The SOS response-associated peptidase (SRAP) domain of YedK catalyzes ring opening of abasic sites and reversal of its DNA-protein crosslink

NEIL3: A unique DNA glycosylase involved in interstrand DNA crosslink repair

Endonuclease VIII-like 3 (NEIL3) is a versatile DNA glycosylase that repairs a diverse array of chemical modifications to DNA. Unlike other glycosylases, NEIL3 has a preference for lesions within single-strand DNA and at single/double-strand DNA junctions. Beyond its canonical role in base excision repair of oxidized DNA, NEIL3 initiates replication-dependent interstrand DNA crosslink repair as an alternative to the Fanconi Anemia pathway. This review outlines our current understanding of NEIL3's biological functions, role in disease, and three-dimensional structure as it pertains to substrate specificity and catalytic mechanism.

Novel Processes Associated with the Repair of Interstrand Cross-Links Derived from Abasic Sites in Duplex DNA: Roles for the Base Excision Repair Glycosylase NEIL3 and the SRAP Protein HMCES

Recent studies have defined a novel pathway for the repair of interstrand cross-links derived from the reaction of an adenine residue with an apurinic/apyrimidinic (AP) site on the opposing strand of DNA (dA-AP ICL). Stalling of a replication fork at the dA-AP ICL triggers TRAIP-dependent ubiquitylation of the CMG helicase that recruits the base excision repair glycosylase NEIL3 to the lesion. NEIL3 unhooks the dA-AP ICL to regenerate the native adenine residue on one strand and an AP site on the other strand. Covalent capture of the abasic site by the SRAP protein HMCES protects against genomic instability that would result from cleavage of the abasic site in the context of single-stranded DNA at the replication fork. After repair synthesis moves the HMCES-AP adduct into the context of double-stranded DNA, the DNA-protein cross-link is resolved by a nonproteolytic mechanism involving dissociation of thiazolidine attachment. The AP site in duplex DNA is then repaired by the base excision repair pathway.

Formation and Properties of DNA Adducts Generated by Reactions of Abasic Sites with 1,2-Aminothiols Including Cysteamine, Cysteine Methyl Ester, and Peptides Containing N-Terminal Cysteine Residues

The reaction of 1,2-aminothiol groups with aldehyde residues in aqueous solution generates thiazolidine products, and this process has been developed as a catalyst-free click reaction for bioconjugation. The work reported here characterized reactions of the biologically relevant 1,2-aminothiols including cysteamine, cysteine methyl ester, and peptides containing N-terminal cysteine residues with the aldehyde residue of apurinic/apyrimidinic (AP) sites in DNA oligomers. These 1,2-aminothiol-containing compounds rapidly generated adducts with AP sites in single-stranded and double-stranded DNA. NMR and MALDI-TOF-MS analyses provided evidence that the reaction generated a thiazolidine product. Conversion of an AP site to a thiazolidine-AP adduct protected against the rapid cleavage normally induced at AP sites by the endonuclease action of the enzyme APE1 and the AP-lyase activity of the biogenic amine spermine. In the presence of excess 1,2-aminothiols, the thiazolidine-AP adducts underwent slow strand cleavage via a β-lyase reaction that generated products with 1,2-aminothiol-modified sugar residues on the 3'-end of the strand break. In the absence of excess 1,2-aminothiols, the thiazolidine-AP adducts dissociated to release the parent AP-containing oligonucleotide. The properties of the thiazolidine-AP adducts described here mirror critical properties of SRAP proteins HMCES and YedK that capture AP sites in single-stranded regions of cellular DNA and protect them from cleavage.

Temporary HMCES-DNA cross-link prevents permanent DNA damage

HMCES, a recently discovered but ancient protein, covalently attaches to damaged single-stranded DNA and shields it from nucleases. Rua-Fernandez and colleagues now show that HMCES catalyzes its own recycling, permitting normal growth and non-mutagenic DNA repair.1.

Self-reversal facilitates the resolution of HMCES DNA-protein crosslinks in cells

Abasic sites are common DNA lesions stalling polymerases and threatening genome stability. When located in single-stranded DNA (ssDNA), they are shielded from aberrant processing by 5-hydroxymethyl cytosine, embryonic stem cell (ESC)-specific (HMCES) via a DNA-protein crosslink (DPC) that prevents double-strand breaks. Nevertheless, HMCES-DPCs must be removed to complete DNA repair. Here, we find that DNA polymerase α inhibition generates ssDNA abasic sites and HMCES-DPCs. These DPCs are resolved with a half-life of approximately 1.5 h. HMCES can catalyze its own DPC self-reversal reaction, which is dependent on glutamate 127 and is favored when the ssDNA is converted to duplex DNA. When the self-reversal mechanism is inactivated in cells, HMCES-DPC removal is delayed, cell proliferation is slowed, and cells become hypersensitive to DNA damage agents that increase AP (apurinic/apyrimidinic) site formation. In these circumstances, proteolysis may become an important mechanism of HMCES-DPC resolution. Thus, HMCES-DPC formation followed by self-reversal is an important mechanism for ssDNA AP site management.

A non-proteolytic release mechanism for HMCES-DNA-protein crosslinks

The conserved protein HMCES crosslinks to abasic (AP) sites in ssDNA to prevent strand scission and the formation of toxic dsDNA breaks during replication. Here, we report a non-proteolytic release mechanism for HMCES-DNA-protein crosslinks (DPCs), which is regulated by DNA context. In ssDNA and at ssDNA-dsDNA junctions, HMCES-DPCs are stable, which efficiently protects AP sites against spontaneous incisions or cleavage by APE1 endonuclease. In contrast, HMCES-DPCs are released in dsDNA, allowing APE1 to initiate downstream repair. Mechanistically, we show that release is governed by two components. First, a conserved glutamate residue, within HMCES' active site, catalyses reversal of the crosslink. Second, affinity to the underlying DNA structure determines whether HMCES re-crosslinks or dissociates. Our study reveals that the protective role of HMCES-DPCs involves their controlled release upon bypass by replication forks, which restricts DPC formation to a necessary minimum.

Repair and tolerance of DNA damage at the replication fork: A structural perspective

The replication machinery frequently encounters DNA damage and other structural impediments that inhibit progression of the replication fork. Replication-coupled processes that remove or bypass the barrier and restart stalled forks are essential for completion of replication and for maintenance of genome stability. Errors in replication-repair pathways lead to mutations and aberrant genetic rearrangements and are associated with human diseases. This review highlights recent structures of enzymes involved in three replication-repair pathways: translesion synthesis, template switching and fork reversal, and interstrand crosslink repair.

Abasic site-peptide cross-links are blocking lesions repaired by AP endonucleases

Apurinic/apyrimidinic (AP) sites are abundant DNA lesions arising from spontaneous hydrolysis of the N-glycosidic bond and as base excision repair (BER) intermediates. AP sites and their derivatives readily trap DNA-bound proteins, resulting in DNA-protein cross-links. Those are subject to proteolysis but the fate of the resulting AP-peptide cross-links (APPXLs) is unclear. Here, we report two in vitro models of APPXLs synthesized by cross-linking of DNA glycosylases Fpg and OGG1 to DNA followed by trypsinolysis. The reaction with Fpg produces a 10-mer peptide cross-linked through its N-terminus, while OGG1 yields a 23-mer peptide attached through an internal lysine. Both adducts strongly blocked Klenow fragment, phage RB69 polymerase, Saccharolobus solfataricus Dpo4, and African swine fever virus PolX. In the residual lesion bypass, mostly dAMP and dGMP were incorporated by Klenow and RB69 polymerases, while Dpo4 and PolX used primer/template misalignment. Of AP endonucleases involved in BER, Escherichia coli endonuclease IV and its yeast homolog Apn1p efficiently hydrolyzed both adducts. In contrast, E. coli exonuclease III and human APE1 showed little activity on APPXL substrates. Our data suggest that APPXLs produced by proteolysis of AP site-trapped proteins may be removed by the BER pathway, at least in bacterial and yeast cells.

Self-reversal facilitates the resolution of HMCES-DNA protein crosslinks in cells

Abasic sites are common DNA lesions that stall polymerases and threaten genome stability. When located in single-stranded DNA (ssDNA), they are shielded from aberrant processing by HMCES via a DNA-protein crosslink (DPC) that prevents double-strand breaks. Nevertheless, the HMCES-DPC must be removed to complete DNA repair. Here, we found that DNA polymerase α inhibition generates ssDNA abasic sites and HMCES-DPCs. These DPCs are resolved with a half-life of approximately 1.5 hours. Resolution does not require the proteasome or SPRTN protease. Instead, HMCES-DPC self-reversal is important for resolution. Biochemically, self-reversal is favored when the ssDNA is converted to duplex DNA. When the self-reversal mechanism is inactivated, HMCES-DPC removal is delayed, cell proliferation is slowed, and cells become hypersensitive to DNA damage agents that increase AP site formation. Thus, HMCES-DPC formation followed by self-reversal is an important mechanism for ssDNA AP site management.

Novel mechanisms for the removal of strong replication-blocking HMCES- and thiazolidine-DNA adducts in humans

Apurinic/apyrimidinic (AP) sites are DNA lesions created under normal growth conditions that result in cytotoxicity, replication-blocks, and mutations. AP sites are susceptible to β-elimination and are liable to be converted to DNA strand breaks. HMCES (5-hydroxymethylcytosine binding, ES cell specific) protein interacts with AP sites in single stranded (ss) DNA exposed at DNA replication forks to generate a stable thiazolidine protein-DNA crosslink and protect cells against AP site toxicity. The crosslinked HMCES is resolved by proteasome-mediated degradation; however, it is unclear how HMCES-crosslinked ssDNA and the resulting proteasome-degraded HMCES adducts are processed and repaired. Here, we describe methods for the preparation of thiazolidine adduct-containing oligonucleotides and determination of their structure. We demonstrate that the HMCES-crosslink is a strong replication blocking adduct and that protease-digested HMCES adducts block DNA replication to a similar extent as AP sites. Moreover, we show that the human AP endonuclease APE1 incises DNA 5' to the protease-digested HMCES adduct. Interestingly, while HMCES-ssDNA crosslinks are stable, the crosslink is reversed upon the formation of dsDNA, possibly due to a catalytic reverse reaction. Our results shed new light on damage tolerance and repair pathways for HMCES-DNA crosslinks in human cells.