Ahmad Faili

Induction of somatic hypermutation in immunoglobulin genes is dependent on DNA polymerase iota

Somatic hypermutation of immunoglobulin genes is a unique, targeted, adaptive process. While B cells are engaged in germinal centres in T-dependent responses, single base substitutions are introduced in the expressed Vh/Vl genes to allow the selection of mutants with a higher affinity for the immunizing antigen. Almost every possible DNA transaction has been proposed to explain this process, but each of these models includes an error-prone DNA synthesis step that introduces the mutations. The Y family of DNA polymerases—pol η, pol ι, pol κ and rev1—are specialized for copying DNA lesions and have high rates of error when copying a normal DNA template. By performing gene inactivation in a Burkitts lymphoma cell line inducible for hypermutation, we show here that somatic hypermutation is dependent on DNA polymerase iota.

DNA Polymerase η Is Involved in Hypermutation Occurring during Immunoglobulin Class Switch Recombination

Base substitutions, deletions, and duplications are observed at the immunoglobulin locus in DNA sequences involved in class switch recombination (CSR). These mutations are dependent upon activation-induced cytidine deaminase (AID) and present all the characteristics of the ones observed during V gene somatic hypermutation, implying that they could be generated by the same mutational complex. It has been proposed, based on the V gene mutation pattern of patients with the cancer-prone xeroderma pigmentosum variant (XP-V) syndrome who are deficient in DNA polymerase η (pol η), that this enzyme could be responsible for a large part of the mutations occurring on A/T bases. Here we show, by analyzing switched memory B cells from two XP-V patients, that pol η is also an A/T mutator during CSR, in both the switch region of tandem repeats as well as upstream of it, thus suggesting that the same error-prone translesional polymerases are involved, together with AID, in both processes.

A Backup Role of DNA Polymerase κ in Ig Gene Hypermutation Only Takes Place in the Complete Absence of DNA Polymerase η

Patients with the variant form of xeroderma pigmentosum (XPV) syndrome have a genetic deficiency in DNA polymerase (Pol) η, and display accordingly an increased skin sensitivity to UV light, as well as an altered mutation pattern of their Ig V genes in memory B cells, alteration that consists in a reduced mutagenesis at A/T bases. We previously suggested that another polymerase with a different mutation signature, Pol κ, is used as backup for Ig gene hypermutation in both humans and mice in cases of complete Pol η deficiency, a proposition supported in this study by the analysis of Pol η × Pol κ double-deficient mice. We also describe a new XPV case, in which a splice site mutation of the first noncoding exon results in a decreased mRNA expression, a mRNA that otherwise encodes a normal Pol η protein. Whereas the Pol η mRNA level observed in patient’s fibroblasts is one-twentieth the value of healthy controls, it is only reduced to one-fourth of the normal level in activated B cells. Memory B cells from this patient showed a 50% reduction in A/T mutations, with a spectrum that still displays a strict Pol η signature. Pol η thus appears as a dominant enzyme in hypermutation, its presence precluding the use of a substitute enzyme even in conditions of reduced availability. Such a dominant behavior may explain the lack of Pol κ signature in Ig gene mutations of some XPV patients previously described, for whom residual Pol η activity might exist.

Competitive repair pathways in immunoglobulin gene hypermutation

This review focuses on the contribution of translesion DNA polymerases to immunoglobulin gene hypermutation, in particular on the roles of DNA polymerase eta (Polη) in the generation of mutations at A/T bases from the initial cytosine-targeted activation-induced cytidine deaminase (AID)-mediated deamination event, and of Polκ, an enzyme of the same polymerase family, used as a substitute when Polη is absent. The proposition that the UNG uracil glycosylase and the MSH2–MSH6 mismatch recognition complex are two competitive rather than alternative pathways in the processing of uracils generated by AID is further discussed.

Ig gene hypermutation: A mechanism is due

Publisher Summary This chapter presents a discussion on Ig gene hypermutation. The chapter reviews (1) the possible participation of some mismatch repair (MMR) components, the mammalian MutS-homologs, (2) the role of Ig gene transcription in the targeting of the process, (3) the occurrence of DNA breaks as specific priming events, (4) the involvement of an error-prone DNA polymerase and the emergence of several new candidate enzymes, and (5) a new partner, activation-induced cytidine deaminase whose molecular contribution is the subject of intense speculation. Several reports have shown that transcription and hypermutation are quantitatively correlated starting with the earlier observation that the hypermutation domain at the heavy chain locus extends over 1 kb downstream from the Ig promoter. From these observations, various models have been proposed in which the transcription complex—as it progresses through the V gene—induces an error-prone repair process. The field of hypermutation is crowded with experiments and models but a precise molecular description of the process is due. The first molecule that seems to be a major player in the process has been discovered after a cDNA subtraction designed to elucidate switch recombination and its putative properties generate many more questions than they bring straightforward explanations.

Proteasomal degradation restricts the nuclear lifespan of AID

Aoufouchi et al. 2008. J. Exp. Med. doi:10.1084/jem.20070950 [OpenUrl][1][Abstract/FREE Full Text][2] [1]: {openurl}?query=rft_id%253Dinfo%253Adoi%252F10.1084%252Fjem.20070950%26rft_id%253Dinfo%253Apmid%252F18474627%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%

DNA polymerase η is the sole contributor of A/T modifications during immunoglobulin gene hypermutation in the mouse

Delbos et al. 2007. J. Exp. Med. doi:10.1084/jem.20062131 [OpenUrl][1][Abstract/FREE Full Text][2] [1]: {openurl}?query=rft_id%253Dinfo%253Adoi%252F10.1084%252Fjem.20062131%26rft_id%253Dinfo%253Apmid%252F17190840%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%