The origin of micronuclei induced by cytosine arabinoside and its synergistic interaction with hydroxyurea in human lymphocytes.

Using human lymphocytes and the cytokinesis-block micronucleus (CBMN) assay we have recently shown that excision-repairable DNA lesions induced by methylnitrosourea (MNU) and ultraviolet (UV) light (254 nm) can be converted to micronuclei (MNi) within one cell cycle if cytosine arabinoside (ARA-C) is added during the G1 phase of the cell cycle after mitogen stimulation. We have proposed that this conversion resulted from the inhibition by ARA-C of the gap-filling step during excision repair which results in the formation of single-stranded breaks at repair sites; these breaks are then converted to chromatid or chromosome breaks and subsequently to MNi on completion of nuclear division. To confirm this hypothesis we have examined the origin of MNi induced by ARA-C by using anti-kinetochore antibodies and found that between 77 and 86% of these MNi are kinetochore-negative which supports the idea that they originate mainly from acentric chromosome fragments. We have also demonstrated that combining ARA-C treatment with hydroxyurea (HU) during G1 provides a synergistic improvement in the conversion of spontaneous (4-fold increment) or MNU-induced (2-fold increment) excision-repairable DNA lesions to MNi when compared to the effects with ARA-C alone. HU treatment on its own did not influence micronucleus expression in untreated cells and cells treated with MNU.