Interferon-α directly inhibits DNA polymerase activity in isolated chromatin nucleoprotein complexes: correlation with IFN-α treatment outcome in patients with chronic myelogenous leukemia

We have developed an in vitro assay to assess and predict the potential efficacy of in vivo interferon-α (IFN-α) treatment (5 × 10 6 units/m 2 per day) for patients with chronic myelogenous leukemia (CML). Although determining the numbers and affinities of IFN-α receptors on CML cells has been developed as a method for predicting treatment response to IFN-α, it fails to predict response in CML. Previously, we and others observed that mitogens, toxins and lectins that bind to cell-surface receptors are endocytosed, escaping endosomes in order to act directly on cellular targets. Therefore, we tested the ability of low concentrations of IFN-α (5–10 units) to act directly on DNA polymerase (Pol) in purified chromatin nucleoprotein complexes (NPC). NPC were prepared by a methodology that uses direct treatment of leukocyte nuclei with MspI to generate six NPC-containing fractions (S1, M1, S2, M2, 0.1K and R). We found three general categories of in vitro DNA synthesis response for the six different NPC fractions isolated from the white blood cells of patients with CML ( n = 19) before their treatment with IFN-α. IFN-α induced either stimulation, inhibition or had no apparent effect on Pol activity in the six different NPC fractions in a blind assay. In most of the NPC fractions isolated from the leukocytes of patients with progressive CML and in those from CML patients who failed to show a clinical response to IFN-α, this cytokine stimulated or had no effect on Pol activity. In contrast, a majority of NPC fractions isolated from the leukocytes of patients that responded clinically to IFN-α, were inhibited in their Pol activities. However, we were not able to correctly predict IFN-α response in CML patients that had undergone prior chemotherapy. These data suggest that cytokines, such as IFN-α, may act directly on DNA synthesis machinery in the nucleus after entering cells, and assays that measure the direct effects of cytokines on Pol activity in NPC fractions could be useful in assessing the response of patients to cytokines in a clinical setting.