The Transcription Factors RUNX1/AML1 and RUNX3/AML2 Protect Bcr-Abl-Transformed B-Cells from Imatinib Induced Apoptosis.

Abstract

The Abl-tyrosine kinase inhibitor Imatinib efficiently targets the Bcr-Abl kinase and produces major cytogenetic responses in most patients with chronic phase CML. In contrast, patients with advanced stage CML or Ph+ ALL frequently become refractory to Imatinib treatment. Resistance arises predominantly from point mutations in the Abl-kinase region, Bcr-Abl amplification or clonal evolution due to secondary genetic aberrations. To screen for genes contributing to clonal evolution, we have employed retroviral insertional mutagenesis in a murine CML/ALL model to identify potential candidate genes leading to Imatinib resistance. We found proviral insertions near the RUNX3/AML2 promoter in Imatinib resistant leukemic clones, leading to upregulation of RUNX3 mRNA expression. To analyze the effects of high RUNX3 levels on Imatinib response, we expressed RUNX3 in a Bcr-Abl-transformed murine pre-B-cell line. Significantly, whereas there was no effect on Imatinib-mediated proliferation inhibition, the cells displayed a marked reduction of apoptosis. A RUNX3R193A mutant carrying a mutation in the DNA-binding domain of RUNX3 did not protect from apoptosis, indicating that transcriptional regulation by RUNX3 was required to induce the anti-apoptotic effects. To allow for a controlled activation of RUNX transcriptional activity and to extend our analysis to other members of the RUNX family of transcription factors, we constructed 4-OH-tamoxifen (TAM) inducible RUNX3/AML2- and RUNX1/AML1-Estrogen receptor (ER) fusion proteins. These fusion proteins readily translocated from the cytoplasm into the nucleus and activated a RUNX-dependent TCRß-luciferase construct upon addition of TAM. Using these constructs, we could demonstrate that activation of RUNX3 as well as RUNX1 protected Bcr-Abl-transformed Ba/F3 cells from Imatinib-induced apoptosis. Furthermore, we found that RUNX1 mRNA levels were significantly upregulated in patients with Ph+ ALL upon resistance development. Taken together, our data indicate that elevated RUNX3 or RUNX1 levels may contribute to Imatinib resistance in Bcr-Abl expressing leukemic cells.

RUNX1 and RUNX3 Genes Expression Level in Adult Acute Lymphoblastic Leukemia—A Case Control Study

“…Furthermore, it was proposed that the weak expression of the RUNX3 gene might collaborate with a preferential genetic mutation, such as BCR-ABL1 [ 40 ]. Other reports stated that the high expression of the RUNX3 gene and protein level in mice and in cell lines with the present Bcr-abl fusion resulted in resistance to treatment with imatinib and this effect was lost in mutant RUNX3 [ 41 , 42 ]. Unfortunately, to the best of our knowledge, there are no other reports as to the possible functional influence of these genes on each other, especially in the material obtained from patients, hence the discussion is limited.…”