Scharff, M. D. | bioRxiv Science

bioRxiv2022

Locus-specific proteomics identifies new aspects of the chromatin context involved in V region somatic hypermutation

Activation-induced cytidine deaminase (AID) somatically hypermutates the immunoglobulin heavy chain variable region (IGHV) gene to create the antibody diversity required to resist infections. This hypermutational process involves many pathways including transcription, DNA structural change and repair. While many of the proteins involved have been identified, their relative abundance, organization and regulation have not been resolved and additional factors and pathways need to be identified. To identify the proteome occupying IGHV, we have utilized dCas9-APEX targeted by guide RNAs to biotinylate and enrich the proteins associated with the mutating V region chromatin in the Ramos human B cell line and compared them to the non-mutating downstream constant region (C) chromatin. We identified hundreds of proteins specifically enriched on the V or C region. We confirmed the functionality of selected factors by examining the changes in the V region-specific proteome after inhibiting transcriptional elongation and somatic mutation with the Dot1L inhibitor EPZ004777. SummaryLocus-specific proteomics using dCas9-APEX identifies new aspects of the chromatin context involved in V region somatic hypermutation (SHM) in the human Ramos B cell line. An inhibitor of Dot1L which participates in SHM is used to identify functional SHM-related factors.

immunology↗

bioRxiv2021

Role of EXO1 nuclease activity in genome maintenance, the immune response and tumor suppression in Exo1D173A mice

DNA damage response pathways rely extensively on nuclease activity to process DNA intermediates. Exonuclease 1 (EXO1) is a pleiotropic evolutionary conserved DNA exonuclease involved in various DNA repair pathways, replication, antibody diversification, and meiosis. But, whether EXO1 facilitates these DNA metabolic processes through its enzymatic or scaffolding functions remains unclear. Here we dissect the contribution of EXO1 enzymatic versus scaffolding activity by comparing Exo1DA/DA mice expressing a proven nuclease-dead mutant form of EXO1 to entirely EXO1-deficient Exo1-/- and EXO1 wild type Exo1+/+ mice. We show that Exo1DA/DA and Exo1-/- mice are compromised in canonical DNA repair processing, suggesting that the EXO1 enzymatic role is important for error-free DNA mismatch and double-strand break repair pathways. However, in non-canonical repair pathways, EXO1 appears to have a more nuanced function. Next-generation sequencing of heavy chain V region in B cells showed the mutation spectra of Exo1DA/DA mice to be intermediate between Exo1+/+ and Exo1-/- mice, suggesting that both catalytic and scaffolding roles of EXO1 are important for somatic hypermutation. Similarly, while overall class switch recombination in Exo1DA/DA and Exo1-/- mice was comparably defective, switch-switch junction analysis suggests that EXO1 might fulfill an additional scaffolding function downstream of class switching. In contrast to Exo1-/- mice that are infertile, meiosis progressed normally in Exo1DA/DA and Exo1+/+ cohorts, indicating that a structural but not the nuclease function of EXO1 is critical for meiosis. However, both Exo1DA/DA and Exo1-/- mice displayed similar mortality and cancer predisposition profiles. Taken together, these data demonstrate that EXO1 has both scaffolding and enzymatic functions in distinct DNA repair processes and suggest a more composite and intricate role for EXO1 in DNA metabolic processes and disease.

cancer biology↗