BCR/ABL Expression Increases the Formation of Chromosomal Translocations after DNA Damage.

BCR/ABL is the protein product of the t(9;22) translocation and is the cause of the hyperproliferation associated with chronic phase chronic myeloid leukemia (CML). However, whether BCR/ABL induces genomic instability leading to blast crisis is controversial. We have previously demonstrated that BCR/ABL translocates to the nucleus after genotoxic damage and associates with the DNA damage sensor, ataxia telangiectasia and rad 3 related (ATR) protein, disrupting the sensing and repair of DNA double strand breaks. Here, we have asked if BCR/ABL expression leads to permanent changes in the DNA after genotoxic stress. For these experiments we have studied the hematopoietic cell line, Ba/F3pTetOn p210, which expresses p210 BCR/ABL after incubation in doxycycline. Cells were incubated in low doses of etoposide for two hours and then allowed to recover for 48 hours in the absence of further DNA damage. Induction of apoptosis in these conditions was consistently less than 5% as demonstrated by annexin V staining of cells. Cells were examined for alterations in the chromosomes using Giemsa banding and spectral karyotyping (SKY). Cells growing in IL3 showed low levels of DNA damage with a few broken chromosomes present in metaphase spreads and an average of 0.5 new chromosomal translocations per cell as revealed by SKY analysis. In contrast, when cells expressing BCR/ABL were treated with the same conditions, a marked number of genetic abnormalities were seen. 75% of cells showed abnormal chromosome forms with ring chromosomes, triradial forms and other abnormalitites. Analysis of BCR/ABL expressing cells by SKY analysis showed frequent abnormalities, averaging at least 6 new translocations per cell. Several cells had greater than 10 translocations present including multiple complex translocations involving more than two chromosomes. The majority of abnormalities consisted of unbalanced translocations. This data demonstrates that BCR/ABL alters the cellular response to DNA damage leading to an increase in chromosomal translocations in cells expressing the oncogene and suggests that BCR/ABL itself is directly responsible for the genomic instability leading to CML blast crisis.