DNA FRAGMENTS OF ALTERED ELECTROPHORETIC MOBILITY IN LEUKEMIA SAMPLESCAN ARISE FROM DOUBLE-STRAND DNA BREAKS AT NUCLEASE HYPERSENSITIVE SITES OF ACTIVE GENES

Chromosome translocations that disrupt or alter gene function have bee n implicated in the pathogenesis of a variety of malignancies. Therefo re, identification of a translocation breakpoint has become a more imp ortant means by which to identify genes involved in cellular transform ation. A common site of translocation in myeloid and lymphoid malignan cies involves 11q23. One human protooncogene, ETS1, has been localized to this chromosomal segment, and several tumors with 11q23 translocat ions have been shown to have altered ETS1 DNA migration after restrict ion enzyme digestion. Two laboratories, however, have recently localiz ed the 11q23 breakpoint region to a small region of DNA telomeric of t he CD3 loci, a region at considerable distance from the ETS1 gene locu s. Therefore, it is difficult to reconcile the studies that suggest al tered migration of fragments associated with-ETS1 and lack of a locali zation of the breakpoint to a region near the ETS1 gene. Recently, in our studies to characterize the promoter/enhancer region of the ETS1 p rotooncogene, we had the opportunity to analyze DNA from 18 patients w ith acute leukemia involving chromosome 11q23 aberrations. We were una ble to demonstrate rearrangement of the ETS1 gene in this group, thus confirming that the 11q23 breakpoint does not involve ETS1 protooncoge ne. In one patient, however, a DNA break in the region of the ETS1 pro moter was detected reproducibly. This DNA break was mapped to the majo r DNaseI hypersensitive site in the ETS1 promoter. Mapping from both s ides of the break demonstrated that the break must have occurred durin g processing of the leukemic cells for DNA analysis. Therefore, artifa ctual DNA breaks can occur at nuclease-hypersensitive sites of active genes. These data suggest that previous reports of chromosomal translo cations involving the ETS1 protooncogene may have resulted from DNA br eaks at nuclease hypersensitive sites. This mechanism may account for sporadic case reports of altered restriction enzyme fragment migration involving genes that are not ultimately shown to be associated with t he chromosome translocation being examined.