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

Abstract

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.

Keywords: 8-oxo-guanine; DNA glycosylases; DNA polymerase beta and lambda; MUTYH; MUTYH-associated polyposis (MAP); MYH; base-excision repair (BER); mutY.

Figure 1

MUTYH-initiated BER of A:8-oxo-G lesions. When ROS attack DNA, they lead to the formation of C:8-oxo-G base pairs through oxidation of G. Left column: These can be recognized by OGG1, which excises the 8-oxo-G and incises the resulting AP-site by β-elimination, giving rise to a 3′ddR5P and a 5′P residue. This 3′ sugar phosphate is then removed by APE1, yielding in a 1 nucleotide gap with a 3′OH and a 5′P. Subsequently, pol β catalyzes the insertion of a G opposite the templating C in this SP-BER pathway, and ligation by XRCC1/DNA ligase I leads to restoration of an intact, correctly base-paired double-stranded DNA again. Middle column: If the C:8-oxo-G base pairs are not recognized before S-phase by OGG1, or they arise through oxidation in S-phase, the replicative pols will often incorporate a wrong A opposite 8-oxo-G, giving rise to A:8-oxo-G mispairs. If these are not corrected, another round of replication will lead to a CG→AT transversion mutation. Right column: The A:8-oxo-G base pairs can be recognized by MUTYH, which catalyzes the excision of the wrong A from opposite 8-oxo-G, leading to the formation of an AP site. This AP site is further processed by APE1, which results in a 1 nt gap with 3′OH and 5′dRP moieties. The incorporation of the correct C opposite 8-oxo-G and one more nucleotide is performed by pol λ in collaboration with the cofactors PCNA and RP-A, thus performing strand displacement of the downstream DNA strand. FEN1 cleaves the 5′ flap, leading to a 5′P moiety, which can be ligated by DNA ligase I to yield an intact C:8-oxo-G containing double-stranded DNA. This C:8-oxo-G is then again substrate for OGG1-mediated removal of 8-oxo-G (left column).

Figure 2

Adenine removal by MUTYH. (A) View of the substrate adenosine interacting with catalytic residues of MUTYH. (B) Proposed glycolytic mechanism based on the structural information of (A). Reproduced form Lee and Verdine (2009). For details see text.