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Biomedical subjects

P Dyson

Publications and source records attributed to P Dyson.

17 recordsLinked to original sources
PubMed1995

Tris-dependent oxidative DNA strand scission during electrophoresis.

The DNA of two Streptomyces species contains site-specific labile modifications. During gel electrophoresis the DNA can undergo Tris-dependent strand scission at the positions of these modifications. Our investigations into the nucleolytic activity which reacts with the modifications implicate a peracid derivative of Tris formed at the anode; the kinetics of production and decay of this activity were followed using both a DNA cleavage assay and a reduced methyl viologen assay to measure oxidant. Anode activation could be chemically mimicked by addition of peracetic acid to Tris buffers. We tested the DNA cleavage activity of several other compounds after anode or chemical activation; we used an analogue of Tris lacking a primary amine group and also several reagents known to promote DNA strand cleavage by amine-catalysis at abasic sites. Anode generation of oxidant could be detected for compounds containing either hydroxyl or carboxyl groups. However, DNA cleavage activity correlated with oxidant formation only for those compounds also containing primary amine groups. These results support a mechanism of DNA strand scission at modification sites via concerted peracid-mediated oxidative and amine-catalysed reactions. The novel finding of Tris-dependent formation of a long-lived reactive oxidant at the anode suggests that this compound is unsuited as an electrophoresis buffer for certain biological macromolecules.

Amines↗
PubMed1995

Wound healing on the lower leg after radiotherapy or cryotherapy of Bowen's disease and other malignant skin lesions.

To investigate lower leg wound healing after treatment of skin tumours, the results of external beam radiotherapy were assessed for 141 lesions in 91 patients. Poor healing or failure to heal occurred in 33%, and was related to the age of the patients, the diameter of the radiotherapy field, and the dose and energy of radiotherapy used. No effect of fractionation on healing was apparent, but only 14% of patients were exposed to regimens with a nominal standard dose of over 1800 rets. Age > 90 years, field diameter > 4 cm and dose > 3000 cGy were all associated with a risk of impaired healing of over 50%. The most frequent single diagnosis (59 lesions) in the radiotherapy patients was Bowen's disease. The results of wound healing in these patients were compared with the results of cryotherapy to 82 lower leg Bowen's disease lesions in 49 dermatology patients. Only 2% of the cryotherapy lesions failed to heal compared with 20% of the radiotherapy wounds, although 6% of cryotherapy cases had local recurrence compared with none in the radiotherapy group. Serial overlapping cryotherapy fields of up to 2 cm diameter were shown to be a valid treatment option for Bowen's disease, as no patients had therapy-related failure to heal. To avoid the requirement for potentially more difficult post-radiotherapy salvage surgery, intra-epidermal non-invasive skin tumours > 4 cm in diameter on the lower leg are probably initially best treated by a primary surgical procedure or by staged cryotherapy. Invasive carcinoma of > 4 cm diameter is best treated by surgical excision.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗
PubMed1994

Characterization of a Streptomyces-lividans-type site-specific DNA modification system in the avermectin-producer Streptomyces avermitilis permits investigation of two novel giant linear plasmids, pSA1 and pSA2.

The degradation of Streptomyces avermitilis DNA samples analysed by conventional pulsed-field gel electrophoresis was shown to be due to Tris-dependent, double-strand cleavage. Using alternative electrophoretic conditions, separation of intact DNA molecules was achieved, permitting the identification of two novel giant linear plasmids: the 100 kb pSA1 and 250 kb pSA2. Use of pSA2 DNA as a probe showed that pSA1 does not cross-hybridize, indicating that the plasmids are not closely related. The site-specificity of the DNA modifications, which render the DNA susceptible to Tris-dependent cleavage, was found to be essentially identical to that of similar modifications found in the DNA of S. lividans.

Bacterial Proteins↗
PubMed1992

Tris-dependent site-specific cleavage of Streptomyces lividans DNA.

During conventional gel electrophoresis, Streptomyces lividans DNA undergoes site-specific double-strand cleavage at the positions of closely opposed unstable modifications introduced into the DNA in vivo. We investigated this electrophoretic instability and demonstrated that it was dependent on Tris. Tris buffer was activated at the anode to generate a nucleolytic species; prior to activation, Tris was not able to cleave the DNA. The nucleolytic species was shown to react with thiourea, which could thus protect the DNA from strand cleavage. Non-degradative electrophoresis of the DNA could also be achieved in an alternative buffer such as Hepes.

Binding Sites↗
PubMed1992

Spinal cord compression: prognosis and implications for treatment fractionation.

A review of 158 cases of metastatic extradural spinal cord compression referred to three radiotherapy departments shows that the median survival time is less than 3 months, that recovery of motor and sphincter function is uncommon, and that low fraction (2-5) regimens have a similar clinical outcome to more protracted regimens.

Actuarial Analysis↗
PubMed1988

Do nonorganic signs help to predict the return to activity of patients with low-back pain?

The value of the nonorganic signs in predicting return to activity following an initial episode of low-back pain was determined. One hundred and twenty patients were assessed within 6 months of the onset of their first episode of disabling low-back pain and again at a mean of 15.3 months following injury. No correlation was found between the presence of nonorganic signs at initial assessment and either return to activity or resolution of the patient's symptoms.

Adolescent↗
PubMed1987

Genetic instability and DNA amplification in Streptomyces lividans 66.

Streptomyces lividans 66 exhibits genetic instability, involving sequential loss of resistance to chloramphenicol (Cams) and subsequent mutation of argG. Associated with this instability is the amplification of a 5.7-kilobase (kb) amplified DNA sequence (ADS). We have characterized a second, independent pathway of genetic instability, involving sequential loss of resistance to tetracycline (Tets) followed by mutation in nitrogen assimilation (Ntr). We detected DNA amplification in many of these mutant strains, as well as other reiterations coresident with the 5.7-kb ADS in Cams Arg mutants. However, in contrast to the 5.7-kb ADS, none of the novel elements were observed to amplify at high frequency. The mutation of argG is due to a deletion, one endpoint of which is defined by the 5.7-kb ADS. This amplification derives from a structure, the tandemly duplicated amplifiable unit of DNA (AUD), present in the wild-type genome. We found that progenitor strains containing just a single-copy AUD failed to reproducibly generate amplification of this element in Cams argG mutants, and DNA deletion endpoints proximal to the element were found to be unspecific. These results suggest that a duplicated AUD structure is required for high-frequency amplification and that this reiteration can subsequently buffer the extent of deletion formation in the relevant chromosomal region.

Base Sequence↗
PubMed1987

Relationship of an unstable argG gene to a 5.7-kilobase amplifiable DNA sequence in Streptomyces lividans 66.

The relationship between an unstable argG gene and a 5.7-kilobase (kb) amplifiable DNA sequence in Streptomyces lividans 66 was investigated. Spontaneous, high-frequency Arg mutants deleted for this gene typically contain 200 to 300 copies of the tandemly reiterated sequence. A library of S. lividans 66 (strain 1326) wild-type genomic DNA was prepared in the vector lambda Charon 35. Chromosome walking over 44 kb established that argG is located 25 kb distant from a duplicated amplifiable DNA structure. A sequence was characterized, located farther distal from the amplifiable structure, containing strong homology with an internal sequence of the amplifiable DNA, which may have a role in the deletion of argG. Genetic mapping showed that argG and the 5.7-kb amplifiable sequence are linked to another unstable gene, determining chloramphenicol resistance (Camr) and that together these genes may be located in a silent chromosomal arc.

Argininosuccinate Synthase↗
PubMed1983

Transposon-encoded site-specific recombination: nature of the Tn3 DNA sequences which constitute the recombination site res.

The tnpR gene of transposon Tn3 encodes a site-specific recombination enzyme that acts at res, a DNA region adjacent to tnpR, to convert co-integrate intermediates of interreplicon transposition to the normal transposition end-products. We have used two complementary approaches to study the nature of the Tn3 recombination region, res. Firstly, the DNA-binding sites for tnpR protein were determined in DNase I protection experiments. These identified a 120-bp region between the tnpA and tnpR genes that can be subdivided into three separate protein-binding sites. Genetic dissection experiments indicate that few, if any, other sequences in addition to this 120-bp region are required for res function. Moreover, we have shown that the two directly repeated res regions within a molecule are unequal partners in the recombination reaction: a truncated res region, which is unable to recombine with a second identical res region, can recombine efficiently with an intact res region. This demonstration, along with the observation that tnpR/res recombination acts efficiently on directly repeated res regions within a molecule but inefficiently both on inverted res regions in the same molecule and in the fusion reaction between res regions in different molecules, leads us to propose that one-dimensional diffusion (tracking) of tnpR protein along DNA is used to locate an initial res region, and then to bring a second directly repeated res region into a position that allows recombination between the res regions.

Bacterial Proteins↗
PubMed1977

Cell kinetics of urethane-induced murine pulmonary adenomas: III. Implications of the disparity between the rates of entry into DNA synthesis and into mitosis.

Metaphase arrest by vincristine in urethane-induced murine pulmonary adenomas became linear after an interval of 60 min. The rate of entry into metaphase was 0-191%/h, which was considerably less than the 1%/h for the rate of entry into DNA synthesis obtained previously by double labelling. The duration of prophase plus metaphase was calculated to be 1-7 h. A growth fraction of 9% and a cell-loss factor of 52% were derived. The disparity between rates of entry into DNA synthesis and into metaphase was investigated by microdensitometry on Feulgen-stained squash preparations of tumours of varying ages. Tne DNA profiles showed an increasing frequency of hyperdiploid nuclei with age. Circumstantial evidence for polyploidy was provided by the presence of many binucleate cells in the tumours. By analogy with the liver, these cells may well represent a stage in the development of polyploidy, and the possible relevance of these findings to the neoplastic process is considered.

Adenoma↗
PubMed1977

The measurement of the cell cycle time in squamous epithelium using the metaphase arrest technique with vincristine.

In squamous epithelia with a single layer of germinative cells, the age distribution of cells in the cell cycle is shown to depend on the direction of the mitotic axis (i.e. a line joining the nuclei of daughter cells) relative to the plane of the basal layer. When axes are in the plane of the basal layer the age distribution is exponential; when cells divide at right angles to the plane of the basal layer, the age distribution is rectangular. When there is a ratio of vertical to horizontal axes, the age distribution is intermediate but can be calculated from knowledge of the proportion of axes in the plane of the layer. Squamous epithelia can be classified according to this arrangement of axes. When there are multiple layers of germinative cells, as in psoriasis, the age distribution is shown to be exponential to a good approximation, whatever the direction of the mitotic axes in the several layers. The importance of these observations is demonstrated by analysing metaphase arrest experiments with vincristine in the single layer of germinative cells in the mouse oesophagus, and in the several layers found in psoriatic epidermis. Choice of the wrong age distribution leads to an error of 6 h in the oesophagus and 23 h in psoriatic epidermis, when the mean cell cycle time is calculated. It is concluded that, in squamous epithelium, it is most important to know the age distribution before calculating the cell cycle time by methods involving measurement of the rate of entry of cells into mitosis or DNA synthesis.

Animals↗
PubMed1976

Cell kinetics of urethane-induced murine pulmonary adenomata: II, the growth fraction and cell loss factor.

Continuous labelling of urethane induced pulmonary adenomata in adult male A2G mice at intervals up to 20 weeks showed that the growth fraction fell progressively from 18% at 7 weeks to 7% at 20 weeks. This fall appears to be wholly responsible for the decrease in production of adenoma cells with age. A fraction labelled mitoses curve was constructed for pulmonary adenomata at 14 weeks post urethane. Only the first peak was apparent, giving median t2 and ts values of 2 and 9 h respectively. The cell cycle time was calculated at 45 h and the growth fraction at 6-2%, whilst the cell loss factor was estimated at 31%. Other cell loss values were calculated from data on the rate of entry into DNA synthesis obtained previously by double labelling. These values remained constant with time at 83-95%, suggesting that cell loss is in some way linked to cell production. However, the development of polyploidy in adenoma cells could not be eliminated. No areas of necrosis were seen in the adenomata at any time although karyorrhexis occurred in isolated cells. The labelling characteristics of alveolar wall cells in the same lung sections as the adenomata did not vary with time and the continuous labelling curve gave a growth fraction of 1-8%, a DNA synthetic time of 10 h and a cell cycle time of 30 h.

Adenoma↗
PubMed1975

Cell kinetics of urethane induced murine pulmonary adenomata: I. The growth rate.

A single injection of urethane into adult male A2G mice produced an increase in the proliferative rate of alveolar wall cells, reaching a peak at 2 weeks post urethan (PU) and declining to control levels by 2 months PU. During this urethane induced proliferative response the single and double labelling indices and the native metaphase index were all elevated although there was no corresponding alteration in the arrested metaphase index. This proliferative response may not be restricted to hyperplasia of potentially neoplastic cells, such as type II epithelium, but may also include type I epithelial cells and alveolar macrophage precursors. However, it was impossible to identify individual cell populations by methods used. The growth rate of adenomata decrease with time and cell kinetic techniques showed that the rates of entry of adenoma cells into DNA synthesis and into metaphase were decreasing concurrently with the growth rate. Thus the rate of cell production falls as adenomata age but how much cell loss contributes to the decrease in growth rate is not yet known. Decreasing cell production could be due to an increased cell cycle time and/or a decreased growth fraction. The duration of DNA synthesis in adenomata increased markedly as the mice survived, suggesting that the cell cycle time might be increased, but further experiments are required to determine whether the growth fraction changes. Attention is drawn to a complication that Colcemid introduces into kinetic studies on alveolar wall cells.

Adenoma↗