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Biomedical subjects

R Holliday

Publications and source records attributed to R Holliday.

At least 55 records · Page 3Linked to original sources
PubMed1996

Neoplastic transformation: the contrasting stability of human and mouse cells.

The probability of a mouse cell becoming fully transformed in vivo or in vitro is enormously greater than that of a human cell. The number of events in tumour progression is similar in rodent and human cells, and it is unlikely that the difference in neoplastic transformation frequency can be explained on the basis of gene mutation in oncogenes and tumour suppressor genes. Instead, it is proposed that mouse cells may be (a) more subject to destabilization of the karyotype, (b) have less efficient check point cell cycle controls after DNA is damaged and/or (c) have less stringent epigenetic controls of gene activity, based on DNA methylation. Much evidence exists that mouse or rat cells are less efficient in DNA repair, maintenance of DNA methylation and other aspects of DNA metabolism. These relate to the difference in longevity in these and other mammalian species. Ageing is likely to be due to the failure of cell and tissue maintenance. Long lived species invest more in various somatic maintenance mechanisms than do short lived ones, and this includes protection against neoplastic transformation. The future study of the basis of the difference between human and mouse or rat cells in resistance to transformation is likely to yield important insights into the sequential events in tumour progression.

Age Factors↗
PubMed1995

Evidence for gene silencing by DNA methylation in normal human diploid fibroblasts.

Human diploid fibroblasts, strain MRC-5, were permeabilized by electroporation and treated with 5-methyl deoxycytidine triphosphate (5-methyl dCTP) in the S phase of the cell cycle. The frequency of TGR HPRT- cells was increased up to 20-fold in comparison to control untreated cultures. Representative TGR clones were unable to grow in HAT, and these were treated with 5-azacytidine (5-aza-CR). In many cases subsequent growth in HAT medium was observed, but in others it is likely that the cells had run out of growth potential. The results provide the first evidence of the silencing and reactivation of a gene in normal diploid mammalian cells.

Azacitidine↗
PubMed1994

Evidence for adenine methylation within the mouse myogenic gene Myo-D1.

Previous studies have indicated that there may be uncleavable TaqI sites (TCGA) within the mouse myogenic gene, Myo-D1. Fragments of DNA bearing most of the presumed insensitive TaqI sites have been reproduced using PCR. The presence of each of the originally uncleavable TaqI sites has been confirmed and each TaqI site has been shown to be sensitive to TaqI hydrolysis in PCR-synthesized genomic DNA. Since TaqI is inhibited by methylation of the adenine residue within its recognition sequence (but not by cytosine methylation), it is suggested that specific adenine bases are methylated in the coding region of Myo-DI and maintained throughout cell division. The same TaqI recognition sequences are insensitive to digestion in genomic DNA isolated from various mouse tissues including fetus, regenerating skeletal muscle and a myogenic cell line, all of which express Myo-D1. Thus, adenine methylation is not a modification of DNA following gametic fusion nor does it appear to play a major role in regulation of Myo-D1 expression.

Adenine↗
PubMed1994

Specificity in splicing.

Considerable information about the process of pre-mRNA splicing has accumulated, but the mechanism by which highly accurate splicing is achieved is unresolved. Fifteen years ago we proposed that accuracy in splicing might depend on small RNA molecules (splicer RNAs) which hybridise across adjacent exon termini, or intron termini. Gene expression, including alternative splicing, could be controlled by the transcription of specific splicer RNA genes. We re-assess our model here, in the light of subsequent developments.

Alternative Splicing↗
PubMed1994

Retardation of the senescence of cultured human diploid fibroblasts by carnosine.

We have examined the effects of the naturally occurring dipeptide carnosine (beta-alanyl-L-histidine) on the growth, morphology, and lifespan of cultured human diploid fibroblasts. With human foreskin cells, HFF-1, and fetal lung cells, MRC-5, we have shown that carnosine at high concentrations (20-50 mM) in standard medium retards senescence and rejuvenates senescent cultures. These late-passage cultures preserve a nonsenescent morphology in the presence of carnosine, in comparison to the senescent morphology first described by Hayflick and Moorhead. Transfer of these late-passage cells in medium containing carnosine to unsupplemented normal medium results in the appearance of the senescent phenotype. The serial subculture of cells in the presence of carnosine does not prevent the Hayflick limit to growth, although the lifespan in population doublings as well as chronological age is often increased. This effect is obscured by the normal variability of human fibroblast lifespans, which we have confirmed. Transfer of cells approaching senescence in normal medium to medium supplemented with carnosine rejuvenates the cells but the extension in lifespan is variable. Neither D-carnosine, (beta-alanyl-D-histidine), homocarnosine, anserine, nor beta-alanine had the same effects as carnosine on human fibroblasts. Carnosine is an antioxidant, but it is more likely that it preserves cellular integrity by its effects on protein metabolism.

Carnosine↗
PubMed1993

DNA methylation and mutation.

5-Methylcytosine (5mC) in DNA is produced by post-synthetic modification of cytosine residues, and it occurs primarily in CpG doublets in the mammalian genome. 5mC is a mutable site, because it can undergo spontaneous deamination to thymine. There is a repair mechanism which specifically recognises G.T mispairs, and replaces thymine with cytosine. However, this repair is not fully efficient, because the 5mC-->T transition mutation occurs about 10 times as frequently as other transitions. Such mutations are frequently seen in inherited diseases, and mutations in the p53 gene in tumours are also very commonly in 5mCpG doublets. As well as mutations, there can also be heritable changes in DNA methylation, known as epimutations, which may be of particular significance in somatic cells. Whereas the pattern of DNA methylation is very constant for any one cell type, the pattern becomes very variable in tumour cells. The breakdown of the normal controls of DNA methylation in tumorigenesis can lead to increased gene expression or to gene silencing. DNA damage increases not only mutation, but also heritable changes in methylation. At present, little is known about the ability of DNA repair to preserve the normal pattern of methylation in somatic cells.

5-Methylcytosine↗
PubMed1993

[Splenectomy by celioscopy. Experience of 20 cases].

We describe the clinical course of 20 patients considered for laparoscopic splenectomy. One patient was excluded on the basis of preoperative angiography findings and two (10.5%) were converted to open surgery. The first patient in the series was converted for a hemorrhagic incident and the eight patient, who had a 34 cm spleen, for dissection difficulties. In the remaining 17 patients who successfully underwent laparoscopic splenectomy, no mortality was reported and there were four postoperative complications (23.5%). Mean operating time was 3 h 40 min (135-300 min). After a mean postoperative stay of four days, all patients, except two, were back to normal activities within 2 weeks of hospital discharge. Preoperative splenic artery embolization was begun with the third patient and was found to significantly reduce operative blood loss and make the procedure easier to perform. Laparoscopic splenectomy has become our procedure of choice for elective removal of normal-sized or moderately enlarged spleens.

Adolescent↗
PubMed1992

Of mice and men.

Explore the source record for details and available documents.

Animals↗
PubMed1991

Quantitative genetic variation and developmental clocks.

It is well-known that most genetic variation affects quantitative traits, and natural or artificial selection can act to change quantitative features of organisms more rapidly than qualitative ones. Surprisingly, variability is not confined to outbred species, but also occurs in inbred mice at a much higher rate than expected from known mutation rates. The size and shape of organisms and their constituent parts are, at least in part, controlled by the number of cell divisions, and there is published evidence for the existence of developmental clocks, which may count cell divisions. A molecular model for a developmental clock was previously proposed. It depends on the DNA methylation of repeated sequences of DNA, where the methylation of each additional sequence is tied to DNA synthesis and therefore cell division. The number of repeats specifies the number of divisions which will occur before a signal is produced which can activate or inactivate one or more genes. It is known that crossing over occurs between sister chromatids, and where tandemly repeated sequences occur unequal exchange can generate a larger or smaller number of repeats. An example of this is seen in the well-known variability of "minisatellite" sequences in human DNA. Unequal sister chromatid exchange can occur in mitotic and meiotic cells in the germ line, and in the case of developmental clock sequences could generate variation in clock length which in turn would directly affect quantitative traits. These events can be regarded as a special case of molecular drive during evolution.

Animals↗
PubMed1991

Gene silencing in mammalian cells by uptake of 5-methyl deoxycytidine-5'-triphosphate.

Chinese hamster ovary (CHO) cells were subjected to electroporation in the presence of 5-methyl deoxycytidine-triphosphate. This treatment increases by 10 to 100-fold the frequency of cells lacking thymidine kinase, hypoxanthine-guanine phosphoribosyltransferase, or adenine phosphoribosyltransferase. The inactivation of the genes coding for these enzymes is thought to occur following the direct incorporation of the methylated nucleotide triphosphate into DNA. The enzyme-deficient clones were stable, but almost all were reactivated at high frequency by the demethylating agent 5-azacytidine, to produce derivatives with enzyme activity. The results indicate that there is a direct relationship between DNA methylation and gene silencing.

Adenine Phosphoribosyltransferase↗