The Effects of BCR/ABL on DNA Damage and Repair Are Dependent on Apoptosis Competence

Chronic myeloid leukemia (CML) is a two-stage disease associated with the t(9;22) translocation. This translocation fuses the BCR gene with the ABL tyrosine kinase, forming the BCR/ABL oncogene. BCR/ABL is a constitutively activated tyrosine kinase, which causes the first stage of CML, chronic phase. However, it is still unknown why patients progress to the second phase, blast crisis, a phase marked by increased chromosomal abnormalities. Previous data from our lab suggests that cells expressing BCR/ABL have an increase in chromosomal translocations after DNA repair compared to control cells, as assessed by spectral karyotyping. We hypothesized that BCR/ABL alters the apoptotic threshold in response to DNA damage, and we sought to determine if cells lacking BCR/ABL that were unable to undergo apoptosis would show similar responses in DNA damage and repair assays to BCR/ABL expressing cells. In order to study this question we have used an interleukin 3 (IL3) dependent cell line that lacks expression of the pro-apoptotic proteins, Bax and Bak. These cells grow in the presence of IL3 but do not undergo apoptosis after cytokine withdrawal or genotoxic stress. We have generated a subclone of these cells that expresses BCR/ABL. This cell line, designated DBA, grows in the absence of IL3. As expected, growth of the cells in the absence of IL3 is suppressed by the ABL kinase inhibitor, dasatinib. BCR/ABL expressing and control cells proliferate at similar rates following DNA damage. After damage, both control cells and DBA cells have a delay in the G2/M phase of the cell cycle, which is modestly prolonged in BCR/ABL expressing cells. Both control and BCR/ABL expressing cells have a similar amount of DNA damage after irradiation as assessed by pulsed field gel electrophoresis. Additionally, the rate of repair of DNA double strand breaks is not significantly different in the two cell types. Spectral karyotype analysis is ongoing to determine whether error prone DNA repair in BCR/ABL cells is secondary to the effects of BCR/ABL on inhibition of apoptosis or reflects other proposed mechanisms of action of the oncogene.