Transcription, β–like DNA polymerases and hypermutation

Abstract

This paper discusses two aspects of immunoglobulin (Ig) gene hypermutation. In the ¢rst approach, a transcription termination signal is introduced in an Ig light chain transgene acting as a mutation substrate, and transgenic lines are generated with control and mutant transgenes integrated in tandem. Analysis of transcription levels and mutation frequencies between mutant and control transgenes clearly dissociates transcription elongation and mutation, and therefore argues against models whereby speci¢c pausing of the RNA polymerase during V gene transcription would trigger an error-prone repair process. The second part reports the identi¢cation of two novel b-like DNA polymerases named Pol l and Pol m, one of which (Pol m) represents a good candidate for the Ig mutase due to its higher lymphoid expression and its similarity with the lymphoid enzyme terminal deoxynucleotidyl transferase. Peculiar features of the expression of this gene, including an unusual splicing variability and a splicing inhibition in response to DNA-damaging agents, are discussed.

Terminal deoxynucleotidyl transferase: The story of a misguided DNA polymerase

“…This activity is based on the preferential expression of pol μ in secondary lymphoid tissues as well as its low fidelity during the replication of undamaged DNA [132]. Additionally, the ability of pol μ to bypass several DNA lesions through a deletion mechanism provides circumstantial evidence for its role as a mutase during somatic hypermutation [133]. A perspective on the role of certain X-family DNA polymerases in regulating nucleic acid integrity is provided in a recent review by Ramadan et al [134].…”

Modulation of Pleurodeles waltl DNA Polymerase mu Expression by Extreme Conditions Encountered during Spaceflight

“…Numerous human and murine Polµ splice variants have been described representing approximately 90% of Polµ mRNA species, in which splicing affected exons 5 to 11, encoding the active site of the polymerase. In most cases, these splicing events induced a premature stop codon and truncated proteins [23], [27], which could function as regulators of the activity of this enzyme [36]. Similarly, the splice variant detected in this study lacks one part of its active site and will consequently likely be nonfunctional.…”

“…Figure 2B also shows that the expression of Polµ in the spleen decreases with age. Some authors have suggested that due to its mutagenic potential, its preferential expression in secondary lymphoid tissues and its expression in the nucleus of centroblasts from human tonsils, Polµ could play a role in somatic hypermutation of Ig genes [24]–[27]. However, an analysis of Polµ-deficient mice did not support this hypothesis [36], [41].…”

“…Several studies suggested that Polµ could also be involved in somatic hypermutation (SHM) of Ig genes that diversify antibody binding sites [24]–[27].…”

Eukaryotic DNA Polymerases: Proposal for a Revised Nomenclature

“…Both human Pol ␤ and human Pol show deoxyribose phosphate lyase activity, indicative of their ability to process intermediates in the DNA glycosylase-initiated repair of damaged bases (35,36). A function for Pol in somatic hypermutation has been proposed based upon its low fidelity of DNA synthesis in vitro and its cell type-specific expression pattern in mammals (5,37). Moreover, a more general role of Pol in non-homologous end joining of double-stranded DNA breaks has also been proposed (38).…”