ploidy status is rarely considered although the presence of homologous chromosomes dramatically impacts the repair capacities of cells. This is especially important for the haploid gametes as they must transfer genetic information to the offspring. An understanding of the different mechanisms monitoring genetic integrity in this context is, therefore, essential as differences in repair pathways exist that differentiate the gamete’s role in transgenerational inheritance. Hence, the oocyte must have the most reliable repair capacity while sperm, produced in large numbers and from many differentiation steps, are expected to carry de novo variations. This review describes the main DNA repair pathways with a special emphasis on ploidy. Differences between Saccharomyces cerevisiae and Schizosaccharomyces pombe are especially useful to this aim as they can maintain a diploid and haploid life cycle respectively.

MUTYH: Not just polyposis

MUTYH is a base excision repair enzyme, it plays a crucial role in the correction of DNA errors from guanine oxidation and may be considered a cell protective factor. In humans it is an adenine DNA glycosylase that removes adenine misincorporated in 7,8-dihydro-8-oxoguanine (8-oxoG) pairs, inducing G:C to T:A transversions. MUTYH functionally cooperates with OGG1 that eliminates 8-oxodG derived from excessive reactive oxygen species production. MUTYH mutations have been linked to MUTYH associated polyposis syndrome (MAP), an autosomal recessive disorder characterized by multiple colorectal adenomas. MAP patients show a greatly increased lifetime risk for gastrointestinal cancers. The cancer risk in mono-allelic carriers associated with one MUTYH mutant allele is controversial and it remains to be clarified whether the altered functions of this protein may have a pathophysiological involvement in other diseases besides familial gastrointestinal diseases. This review evaluates the role of MUTYH, focusing on current studies of human neoplastic and non-neoplastic diseases different to colon polyposis and colorectal cancer. This will provide novel insights into the understanding of the molecular basis underlying MUTYH-related pathogenesis. Furthermore, we describe the association between MUTYH single nucleotide polymorphisms (SNPs) and different cancer and non-cancer diseases. We address the utility to increase our knowledge regarding MUTYH in the light of recent advances in the literature with the aim of a better understanding of the potential for identifying new therapeutic targets. Considering the multiple functions and interactions of MUTYH protein, its involvement in pathologies based on oxidative stress damage could be hypothesized. Although the development of extraintestinal cancer in MUTYH heterozygotes is not completely defined, the risk for malignancies of the duodenum, ovary, and bladder is also increased as well as the onset of benign and malignant endocrine tumors. The presence of MUTYH pathogenic variants is an independent predictor of poor prognosis in sporadic gastric cancer and in salivary gland secretory carcinoma, while its inhibition has been shown to reduce the survival of pancreatic ductal adenocarcinoma cells. Furthermore, some MUTYH SNPs have been associated with lung, hepatocellular and cervical cancer risk. An additional role of MUTYH seems to contribute to the prevention of numerous other disorders with an inflammatory/degenerative basis, including neurological and ocular diseases. Finally, it is interesting to note that MUTYH could be a new therapeutic target and future studies will shed light on its specific functions in the prevention of diseases and in the improvement of the chemo-sensitivity of cancer cells.

MutY-glycosylase: an overview on mutagenesis and activities beyond the GO system

MutY is a glycosylase known for its role in DNA base excision repair (BER). It is critically important in the prevention of DNA mutations derived from 7,8-dihydro-8-oxoguanine (8-oxoG), which are the major lesions resulting from guanine oxidation. MutY has been described as a DNA repair enzyme in the GO system responsible for removing adenine residues misincorporated in 8-oxoG:A mispairs, avoiding G:C to T:A mutations. Further studies have shown that this enzyme binds to other mispairs, interacts with several enzymes, avoids different transversions/transitions in DNA, and is involved in different repair pathways. Additional activities have been reported for MutY, such as the repair of replication errors in newly synthesized DNA strands through its glycosylase activity. Moreover, MutY is a highly conserved enzyme present in several prokaryotic and eukaryotic organisms. MutY defects are associated with a hereditary colorectal cancer syndrome termed MUTYH-associated polyposis (MAP). Here, we have reviewed the roles of MutY in the repair of mispaired bases in DNA as well as its activities beyond the GO system.

A role for Myh1 in DNA repair after treatment with strand-breaking and crosslinking chemotherapeutic agents

The highly conserved DNA glycosylase MutY is implicated in repair of oxidative DNA damage, in particular in removing adenines misincorporated opposite 7,8-dihydro-8-oxoguanine (8-oxo-G). The MutY homologues (MutYH) physically associate with proteins implicated in replication, DNA repair, and checkpoint signaling, specifically with the DNA damage sensor complex 9-1-1 proteins. Here, we ask whether MutYH could have a broader function in sensing and repairing different types of DNA damage induced by conventional chemotherapeutics. Thus, we examined if deletion of the Schizosaccharomyces pombe MutY homologue, Myh1, alone or in combination with deletion of either component of the 9-1-1 sensor complex, influences survival after exposure to different classes of DNA damaging chemotherapeutics that do not act primarily by causing 8-oxoG lesions. We show that Myh1 contributes to survival on genotoxic stresses induced by the oxidizing, DNA double strand break-inducing, bleomycins, or the DNA crosslinking platinum compounds, particularly in a rad1 mutant background. Exposure of cells to cisplatin leads to a moderate overall accumulation of Myh1 protein. Interestingly, we found that DNA damage induced by phleomycin results in increased chromatin association of Myh1. Further, we demonstrate that Myh1 relocalizes to the nucleus after exposure to hydrogen peroxide or chemotherapeutics, most prominently seen after phleomycin treatment. These observations indicate a wider role of Myh1 in DNA repair and DNA damage-induced checkpoint activation than previously thought.

MUTYH DNA glycosylase: the rationale for removing undamaged bases from the DNA

Maintenance of genetic stability is crucial for all organisms in order to avoid the onset of deleterious diseases such as cancer. One of the many proveniences of DNA base damage in mammalian cells is oxidative stress, arising from a variety of endogenous and exogenous sources, generating highly mutagenic oxidative DNA lesions. One of the best characterized oxidative DNA lesion is 7,8-dihydro-8-oxoguanine (8-oxo-G), which can give rise to base substitution mutations (also known as point mutations). This mutagenicity is due to the miscoding potential of 8-oxo-G that instructs most DNA polymerases (pols) to preferentially insert an Adenine (A) opposite 8-oxo-G instead of the appropriate Cytosine (C). If left unrepaired, such A:8-oxo-G mispairs can give rise to CG→AT transversion mutations. A:8-oxo-G mispairs are proficiently recognized by the MutY glycosylase homologue (MUTYH). MUTYH can remove the mispaired A from an A:8-oxo-G, giving way to the canonical base-excision repair (BER) that ultimately restores undamaged Guanine (G). The importance of this MUTYH-initiated pathway is illustrated by the fact that biallelic mutations in the MUTYH gene are associated with a hereditary colorectal cancer syndrome termed MUTYH-associated polyposis (MAP). In this review, we will focus on MUTYH, from its discovery to the most recent data regarding its cellular roles and interaction partners. We discuss the involvement of the MUTYH protein in the A:8-oxo-G BER pathway acting together with pol λ, the pol that can faithfully incorporate C opposite 8-oxo-G and thus bypass this lesion in a correct manner. We also outline the current knowledge about the regulation of MUTYH itself and the A:8-oxo-G repair pathway by posttranslational modifications (PTM). Finally, to achieve a clearer overview of the literature, we will briefly touch on the rather confusing MUTYH nomenclature. In short, MUTYH is a unique DNA glycosylase that catalyzes the excision of an undamaged base from DNA.

Early Steps in the DNA Base Excision Repair Pathway of a Fission Yeast Schizosaccharomyces pombe

DNA base excision repair (BER) accounts for maintaining genomic integrity by removing damaged bases that are generated endogenously or induced by genotoxic agents. In this paper, we describe the roles of enzymes functioning in the early steps of BER in fission yeast. Although BER is an evolutionarily conserved process, some unique features of the yeast repair pathway were revealed by genetic and biochemical approaches. AP sites generated by monofunctional DNA glycosylases are incised mainly by AP lyase activity of Nth1p, a sole bifunctional glycosylase in yeast, to leave a blocked 3′ end. The major AP endonuclease Apn2p functions predominantly in removing the 3′ block. Finally, a DNA polymerase fills the gap, and a DNA ligase seals the nick (Nth1p-dependent or short patch BER). Apn1p backs up Apn2p. In long patch BER, Rad2p endonuclease removes flap DNA containing a lesion after DNA synthesis. A UV-specific endonuclease Uve1p engages in an alternative pathway by nicking DNA on the 5′ side of oxidative damage. Nucleotide excision repair and homologous recombination are involved in repair of BER intermediates including the AP site and single-strand break with the 3′ block. Other enzymes working in 3′ end processing are also discussed.

Evolutionary loss of 8-oxo-G repair components among eukaryotes

BACKGROUND

We have examined the phylogenetic pattern among eukaryotes of homologues of the E. coli 7,8-dihydro-8-oxoguanine (8-oxo-G) repair enzymes MutY, MutM, and MutT.

RESULTS

These DNA repair enzymes are present in all large phylogenetic groups, with MutM homologues being the most universally conserved. All chordates and echinoderms were found to possess all three 8-oxo-G repair components. Likewise, the red and green algae examined have all three repair enzymes, while all land-living plants have MutY and MutM homologues, but lack MutT. However, for some phyla, e.g. protostomes, a more patchy distribution was found. Nematodes provide a striking example, where Caenorhabditis is the only identified example of an organism group having none of the three repair enzymes, while the genome of another nematode, Trichinella spiralis, instead encodes all three. The most complex distribution exists in fungi, where many different patterns of retention or loss of the three repair components are found. In addition, we found sequence insertions near or within the catalytic sites of MutY, MutM, and MutT to be present in some subgroups of Ascomycetes.

CONCLUSION

The 8-oxo-G repair enzymes are ancient in origin, and loss of individual 8-oxo-G repair components at several distinct points in evolution appears to be the most likely explanation for the phylogenetic pattern among eukaryotes.

Functional characterization of human MutY homolog (hMYH) missense mutation (R231L) that is linked with hMYH-associated polyposis

The MutY homolog (MYH) can excise adenines misincorporated opposite to guanines or 7,8-dihydro-8-oxo-guanines (8-oxoG) during DNA replication; thereby preventing G:C to T:A transversions. Germline mutations in the human MYH gene are associated with recessive inheritance of colorectal adenomatous polyposis (MAP). Here, we characterize one newly identified MAP-associated MYH missense mutation (R231L) that lies adjacent to the putative hMSH6 binding domain. The R231L mutant protein has severe defects in A/GO binding and in adenine glycosylase activities. The mutant fails to complement mutY-deficiency in Escherichia coli, but does not affect binding to hMSH6. These data support the role of the hMYH pathway in carcinogenesis.

Role of a MutY DNA glycosylase in combating oxidative DNA damage in Helicobacter pylori

MutY is an adenine glycosylase that has the ability to efficiently remove adenines from adenine/7,8-dihydro-8-oxoguanine (8-oxo-G) or adenine/guanine mismatches, and plays an important role in oxidative DNA damage repair. The human gastric pathogen Helicobacter pylori has a homolog of the MutY enzyme. To investigate the physiological roles of MutY in H. pylori, we constructed and characterized a mutY mutant. H. pylori mutY mutants incubated at 5% O2 have a 325-fold higher spontaneous mutation rate than its parent. The mutation rate is further increased by exposing the mutant to atmospheric levels of oxygen, an effect that is not seen in an E. coli mutY mutant. Most of the mutations that occurred in H. pylori mutY mutants, as examined by rpoB sequence changes that confer rifampicin resistance, are GC to TA transversions. The H. pylori enzyme has the ability to complement an E. coli mutY mutant, restoring its mutation frequency to the wild-type level. Pure H. pylori MutY has the ability to remove adenines from A/8-oxo-G mismatches, but strikingly no ability to cleave A/G mismatches. This is surprising because E. coli MutY can more rapidly turnover A/G than A/8-oxo-G. Thus, H. pylori MutY is an adenine glycosylase involved in the repair of oxidative DNA damage with a specificity for detecting 8-oxo-G. In addition, H. pylori mutY mutants are only 30% as efficient as wild-type in colonizing the stomach of mice, indicating that H. pylori MutY plays a significant role in oxidative DNA damage repair in vivo.