R399H Mutated ETS Domain Leads to a Functional ETV6 Double-Knock-Out in Acute Leukemia and Completely Alters the Physiological Function of ETV6.

ETV6 is the only known transcriptional repressor which plays an important role in the malignant transformation of haematopoietic cells in different leukemias.

In most cases of ETV6-AML1 + leukemias, the second ETV6 allele is knocked out by deletions. Other mechanisms of knocking out the second ETV6 allele in acute leukemias carrying ETV6 fusion genes, have not been described as of yet. Recently, we have published that the MT-ALL cell line with multilineage differentiation potential carries an ETV6-ARG fusion gene. The second ETV6 allele carries a point mutation in the highly conserved ETS-domain (G to A, bp 1220), leading to the replacement of arginine, normally directly binding to DNA, by histidine (R399H). Therefore, we investigated whether the R399H point mutation leads to a functional double knock-out and will reduce the DNA binding affinity of ETV6.

For the ETV6 binding studies the single point mutated and wild type ETS-domains were amplified and the proteins were expressed. Electro mobility shift assays (EMSA) were performed with the EBS (ETS binding site) carrying the specific GGAA core motif. The wild type ETS domain (ETSwt) incubated with the EBS showed a clear shift of the protein DNA complex loaded on a gel, but not the point mutated ETS domain (ETSmt). In order to quantify the binding of the ETSwt and mt-domains to EBS we established a transient transfection reporter gene assay. For this assay we constructed vectors which contain the EBS fused to the TK promoter, firefly luciferase serves as reporter gene. After transient cotransfections of HeLa cells with plasmids which expressed the ETSwt- or ETSmt-domain together with the vector containing the EBS(wt or mt), the activities of luciferase were measured in a tube luminometer. As expected, cotransfection of ETSwt and EBSwt reduced luciferase activity up to 75% compared with control vector and EBSwt cotransfection. Cotransfection of ETSmt with EBSwt did not lead to a reduction of luciferase activity. The specificity of the reporter gene analysis was controlled by using EBSmt.

These results show that the transcriptional repressor function of ETV6 is dependent on binding of ETV6-ETS domain to EBS and that the point mutation completely alters the physiological function of ETV6. Further studies would explain whether the multilineage differentiation potential of MT-ALL is dependent on ETV6 double-knock-out.