Data from BCR/ABL Inhibits Mismatch Repair to Protect from Apoptosis and Induce Point Mutations

Abstract

<div>Abstract<p>BCR/ABL kinase–positive chronic myelogenous leukemia (CML) cells display genomic instability leading to point mutations in various genes including <i>bcr/abl</i> and <i>p53</i>, eventually causing resistance to imatinib and malignant progression of the disease. Mismatch repair (MMR) is responsible for detecting misincorporated nucleotides, resulting in excision repair before point mutations occur and/or induction of apoptosis to avoid propagation of cells carrying excessive DNA lesions. To assess MMR activity in CML, we used an <i>in vivo</i> assay using the plasmid substrate containing <i>enhanced green fluorescent protein</i> (<i>EGFP</i>) gene corrupted by T:G mismatch in the start codon; therefore, MMR restores EGFP expression. The efficacy of MMR was reduced ∼2-fold in BCR/ABL-positive cell lines and CD34<sup>+</sup> CML cells compared with normal counterparts. MMR was also challenged by <i>N</i>-methyl-<i>N</i>'-nitro-<i>N</i>-nitrosoguanidine (MNNG), which generates <i>O</i><sup>6</sup>-methylguanine and <i>O</i><sup>4</sup>-methylthymine recognized by MMR system. Impaired MMR activity in leukemia cells was associated with better survival, accumulation of p53 but not of p73, and lack of activation of caspase 3 after MNNG treatment. In contrast, parental cells displayed accumulation of p53, p73, and activation of caspase 3, resulting in cell death. Ouabain-resistance test detecting mutations in the Na<sup>+</sup>/K<sup>+</sup> ATPase was used to investigate the effect of BCR/ABL kinase–mediated inhibition of MMR on mutagenesis. BCR/ABL-positive cells surviving the treatment with MNNG displayed ∼15-fold higher mutation frequency than parental counterparts and predominantly G:C→A:T and A:T→G:C mutator phenotype typical for MNNG-induced unrepaired lesions. In conclusion, these results suggest that BCR/ABL kinase abrogates MMR activity to inhibit apoptosis and induce mutator phenotype. [Cancer Res 2008;68(8):2576–80]</p></div>