Mismatch-mediated error prone repair at the immunoglobulin genes

Abstract

The generation of effective antibodies depends upon somatic hypermutation (SHM) and class-switch recombination (CSR) of antibody genes by activation induced cytidine deaminase (AID) and the subsequent recruitment of error prone base excision and mismatch repair. While AID initiates and is required for SHM, more than half of the base changes that accumulate in V regions are not due to the direct deamination of dC to dU by AID, but rather arise through the recruitment of the mismatch repair complex (MMR) to the U:G mismatch created by AID and the subsequent perversion of mismatch repair from a high fidelity process to one that is very error prone. In addition, the generation of double-strand breaks (DSBs) is essential during CSR, and the resolution of AID-generated mismatches by MMR to promote such DSBs is critical for the efficiency of the process. While a great deal has been learned about how AID and MMR cause hypermutations and DSBs, it is still unclear how the error prone aspect of these processes is largely restricted to antibody genes. The use of knockout models and mice expressing mismatch repair proteins with separation-of-function point mutations have been decisive in gaining a better understanding of the roles of each of the major MMR proteins and providing further insight into how mutation and repair are coordinated. Here, we review the cascade of MMR factors and repair signals that are diverted from their canonical error free role and hijacked by B cells to promote genetic diversification of the Ig locus. This error prone process involves AID as the inducer of enzymatically-mediated DNA mismatches, and a plethora of downstream MMR factors acting as sensors, adaptors and effectors of a complex and tightly regulated process from much of which is not yet well understood.

Somatic Hypermutation at A/T-Rich Oligonucleotide Substrates Shows Different Strand Polarities in Ung-Deficient or -Proficient Backgrounds

“…As for Ung, Pms2 and Mlh1 deficiencies have been reported to have no impact on A/T mutagenesis (9). More recent data, obtained using catalytic knock-in mutants, further addressed the specific contribution of the endonuclease activity of Pms2 and did not report any major impact of its inactivation on Ig gene hypermutation (21).…”

“…This pathway generates mainly mutations at A/T bases distal to the initial deamination sites, because of the specific error-proneness of Pol, which copies Ts with low fidelity. This patch repair is unusual in that the second part of the mismatch repair complex, MutL␣, composed of Pms2 and Mlh1, appears to be dispensable and relies on the sole Msh2-Msh6 complex (MutS␣) together with ExoI (8,9). Therefore, which endonuclease activity provides the DNA incision required to initiate error-prone synthesis is not completely clear.…”

Postreplicative Mismatch Repair

“…It now appears that PCNA monoubiquitylation is generally induced when DNA damage (uracil, O 6 -methylguanine) occurring outside of S-phase activates MMR, which generates long excision tracts that cannot be readily filled in by high-fidelity polymerases . The latter findings provide a possible explanation for the involvement of MMR proteins in antibody maturation, where-counterintuitively-they are required for immunoglobulin mutagenesis, rather than for its prevention (Chahwan et al 2011;.…”

Mammalian mismatch repair: error-free or error-prone?

“…Mouse genetic experiments clearly demonstrated that SHM requires, in addition to MSH2 [50], MSH6 [51], EXO1 [52] and polymerase-η [53,54] also mono-ubiquitylated PCNA [55]. As SHM mutations at T/A base pairs are reduced to a similar extent in knock-in mice expressing a non-ubiquitylatable PCNA K164R variant, and in Msh2 -/-or Msh6 -/-animals [55], the origin of the mutations appears to be monoubiquitylated PCNA-mediated recruitment of error-prone DNA polymerases to MMR-generated gaps (see for [56] recent review). Why the processing of AID-generated U/G mispair might trigger PCNA ubiquitylation and error-prone DNA synthesis, rather than deploy error-free replicative polymerases is currently the subject of intense study in several laboratories.…”