| Accumulating evidence suggests that the non-random nature of spatial genome organization contributes to the formation of chromosomal translocations; specifically, that spatial proximity of potential translocation partners influences translocation potential. Purified subpopulations of non-malignant cells from affected patients have not been studied, or compared to sorted subsets from healthy donors (HD). Proximity of loci in patient-derived normal cells may closely reflect loci positioning for the original cell in which the translocation initially formed, and may favour translocations between proximal loci in a patient-specific manner. In multiple myeloma (MM), recurrent translocations in plasma cells (PC) involve IGH and FGFR3 t(4;14) or CCND1 t(11;14), or less frequently c-MAF t(14;16). Utilizing 3-D FISH and 3-D analysis techniques, we analyzed the radial and relative positioning of translocation-prone and control loci in the nuclei of hematopoietic progenitor cells (HPC) and B-cells from patients with MM and from HD. The radial and relative positioning was not different among HD HPC, HD B-cells, and HPC from MM patients. On average, in all subsets, IGH, FGFR3, and CCND1 are centrally located in the nucleus, while c-MAF occupies a more peripheral position. In non-malignant B-cells from t(4;14) and t(11;14) patients, IGH and FGFR3, or IGH and CCND1 are preferentially positioned in spatial proximity: IGH and c-MAF are not. This pattern of positioning differs in B-cells from t(14;16) patients or HD: IGH is equivalently distal to FGFR3, CCND1, c-MAF, and the control locus, TGFBR2. Our results suggest that spatial proximity and radial position in the nucleus influence translocation potential of specific loci, and are specific to some patient subsets. Furthermore, FGFR3, and CCND1 frequently position outside their respective chromosome territories (CTs), but c-MAF rarely does. The frequency of extra-territorial positioning (ETP) reflects the clinical frequencies in MM and suggests that ETP may influence translocation potential. Furthermore, extra-territorial FGFR3, and CCND1 occasionally co-localize with IGH. The finding that some translocation-prone gene loci (TPGL) frequently extend beyond their CTs, provides evidence for a previously undocumented mechanism that brings TPGL from different chromosomes into spatial proximity with one another. |
