UC San Diego

The immunoglobulin heavy chain (Igh) locus consists of multiple variable (VH), diversity (DH), and joining (JH) elements. During V(D)J recombination the VH, DH, and JH segments recombine in a combinatorial fashion to generate a diverse repertoire of antibodies. The mechanism through which each cell utilizes a unique set of VH, DH, and JH segments has been a topic of intense study. Within a population of B-lineage cells the Igh locus adopts a wide spectrum of trajectories permitting VH regions to encounter DHJH segments with appropriate frequencies. We have established a novel approach to track Igh locus motion in live cells and to describe the trajectories adopted by the VH and DHJH elements in physical terms. Specifically, we generated stable lines of mice that carry arrays of Tet-operator binding sites in the Igh locus within the VH region as well as down-stream of the DHJH elements. Through expression of a fluorescently tagged Tet -repressor protein, we have visualized the trajectories adopted by regions of the Igh locus at distinct stages of B cell development. Results indicate that the structure of the Igh locus is dynamic and that motion of individual regions of the locus is subdiffusive. Furthermore, variations in mobility throughout B cell development are consistent with a model that predicts infrequent interactions between VH and DHJH elements of the Igh locus prior to the formation of the DH to JH joint and more frequent interactions after DH to JH joining. Additionally, generation of leukemic cell lines through BCR-ABL transduction leads to an increase of Igh mobility. We suspect this increase in mobility is not unique to the Igh locus and that it may provide a mechanism to explain the genomic instability associated with BCR-ABL expressing cells. In sum, we have established the experimental procedures to track with high accuracy Igh locus trajectories in developing B cells and to describe such trajectories in physical terms