A novel pax5-binding regulatory element in the igκ locus

Enhancers as regulators of antigen receptor loci three-dimensional chromatin structure

Enhancers are defined as regulatory elements that control transcription in a cell-type and developmental stage-specific manner. They achieve this by physically interacting with their cognate gene promoters. Significantly, these interactions can occur through long genomic distances since enhancers may not be near their cognate promoters. The optimal coordination of enhancer-regulated transcription is essential for the function and identity of the cell. Although great efforts to fully understand the principles of this type of regulation are ongoing, other potential functions of the long-range chromatin interactions (LRCIs) involving enhancers are largely unexplored. We recently uncovered a new role for enhancer elements in determining the three-dimensional (3D) structure of the immunoglobulin kappa (Igκ) light chain receptor locus suggesting a structural function for these DNA elements. This enhancer-mediated locus configuration shapes the resulting Igκ repertoire. We also propose a role for enhancers as critical components of sub-topologically associating domain (subTAD) formation and nuclear spatial localization.

Gain-of-function analysis of cis-acting diversification elements in DT40 cells

Activation-induced cytidine deaminase (AID) is required for the immunoglobulin diversification processes of somatic hypermutation, gene conversion and class-switch recombination. The targeting of AID's deamination activity is thought to be a combination of cis- and trans-acting elements, but has not been fully elucidated. Deletion analysis of putative proximal cis-regulatory motifs, while helpful, fails to identify additive versus cumulative effects, redundancy, and may create new motifs where none previously existed. In contrast, gain-of-function analysis can be more insightful with fewer of the same drawbacks and the output is a positive result. Here, we show five defined DNA regions of the avian Igλ locus that are sufficient to confer events of hypermutation to a target gene. In our analysis, the essential cis-targeting elements fully reconstituted diversification of a transgene under heterologous promotion in the avian B-cell line DT40. Furthermore, to the best of our knowledge two of the five regions we report on here have not previously been described as individually having an influence on somatic hypermutation.

Epigenetic regulation of monoallelic rearrangement (allelic exclusion) of antigen receptor genes

While most genes in the mammalian genome are transcribed from both parental chromosomes in cells where they are expressed, approximately 10% of genes are expressed monoallelically, so that any given cell will express either the paternal or maternal allele, but not both. The antigen receptor genes in B and T cells are well-studied examples of a gene family, which is expressed in a monoallelic manner, in a process coined "allelic exclusion." During lymphocyte development, only one allele of each antigen receptor undergoes V(D)J rearrangement at a time, and once productive rearrangement is sensed, rearrangement of the second allele is prevented. In this mini review, we discuss the epigenetic processes, including asynchronous replication, nuclear localization, chromatin condensation, histone modifications, and DNA methylation, which appear to regulate the primary rearrangement of a single allele, while blocking the rearrangement of the second allele.