Translocation frequencies and chromosomal proximities for selected mouse chromosomes in primary B lymphocytes

Chromosome positions within the nucleus of mammalian cells are nonrandom and it is assumed that chromosomal neighborhoods affect the probability of translocations. Four chromosomes can be involved in c-myc-activating chromosomal translocations in mouse plasmacytoma (PCT): the c-myc gene on mouse chromosome 15 can be juxtaposed to either one of the immunoglobulin (Ig) loci on chromosomes 12 (IgH), 16 (Igλ), or 6 (Igκ). In the BALB/c mouse, the translocation between chromosomes 12 and 15, T(12;15), is most common (90%) while the other two possible translocations, T(6;15) and T(16;15), are much less common (<10%). In contrast, in the BALB/cRb6.15 mouse, T(6;15) is found with the same frequency as T(12;15). We, therefore, examined the distance between chromosomes 15 and 12, 6, and 16 in primary mouse B lymphocytes in order to examine the effect of the chromosome proximity on the translocation frequency. We performed three-dimensional fluorescent in situ hybridization (3D-FISH) with chromosome paints. We acquired three-dimensional image stacks with 90 slices per stack and used constrained iterative deconvolution. The nucleus and chromosomes were segmented from this image stack and the interchromosomal distances were measured. Chromosomes 6 and 15 were found in close proximity in BALB/cRb6.15 mice (82%), whereas they did not share this neighborhood relationship in BALB/c mice. No other chromosome combinations showed such a high percentage of close proximities in either mouse strain. Chromosome positions contribute to translocation frequencies in mouse PCTs. The BALB/cRb6.15 mouse data argue for a proximity relationship of chromosomes that engage in illegitimate recombination. These positions are not, however, the only contributing factor as the T(12;15) translocation preference in BALB/c mice could not be supported by significantly elevated proximity of chromosomes 12 and 15 versus 12 and 16 or 12 and 6. Moreover, while there is a significant increase in T(6;15) in BALB/cRb6.15 mice, T(12;15) still occurs in this mouse strain.