High Levels of Nm23-H2 Protein Are a Specific and Invariable Feature of CML-CP Cells Due to Post Transcriptional Control Via a Bcr-Abl Dependent Pathway

We have previously identified Nm23-H2 as a tumor antigen in a case of chronic myeloid leukemia. In the following project we investigated the molecular background for the increased expression of Nm23-H2 in CML cells from a large number of patients.

Clinical samples from 30 patients with CML in chronic phase at diagnosis [Bcr-Abl/Abl transcript levels from 69.5 to 93% IS (international scale)], in addition to patients with molecular response to imatinib [n=10; median 2 (range 0.1–11) % IS Bcr-Abl/Abl transcript level] and to imatinib resistant patients [n=10; median 66 (range 37–100) % IS Bcr-Abl/Abl transcript level] were investigated. Subpopulations were isolated from the blood (CD3⁺, CD14⁺) or bone marrow (CD34⁺) by FACS sorting. AML samples were obtained from 12 patients with de novo AML and 3 patients with secondary AML at diagnosis. Ba/F3 cells expressing either wild-type Bcr-Abl (Bcr-Abl^wt Ba/F3), the T315I mutant of Bcr-Abl (Bcr-Abl^T315I Ba/F3) or the E255K mutant of Bcr-Abl (Bcr-Abl^E255K Ba/F3) were used in the cell line model.

Messenger RNA levels of Nm23-H2 were measured using the QuantiTect Primer Assay (Qiagen). Results were normalized to mRNA for the ribosomal protein RPLP0. Western blotting for Nm23-H2 was performed by standard techniques using chemiluminescence and immunocytochemistry was performed using Nm23-H2 and ß-Actin antibodies. Mean fluorescence intensity was determined using the LSM Image Browser software.

Nm23-H2 protein is over-expressed in the cytoplasm of mononuclear cells from 30/30 CML patients at diagnosis and 10/10 patients with resistance to imatinib. In contrast, patients responding to Imatinib had markedly reduced levels of Nm23-H2 protein. Over-expression is limited to Bcr-Abl+ progenitor and myeloid populations and is absent from (Bcr-Abl-) T lymphoid cells, normal donors and 14/15 acute myeloid leukemia samples. The increase in Nm23-H2 protein is due to post-transcriptional regulation, with no detectable increase in mRNA levels. The increase in Nm23-H2 can be reversed by imatinib or nilotinib treatment of Ba/F3 cells expressing wild-type Bcr-Abl, but not of cells expressing the respective resistant Bcr-Abl mutants.

The post-transcriptional up-regulation of Nm23-H2 is therefore a common and specific property of CML closely associated with Bcr-Abl activity, making Nm23-H2 a potential target for the development of novel immunotherapeutic or pharmaceutical therapies for CML.

Al-Ali:Novartis: Consultancy, Honoraria. Niederwieser:Bristol-Myers Squibb: Speakers Bureau; Novartis: Speakers Bureau.