4 research outputs found
Disparate roles of ATR and ATM in immunoglobulin class switch recombination and somatic hypermutation
13 pages, 3 figures, 4 tables.-- et al.Class switch recombination (CSR) and somatic hypermutation (SHM) are mechanistically related processes initiated by activation-induced cytidine deaminase. Here, we have studied the role of ataxia telangiectasia and Rad3-related protein (ATR) in CSR by analyzing the recombinational junctions, resulting from in vivo switching, in cells from patients with mutations in the ATR gene. The proportion of cells that have switched to immunoglobulin (Ig)A and IgG in the peripheral blood seems to be normal in ATR-deficient (ATRD) patients and the recombined S regions show a normal âblunt end-joining,â but impaired end joining with partially complementary (1â3 bp) DNA ends. There was also an increased usage of microhomology at the ÎŒ-α switch junctions, but only up to 9 bp, suggesting that the end-joining pathway requiring longer microhomologies (â„10 bp) may be ATR dependent. The SHM pattern in the Ig variable heavy chain genes is altered, with fewer mutations occurring at A and more mutations at T residues and thus a loss of strand bias in targeting A/T pairs within certain hotspots. These data suggest that the role of ATR is partially overlapping with that of ataxia telangiectasiaâmutated protein, but that the former is also endowed with unique functional properties in the repair processes during CSR and SHM.This work was supported by the Swedish Research Council, the Swedish Society for Medical Research (SSMF), and the Swedish Doctors Association.Peer reviewe
Role of DNA repair in class switch recombination and somatic hypermutation
Class switch recombination (CSR) and somatic hypermutation (SHM), occurring in the
germinal center, are two important processes for B cell development. Both are initiated by
activation-induced cytidine deaminase (AID), through deamination of the C residues in the
variable and switch regions of the immunoglobulin locus, resulting in either in single stranded or
double stranded DNA breaks. At least three pathways (nonhomologous end joining (NHEJ), base
excision repair (BER), and mismatch repair (MMR)) have been implicated in processing, repair
and ligation of the DNA breaks during CSR and SHM. However, the way in which these pathways
are regulated and coordinated to mediate CSR and /or SHM is still not well understood.
To explore the potential role of several proteins in CSR and SHM, including Ataxiatelangiectasia
and Rad3-related (ATR), Artemis, Cernunnos and the Mre11-Rad50-NBS1 (MRN)
complex, CSR junctions and SHM patterns in the immunoglobulin variable region gene were
analyzed in patients with deficiency in these factors.
We first studied the role of ATR during CSR and SHM. CSR junctions obtained from ATR
deficiency (ATRD) patients showed significantly increased usage of microhomology, but
impaired end joining with partially complementary (1-3 bp) DNA ends. The SHM pattern was
also altered, with fewer mutations occurring at A but more at T residues, representing a loss of
strand bias in targeting A/T pairs within certain hotspots. The function of ATR and ATM in CSR
and SHM was also compared. Our data suggest that the role of ATR is partially overlapping with
ATM, whereas ATR is also endowed with unique functional properties in the repair processes
during CSR and SHM.
We further studied the CSR junctions in Artemis deficient patients. A significantly increased
usage of microhomology of â„10 bp and an absolute absence of blunt-end joining were observed in
SΌ-Sα junctions in the patients. However, the SΌ-Sα junctions obtained from a patient who carried
âhypomorphicâ mutations appeared to be largely normal in their usage of microhomology,
although an unusual type of sequential switching was observed more frequently than expected.
These findings suggest that varying modes of CSR junction resolution were used for different S
regions, when the function of Artemis is impaired. The altered pattern of CSR junctions also
strongly link Artemis to the predominant NHEJ pathway during CSR.
CSR junctions were also studied in B cells from Cernunnos deficient patients. These
junctions were characterized by a significantly increased usage of microhomology of â„10 bp and a
significantly decreased usage of âdirect end joiningâ. This pattern has previously been observed in
B cells deficient for DNA Ligase IV (Lig4), XRCC4, Artemis and ATM, suggesting that
Cernunnos is likely to be involved in the DNA Lig4 dependent NHEJ pathway during CSR.
One somatically acquired missense mutation (p.Q227R) was also observed in the Cernunnos
encoding gene in a germinal B cell like (GCB) diffuse large B cell lymphoma (DLBCL) sample.
Two types of translocations (IGH/BCL2 and MYC/IGH) were detected in this tumor sample and
one of the switch Îł regions appeared to be disrupted during translocation. Clonal-like, dynamic
IgA switching activities were also observed, suggesting a link between defects in the Cernunnos
dependent NHEJ pathway and aberrant CSR/switch translocations during the development of B
cell malignancies.
Mutations in Mre11 and NBS1 gene can cause Ataxia-telangiectasia-like disorder (ATLD)
and Nijmegen breakage syndrome (NBS) respectively. SHM patterns in cells from these patients
were furthermore analyzed. The frequency and distribution of mutations obtained from both
patient groups were largely similar to that observed in controls. The mutation pattern from ATLD
patients was only slightly changed, with a small increase of the frequency of A to C transversions,
suggesting that Mre11 is unlikely to be the major nuclease involved in cleavage of the abasic sites
during SHM. The mutation pattern from NBS patients was however, altered with a significantly
increased number of G transversions (GâC and GâT), which mainly occurred in AID and/or
SHM hotspot motifs. NBS1 might thus have a specific role in regulating the strand-biased repair
during phase Ib mutagenesis
Disparate roles of ATR and ATM in immunoglobulin class switch recombination and somatic hypermutation
Class switch recombination (CSR) and somatic hypermutation (SHM) are mechanistically related processes initiated by activation-induced cytidine deaminase. Here, we have studied the role of ataxia telangiectasia and Rad3-related protein (ATR) in CSR by analyzing the recombinational junctions, resulting from in vivo switching, in cells from patients with mutations in the ATR gene. The proportion of cells that have switched to immunoglobulin (Ig)A and IgG in the peripheral blood seems to be normal in ATR-deficient (ATRD) patients and the recombined S regions show a normal âblunt end-joining,â but impaired end joining with partially complementary (1â3 bp) DNA ends. There was also an increased usage of microhomology at the ÎŒ-α switch junctions, but only up to 9 bp, suggesting that the end-joining pathway requiring longer microhomologies (â„10 bp) may be ATR dependent. The SHM pattern in the Ig variable heavy chain genes is altered, with fewer mutations occurring at A and more mutations at T residues and thus a loss of strand bias in targeting A/T pairs within certain hotspots. These data suggest that the role of ATR is partially overlapping with that of ataxia telangiectasiaâmutated protein, but that the former is also endowed with unique functional properties in the repair processes during CSR and SHM