Molecular characterization of jumping translocations reveals spatial and temporal breakpoint heterogeneity.

Jumping translocations (JT) are characterized by the relocalization of the same part of a donor to several recipient chromosomes. Although JT occasionally are constitutional, most are associated with hematologic malignancies. In such cases, JT usually arise during disease progression and are associated with poor prognosis. Despite its clinical importance, this cytogenetic phenomenon has not been characterized at the molecular level. We have analyzed JT in a juvenile chronic myelomonocytic leukemia that subsequently transformed to an acute myeloid leukemia. Detailed fluorescence in situ hybridization (FISH) analyses showed that the cytogenetically identical donor breakpoint at 3q21 was highly heterogeneous. In fact, more than 10 distinct breakpoints, four of which mapped within YACs, were identified. Analyses of samples during disease progression showed that the breakpoint complexity decreased, indicating clonal selection. Hence, the 3q21 breakpoints displayed a spatial as well as a temporal heterogeneity, revealing that JT are highly unstable, showing great variation in the size of donor segment. The breaks at the recipient chromosomes were mapped within the subtelomeric regions. The general telomere length was not affected and an underlying replication error resulting in microsatellite instability was excluded. We conclude that the emergence of JT is unlikely to cause fusion genes or to affect the expression of genes located in the breakpoint regions. The identification of YACs spanning the breakpoints, ie, YACs 913c7, 937g5, 948c2 and 955g1, may facilitate the isolation of DNA sequences leading to a genetic instability associated with the origin of multiple translocations.