ETS1 Expression Is Upregulated in Chronic Myeloid Leukemia (CML) and the ETS1 Transcriptional Program Correlates with Disease Progression Towards Blast Crisis

Chronic myeloid leukemia is a clonal myeloproliferative neoplasm characterized by the occurrence of the Ph1 chromosome in a primitive hematopoietic stem cell with a major amplification of myeloid compartment. Despite the major progress obtained by the use of tyrosine kinase inhibitors (TKI), resistances to these drugs occur with progression towards blast crisis (BC) during which there is an increase of BCR-ABL expression. To evaluate the potential targets of BCR-ABL in this context, a gene profiling analysis of the UT7-11 cell line overexpressing BCR-ABL fusion protein was performed using a whole transcriptome microarray. One of the highly upregulated genes was ETS1 with a fold change of +35.86. ETS1 proto-oncogene, the founder member of the ETS-domain family of transcription factors such as TEL and FLI1, is the human homolog of the avian erythroblastosis virus E26. They play a crucial role in stem cell biology, tumorigenesis and ETS1 has been shown to be involved in the regulation of granulocytic differentiation. Its role in CML pathophysiology has not been studied so far. We first showed by Western blots that ETS1 protein was highly increased in UT7-11 cells as compared to parental UT7 cells. Using a DOX-Inducible BCR-ABL system, we have shown that ETS1 expression was downregulated upon inhibition of BCR-ABL expression. ETS1 expression was a tyrosine kinase dependent event as its expression was reduced in the presence of TKI. To determine if ETS1 expression is upregulated in primary leukemic cells, we have analyzed leukemic cells of CML patients at diagnosis (n= 40) as compared to healthy controls (n= 30) showing an increase of ETS1 mRNA expression in primary CML cells in a highly significant manner (p < 0.0007). We then analyzed ETS1 targets using chip-sequencing in K562 cells, performed on HG19 human genome allowing to predict 3209 proximal promoters (-3000 upstream pb, + 50pb around Transcription Starting Sites) which represents a promoter enrichment of +33.68 (p-value=4.9E-324) as compared to the distribution of peaks in the whole genome. Chromosomes 19, 16, 17, 11, 12 and 1 were found over-represented with ETS1 bound events in Chip-sequencing. Integration of BCR-ABL transcriptome with ETS1 promoters identified by Chip-Sequencing led to the identification of 130 ETS1-targets activated by BCR-ABL, allowing reclassification of BCR-ABL transfected samples with transcriptome data by unsupervised classification. ETS1 transcriptional program regulated by BCR-ABL analysis performed on CD34+ from CML patients during progression of the disease (GSE47927) allowed discrimination of the different states of the progression (from the chronic phase to accelerated phase and BC p-value=1.63E-33). This analysis was also found significant by Gene Set Enrichment Analysis ETS1_BC specific geneset (NES=+1.52, p-value<0.0001). Among this ETS1 program specific of BC, we identified 2 targets that are discriminant for CD34+ cells of CML patients without cytogenetic response under Imatinib therapy. These two genes were Dynamin 3 (DNM3) and LIMS1/ PINCH1. Dynamin 3 is a member of the motor proteins implicated in cell motility and cytokinesis and it is implicated in megakaryocyte development whereas LIMS1 / PINCH1 is essentially involved in cell migration and adhesion. In order to validate ETS1 and its target expressions in a novel cohort of CML patients, we have analyzed the expression of DNM3 and PINCH1 in whole blood samples in CML patients ( n= 60 ) as compared to healthy controls (n= 32). We have found that, similar to ETS1, DNM3 and LIMS1 / PINCH1 mRNA were highly upregulated in primary CML and this increase was highly significant. In this new cohort of patients, DNM3 transcript levels were inversely correlated to WBC count (r=-0.32, p-value=0.011) and especially in male gender (r=-0.43, p-value=0.012). The expression of LIMS1 was inversely correlated with the age at diagnosis and the major molecular response at 12 months. Thus, our results show for the first time the major upregulation of ETS1 transcriptional program in CML. ETS1 transcriptional program has been found to correlate with the gene expression pattern involved in the progression of the disease in the transcriptomic analysis of CML CD34+ cells. The novel druggable targets that we identified in the BCR-ABL-activated transcriptional program is currently under investigation to determine their use in patients with resistance to targeted therapies.