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Activation of the RAS Pathway Is Predictive for a Chemosensitive Phenotype of Acute Myelogenous Leukemia Blasts

Authors' Affiliation: ¹ Medical Clinic and Policlinic I, University Hospital of the Technical University, Dresden, Germany and ² Department Internal Medicine, Hospital of Philipps University, Marburg, Germany

Requests for reprints: Thomas Illmer, Medical Clinic I-Hematology Department of the Technical University, University Hospital of the Technical University Dresden, Dresden Sachsen, Germany 01307. Phone: 49-351-4585602; Fax: 49-351-4585362; E-mail: illmer{at}mk1.med.tu-dresden.de.

Purpose: Activation of the RAS pathway plays a major role in cancer cells. In acute myeloid leukemia (AML), mutations of the RAS genes cause an intrinsic activation of this pathway. Until now, clinical studies could not find clear association of RAS mutations with the clinical outcome after AML therapy. This could be due to alternative initiating events for activation of the RAS pathway like constitutive tyrosine kinase activation or mutations in Ras-regulating genes.

Experimental Design: In total, 191 AML patients (126 as training population and 65 as test population) were studied for Ras activity with a glutathione S-transferase pull-down assay using Raf binding of activated Ras.

Results: AML samples showed a wide range of Ras activity values, which was in contrast to normal bone marrow donors who showed no or very limited Ras activity. Using a Ras binding score based on semiquantitative Western blotting, we defined patients with strong Ras activity and compared Ras activity with RAS mutation. Surprisingly, only a minority of RAS mutated AML samples (22.2%) showed strong Ras activity, whereas 25 patients presented strong Ras activity in the absence of RAS mutations. Clinical outcome did not show differences according to RAS mutations. In contrast, Ras activity predicted for a high response rate (P < 0.05) and proved to be an independent factor for overall survival rate (P < 0.05) in younger AML patients receiving high-dose 1-ß-D-arabinofuranosylcytosine as induction therapy.

Conclusion: The data highlight the role for alternative pathways of Ras activation without RAS mutations. Intrinsically activated Ras seems to increase sensitivity of the AML blast to high-dose 1-ß-D-arabinofuranosylcytosine therapy.

Key Words: proliferation • multidrug resistance • AML • therapy outcome

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