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Inhibition of Bcl-x_L Deamidation Pathway in Myeloproliferation

Mutant tyrosine kinases in chronic myeloid leukemia and polycythemia vera block apoptotic responses to DNA damage.

Chronic myeloid leukemia (CML) and polycythemia vera (PV) are clonal diseases characterized by progressive accumulation of myeloid elements, occasional transformation to acute myeloid leukemia, activation of specific tyrosine kinases (BCR-ABL fusion kinase in CML and JAK2 mutation in PV), and upregulation of the antiapoptotic protein Bcl-x_L. Recently, the pathway for inactivation of Bcl-x_L has been elucidated: DNA damage increases activity of sodium-hydrogen exchanger (NHE-1), which leads to a rise in intracellular pH (alkalinization). In turn, increased pH promotes the removal of amide groups from Bcl-x_L, which prevents Bcl-x_L from inhibiting proapoptotic proteins, thus blocking apoptosis. Now investigators have examined this pathway in patients with CML and PV.

To establish the effects of DNA damage in normal myeloid cells, the researchers exposed these cells to etoposide or radiation and observed increased alkalinization, apoptosis, and levels of NHE-1 and deamidated Bcl-x_L. Conversely, in granulocytes from patients with CML or PV, no such effects were observed, and significantly less apoptosis occurred in CML granulocytes than in exposed normal granulocytes. Of note, diminished responsiveness to DNA injury was found in granulocytes but not lymphocytes from CML and PV patients, consistent with the clinical observation that cells in the granulocytic series, but not the lymphocytic series, accumulate in these myeloproliferative diseases.

Experiments showed that normal responsiveness to DNA damage could be restored by transfecting cells with the NHE-1 transcript or by eliciting cellular alkalinization. To confirm that abnormal tyrosine kinases were responsible for resistance to apoptosis, the investigators examined various cell lines. Cells carrying the BCR-ABL fusion gene from a CML patient in blast crisis (but not cells from patients without this specific mutation) showed inhibition of the Bcl-x_L deamidation pathway. In cells from patients with myelofibrosis, the pathway was inhibited only in those cells with the JAK2 mutation. In other experiments, extent of inhibition of the Bcl-x_L deamidation pathway correlated with levels of BCR-ABL and JAK2 expression. Inhibition of the Bcl-x_L deamidation pathway occurred in multipotent stem cells (CD34⁺ cells) from patients with CML, but not in CD34⁺ cells from healthy controls. The pathway inhibition was reversed by blockers of BCR-ABL in patients with CML or of JAK2 in patients with PV. That blocking BCR-ABL — and not other kinases — was responsible for inhibition of the Bcl-x_L deamidation pathway in patients with CML was demonstrated by showing that cells from a CML patient who was resistant to the BCR-ABL kinase inhibitor imatinib (Gleevec) did not have increased NHE-1 levels after exposure to imatinib, but deamidation of Bcl-x_L could be restored in these cells if NHE-1 was overexpressed or if the pH of the cytosol was raised.

Comment: These authors have delineated a major mechanism leading to cell accumulation in CML and PV: The mutated tyrosine kinases (BCR-ABL and JAK2, respectively) expressed in these diseases promote resistance to apoptosis induced by DNA damage. This resistance allows cells to survive despite DNA damage, which might be a factor in the transformation to acute leukemia. The authors also have shown that by reversing kinase-induced inhibition of Bcl-x_L deamidation (either by enhancing the activity of NHE-1 or by raising intracellular pH), normal apoptosis can be restored and cell accumulation controlled.

— David Green, MD, PhD

Published in Journal Watch Oncology and Hematology January 6, 2009

Citation(s):

Zhao R et al. Inhibition of the Bcl-x_L deamidation pathway in myeloproliferative disorders. N Engl J Med 2008 Dec 25; 359:2778.

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