Interallelic class switch recombination can reverse allelic exclusion and allow trans‐complementation of an IgH locus switching defect

Abstract

The predominant path of immunoglobulin class switch recombination follows the paradigm of intra-chromosomal deletion enabling expression of another heavy chain instead of l and d. This was, however, challenged by observations of inter-allelic class switch recombination in rabbit or mouse IgG3-or IgA-producing B cells. Assuming that the conditions of inter-chromosomal exchange are likely present at any target S regions in stimulated B cells, we explored trans-association of VH and C genes in a model allowing all C genes to be checked simultaneously. Heterozygous mutant mice are thus studied, which carry one non-functional IgH allele inactivated by a non-translatable mutation of VDJ-CH transcripts, while the functional allele is deficient for class switching due to a truncated 3 0 regulatory region. A fair level of switching to all Ig classes is restored in heterozygous mice despite the fact that cis-recombination is either non productive on one allele or deficient on the other. Molecular evidence at the DNA level of trans-CSR to IgG3 was demonstrated by cloning and sequencing Sl-Sc3 hybrid junctions. These data demonstrate that inter-allelic recombination may broadly rescue the production of various class-switched isotypes and allow complementation between mutations located at both ends of the IgH constant gene cluster.

Reassessment of the Role of Mut S Homolog 5 in Ig Class Switch Recombination Shows Lack of Involvement in cis- and trans-Switching

“…S μ -S γ 3 junctions were PCR amplified using Expand Long Template Taq polymerase (Roche) and the primers S μ -nest2F (5′-CCAGCCACAGTAATGACCCAG-3′) and S γ 3-nest2R (5′-CTGACCCAGGAGCTGCATAAC-3′), as previously described (26). PCR products were cloned into the pCR4-TOPO vector and sequenced by Macrogen.…”

“…Most switch recombination occurs by an intrachromosomal deletion event, although recently it was demonstrated that 7-17% of the CSR events occur between the two homologous chromosomes (24-26). Because trans -chromosomal CSR would require alignment of the two chromosome homologs, we hypothesized that Msh5 might regulate this trans -chromosomal switch.…”

Ikaros controls isotype selection during immunoglobulin class switch recombination

“…9 and Table S1). Importantly, because interallelic CSR occurs efficiently in WT cells (40,41), SC-RT-PCR was performed without regard for allele. In LPS-stimulated WT cells, 97.1% of switching competent cells (e.g., aicda + ) expressed GLTs for 2b (90.1%) and/or 3 (78.5%), which correlated with switching to these two isotypes (Fig.…”

The IgH locus 3′ cis-regulatory super-enhancer co-opts AID for allelic transvection

“…Homozygous 3′RR deletions suppress CSR-accessibility of downstream switch (S) regions but have no effect on Sµ (and barely any on Sγ1) [17, 29]. This is partly rescued by heterozygosity, where trans- CSR is maintained and joins the accessible Sµ from the 3′RR-less allele to downstream S regions from the wt allele [24]. The IgH a locus (from SV129 strain) drives expression of IgM a , IgG1 a , IgG2a, IgE a and IgA a (for which some allotype-specific antisera are available) instead of IgM b , IgG1 b , IgG2c, IgE b and IgA b for the IgH b locus (from C57BL/6 strain).…”

“…In this regard, while illegitimate off-target CSR-like junctions with non Ig genes remain exceptional during AID expression, legitimate junctions repairing CSR breaks are known to occur actively in both cis and trans [24–27]. Promiscuous trans -CSR junctions strongly suggest that IgH biallelic co-localization has functional implications at stages where proximity might secure legitimate synapsis between concomitantly generated biallelic DNA breaks.…”