AID function & regulation
AID is necessary for generating protective antibodies against infections. However, because it is mutagenic, this enzyme also carries a risk of off-target mutagenesis that can predispose to lymphoma. We use structure-function, proteomics and a number of biochemical, immunological and molecular biology techniques to discover and understand the mechanisms that balance the physiological and pathological activities of AID.
License to mutate: AID link to transcription elongation
AID mutates the antibody genes and, as a side effect, a relatively small number of other genes but most of the genome is spared. We have found that, even though AID is widely associated to chromatin, its ability to mutate a gene depends on a specific motif in AID, which couples AID to transcription elongation and thereby licenses it for mutagenesis.
Methot et al. Nat. Commun. 9 (1):1248, (2018).
Mechanisms regulating AID activity via subcellular localization.
We have identified mechanisms that regulate AID subcellular localization. AID requires active nuclear import because a mechanism of cytoplasmic retention prevents its diffusion into the nucleus. This mechanism depends on the eEF1A translation factor. The combined action of these mechanisms strike a balance that results in antibody gene diversification, while limiting the off target effects of AID. We have also identified drugs inhibiting the interaction between AID and eEF1A. This is the first pharmacological approach to increase AID function, which can have biotechnological and perhaps clinical applications.
Methot et al. J. Exp. Med. 212, 581–596 (2015) ; Patenaude et al. Nat. Struct. Mol. Biol. 16, 517–527 (2009).
A pathway stabilizing AID protein that can be exploited to decrease its activity.
We discovered that immature AID is protected from degradation by an HSP90/DNAJA1 molecular chaperoning pathway in the cytoplasms. This pathway directly impacts the efficiency of somatic hypermutation and class switching, as well as the oncogenic activity of AID. We exploited these findings to describe pharmacological approaches to reduce AID activity in vitro and in vivo; by either using inhibitors of HSP90 or inhibitors that prevent DNAJA1 farnesylation, which is necessary for the interaction with AID.
Montamat-Sicotte et al. Eur J Immunol 45, 2365-76 (2015) ; Orthwein et al. EMBO J 31, 679-91 (2012) ; Orthwein et al. J. Exp. Med. 207, 2751-65 (2010).
The role of DNA repair pathways in antibody responses.
We have shown that the C-terminus of AID links DNA damage by AID to DNA repair by non-homologous end joining during class switch recombination. Our results provide an explanation to an autosomal dominant immunodeficiency syndrome in patients expressing truncated forms of AID.
We have also a long interest in uracil DNA N-glycosylase (UNG), the major enzyme recognizing the Uracils made by AID. We have shown that UNG is critical for an efficient antibody response against acute antigenic exposure. A backup pathway mediated by MSH2/6 allows generating switched antibodies in the long term but the kinetics of isotype switching by MSH2/6 are too slow for an effective acute immune response.
In collaboration with Dr Ramiro Verdun (U. of Miami) we have also shown that UNG protects the telomeres of B cells from AID-induced damage. In the absence of UNG, MSH2/6-dependent damage leads to sudden telomere loss. Since inhibiting UNG in B cell lymphoma cells that express AID can reduce their proliferation, we posit that UNG could be a drug target against lymphoma cells expressing AID.
Zahn et al. Proc Natl Acad Sci U S A.111, E988-E997 (2014); Cortizas et al. J Exp Med. 213, 2459-2472 (2016); Cortizas et al. J immunol. 191, 5751-63 (2013); Zahn et al. J. Immunol. 190, 5949–5960 (2013).
Germinal center function
Germinal centers are the major anatomical locations of AID function. Once B cells recognize cognate antigen, they can form GC, where they undergo somatic hypermutation by AID and selection according to the affinity of their antibody molecules. The GC segregates mutation and selection in two distinct zones, in which the GC B cells exist in different stages. B cell centroblasts in the dark zone express AID and mutate their antibody genes; while B cell centrocytes in the light zone compete for T cell help by using their antibody molecules, as the B cell receptor, to capture antigen and present it to the T cells. Centrocytes can either go back to the dark zone and recycle in the GC or differentiate into memory or antibody-producing cells. We are interested in identifying molecules that are important to regulate the GC dynamics (i.e. that regulate the stage transitions and differentiation of GC B cells).
We have found that the protein arginine methyl transferase 5 (PRMT5) has a major role in B cell biology, being required for B cell development in the bone marrow, B cell activation after the first antigen encounter, and for adequate GC B cell dynamics. PRMT5 deficiency in GC B cells results in altered GC composition and prevents the generation of antigen specific genes. Mechanistically, PRMT5 has multiple functions, but we have characterized its function in enforcing the fidelity of mRNA splicing.
Litzler et al. Nature Communications, 2019 Jan 3;10(1):22.