The nuclear topography of ABL, BCR, PML, and RAR alpha genes: Evidence forgene proximity in specific phases of the cell cycle and stages of hematopoietic differentiation

The mechanisms whereby chromosomal translocations are consistently associat ed with specific tumor types are largely unknown. A generally accepted hypo thesis is that the physical proximity of the involved chromosomal regions m ay 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 a nd cell-differentiation stage-specific manner. In this work, we have addres sed this issue using as models the ABL and BCR genes of t(9;22) and the PML and RAR alpha genes of t(15;17). By using in situ hybridization and confoc al microscopy, we have measured the distances between these two pairs of ge nes in three-dimensionally preserved hematopoietic cells belonging to diffe rent cell lineages, at various stages of differentiation, and at various st ages of the cell cycle, with the following results. (1) Intergenic distance s vary periodically during the cell cycle and a significant association of ABL with BCR and of PML with RAR alpha is seen at the transition between S and G2, which persists during G2 and prophase (such a behavior is not obser ved for distances between ABL or PML and the beta-globin genes, used as a c ontrol). (2) The proportion of cells in which PML and RAR alpha 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 beta-globi n genes are not affected by cell type). (3) When intergenic distances in un stimulated bone marrow CD34(+) cells were compared with those in CD34+ cell s 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 alpha genes in t he latter is observed. (4) Analysis of B-lymphoid cells during mitosis show s that intergenic distances at metaphase are strongly influenced by physica l constraints imposed by the chromosomal location of the gene, by the size of the respective chromosome, and by the geometry of the metaphase plate. T hese findings suggest that intrinsic spatial dynamics, established early in hematopoiesis and perpetuated differentially in distinct cell lineages, ma y facilitate the collision of individual genes and thus reciprocal recombin ation between them at subsequent stages of hematopoietic differentiation. ( C) 1999 by The American Society of Hematology.