Biomedical subjects
Publications and source records attributed to M H Caruthers.
A method of accurate mass determination of phosphoramidites is described. The commonly used methanol/water/acid system was replaced by LiCl-containing acetonitrile and the concentrations of LiCl, poly(ethylene glycol), and phosphoramidite samples were optimized.
Single-atom substrate modifications have revealed an intricate network of transition state interactions in the Tetrahymena ribozyme reaction. So far, these studies have targeted virtually every oxygen atom near the reaction center, except one, the 5'-bridging oxygen atom of the scissile phosphate. To address whether interactions with this atom play any role in catalysis, we used a new type of DNA substrate in which the 5'-oxygen is replaced with a methylene (-CH2-) unit. Under (kcat/Km)S conditions, the methylene phosphonate monoester substrate dCCCUCUT(mp)TA4 (where mp indicates the position of the phosphonate linkage) unexpectedly reacts approximately 10(3)-fold faster than the analogous control substrates lacking the -CH2- modification. Experiments with DNA-RNA chimeric substrates reveal that the -CH2- modification enhances docking of the substrates into the catalytic core of the ribozyme by approximately 10-fold and stimulates the chemical cleavage by approximately 10(2)-fold. The docking effect apparently arises from the ability of the -CH2- unit to suppress inherently deleterious effects caused by the thymidine residue that immediately follows the cleavage site. To analyze the -O- to -CH2- modification in the absence of this thymidine residue, we prepared oligonucleotide substrates containing methyl phosphate or ethyl phosphonate at the reaction center, thereby eliminating the 3'-terminal TA4 nucleotidyl group. In this context, the -O- to -CH2-modification has no effect on docking but retains the approximately 10(2)-fold effect on the chemical step. To investigate further the stimulatory influence on the chemical step, we measured the "intrinsic" effect of the -O- to -CH2- modification in nonenzymatic reactions with nucleophiles. We found that in solution, the -CH2- modification stimulates chemical reactivity of the phosphorus center by <5-fold, substantially lower in magnitude than the stimulatory effect in the catalytic core of the ribozyme. The greater stimulatory effect of the -CH2- modification in the active site compared to in solution may arise from fortuitous changes in molecular geometry that allow the ribozyme to accommodate the phosphonate transition state better than the natural phosphodiester transition state. As the -CH2- unit lacks lone pair electrons, its effectiveness in the ribozyme reaction suggests that the 5'-oxygen of the scissile phosphate plays no role in catalysis via hydrogen bonding or metal ion coordination. Finally, we show by analysis of physical organic data that such interactions in general provide little catalytic advantage to RNA and protein phosphoryl transferases because the 5'-oxygen undergoes only a small buildup of negative charge during the reaction. In addition to its mechanistic significance for the Tetrahymena ribozyme reaction and phosphoryl transfer reactions in general, this work suggests that phosphonate monoesters may provide a novel molecular tool for determining whether the chemical step limits the rate of an enzymatic reaction. As methylene phosphonate monoesters react modestly faster than phosphate diesters in model reactions, a similarly modest stimulatory effect on an enzymatic reaction upon -CH2- substitution would suggest rate-limiting chemistry.
Under standard reaction conditions, a hammerhead ribozyme with a phosphorodithioate linkage at the cleavage site cleaved to the expected products with a rate about 500-fold slower than the corresponding phosphodiester linkage. When the greater stability of the dithioate linkage to nonenzymatic nucleophilic attack is taken into account, the hammerhead is remarkably effective at cleaving the dithioate linkage considering that the R(P)-phosphoromonothioate linkage is virtually inactive. On the basis of experiments determining the Mg(2+) concentration dependence of the cleavage rate and the stimulation of cleavage by thiophilic Cd(2+) ion, the lesser catalytic rate enhancement of the dithioate linkage is primarily due to the loss of a single Mg(2+) ion bound near the cleavage site. These results are qualitatively similar to, but quantitatively different from, similar experiments examining the hammerhead cleavage properties of the R(P)-phosphoromonothioate linkage. The dithioate linkage thus promises to be a valuable alternative phosphate analogue to the monothioate linkage in studying the mechanisms of RNA catalysis.
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Antisense activity against erbB-2 of a variety of sulfur-modified oligonucleotides was examined in a breast cancer cell line which overexpresses this oncogene. Using a 15 base anti-erbB-2 sequence previously shown to be effective, various backbone configurations containing phosphoromonothioate or phosphorodithioate linkages were evaluated for antisense activity by a two-color flow cytometric assay. This sequence was effective in inhibiting the production of erbB-2 protein when it was configured as a monothioate at each linkage and as an alternating dithioate/phosphodiester. Both of these compounds were also able to specifically inhibit erbB-2 mRNA expression, indicative of RNase H-mediated activity. The same sequence protected by either three dithioate or three monothioate linkages at each end was ineffective as an antisense reagent, suggesting that endonuclease activity is a significant determinant of the stability of oligonucleotides. Finally, the erbB-2 sequence target was shifted in an effort to improve antisense activity. A new lead sequence was identified that was significantly more effective in inhibiting erbB-2 protein levels and retained activity at lower concentrations.
The biochemical and physicochemical properties of DNA oligomers containing phosphorodithioate linkages in various configurations were evaluated. Duplex stability studies, which were carried out by thermal denaturation analysis with complementary unmodified DNA, indicated a highly cooperative process similar to completely unmodified duplexes. Oligomers containing phosphorodithioate linkages were found to have reduced melting temperatures relative to unmodified duplexes, with the degree of Tm depression paralleling the percent phosphorodithioate composition of the oligomer. Relative to activation of RNase H, DNA oligomers containing up to 50% phosphorodithioate linkages were able to direct RNase H degradation with the same efficiency as unmodified DNA while those containing from 50 to 100% acted with somewhat reduced efficiency. At limiting concentrations, an oligomer containing alternating phosphorodithioate and phosphate linkages was able to direct RNase H degradation of the target RNA in an extended incubation, while an unmodified oligomer did not. The nuclease resistance of phosphorodithioate-containing oligomers was evaluated in HeLa cell nuclear and cytoplasmic extracts, in human serum, and with nucleases S1 and DNase I. Oligomers containing alternating phosphorodithioate and phosphate were highly resistant to degradation in all systems. However, oligomers having more than one unmodified linkage separating phosphorodithioates were degraded rapidly by DNase I, while demonstrating stability to degradation in all other systems tested. These results indicate that phosphorodithioate-containing DNA oligomers are highly nuclease-resistant, are able to form stable duplexes with complementary nucleic acid sequences, and efficiently direct RNase H degradation of target RNA.
A causal role has been inferred for ERBB2 overexpression in the etiology of breast cancer and other epithelial malignancies. The development of therapeutics that inhibit this tyrosine kinase cell surface receptor remains a high priority. This report describes the specific downregulation of ERBB2 protein and mRNA in the breast cancer cell line SK-BR-3 by using antisense DNA phosphorothioates. An approach was developed to examine antisense effects which allows simultaneous measurements of antisense dose and gene specific regulation on a per cell basis. A fluorescein isothiocyanate end-labeled tracer oligonucleotide was codelivered with antisense DNA followed by immunofluorescent staining for ERBB2 protein expression. Two-color flow cytometry measured the amount of both intracellular oligonucleotide and ERBB2 protein. In addition, populations of cells that received various doses of nucleic acids were physically separated and studied. In any given transfection, a 100-fold variation in oligonucleotide dosage was found. ERBB2 protein expression was reduced greater than 50%, but only in cells within a relatively narrow uptake range. Steady-state ERBB2 mRNA levels were selectively diminished, indicating a specific antisense effect. Cells receiving the optimal antisense dose were sorted and analyzed for cell cycle changes. After 2 days of ERBB2 suppression, breast cancer cells showed an accumulation in the G1 phase of the cell cycle.
A series of novel 3'-C-branched 2',3'-dideoxynucleosides have been synthesized and evaluated as anti-HIV agents. Hydroboration of 2',3'-dideoxy-3'-C-methylene nucleoside proceeded regio- and stereoselectively to give 1-(2,3-dideoxy-3-C-hydroxy methyl-5-O-trityl-beta-D-threo-pentofuranosyl)thymine (5) after oxidation. 3'-C-Chloromethyl and 3'-C-iodomethyl 5'-O-protected 2',3'-dideoxynucleosides 9 and 12 were obtained from 5 by reaction with carbon tetrachloride/triphenylphosphine and methyltriphenoxyphosphonium iodide, respectively. Arbuzov reaction of 12 with triethyl phosphite afforded 3'-C-(diethyl-phosphono)methyl 5'-O-protected 2',3'-dideoxynucleoside 14. Compounds 5, 9, 12 and 14 were detritylated to give 1-(3-C-chloromethyl-2,3-dideoxy-beta-D-threo-pentofuranosyl)thymine (10), 1-(2,3-dideoxy-3-C-hydroxymethyl-beta-D-threo-pentofuranosyl)-thymine (11), 1-(2,3-dideoxy-3-C-iodomethyl-beta-D-threo-pentofuranosyl)thymine (13) and 1-(2,3-dideoxy-3-C-(O,O'-diethylphosphono)methyl-beta-D-threo- pentofuranosyl)thymine (15), respectively. These nucleoside analogues were evaluated for antiviral activity against human immunodeficiency virus type 1 (HIV-1) and herpes simplex virus type 1 (HSV-1) in vitro. Compound 5 demonstrated selective antiviral activity against HIV-1 but not HSV-1.
Phosphorodithioate (PS2) oligodeoxynucleotides (oligos) represent a relatively new class of backbone-modified oligo that have potential use as antisense agents. PS2 oligos are isoelectronic with phosphodiester (PO) and phosphoromonothioate (PS) oligos, and are nuclease resistant. However, unlike their PS congeners, PS2 oligos do not contain chiral centers. Little is known about the manner in which PS2 oligos interact with biological systems. In this study, we compare the cellular pharmacology of PS and PS2 oligos in HL60 cells. Cell surface binding, internalization, and compartmentalization are examined. Furthermore, the ability of PS and PS2 oligos to bind to rsCD4 and bFGF and to inhibit the activity of protein kinase C (PKC) is examined. Although the behavior of PS2 oligos closely parallels that of PS oligos, PS2 oligos appear to interact with some biological systems in a slightly different manner than PS oligos. These results indicate that PS2 oligos may have therapeutic potential other than as antisense agents.
This article summarizes methods for the synthesis of phosphorodithioate-linked deoxyoligonucleotides and details an analysis of one of the distinctive properties of phosphorodithioate DNA oligomers, their ability to strongly inhibit human immunodeficiency virus type-1 reverse transcriptase (HIV-1 RT). Mechanistic studies indicate that oligomers of this type interfere with enzyme function by binding tightly to the active site for primer-template, which results in low or subnanomolar inhibitory constants. Although many of these studies have used deoxyoligocytidine analogs, a rationally designed approach has led to the discovery of a very active phosphorodithioate deoxyoligonucleotide inhibitor. This type of inhibitor, which binds strongly to the primer-template active site of HIV-1 RT, provides another type of potential therapeutic agent against HIV-1.
All catalytic RNAs (ribozymes) require or are stimulated by divalent metal ions, but it has been difficult to separate the contribution of these metal ions to formation of the RNA tertiary structure from a more direct role in catalysis. The Tetrahymena ribozyme catalyses cleavage of exogenous RNA or DNA substrates with an absolute requirement for Mg2+ or Mn2+ (ref. 6). A DNA substrate, in which the bridging 3' oxygen atom at the cleavage site is replaced by sulphur, is cleaved by the ribozyme about 1,000 times more slowly than the corresponding unmodified DNA substrate when Mg2+ is present as the only divalent metal ion. But addition of Mn2+ or Zn2+ to the reaction relieves this negative effect, with the 3' S-P bond being cleaved nearly as fast as the 3' O-P bond. Considering that Mn2+ and Zn2+ coordinate sulphur more strongly than Mg2+ does, these results indicate that the metal ion contributes directly to catalysis by coordination to the 3' oxygen atom in the transition state, presumably stabilizing the developing negative charge on the leaving group. We conclude that the Tetrahymena ribozyme is a metalloenzyme, with mechanistic similarities to several protein enzymes.
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Phosphorothioate oligodeoxynucleotides exert a sequence-independent cytoprotective effect against human immunodeficiency virus type 1 (HIV-1). We now report that phosphorodithioate-containing oligodeoxycytidines are very potent inhibitors of HIV-1 reverse transcriptase in vitro, as they exhibit an increasing inhibitory effect with length and number of phosphorodithioate internucleotide linkages. This inhibitory effect can be at least 30-fold greater with phosphorodithioate oligodeoxycytidine than for the corresponding phosphorothioate analog of similar length. In cell culture, phosphorodithioate oligodeoxycytidines are active inhibitors of syncytia formation and effectively inhibit de novo infection of target cells by HIV-1. Moreover, comparative experiments show that a deoxycytidine phosphorodithioate 14-mer is as effective an inhibitor of de novo infection as a phosphorothioate-containing 28-mer. Such potent inhibition by oligomers of relatively short length makes dithioate analogs an additional class of potential therapeutic agents against acquired immunodeficiency syndrome.
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Dithioate DNA was synthesized and used for various biochemical studies. Results from these studies indicate that dithioate DNA is a potent inhibitor of HIV Reverse Transcriptase, activates endogenous RNase H in HeLa cell nuclear extracts, and is a useful probe for studying protein-DNA interactions.
Mild heat treatment of HeLa cell nuclear extracts (NE) selectively inhibits pre-mRNA splicing. Heat-inactivated extracts can be complemented by a small amount of untreated NE. Utilizing this complementation assay and a combination of ion-exchange, affinity, and hydrophobic chromatography, a heat reversal factor (HRF) was purified from NE that is required to rescue pre-mRNA splicing from a heat-inactivated extract. This activity in its most purified form consistently copurified in a fraction containing two 70-kDa proteins and a minor polypeptide of approximately 100 kDa. It was free of the major small nuclear RNAs, sensitive to protease, and required to rescue spliceosome formation from a heat-inactivated nuclear extract. These results suggest that this factor is a protein that may be an important component in pre-mRNA splicing, or alternatively, it may be involved in renaturation of a heat-sensitive splicing factor.
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