The Nuclear Topography of ABL, BCR, PML, and RAR Genes: Evidence for Gene Proximity in Specific Phases of the Cell Cycle and Stages of Hematopoietic Differentiation

Abstract

The mechanisms whereby chromosomal translocations are consistently associated with specific tumor types are largely unknown. A generally accepted hypothesis is that the physical proximity of the involved chromosomal regions may be one important factor in the genesis of these phenomena. Accordingly, a likely possibility is that such a proximity may occur in a cell-lineage and cell-differentiation stage-specific manner. In this work, we have addressed this issue using as models the ABL and BCR genes of t(9;22) and the PML and RAR genes of t(15;17). By using in situ hybridization and confocal microscopy, we have measured the distances between these two pairs of genes in three-dimensionally preserved hematopoietic cells belonging to different cell lineages, at various stages of differentiation, and at various stages of the cell cycle, with the following results. (1) Intergenic distances vary periodically during the cell cycle and a significant association of ABL with BCR and of PML with RAR is seen at the transition between S and G2, which persists during G2 and prophase (such a behavior is not observed for distances between ABL or PML and the β-globin genes, used as a control). (2) The proportion of cells in which PML and RAR or ABL and BCR are closely associated is higher in hematopoietic precursors than in B-lymphoid cells (whereas the distances between ABL or PML and the β-globin genes are not affected by cell type). (3) When intergenic distances in unstimulated bone marrow CD34+ cells were compared with those in CD34+ cells treated with interleukin-3 (IL-3), a trend towards a higher proximity of the ABL and BCR genes in the former and of the PML and RAR genes in the latter is observed. (4) Analysis of B-lymphoid cells during mitosis shows that intergenic distances at metaphase are strongly influenced by physical constraints imposed by the chromosomal location of the gene, by the size of the respective chromosome, and by the geometry of the metaphase plate. These findings suggest that intrinsic spatial dynamics, established early in hematopoiesis and perpetuated differentially in distinct cell lineages, may facilitate the collision of individual genes and thus reciprocal recombination between them at subsequent stages of hematopoietic differentiation.