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Biomedical subjects

A H Wyllie

Publications and source records attributed to A H Wyllie.

At least 37 records · Page 2Linked to original sources
PubMed1999

Identification of a region of frequent loss of heterozygosity at 11q24 in colorectal cancer.

Loss of heterozygosity (LOH) at 11q23-qter occurs frequently in ovarian and other cancers, but for colorectal cancer, the evidence is conflicting. Seven polymorphic loci were analyzed between D11S897 and D11S969 in 50 colorectal tumors. Two distinct LOH regions were detected, suggesting possible sites for tumor-suppressor genes involved in colorectal neoplasia: a large centromeric region between D11S897 and D11S925, and a telomeric 4.9-Mb region between D11S912 and D11S969. There was no correlation with clinicopathological features. This analysis describes a region of LOH in the region 11q23.3-24.3 for the first time in colorectal cancer and provides complementary evidence for the ongoing effort to identify the gene(s) involved.

Aged↗
PubMed1999

Msh2 status modulates both apoptosis and mutation frequency in the murine small intestine.

Deficiency in genes involved in DNA mismatch repair increases susceptibility to cancer, particularly of the colorectal epithelium. Using Msh2 null mice, we demonstrate that this genetic defect renders normal intestinal epithelial cells susceptible to mutation in vivo at the Dlb-1 locus. Compared with wild-type mice, Msh2-deficient animals had higher basal levels of mutation and were more sensitive to the mutagenic effects of temozolomide. Experiments using Msh2-deficient cells in vitro suggest that an element of this effect is attributable to increased clonogenicity. Indeed, we show that Msh2 plays a role in the in vivo initiation of apoptosis after treatment with temozolomide, N-methyl-N'-nitro-N-nitrosoguanidine, and cisplatin. This was not influenced by the in vivo depletion of O6-alkylguanine-DNA-alkyltransferase after administration of O6-benzylguanine. By analyzing mice mutant for both Msh2 and p53, we found that the Msh2-dependent apoptotic response was primarily mediated through a p53-dependent pathway. Msh2 also was required to signal delayed p53-independent death. Taken together, these studies characterize an in vivo Msh2-dependent apoptotic response to methylating agents and raise the possibility that Msh2 deficiency may predispose to malignancy not only through failed repair of mismatch DNA lesions but also through the failure to engage apoptosis.

Animals↗
PubMed1999

Role of BAX mutations in mismatch repair-deficient colorectal carcinogenesis.

BAX gene mutations occur in approximately 50% of RER+ colorectal cancers. To determine the role of these mutations in tumour progression we analysed multiple different tumour sites from RER+ colorectal cancers for BAX mutations. Sixty colorectal carcinomas were analysed for microsatellite instability at loci BAT-26, L-myc, TGF betaRII, D13S160 and D2S123. Twelve out of 60 tumours (20%) were RER+. Forty-five different tumour sites from the 12 RER+ carcinomas were analysed for BAX mutations at the [(G)8] tract in exon 3. Six out of 12 (50%) RER+ tumours showed BAX mutations, four of which showed a homogenous pattern of such mutations detected in all tumour sites. In the other two cases, BAX mutations were present in some but not all tumour sites sampled from the same patient. In contrast, TGF betaRII mutations were found in 9/12 cases (75%) and in each of these were present in all the sampled sites. Two cases showed neither BAX nor TGF betaRII mutation. These data suggest that mutations in TGF betaRII may occur at a very early stage in tumour progression, perhaps in the founder clone. BAX mutations, however, are clearly not necessary for formation of the founder clone and can occur for the first time later in tumour progression.

Adult↗
PubMed1999

P53-dependent and -independent links between DNA-damage, apoptosis and mutation frequency in ES cells.

The hypothesis that p53 deficiency enhances the survival of DNA-damage bearing cells was investigated in wild-type and p53 mutant embryonic stem (ES) cells. Following UV-C irradiation, p53 is rapidly induced in wild-type cells and p53-dependent apoptosis follows within 8 h, resulting in the death of the majority of cells within 36 h. Increasing doses of UV-irradiation resulted in enhanced clonogenic survival of null cells as compared to wild-type. Amongst surviving clones, the Hprt mutation frequency was found to be dependent upon UV dose and influenced by p53 status. Treatment with ionizing radiation led to enhanced expression of p53 but resulted in little induction of apoptosis irrespective of p53 status. However, clonogenic potential was considerably reduced, particularly in wild-type cells which showed a tenfold lower survival than null cells. In contrast to the effects of UV-irradiation, the incidence of Hprt mutation did not differ significantly between wild-type and p53 null survivors. The data confirm that p53 restricts the numbers of cells bearing mutations that survive DNA damage induced by either agent, albeit by different mechanisms.

Animals↗
PubMed1999

DNA chip technology.

A mini-revolution is sweeping the world of science and medicine. DNA chip or microarray technology will have a more profound impact than other recent major advances, including DNA sequencing and the polymerase chain reaction (PCR). This mini-review explains the technology, its scope, and impact on science and medicine, as well as its cost and possible limitations.

Chromosome Mapping↗
PubMed1999

Microsomal epoxide hydrolase gene polymorphism and susceptibility to colon cancer.

We examined polymorphisms in exons 3 and 4 of microsomal epoxide hydrolase in 101 patients with colon cancer and compared the results with 203 control samples. The frequency of the exon 3 T to C mutation was higher in cancer patients than in controls (odds ratio 3.8; 95% confidence intervals 1.8-8.0). This sequence alteration changes tyrosine residue 113 to histidine and is associated with lower enzyme activity when expressed in vitro. This suggests that putative slow epoxide hydrolase activity may be a risk factor for colon cancer. This appears to be true for both right- and left-sided tumours, but was more apparent for tumours arising distally (odds ratio 4.1; 95% confidence limits 1.9-9.2). By contrast, there was no difference in prevalence of exon 4 A to G transition mutation in cancer vs controls. This mutation changes histidine residue 139 to arginine and produces increased enzyme activity. There was no association between epoxide hydrolase genotype and abnormalities of p53 or Ki-Ras.

Adolescent↗
PubMed1999

Caspase-mediated cleavage of APC results in an amino-terminal fragment with an intact armadillo repeat domain.

During the effector phase of apoptosis, caspase activation appears to be responsible for the distinctive structural changes of apoptosis and perhaps for some of the changes in function of the doomed cells. There is therefore interest in identifying caspase substrates and the details of the cleavage events. Here we define precisely the event responsible for generation of a stable 90 kDa fragment from the oncosuppressor protein adenomatous polyposis coli (APC). Using synthetic radiolabeled APC peptides as substrate, we demonstrate cleavage by cytosolic extracts from preapoptotic cells. This cleavage was reproduced by recombinant caspase-3 and blocked by a tetrapeptide inhibitor Ac-DEVD-CHO, which is specific for caspase-3 family members. Inhibitors specific for caspase-1 and -8 however, were less effective in blocking APC cleavage. Mutation of a candidate DNID caspase-3 target site completely abolished cleavage. This cleavage may be of biological importance since the 90 kDa fragment consists of a sequence that is highly conserved in the human, rat, mouse, Xenopus, and Drosophila APC, although wide sequence divergence is observed in Drosophila immediately carboxy-terminal to the DNID site. Furthermore, cleavage at this site separates two significant functional domains: an amino-terminal armadillo repeat and an adjacent series of beta-catenin binding sites. Further circumstantial evidence for the significance of APC-related pathways in apoptosis is provided by the observation that apoptosis also induces cleavage of beta-catenin itself, a protein known to accumulate in cells depleted in functional APC and that appears to link cell-cell signaling to changes in transcription and cell movement.

Adenomatous Polyposis Coli↗
PubMed1999

Apoptosis and carcinogenesis.

Apoptosis has long been known to be effected through a common sequence of structural changes, despite the wide variety of initiating stimuli. These common structural events appear to depend upon activation of a set of enzymes (caspases) which direct a strongly conserved, terminal effector pathway. The regulation of this pathway, and in particular its coupling to DNA damage, appears to be critical in maintaining at low levels the number of mutated cells within tissues. The frequency with which tumours (experimental and human) bear deficiency in p53 or MSH-2 repair function may indicate the importance of these proteins in coupling DNA damage to apoptosis.

Animals↗
PubMed1998

p53-independent DNA repair and cell cycle arrest in embryonic stem cells.

The role of p53 in DNA repair and cell cycle checkpoint after ultraviolet irradiation was investigated in an embryonic stem cell line homozygous for a targeted deletion of p53. Results indicate that loss of p53 does not alter the capacity of ES cells to respond to DNA damage. Wild-type and p53-deficient cells showed similar cessation of DNA synthesis after UV damage and similar ultimate capacity to repair a transiently transfected reporter plasmid. Interestingly, in the absence of DNA damaging treatment, the transit of p53-deficient cells through S phase was slower than wild-type cells. We suggest that this may result from the absence of a p53-dependent response to endogenous DNA damage: without p53 sensing endogenous damage leading to immediate repair, such damage may persist and thus delay DNA synthesis.

Animals↗
PubMed1998

N-acetyl transferase 1: two polymorphisms in coding sequence identified in colorectal cancer patients.

Increased cancer risk has been associated with functional polymorphisms that occur within the genes coding for the N-acetyltransferase enzymes NAT1 and NAT2. We detected two NAT1 polymorphisms in colorectal cancer patients by heteroduplex analysis. DNA sequencing revealed the wild-type sequence (NAT1*4) and two single base substitutions at adjacent positions 999 bp (C to T, NAT1*14) and 1000 bp (G to A, NAT1*15) of the gene, changing Arg187 to a stop codon and Arg187 to Gln respectively. NAT1 alleles NAT1*4 (0.98) and NAT1*15 (0.02) were present at a similar frequency in patients with colorectal cancer (n=260) and in a Scottish control group (n=323). The third allele, NAT1*14, was present only in the colorectal cancer group at a frequency of 0.006. NAT1 genotype NAT1*4/ NAT1*15 was significantly less frequent in individuals that had a slow NAT2 genotype. This was observed in both cancer and control groups and suggests that this association was unrelated to cancer risk. We conclude that polymorphisms within the coding region of the NAT1 gene are infrequent and do not appear to have an independent association with colorectal cancer risk. However, the relationship between NAT1 and NAT2 polymorphisms appears non-random, suggesting a linkage between these enzymes.

Acetyltransferases↗
PubMed1997

Absence of p53 permits propagation of mutant cells following genotoxic damage.

Much evidence has been gathered in support of a critical role for p53 in the cellular response to DNA damage. p53 dysfunction is associated with progression and poor prognosis of many human cancers and with a high incidence of tumours in p53 knockout mice. The absence of a p53-dependent G1 arrest that facilitates DNA repair or apoptosis might impact critically on clinical cancer in two ways. First, by abrogating the impact on therapy that operates via genotoxic damage and apoptosis; and second, by encouraging progression either by inducing genomic instability and DNA mis-repair or by permitting survival of mutants. However, experiments examining the relationship between p53 deficiency and mutation frequency have so far failed to confirm these predictions. The precise role played by p53 is therefore unclear. We now report use of a short term in vitro approach to assess the influence of p53 on radiation-induced mutations at the hprt locus in murine B cell precursors that are normally radiation ultrasensitive. We find a high number of hprt mutants among X-irradiated p53 null cells, which results from preferential survival as clonogenic mutants rather than from a p53-dependent increase in mutation rate. This result has important implications for genotoxic cancer therapy.

Animals↗
PubMed1997

The pattern of K-ras mutation in pulmonary adenocarcinoma defines a new pathway of tumour development in the human lung.

Codon 12 of the K-ras oncogene was screened for mutations in 65 surgically-resected primary pulmonary adenocarcinomas and in 32 tissue foci of alveolar atypical hyperplasia (AAH) by a polymerase chain reaction (PCR)-based method. Mutations in either position 1 or position 2 of codon 12 were detected in 16 tumours (25 per cent). When analysed by site of origin, mutations were seen in 9/26 (35 per cent) parenchymal and in 0/12 bronchial adenocarcinomas (P < 0-02), K-ras mutations were seen in five AAH lesions from four patients. DNA sequencing showed that the great majority of mutations in both adenocarcinomas and AAH were G-T transversions. These findings provide support for the classification of pulmonary adenocarcinomas into bronchial and parenchymal subtypes and also provide molecular evidence to support the importance of AAH in the development of parenchymal cancers.

Adenocarcinoma↗
PubMed1997

APC expression in normal human tissues.

The tumour suppressor gene APC codes for a 2843-amino acid protein whose precise functions are still poorly understood. This paper describes the development of two new antisera to APC (to amino- and carboxy-terminal epitopes) which permit localization of the protein by immunohistochemistry in archival paraffin sections. The protein is expressed in a wide variety of normal epithelial tissues. Its distribution frequently coincides with the location of post-replicative cells within tissues. Staining patterns demonstrate that the APC protein, although often diffusely cytoplasmic in distribution, may also accumulate in the apical and immediately subapical regions, or along the lateral margins of certain cells. These results indicate that APC is significant in many tissues in addition to the colorectal epithelium. They are compatible with a function related to signalling at the adherens junction and possibly with other more complex roles in cells committed to terminal differentiation.

Adenomatous Polyposis Coli Protein↗
PubMed1997

UV but not gamma-irradiation induces specific transcriptional activity of p53 in primary hepatocytes.

The mechanisms are poorly understood by which p53 can stimulate different downstream events, including growth arrest, DNA repair, and apoptosis, after DNA damage. Changes in protein levels do not predict a particular p53 response, but it is possible that differences in functional activities such as transactivation are important. The present report describes the successful use of a specific p53 reporter plasmid transfected into primary murine hepatocytes to evaluate p53 transactivation activity over time after two different genotoxic injuries (gamma-irradiation, 15 Gy and UV-c irradiation, 10 J/m2) known to produce p53-dependent growth arrest in this cell type. The results show that UV injury to hepatocytes was followed by a transient increase in transcriptional activation of the reporter plasmid by p53 and that this response preceded changes in p53 protein levels, as assessed by immunocytochemistry. By contrast, gamma-irradiation injury failed to induce detectable changes in either transactivation activity or hepatocyte p53 protein levels. The data show that p53 responses to DNA damage are dependent on both cell and injury type and suggest that in hepatocytes they can be independent of protein concentration and specific transcriptional activity. The results have implications for how particular dysfunctional p53 mutations in carcinogenesis could alter hepatocyte responses to different DNA injuries.

Animals↗