Form follows function - the three-dimensional structure of antigen receptor gene loci

Distinguishing cell phenotype using cell epigenotype

The relationship between microscopic observations and macroscopic behavior is a fundamental open question in biophysical systems. Here, we develop a unified approach that-in contrast with existing methods-predicts cell type from macromolecular data even when accounting for the scale of human tissue diversity and limitations in the available data. We achieve these benefits by applying a k-nearest-neighbors algorithm after projecting our data onto the eigenvectors of the correlation matrix inferred from many observations of gene expression or chromatin conformation. Our approach identifies variations in epigenotype that affect cell type, thereby supporting the cell-type attractor hypothesis and representing the first step toward model-independent control strategies in biological systems.

Regulated large-scale nucleosome density patterns and precise nucleosome positioning correlate with V(D)J recombination

We show that the physical distribution of nucleosomes at antigen receptor loci is subject to regulated cell type-specific and lineage-specific positioning and correlates with the accessibility of these gene segments to recombination. At the Ig heavy chain locus (IgH), a nucleosome in pro-B cells is generally positioned over each IgH variable (VH) coding segment, directly adjacent to the recombination signal sequence (RSS), placing the RSS in a position accessible to the recombination activating gene (RAG) recombinase. These changes result in establishment of a specific chromatin organization at the RSS that facilitates accessibility of the genomic DNA for the RAG recombinase. In contrast, in mouse embryonic fibroblasts the coding segment is depleted of nucleosomes, which instead cover the RSS, thereby rendering it inaccessible. Pro-T cells exhibit a pattern intermediate between pro-B cells and mouse embryonic fibroblasts. We also find large-scale variations of nucleosome density over hundreds of kilobases, delineating chromosomal domains within IgH, in a cell type-dependent manner. These findings suggest that developmentally regulated changes in nucleosome location and occupancy, in addition to the known chromatin modifications, play a fundamental role in regulating V(D)J recombination. Nucleosome positioning-which has previously been observed to vary locally at individual enhancers and promoters-may be a more general mechanism by which cells can regulate the accessibility of the genome during development, at scales ranging from several hundred base pairs to many kilobases.