Correction: The BRCT Domain of PARP-1 Is Required for Immunoglobulin Gene Conversion.

[This corrects the article DOI: 10.1371/journal.pbio.1000428.].

The BRCT domain of PARP-1 is required for immunoglobulin gene conversion.

Genetic variation at immunoglobulin (Ig) gene variable regions in B-cells is created through a multi-step process involving deamination of cytosine bases by activation-induced cytidine deaminase (AID) and their subsequent mutagenic repair. To protect the genome from dangerous, potentially oncogenic effects of off-target mutations, both AID activity and mutagenic repair are targeted specifically to the Ig genes. However, the mechanisms of targeting are unknown and recent data have highlighted the role of regulating mutagenic repair to limit the accumulation of somatic mutations resulting from the more widely distributed AID-induced lesions to the Ig genes. Here we investigated the role of the DNA damage sensor poly-(ADPribose)-polymerase-1 (PARP-1) in the repair of AID-induced DNA lesions. We show through sequencing of the diversifying Ig genes in PARP-1(-/-) DT40 B-cells that PARP-1 deficiency results in a marked reduction in gene conversion events and enhanced high-fidelity repair of AID-induced lesions at both Ig heavy and light chains. To further characterize the role of PARP-1 in the mutagenic repair of AID-induced lesions, we performed functional analyses comparing the role of engineered PARP-1 variants in high-fidelity repair of DNA damage induced by methyl methane sulfonate (MMS) and the mutagenic repair of lesions at the Ig genes induced by AID. This revealed a requirement for the previously uncharacterized BRCT domain of PARP-1 to reconstitute both gene conversion and a normal rate of somatic mutation at Ig genes, while being dispensable for the high-fidelity base excision repair. From these data we conclude that the BRCT domain of PARP-1 is required to initiate a significant proportion of the mutagenic repair specific to diversifying antibody genes. This role is distinct from the known roles of PARP-1 in high-fidelity DNA repair, suggesting that the PARP-1 BRCT domain has a specialized role in assembling mutagenic DNA repair complexes involved in antibody diversification.

Fusobacterium liver abscess.

Fusobacterium is well characterized as an oropharyngeal pathogen that may induce a septic thrombophlebitis by direct extension of abscess into an adjacent neck vessel (Lemierre's syndrome); its potential for visceral abscess formation, however, remains under-recognized. A 65-year-old man with a recent history of multiple rim-enhancing liver lesions presented to the emergency room with fever and abdominal pain. Based on interval increase in the size of the lesions, abscess was suspected. A liver biopsy was performed, and although no organism could be identified on routine microscopy, Warthin-Starry stain revealed Gram-negative bacilli consistent with an anaerobic Fusobacterium species as the underlying etiology of liver abscess formation. Subsequent anaerobic culture results confirmed the diagnosis. This case highlights the importance of consideration for Fusobacterium infection in the setting of liver abscess if anaerobic organisms have not yet been excluded on initial culture evaluation.

Requirements for DNA unpairing during displacement synthesis by HIV-1 reverse transcriptase.

DNA displacement synthesis by reverse transcriptase during retroviral replication is required for the production of the linear precursor to integration. The sensitivity of unpaired thymines to KMnO(4) oxidation was used to probe for the extent of DNA melting by human immunodeficiency virus, type 1 (HIV-1) reverse transcriptase in front of the primer terminus in model oligonucleotide-based displacement constructs. Unpairing of the two base pairs downstream of the primer (+1 and +2 positions) requires the presence of the next correct dNTP, indicating that DNA melting only occurs after the formation of the ternary complex with the enzyme tightly clamped around the DNA. The amount or extent of DNA melting is not significantly affected by the length of the already-displaced strand or the base composition of the DNA beyond the +2 position. The F61W mutant form of HIV-1 reverse transcriptase, which is partially impaired for displacement synthesis, exhibits a reduction in the amount of melting at the +1 and +2 positions. These results demonstrate the importance of the observed melting to displacement synthesis and suggest that the unpairing reaction is mediated by an intimate association between the fingers region of the enzyme and the DNA in the closed clamp conformation of the protein.