DNA Damage Tolerance Mechanisms Revealed from the Analysis of Immunoglobulin V Gene Diversification in Avian DT40 Cells

Dominant roles of BRCA1 in cellular tolerance to a chain-terminating nucleoside analog, alovudine

Alovudine is a chain-terminating nucleoside analog (CTNA) that is frequently used as an antiviral and anticancer agent. Generally, CTNAs inhibit DNA replication after their incorporation into nascent DNA during DNA synthesis by suppressing subsequent polymerization, which restricts the proliferation of viruses and cancer cells. Alovudine is a thymidine analog used as an antiviral drug. However, the mechanisms underlying the removal of alovudine and DNA damage tolerance pathways involved in cellular resistance to alovudine remain unclear. Here, we explored the DNA damage tolerance pathways responsible for cellular tolerance to alovudine and found that BRCA1-deficient cells exhibited the highest sensitivity to alovudine. Moreover, alovudine interfered with DNA replication in two distinct mechanisms: first: alovudine incorporated at the end of nascent DNA interfered with subsequent DNA synthesis; second: DNA replication stalled on the alovudine-incorporated template strand. Additionally, BRCA1 facilitated the removal of the incorporated alovudine from nascent DNA, and BRCA1-mediated homologous recombination (HR) contributed to the progressive replication on the alovudine-incorporated template. Thus, we have elucidated the previously unappreciated mechanism of alovudine-mediated inhibition of DNA replication and the role of BRCA1 in cellular tolerance to alovudine.

CTF18-RFC contributes to cellular tolerance against chain-terminating nucleoside analogs (CTNAs) in cooperation with proofreading exonuclease activity of DNA polymerase ε

Chemotherapeutic nucleoside analogs, such as cytarabine (Ara-C), are incorporated into genomic DNA during replication. Incorporated Ara-CMP (Ara-cytidine monophosphate) serves as a chain terminator and inhibits DNA synthesis by replicative polymerase epsilon (Polε). The proofreading exonuclease activity of Polε removes the misincorporated Ara-CMP, thereby contributing to the cellular tolerance to Ara-C. Purified Polε performs proofreading, and it is generally believed that proofreading in vivo does not need additional factors. In this study, we demonstrated that the proofreading by Polε in vivo requires CTF18, a component of the leading-strand replisome. We found that loss of CTF18 in chicken DT40 cells and human TK6 cells results in hypersensitivity to Ara-C, indicating the conserved function of CTF18 in the cellular tolerance of Ara-C. Strikingly, we found that proofreading-deficient POLE1^D269A/-, CTF18^-/-, and POLE1^D269A/-/CTF18^-/- cells showed indistinguishable phenotypes, including the extent of hypersensitivity to Ara-C and decreased replication rate with Ara-C. This observed epistatic relationship between POLE1^D269A/- and CTF18^-/- suggests that they are interdependent in removing mis-incorporated Ara-CMP from the 3' end of primers. Mechanistically, we found that CTF18^-/- cells have reduced levels of chromatin-bound Polε upon Ara-C treatment, suggesting that CTF18 contributes to the tethering of Polε on fork at the stalled end and thereby facilitating the removal of inserted Ara-C. Collectively, these data reveal the previously unappreciated role of CTF18 in Polε-exonuclease-mediated maintenance of the replication fork upon Ara-C incorporation.

BRCA1 deficiency specific base substitution mutagenesis is dependent on translesion synthesis and regulated by 53BP1

Defects in BRCA1, BRCA2 and other genes of the homology-dependent DNA repair (HR) pathway cause an elevated rate of mutagenesis, eliciting specific mutation patterns including COSMIC signature SBS3. Using genome sequencing of knock-out cell lines we show that Y family translesion synthesis (TLS) polymerases contribute to the spontaneous generation of base substitution and short insertion/deletion mutations in BRCA1 deficient cells, and that TLS on DNA adducts is increased in BRCA1 and BRCA2 mutants. The inactivation of 53BP1 in BRCA1 mutant cells markedly reduces TLS-specific mutagenesis, and rescues the deficiency of template switch-mediated gene conversions in the immunoglobulin V locus of BRCA1 mutant chicken DT40 cells. 53BP1 also promotes TLS in human cellular extracts in vitro. Our results show that HR deficiency-specific mutagenesis is largely caused by TLS, and suggest a function for 53BP1 in regulating the choice between TLS and error-free template switching in replicative DNA damage bypass.

Division of labor of Y-family polymerases in translesion-DNA synthesis for distinct types of DNA damage

Living organisms are continuously under threat from a vast array of DNA-damaging agents, which impact genome DNA. DNA replication machinery stalls at damaged template DNA. The stalled replication fork is restarted via bypass replication by translesion DNA-synthesis polymerases, including the Y-family polymerases Polη, Polι, and Polκ, which possess the ability to incorporate nucleotides opposite the damaged template. To investigate the division of labor among these polymerases in vivo, we generated POLη^−/−, POLι^−/−, POLκ^−/−, double knockout (KO), and triple knockout (TKO) mutants in all combinations from human TK6 cells. TKO cells exhibited a hypersensitivity to ultraviolet (UV), cisplatin (CDDP), and methyl methanesulfonate (MMS), confirming the pivotal role played by these polymerases in bypass replication of damaged template DNA. POLη^−/− cells, but not POLι^−/− or POLκ^−/− cells, showed a strong sensitivity to UV and CDDP, while TKO cells showed a slightly higher sensitivity to UV and CDDP than did POLη^−/− cells. On the other hand, TKO cells, but not all single KO cells, exhibited a significantly higher sensitivity to MMS than did wild-type cells. Consistently, DNA-fiber assay revealed that Polη plays a crucial role in bypassing lesions caused by UV-mimetic agent 4-nitroquinoline-1-oxide and CDDP, while all three polymerases play complementary roles in bypassing MMS-induced damage. Our findings indicate that the three Y-family polymerases play distinctly different roles in bypass replication, according to the type of DNA damage generated on the template strand.

Pold4, the fourth subunit of replicative polymerase δ, suppresses gene conversion in the immunoglobulin-variable gene in avian DT40 cells

The replicative polymerase δ (Polδ), consisting of four subunits, plays a pivotal role in chromosomal replication. Pold4, the smallest subunit of Polδ, is believed to contribute to the regulation of replication by facilitating repair in response to DNA damage. However, that contribution has not been fully elucidated. We here show that Pold4 contributes to the suppression of gene conversion in immunoglobulin-variable (IgV) gene diversification in the chicken DT40 lymphocyte cell line, where gene conversion diversifies the IgV gene through intragenic homologous recombination (HR) between diverged pseudo-V segments. IgV gene conversion is initiated by activation-induced cytidine deaminase-mediated uracil formation in the IgV gene, which in turn converts into an abasic site, leading to replication arrest. POLD4^-/- cells exhibited an increased rate of IgV gene conversion. Moreover, the gene-conversion tract was lengthened and the usage of pseudo-V segments was altered, showing a preference, to use the diverged sequence as a donor in POLD4^-/- cells. These data suggest that Pold4 is involved in the regulation of HR-mediated gene conversion in IgV diversification. By contrast, the rate in HR-mediated, sister-chromatid exchange and gene-targeting induced by an I-SceI endonclease-mediated DNA double-strand break exhibited by POLD4^-/- cells was indistinguishable from that by wild-type cells. These findings indicate that the functionality of general HR is preserved in POLD4^-/- cells. In conclusion, Pold4 is involved in the suppression of IgV-gene conversion without affecting the general functionality of HR.

TET3 dioxygenase modulates gene conversion at the avian immunoglobulin variable region via demethylation of non-CpG sites in pseudogene templates

Diversification of the avian primary immunoglobulin (Ig) repertoire is achieved in developing B cells by somatic hypermutation (SHM) and gene conversion (GCV). GCV is a type of homologous recombination that unidirectionally transfers segments of Ig pseudogenes to Ig variable domains. It is regulated by epigenetic mechanisms like histone modifications, but the role of DNA methylation remains unclear. Here, we demonstrate that the chicken B‐cell line DT40 lacking TET3, a member of the TET (Ten‐eleven translocation) family dioxygenases that facilitate DNA demethylation, exhibited a marked reduction in GCV activity in Ig variable regions. This was accompanied by a drop in the bulk levels of 5‐hydroxymethylcytosine, an oxidized derivative of 5‐methylcytosine, whereas TET1‐deficient or TET2‐deficient DT40 strains did not exhibit such effects. Deletion of TET3 caused little effects on the expression of proteins required for SHM and GCV, but induced hypermethylation in some Ig pseudogene templates. Notably, the enhanced methylation occurred preferably on non‐CpG cytosines. Disruption of both TET1 and TET3 significantly inhibited the expression of activation‐induced cytidine deaminase (AID), an essential player in Ig diversification. These results uncover unique roles of TET proteins in avian Ig diversification, highlighting the potential importance of TET3 in maintaining hypomethylation In Ig pseudogenes.

Mutating for Good: DNA Damage Responses During Somatic Hypermutation

Somatic hypermutation (SHM) of immunoglobulin (Ig) genes plays a key role in antibody mediated immunity. SHM in B cells provides the molecular basis for affinity maturation of antibodies. In this way SHM is key in optimizing antibody dependent immune responses. SHM is initiated by targeting the Activation-Induced Cytidine Deaminase (AID) to rearranged V(D)J and switch regions of Ig genes. The mutation rate of this programmed mutagenesis is ~10^-3 base pairs per generation, a million-fold higher than the non-AID targeted genome of B cells. AID is a processive enzyme that binds single-stranded DNA and deaminates cytosines in DNA. Cytosine deamination generates highly mutagenic deoxy-uracil (U) in the DNA of both strands of the Ig loci. Mutagenic processing of the U by the DNA damage response generates the entire spectrum of base substitutions characterizing SHM at and around the initial U lesion. Starting from the U as a primary lesion, currently five mutagenic DNA damage response pathways have been identified in generating a well-defined SHM spectrum of C/G transitions, C/G transversions, and A/T mutations around this initial lesion. These pathways include (1) replication opposite template U generates transitions at C/G, (2) UNG2-dependent translesion synthesis (TLS) generates transversions at C/G, (3) a hybrid pathway comprising non-canonical mismatch repair (ncMMR) and UNG2-dependent TLS generates transversions at C/G, (4) ncMMR generates mutations at A/T, and (5) UNG2- and PCNA Ubiquitination (PCNA-Ub)-dependent mutations at A/T. Furthermore, specific strand-biases of SHM spectra arise as a consequence of a biased AID targeting, ncMMR, and anti-mutagenic repriming. Here, we review mammalian SHM with special focus on the mutagenic DNA damage response pathways involved in processing AID induced Us, the origin of characteristic strand biases, and relevance of the cell cycle.