Abstract
BCR/ABL kinase transforms hematopoietic stem cells (HSCs) to induce chronic myelogenous leukemia in chronic phase (CML-CP), which eventually evolves into fatal blast crisis (CML-BC). CML is a stem cell-derived but a progenitor-driven disease. In CML-CP leukemia stem (LSCs) and progenitor (LPCs) cells reside in CD34+CD38− and CD34+CD38+ populations, respectively, whereas in CML-BC LSCs are found also in CD34+CD38+ population. BCR/ABL kinase stimulates genomic instability causing imatinib-resistant point mutations and chromosomal aberrations associated with progression to CML-BC. Genomic instability may result from enhanced DNA damage and/or aberrant DNA repair mechanisms. We showed that CD34+ stem/progenitor CML cells contain higher levels of reactive oxygen species (ROS) than these from healthy donors (CML-BC>CML-CP>Normal). In addition, ROS were elevated in CD34+CD38− and CD34+CD38+ sub-populations isolated from CML-BC and CML-CP patients in comparison to cells from healthy donor. Higher ROS levels induced more oxidative DNA lesions such as oxidized bases (e.g., 8-oxoG) and DNA double-strand breaks (DSBs). ROS and oxidative DNA damage in CML stem/progenitor cells could be diminished by an antioxidant N-acetyl-cysteine. Moreover, inhibition of ROS by vitamin E reduced the frequency of imatinib-resistant BCR/ABL point mutants and chromosomal aberrations in leukemia cells in SCID mice. Cellular DNA repair systems act to remove DNA damage and ultimately preserve the informational integrity of the genome. Base excision repair (BER) and mismatch repair (MMR) are responsible for removal of oxidized bases. BER was assessed using single- and double-stranded DNA substrates containing 5-OH-U (a derivative of ROS-damaged hydroxy-deoxycytidine). MMR activity was measured by restoration of the expression of GFP from the construct containing T-G mismatch in the start codon. BCR/ABL kinase severely inhibited BER and MMR in cell lines and CD34+ CML cells, and promoted accumulation of point mutations in genes encoding BCR/ABL kinase and Na+/K+ ATPase. Inhibition of BCR/ABL kinase by imatinib restored BER and MMR activities. Oxidized bases, if not repaired, may lead to accumulation of DSBs observed in LSCs and LPCs. DSBs may be processed by homologous recombination (HR), non-homologous and-joininig (NHEJ), and single-strand annealing (SSA). HR represents faithful repair, NHEJ usually produces small deletions, and SSA causes very large deletions. Genome-integrated repair-specific reporter cassettes containing two disrupted fragments of the gene encoding GFP were used where a single DSB induced by I-SceI endonuclease in one of the fragments stimulated HR, NHEJ, or SSA. In general, BCR/ABL kinase enhanced DSBs repair activities, however at the expense of their fidelity. Numerous point mutations were introduced in HR repair products. NHEJ generated larger than usual deletions. SSA, rather rare but very unfaithful, was also induced in BCR/ABL-positive leukemia cells. In summary, BCR/ABL kinase enhanced ROS-mediated oxidative DNA damage in LSCs and LPCs. In addition, BCR/ABL inhibited BER and MMR of usually non-lethal oxidized DNA lesions leading to accumulation of point mutations. Moreover, BCR/ABL kinase stimulated HR, NHEJ and SSA of lethal DSBs, but compromised the fidelity of repair.
Distinct roles of XRCC1 in genome integrity in Xenopus egg extracts
