Clitocine

Abstract: The total synthesis of clitocine [6-amino-5-nitro-4-(beta-D-ribofuranosylamino)pyrimidine] (1), a nucleoside recently isolated from the mushroom Clitocybe inversa, has been accomplished. Glycosylation of 4,6-diamino-5-nitropyrimidine (4) with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose afforded the protected nucleoside 6-amino-5-nitro-4-[(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl) amino]pyrimidine (5) in good yield exclusively as the beta-anomer. Deprotection of 5 with NaOMe/MeOH gave 1 as an 11.5:1 mixture of the beta- and alpha-anomers, respectively. Recrystallization from MeOH, followed by chromatography, afforded 1 containing less than 1% of its alpha-anomer. X-ray crystal data revealed a planar aglycon moiety in clitocine with each oxygen atom of the nitro group intramolecularly hydrogen bonded to the hydrogen atoms of the two adjacent amino functions. Clitocine inhibited L1210 cells in vitro with an ID50 of 3 X 10(-8) M. Clitocine was also found to be a substrate and inhibitor of adenosine kinase with a Ki value of 3 X 10(-6) M.

Abstract: Nonsense mutations resulting in a premature stop codon in an open reading frame occur in critical tumor suppressor genes in a large number of the most common forms of cancers and are known to cause or contribute to the progression of disease. Low molecular weight compounds that induce readthrough of nonsense mutations offer a new means of treating patients with genetic disorders or cancers resulting from nonsense mutations. We have identified the nucleoside analog clitocine as a potent and efficacious suppressor of nonsense mutations. We determined that incorporation of clitocine into RNA during transcription is a prerequisite for its readthrough activity; the presence of clitocine in the third position of a premature stop codon directly induces readthrough. We demonstrate that clitocine can induce the production of p53 protein in cells harboring p53 nonsense-mutated alleles. In these cells, clitocine restored production of full-length and functional p53 as evidenced by induced transcriptional activation of downstream p53 target genes, progression of cells into apoptosis, and impeded growth of nonsense-containing human ovarian cancer tumors in xenograft tumor models. Thus, clitocine induces readthrough of nonsense mutations by a previously undescribed mechanism and represents a novel therapeutic modality to treat cancers and genetic diseases caused by nonsense mutations.