Genetic Control of Replication through N1-methyladenine in Human Cells

Abstract

N1-methyladenine (1-MeA)3 is formed in DNA by reaction with S n 2 methylating agents such as methyl methanesulfonate and naturally occurring methyl halides (1-3). The S n 2 methyl halides are among the most abundant environmental methylating agents released from biomass burning and from decaying vegetation. Exposure to alkylating agents could also occur from food, occupational hazards, and chemotherapeutic treatments (1).1-MeA is highly cytotoxic because the N1 atom is engaged in Watson-Crick (W-C) base pairing and its modification by a methyl group impairs W-C base pairing and blocks normal DNA replication. In Escherichia coli, AlkB repairs 1-MeA by oxidative demethylation, which liberates formaldehyde from the methylated base and results in complete reversal of the damage (4, 5). In humans, there are nine potential AlkB homologs, two of which, ABH2 and ABH3, can repair the same spectrum of DNA lesions as AlkB (6, 7); ABH2, however, is the primary housekeeping enzyme in humans for repairing 1-MeA (8). Mouse embryonic fibroblast lines derived from ABH2 null mice are highly defective in repairing 1-MeA residues generated in response to methyl methanesulfonate treatment. Because in the absence of any exposure to alkylating agents, 1-MeA residues accumulate over time in the genomic DNA of livers from ABH2 null mice, endogenous DNA methylation contributes to their generation (8).Previously, we reported on the genetic control of translesion synthesis (TLS) opposite UV-induced cyclobutane pyrimidine dimers and (6-4) pyrimidine-pyrimidone photoproducts and opposite thymine glycol (Tg), which is the most common oxidation product of thymine (9 -12). Of the two UV lesions, cyclobutane pyrimidine dimer does not significantly affect the ability of two pyrimidines to form a correct W-C base pair with the purine bases, and it has only a modest effect on DNA structure (13); by contrast, a (6-4) pyrimidine-pyrimidone photoproduct induces a large structural distortion in DNA. It confers a 44º bend in the DNA helix and the 3ЈT is oriented perpendicular to the 5ЈT in the (6-4) TT photoproduct (14 -16). The Tg lesion also has no significant effect on the ability of oxidized T to form a correct base pair with an A; however, because of the addition of hydroxyl groups at C5 and C6 on Tg, the damaged base becomes non-planar and that prevents the base 5Ј to Tg from stacking above it (17-20). Consequently, Tg presents a strong block to extension of synthesis from the Tg:A base pair. Despite the fact that these DNA lesions differ vastly in their effects on DNA structure and on base pairing, they generate only ϳ2% mutagenic TLS products in human cells (10 -12). This is rather surprising in view of the fact that the various TLS DNA polymerases (Pols) synthesize DNA with a low fidelity (21).Here we identify the TLS Pols that promote replication through the 1-MeA lesion in human cells and show that TLS opposite this lesion is mediated by three independent pathways, involving Pols and in one pathway and Pols and , respectively, in the other tw...

Genetic control of predominantly error-free replication through an acrolein-derived minor-groove DNA adduct

“…In human cells, TLS opposite (r) -HOPdG occurs with a high fidelity, generating only ~1% mutations, similar to that observed for TLS opposite DNA lesions such as cis-syn TT dimer (16), (6-4) TT photoproduct (17), thymine glycol (33), N1-methyl adenine (34), and N3-methyl adenine (35) where at most ~1-2% of the TLS products harbor mutations. This is in spite of the fact that TLS Pols synthesize DNA opposite DNA lesions with a poor fidelity.…”

DNA polymerase θ accomplishes translesion synthesis opposite 1,N6-ethenodeoxyadenosine with a remarkably high fidelity in human cells

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“…Thanks to the high-throughput sequencing methods, recent years have witnessed a burst of researches on N 6 -methyladenine (m 6 A), N 1 -methyladenine (m 1 A), N 7 -methylguanosine (m 7 G), and 5-methylcytidine (m 5 C), etc. (Conde et al, 2015;Chen et al, 2019;Pian et al, 2019;Yuan et al, 2019). Another kind of RNA modification, called 5-hydroxymethylcytosine (5hmC) is formed by TET-mediated oxidation of m5C (Fu et al, 2014).…”