Remodeling of immunoglobulin genes by activation-induced deaminase (AID) is required for affinity maturation and class-switch recombination in mature B lymphocytes. In the immunoglobulin heavy chain locus, these processes are predominantly controlled by the 3' cis-regulatory region. We now show that this region is transcribed and undergoes AID-mediated mutation and recombination around phylogenetically conserved switchlike DNA repeats. Such recombination, which we term locus suicide recombination, deletes the whole constant region gene cluster and thus stops expression of the immunoglobulin of the B cell surface, which is critical for B cell survival. The frequency of this event is approaching that of class switching and makes it a potential regulator of B cell homeostasis.

Material

Immunoglobulin (Ig) genes naturally acquire frequent premature termination codons during the error-prone V(D)J recombination process. Although B cell differentiation is linked to the expression of productive Ig alleles, the transcriptional status of nonfunctionally recombined alleles remains unclear. Here, we tracked transcription and posttranscriptional regulation for both Ig heavy-chain (IgH) alleles in mice carrying a nonfunctional knock-in allele. We show that productively and nonproductively VDJ-rearranged alleles are transcribed throughout B cell development, carry similar active chromatin marks, and even display equivalent RNA polymerase II (RNAPII) loading after B cell stimulation. Hence, these results challenge the idea that the repositioning of one allele to heterochromatin could promote the silencing of nonproductive alleles. Interestingly, the efficiency of downstream RNA surveillance mechanisms fluctuates according to B cell activation and terminal differentiation: unspliced nonfunctional transcripts accumulate in primary B cells, while B cell activation promotes IgH transcription, RNA splicing, and nonsense-mediated mRNA decay (NMD). Altogether, IgH transcription and RNA splicing rates determine by which RNA surveillance mechanisms a B cell can get rid of nonproductive IgH mRNAs. Development of the primary immunoglobulin (Ig) repertoire involves DNA recombination between variable (V), diversity (D), and joining (J) segments, and this diversity is extended by the imprecision of VDJ junctions. A collateral effect of this random process is the occurrence of out-of-frame rearrangements inherently associated with premature termination codons (PTCs) in two-thirds of cases. Previous reports documented that around 40 to 50% of B cells carry VDJ rearrangements (VDJ ϩ /VDJ Ϫ ) on both Ig heavy-chain (IgH) alleles, while the remainder retains incomplete DJ rearrangements on the nonproductive allele (VDJ ϩ / DJ) (18,24,49).Although B cell receptor (BCR) signaling, and, hence, the expression of productively rearranged Ig alleles, governs B cell development and survival, the transcriptional status of nonfunctional alleles remains unclear. If translated, these PTC-containing alleles might encode potentially harmful truncated Ig proteins that could disrupt the normal assembly of the BCR or elicit the unfolded protein response (UPR) (13, 32). Recently, it was demonstrated that the stability of untranslated nonsense H mRNAs that escape degradation by nonsense-mediated mRNA decay (NMD) impairs IgH allelic exclusion and pro-B cell differentiation (36). Although nonsense mutations (in the leader exon) generating stable and untranslated H mRNAs should not exist in pro-B cells, this model suggest that the persistence of nonsense H mRNAs per se could be detrimental in early B cell development, yet the underlying mechanisms are currently unknown.For activated B cells, it was reported previously that one IgH allele was localized mainly in heterochromatin domains (47), leading to the assumption that an asymmetric nuclear loc...