Hypusine-Dependent Messenger Nucleic Acids (hymns): Deferiprone dislocates specific mRNAs from the polysomes. † 678

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Deferiprone, a 3-hydroxypyrid-4-one, is an orally active chelator employed in transfusional iron overload of children, e.g. in thalassemia patients. An attractive alternative to the orally inactive deferoxamine, the drug is under FDA-sponsored investigation, but it is already therapeutically available abroad. The major clinically relevant adverse effect of prolonged administration is agranulocytosis, reversible upon discontinuation. In culture, deferiprone has a unique effect on cell cycle progression, causing a reversible arrest at the G1-S boundary without affecting general protein biosynthesis or viability (Hoyes et al., Cancer Res. 52, 4591-4599; 1992. Pattanpanyasat et al., Brit. J. Haem. 82, 13-19; 1992). This pattern was first described for mimosine, another 3-hydroxypyrid-4-one (Watson et al., Cytometry 12, 242-246; Hoffman et al., Cytometry 12, 26-32; both 1991). The molecular mechanism underlying mimosine's extraordinary cytokinetic effect apparently is reversible inhibition of deoxyhypusyl hydroxylase [DOHH; EC], which forms the critical hypusine residue of the eukaryotic translation initiation factor 5A (eIF-5A)(Hanauske-A bel et al., BBA 1221, 115-124; 1994). eIF-5A is implicated in polysomal loading of a subset of mRNAs collectively termed hymns, whose translation in turn generates pivotal proteins required for initiation of DNA replication and thus, enables G1 exit and entry into S phase (Hanauske-A bel et al., FEBS Lett. 366, 92-98; 1995). We hypothesized that in susceptible pediatric patients, deferiprone may activate a similar molecular cascade, with the clinically discernible end point of agranulocytosis.

To assess this hypothesis, we used cultured human cells as model system to test the effects of the drug and of its withdrawal on i) DOHH activity; ii) cell cycle transit; and iii) polysomal mRNA loading. The drug caused a dose-dependent decrease in hypusine formation, coincident with both an increasing block at the G1-S boundary and a selective dislocation of specific mRNAs away from the polysomes. By contrast, withdrawal of the drug within 45 minutes caused resumption of hypusine formation and polysomal relocation of susceptible mRNAs, followed within hours by synchronized entry of cells into S phase. To identify the putative hymns involved, we used RT-PCR and differential cDNA display to isolate these nucleic acids. Among the initial sequences identified is a fragment of 435 nt with a sequence identity of >95% to the human splicing factor PSF (Accession X70944). Our findings suggest the existence of cell cycle-relevant, deferiprone-sensitive hymn candidates, and identifies a rational strategy to develop 3-hydroxypyrid-4-one analogs as orally active chelators that lack the antiproliferative activity of deferiprone.

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