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D M Tidd

Publications and source records attributed to D M Tidd.

36 records · Page 2Linked to original sources
PubMed1995

Detection of ribonuclease H-generated mRNA fragments in human leukemia cells following reversible membrane permeabilization in the presence of antisense oligodeoxynucleotides.

The involvement of ribonuclease H (RNase H) in antisense phenomena in intact cells has, to date, only been adequately demonstrated for microinjected Xenopus systems. The significance of RNase H for the antisense effects of oligodeoxynucleotides observed in human and other mammalian cell cultures has remained obscure, in part because of inadequate analytic methods. In this report we show that the "reverse ligation-mediated PCR" (RL-PCR) procedure permits amplification of RNA fragments produced by oligodeoxynucleotide-directed RNase H activity. We have used this procedure to demonstrate RNase H-dependent antisense effects in irreversibly permeabilized (dead) cells and reversibly permeabilized (live) cells.

Animals↗
PubMed1993

Chimeric oligodeoxynucleotide analogues: enhanced cell uptake of structures which direct ribonuclease H with high specificity.

A previous report has demonstrated that normal phosphodiester oligodeoxynucleotides could direct extensive non-targeted ribonuclease (RNase) H-dependent effects, and that greatly enhanced specificity could be achieved upon methylphosphonodiester substitution of terminal phosphodiester residues. In this report, we extend our previous observations to show that phosphorothioate oligodeoxynucleotides also direct substantial inappropriate RNase H-mediated hydrolysis of non-targeted RNA. Chimeric methylphosphonodiester/phosphodiesters were found to be capable of efficiently directing RNase H when the central phosphodiester section was reduced to just two contiguous internucleoside linkages. Furthermore, cleavage of non-target RNA sites was found to be undetectable, or minimal in extent, when RNase H was directed by such chimeras. In addition, we show that analogue structures which contain three, or fewer, phosphodiester residues in otherwise methylphosphonodiester molecules were imported into cells via the comparatively more efficient route taken by methylphosphonates, rather than by receptor-mediated endocytosis, which is generally characteristic for polyanionic structures. Evidence is presented that the primary process responsible for enhanced uptake is an active mechanism. Nevertheless, a proportion of the applied oligodeoxynucleotide analogues, which demonstrate augmented uptake, appear to have penetrated into the cytoplasmic cellular compartment. The present results suggest that chimeric molecules of the type we describe here may show considerable utility as antisense effectors due to their increased cellular import, access to the intracellular compartments, and their highly efficient and specific direction of RNase H.

Base Sequence↗
PubMed1992

Abrogation of c-MYC protein degradation in human lymphocyte lysates by prior precipitation with perchloric acid.

Conventional lysis buffers, though containing cocktails of protease inhibitors, did not prevent the degradation of c-MYC recombinant protein added immediately prior to lysis to cell pellets from human mixed lymphocyte cultures. Treatment of the cells with 4.2% perchloric acid, however, prevented protein degradation and facilitated the detection of c-MYC protein by Western blotting even in unstimulated lymphocytes, where previously it had been reported to be undetectable or barely detectable using this technique. PHA stimulation of lymphocytes induced an approximately six fold increase in measured c-MYC protein within 5 h if cell extracts were prepared using perchloric acid precipitation. However, using conventional lysis buffer the proto-oncogene protein was undetectable until 48-72 h after mitogen addition. Pretreatment with perchloric acid may be useful for Western blotting analysis of protein in other systems where it may be desirable to dispense with the use of toxic protease inhibitors or where these may be incompletely effective.

Blotting, Western↗
PubMed1992

Increased specificity for antisense oligodeoxynucleotide targeting of RNA cleavage by RNase H using chimeric methylphosphonodiester/phosphodiester structures.

One of the inherent problems in the use of antisense oligodeoxynucleotides to ablate gene expression in cell cultures is that the stringency of hybridization in vivo is not subject to control and may be sub-optimal. Consequently, phosphodiester or phosphorothioate antisense effectors and non-targeted cellular RNA may form partial hybrids which are substrates for RNase H. Such processes could promote the sequence dependent inappropriate effects recently reported in the literature. We have attempted to resolve this problem by using chimeric methylphosphonodiester/phosphodiester oligodeoxynucleotides. In contrast to the extensive RNA degradation observed with all-phosphodiester oligodeoxynucleotides, highly modified chimeric antisense effectors displayed negligible, or undetectable, cleavage at non-target sites without significantly impaired activity at the target site. We also note that all of the all-phosphodiester oligodeoxynucleotides tested demonstrated inappropriate effects, and that such undesirable activity could vary widely between different sequences.

Amino Acid Sequence↗
PubMed1992

Enhanced RNase H activity with methylphosphonodiester/phosphodiester chimeric antisense oligodeoxynucleotides.

Chimeric oligodeoxynucleotides, comprised of internal phosphodiester and terminal methylphosphonodiester sections, possess many beneficial characteristics as antisense effectors. We have investigated the effects of progressive replacement of phosphodiester by methylphosphonodiester linkages on hybrid stability with complementary RNA and DNA. The melting temperatures (Tms) of oligodeoxynucleotide/RNA heteroduplexes were found to decrease dramatically with increasing methylphosphonate substitution. In contrast, a smaller reduction in Tm was observed for comparable DNA heteroduplexes. This disparate reduction in hybrid stability was found with both the G + C-rich human c-myc and A + T-rich human c-Ha-ras sequences used, suggesting that methylphosphonate oligodeoxynucleotide analogues generally hybridize with less affinity to RNA than DNA. RNase H assays were employed to determine if the noted decreases in Tm impaired the ability of chimeric oligodeoxynucleotides to direct the degradation of RNA. Contrary to expectation, increasing methylphosphonate substitution gave rise to increasing rates of RNA degradation for both the c-myc and c-Ha-ras series. The present results suggest that chimeric oligodeoxynucleotide analogues may be of considerable utility as antisense agents in systems where RNase H is thought to make a major contribution to inhibition of gene expression.

Amino Acid Sequence↗
PubMed1992

The uptake kinetics of chimeric oligodeoxynucleotide analogues in human leukaemia MOLT-4 cells.

Chimeric oligodeoxynucleotides with terminal nonionic methylphosphonate analogue sections and internal phosphodiester regions offer several advantages as antisense effectors over either structure alone. These include enhanced biological stability relative to all-phosphodiester molecules, increased activity in directing ribonuclease H mediated destruction of target RNA, increased specificity and reduced non-specific toxicity. However, another important parameter, the ability of these molecules to enter intact mammalian cells, has not previously been investigated. Therefore, oligodeoxynucleotides were tagged at their 5'-termini with fluorescein reporter groups and a detailed study of uptake kinetics in human leukaemia MOLT-4 cells undertaken by calibrated flow cytometry. Baseline measurements with all-phosphodiester and all-methylphosphonate molecules confirmed that uptake of oligodeoxynucleotides by intact cells is a highly inefficient process. The kinetic data were in agreement with previous reports of mechanisms of cell uptake involving receptor mediated endocytosis in the case of phosphodiester molecules and simple diffusion for methylphosphonates. Chimeric oligodeoxynucleotides exhibited saturable cell uptake kinetics similar to all-phosphodiester oligodeoxynucleotides, suggesting that uptake was receptor-mediated and distinct from concentration-dependent uptake of all-methylphosphonate molecules. Similarly, chimeric molecules were apparently confined to the endosomal compartment within cells. These results imply that reversible masking of the negative charges of the phosphodiester sections of chimeric oligodeoxynucleotides may be required to change the uptake mechanism back to simple diffusion and allow intracellular delivery to the site of the target RNA.

Amino Acid Sequence↗
PubMed1989

Partial protection of oncogene, anti-sense oligodeoxynucleotides against serum nuclease degradation using terminal methylphosphonate groups.

Under certain circumstances sequence-specific inhibition of gene expression may be achieved in intact cells using exogenous anti-sense oligodeoxynucleotides. The efficacy of this approach to investigating gene function is limited in part by the rapid serum nuclease mediated degradation of oligodeoxynucleotides in culture media. In order to determine the relative contributions of 3'-exonuclease, 5'-exonuclease and endonuclease activity in fetal calf serum to oligodeoxynucleotide destruction, we have tested chimeric N-ras anti-sense sequence molecules protected against exonuclease attack with terminal methylphosphonate diester linkages. An 18-mer with two methylphosphonate diester linkages at the 3'-terminus, a 20-mer with two methylphosphonate diester groups at both ends, and the 16-mer 3'-methylphosphonate monoester components of their respective piperidine hydrolysates were totally resistant to venom phosphodiesterase, whereas the 16-mer 3'-hydroxyl components of the hydrolysates were rapidly degraded. Both the chimeric oligodeoxynucleotides and 3'-methylphosphonate monoesters were considerably more stable than normal 3'-hydroxyl oligodeoxynucleotides at 37 degrees C in McCoy's 5A medium containing 15% heat inactivated fetal calf serum. Typically 20-30% of the former (initial concentration 10-100 microM) remained intact at 20 h as compared to the latter which were 88-100% degraded in 4 h and undetectable at 20 h. We conclude that a 3'-phosphodiesterase activity is a predominant nuclease responsible for oligodeoxynucleotide degradation by fetal calf serum, and that for cell culture studies, significant protection of oligodeoxynucleotides may be achieved by incorporating 3'-terminal methylphosphonate diester or even monoester end groups.

Drug Stability↗
PubMed1988

Evaluation of N-ras oncogene anti-sense, sense and nonsense sequence methylphosphonate oligonucleotide analogues.

We have investigated the potential for using anti-sense non-ionic methylphosphonate oligonucleotide analogues to study the relationship between oncogene expression and maintenance of the transformed phenotype in malignant cells. Our results confirmed that the methylphosphonates are resistant to biochemical degradation and are devoid of non-specific toxicity towards cultured human HT29 cells. At low temperature (less than 5 degrees C) both N-ras anti-sense and nonsense analogue 9-mers formed 1:1 complexes in solution with an N-ras sense phosphodiester oligodeoxynucleotide 20-mer, but these were largely dissociated at 25 degrees C. Only a fraction (10-20%) of the anti-sense molecules formed stable sequence specific hybrids (Tm 34 degrees C) with the 20-mer. The biological activity of the oligonucleotide analogues was tested in cell culture at 37 degrees C using T15 cells, a line of NIH 3T3 cells transfected with multiple copies of the human N-ras oncogene under control of the glucocorticoid inducible MMTV promoter. On balance the N-ras anti-sense methylphosphonate 9-mer (20-80 microM) had no effect on these cells. In only one of five experiments was an apparent reduction in dexamethasone-induced p21N-ras protein accumulation observed in the presence of the oligonucleotide analogue. Also without effect was an anti-sense 20-mer consisting of a phosphodiester sequence bounded by two methylphosphonate linkages at each end (25-50 microM in culture media; 4.8 microM by microinjection). We conclude from these experiments that, in order to achieve pronounced effects on oncogene expression, it may be necessary to use longer anti-sense methylphosphonate chains, affinity purified for their ability to hybridize with the target sequences.

Base Sequence↗
PubMed1985

The effects of 6-mercaptopurine nucleotide derivatives on the growth and survival of 6-mercaptopurine-sensitive and -resistant cell culture lines.

6-Mercaptopurine (MP)-sensitive and -resistant cell culture lines were used to further characterize the apparent ability of MP nucleotide derivatives to overcome resistance to the parent drug. 6-Mercaptopurine-9-beta-D-ribofuranoside 5'-monophosphate [MPRP], bis(6-mercaptopurine-9-beta-D-ribofuranoside)-5', 5"'-monophosphate [bis(MPR)P], bis(O2',O3'-dibutyryl-6-mercaptopurine-9-beta-D-ribofuranoside)-5', 5"'-monophosphate [bis(dibut.MPR)P], and O2',O3'-dibutyryl-6-mercaptopurine-9-beta-D-ribofuranoside 5'-monophosphate [dibut.MPRP] were tested for cytotoxic and/or growth inhibitory effects against MP-resistant sublines of V79 Chinese hamster lung fibroblasts (CH/TG) and L1210 mouse leukaemia cells (L1210/MPR) in which deficiencies of hypoxanthine-guanine phosphoribosyltransferase, and hence drug nucleotide forming capacity were the basis of resistance. L1210/MPR cells were totally resistant to 1 mM 6-mercaptopurine-9-beta-D-ribofuranoside [MPR] and 2 mM MPRP, but were inhibited by high concentrations (greater than 0.25 mM) of bis(MPR)P. These results suggested that bis(MPR)P was taken up by cells as the intact molecule since MPR and MPRP were its extracellular breakdown products. L1210/MPR cells were much more sensitive to the lipophilic bis(dibut.MPR)P derivative which had a predominantly cytotoxic action as judged by trypan blue staining and the ability of treated cells to produce macroscopic colonies in soft agar medium. However, cells killed by bis(dibut.MPR)P did not disintegrate appreciably over periods of up to 10 days. The effects of bis(dibut.MPR)P were probably the result of cellular uptake of the intact molecule. Dibut.MPRP showed minimal ability to inhibit L1210/MPR cells although this compound was a possible breakdown product of bis(dibut.MPR)P and a source of the same extracellular degradation products. The median cell size decreased in L1210/MPR cultures during exposure to both bis(MPR)P and bis(dibut.MPR)P. This effect was elicited more rapidly and at lower concentration by bis(dibut.MPR)P than by bis(MPR)P. In contrast, sodium butyrate, a breakdown product of bis(dibut.MPR)P induced increases in cell size at high concentration. Bis (dibut.MPR)P was also cytotoxic to MP-resistant CH/TG cells and was approximately 300 times more effective than bis(MRP)P and MPR which exhibited similar activity against this cell line. Bis(dibut.MPR)P and dibut.MPRP were equivalent and less active than MPR in their effects on MP-sensitive L1210/0 cells where their predominant mechanism of action was via degradation to release MPR.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1978

Specific and sensitive combined high-performance liquid chromatographic-flow fluorometric assay for intracellular 6-thioguanine nucleotides metabolites of 6-mercaptopurine and 6-thioguanine.

A new non-radioisotopic technique is described for measuring rates of intracellular formation by human leukemic blasts of 6-thioguanine nucleotide metabolites, obligatory intermediates in the antineoplastic action of both 6-mercaptopurine and 6-thioguanine itself. The method is both specific and sensitive, and involves combined high-performance liquid chromatography and flow fluormetric detection of oxidized 6-thioquanine nucleotides in alkaline permanganate-treated cell extracts. Non-metabolized 6-thioguanine and 6-thioxanthine are also separated and quantitated in this system, permitting complementary in vivo pharmacokinetic analysis. The assay may be applied to detect resistant disease at an early stage in therapy, and thereby provides the opportunity for alternative treatments to be instituted.

Body Fluids↗
PubMedDate not supplied

A potential role for antisense oligonucleotide analogues in the development of oncogene targeted cancer chemotherapy.

The identification of activated oncogenes as the basic biochemical difference between tumour cells and normal cells has opened up the possibility for development of truly tumour specific chemotherapy. It may be hypothesized that malignant cells would revert to a more normal phenotype and might even be triggered into terminal differentiation if expression of the appropriate oncogenes were inhibited. Although, at present, it is not possible to anticipate what form future therapy based upon this approach would take, it is clear that the immediate priority must be to establish the general validity of the hypothesis with a variety of tumour cell types in vitro. For this purpose antisense oligonucleotide analogues appear to offer considerable promise as sequence specific inhibitors of oncogene expression. However, no analogue structure yet devised fulfils all the requirements of an ideal antisense effector in terms of biological stability, cell uptake, non-toxicity, hybridization efficiency and mechanism of action on target nucleic acids. Inhibition of oncogene expression in certain cell types has been reported using antisense oligonucleotides but the technique is not universally applicable and more detailed biochemical investigations of the interactions of oligonucleotides with intact cells are required before improved structures may be developed.

Animals↗