Physiological and druggable skipping of immunoglobulin variable exons in plasma cells

The error-prone V(D)J recombination process generates considerable amounts of nonproductive immunoglobulin(Ig)pre-mRNAs.We recently demonstrated that aberrant Ig chains lacking variable(V)domains can be produced after nonsense-associated altered splicing(NAS)events.Remarkably,the expression of these truncated Ig polypeptides heightens endoplasmic reticulum stress and shortens plasma cell(PC)lifespan.Many questions remain regarding the molecular mechanisms underlying this new truncated Ig exclusion(TIE-)checkpoint and its restriction to the ultimate stage of B-cell differentiation.To address these issues,we evaluated the extent of NAS of Ig pre-mRNAs using an Ig heavy chain(IgH)knock-in model that allows for uncoupling of V exon skipping from TIE-induced apoptosis.We found high levels of V exon skipping in PCs compared with B cells,and this skipping was correlated with a biallelic boost in IgH transcription during PC differentiation.Chromatin analysis further revealed that the skipped V exon turned into a pseudo-intron.Finally,we showed that hypertranscription of Ig genes facilitated V exon skipping upon passive administration of splice-switching antisense oligonucleotides(ASOs).Thus,V exon skipping is coupled to transcription and increases as PC differentiation proceeds,likely explaining the late occurrence of the TIE-checkpoint and opening new avenues for ASO-mediated strategies in PC disorders.

A dual function for the chromatin organizer Special A-T rich Binding Protein 1 in B-lineage cells

SATB1(Special A-T rich Binding protein 1)is a cell type-specific factor that regulates the genetic network in developing T cells and neurons.In T cells,SATB1 is required for lineage commitment,VDJ recombination,development and maturation.Considering that its expression varies during B-cell differentiation,the involvement of SATB1 needs to be clarified in this lineage.Using a KO mouse model in which SATB1 was deleted from the pro-B-cell stage,we examined the consequences of SATB1 deletion in naive and activated B-cell subsets.Our model indicates first,unlike its essential function in T cells,that SATB1 is dispensable for B-cell development and the establishment of a broad IgH repertoire.Second,we show that SATB1 exhibits an ambivalent function in mature B cells,acting sequentially as a positive and negative regulator of Ig gene transcription in naive and activated cells,respectively.Third,our study indicates that the negative regulatory function of SATB1 in B cells extends to the germinal center response,in which this factor limits somatic hypermutation of Ig genes.

Mediator contributes to IgH locus VDJ rearrangements by promoting usage of most distal V segments

INTRODUCTION The immunoglobulin heavy(IgH)chain locus includes a large cluster of V,D and J segments spanning thousands of kilobases in mammals and its expression implies long-range 3D-regulation of various gene modifications.1 V(D)J recombination occurs during early B-cell ontogeny.2 It is regulated by multiple transcription factors and cis-regulatory elements,and most notably by the intronic 5′Eμelement located in the JH to Cμintron.3 The intergenic control region 1(IGCR1)located between the VH and D gene clusters inhibits rearrangement of the most DH-proximal VH gene segments,while promoting usage of distal VH gene segments.4 Other downstream elements of the locus,flanking its 3′end might have more subtle roles:while the large 3′regulatory region(3′RR)shows strong internal synergies2 and behaves as a superenhancer in mature B cells.Its deletion showed no influence on the VDJ repertoire and rather increased transcription of unrearranged V segments.3–5 Insulators binding CTCF farther downstream of the 3′RR were shown,however,to promote usage of distal VH segments.6 Altogether,there are strong indications that elements involved in long-distance regulation of transcription might also influence VDJ recombination.In-frame splicing of VDJ exons is also needed for full Ig expression.7 Given the functional importance of gene architecture and long-range interactions for the IgH locus physiology,we explored the dependence of the VDJ repertoire on Med1,a known actor of long-range enhancer interactions.