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Biomedical subjects

A J Bishop

Publications and source records attributed to A J Bishop.

At least 19 recordsLinked to original sources
PubMed2001

Homologous recombination as a mechanism of carcinogenesis.

Cancer develops when cells no longer follow their normal pattern of controlled growth. In the absence or disregard of such regulation, resulting from changes in their genetic makeup, these errant cells acquire a growth advantage, expanding into pre-cancerous clones. Over the last decade many studies have revealed the relevance of genomic mutation in this process, be it by misreplication, environmental damage or a deficiency in repairing endogenous and exogenous damage. Here we discuss homologous recombination as another mechanism that can result in loss of heterozygosity or genetic rearrangements. Some of these genetic alterations may play a primary role in carcinogenesis, but they are more likely to be involved in secondary and subsequent steps of carcinogenesis by which recessive oncogenic mutations are revealed. Patients whose cells display an increased frequency of recombination also have an elevated frequency of cancer, further supporting the link between recombination and carcinogenesis. In addition, homologous recombination is induced by a wide variety of carcinogens, many of which are classically considered to be efficiently repaired by other repair pathways. Overall, homologous recombination is a process that has been widely overlooked but may be more central to the process of carcinogenesis than previously described.

Animals↗
PubMed2001

Susceptibility of proliferating cells to benzo[a]pyrene-induced homologous recombination in mice.

The pink-eyed unstable mutation, p(un), is the result of a 70 kb tandem duplication within the murine pink-eyed, p, gene. Deletion of one copy of the duplicated region by homologous deletion/recombination occurs spontaneously in embryos and results in pigmented spots in the fur and eye. Such deletion events are inducible by a variety of DNA damaging agents, as we have observed previously with both fur- and eye-spot assays. Here we describe a study of the effect of exposure to benzo[a]pyrene (B[a]P) at different times of development on reversion induction in the eye. Previously we, among others, have reported that the retinal pigment epithelium (RPE) displays a position effect variegation phenotype in the pattern of pink-eyed unstable reversions. Following an acute exposure to B[a]P or X-rays on the tenth day of gestation an increased frequency of reversion events was detected in a distinct region of the adult RPE. Examining exposure at different times of eye development reveals that both B[a]P and X-rays result in an increased frequency of reversion events, though the increase was only significant following B[a]P exposure, similar to our previous report limited to exposure on the tenth day of gestation. Examination of B[a]P-exposed RPE in the present study revealed distinct regions where the induced events lie and that the positions of these regions are found at increasing distances from the optic nerve the later the time of exposure. This position effect directly reflects the previously observed developmental pattern of the RPE, namely that cells in the regions most distal from the optic nerve are proliferating most vigorously. The numbers and positions of RPE cells displaying the transformed (pigmented) phenotype strongly advocate the proposal that dividing cells are at highest risk to deletions induced by carcinogens.

Animals↗
PubMed2000

Benzo(a)pyrene and X-rays induce reversions of the pink-eyed unstable mutation in the retinal pigment epithelium of mice.

The pink-eyed unstable (p(un)) mutation is the result of a 70kb tandem duplication within the murine p gene. Homologous deletion/recombination of the locus to wild-type occurs spontaneously in embryos and results in pigmented spots in the fur and eye that persist for life. Such deletion events are also inducible by a variety of DNA damaging agents, as we have observed previously with the fur spot assay. Here, we describe the use of the retinal pigment epithelium (RPE) of the eye to detect reversion events induced with two differently acting agents. Benzo(a)pyrene (B(a)P) induces a high frequency, and X-ray exposure a more modest increase, of p(un) reversion in both the fur and the eye. The eye-spot assay requires fewer mice for significant results than the fur spot assay. Previous work had elucidated the cell proliferation pattern in the RPE and a position effect variegation phenotype in the pattern of p(un) reversions, which we have confirmed. Acute exposure to B(a)P or X-rays resulted in an increased frequency of reversion events. The majority of the spontaneous reversions lie toward the periphery of the RPE whereas induced events are found more centrally, closer to the optic nerve head. The induced distribution corresponds to the major sites of cell proliferation in the RPE at the time of exposure, and further advocates the proposal that dividing cells are at highest risk to develop deletions.

Animals↗
PubMed2000

Atm deficiency causes an increased frequency of intrachromosomal homologous recombination in mice.

Ataxia telangiectasia (AT) patients have inactivating mutations in both copies of the ATM gene. The ATM protein that the gene encodes is involved in DNA double-strand break (DSB) recognition; in its absence, p53 response to DSBs is delayed and reduced. In addition, AT patients have a high propensity for cancer, and cells from these patients show chromosomal instability. Here, using an in vivo mouse model system with the pink-eyed unstable mutation, we demonstrate that the absence of functional Atm results in a significantly elevated frequency of intrachromosomal recombination resulting in deletion events (wild-type 17.73%, heterozygous Atm 15.72%, and mutant Atm 30.33%). No such increase was observed in mice heterozygous for Atm. These results further advocate the role of ATM in maintaining genomic integrity after the onset of endogenous damage. This system relies on the initiation of events during a relatively short time frame to produce an observable deletion product. AT patients have a lifelong exposure to endogenous damage and perhaps similarly acting external agents. Because 25% of our genome consists of repeated elements, genomic instability due to an increased level of homologous recombination between such repeats, as observed here, may contribute to carcinogenesis in AT patients.

Animals↗
PubMed2000

Minisatellite variants generated in yeast meiosis involve DNA removal during gene conversion.

Two yeast minisatellite alleles were cloned and inserted into a genetically defined interval in Saccharomyces cerevisiae. Analysis of flanking markers in combination with sequencing allowed the determination of the meiotic events that produced minisatellites with altered lengths. Tetrad analysis revealed that gene conversions, deletions, or complex combinations of both were involved in producing minisatellite variants. Similar changes were obtained following selection for nearby gene conversions or crossovers among random spores. The largest class of events involving the minisatellite was a 3:1 segregation of parental-size alleles, a class that would have been missed in all previous studies of minisatellites. Comparison of the sequences of the parental and novel alleles revealed that DNA must have been removed from the recipient array while a newly synthesized copy of donor array sequences was inserted. The length of inserted sequences did not appear to be constrained by the length of DNA that was removed. In cases where one or both sides of the insertion could be determined, the insertion endpoints were consistent with the suggestion that the event was mediated by alignment of homologous stretches of donor/recipient DNA.

Alleles↗
PubMed2000

Homologous recombination as a mechanism for genome rearrangements: environmental and genetic effects.

Novel findings over the last 2 years have led to an increased emphasis on homologous recombination (HR) as both a pathway for DNA repair and a cause for genomic rearrangements. Indeed, environmental carcinogens increase the frequency of HR, as can be observed when two copies of a duplicated sequence recombine to delete the intervening sequences. Such HR events between dispersed homologous sequences may result in not only deletions, but also gene duplications or translocations. These types of genomic rearrangement have been observed to be the cause of several different genetic diseases, including cancer. In reflection of this, several genes have been identified that, when mutant, predispose an individual to an increased frequency of cancer. These genes have been shown to be either directly or indirectly involved in HR. In addition, HR is induced by a wide variety of carcinogens, preferentially in proliferating cells. This fits the most current models of recombination and its involvement in reinitiating stalled replication forks. Thus, 'correct' HR repair may act with high fidelity, an important issue for proliferating cells, but in the context of alternative homologous partner sequences, 'aberrant' HR can cause genomic rearrangements with dire consequences.

Animals↗
PubMed1999

Involvement of p53 in X-ray induced intrachromosomal recombination in mice.

The tumor suppressor gene Trp53 (also known as p53) is the most frequently mutated gene in human cancers. p53 is induced in response to DNA damage and effects a G(1) cell cycle arrest. It is believed that p53 plays a key role in maintaining genomic integrity following exposure to DNA-damaging agents. We determined the frequency of spontaneous and DNA damage-induced homologous intrachromosomal recombination in p53-deficient mouse embryos. Homologous intrachromosomal recombination events resulting in deletions at the pink eyed unstable (p(un)) locus result in reversion to the p gene. Reversions occurring in embryonic premelanocytes give rise to black spots on the gray fur of the offspring. Pregnant C57BL/6J p(un)/p(un) p53(+/-) mice were exposed to X-rays (1 Gy) or administered benzo¿apyrene (B¿aP; 30 or 150 mg/kg i.p.) 10 days after conception. Frequencies of spontaneous p(un) reversions in p53(-/-) and p53(+/-) animals were not significantly different compared with their wild-type littermates. X-ray treatment increased the recombination frequency in wild-type and p53(+/-), but surprisingly not in p53(-/-) offspring. In contrast, B¿aP treatment caused a dose-dependent increase in p(un) reversion frequencies in all three genotypes. Western blot analysis of embryos indicated that p53 protein levels increased approximately 3-fold following X-ray treatment, while B¿aP had no effect on p53 expression. These results are in agreement with the proposal that p53 is involved in the DNA damage response following X-ray exposure and suggest that X-ray-induced double-strand breaks are processed differently in p53(-/-) animals.

Animals↗
PubMed1998

Validation of a new intraoperative technique to evaluate load-independent indices of right ventricular performance in patients undergoing cardiac operations.

BACKGROUND: Assessment of right ventricular performance in the perioperative period is difficult because there is no generally accepted method of measuring right ventricular volume. We set out to determine whether conductance technology could provide a valuable technique for the investigation of intraoperative right ventricular function. METHODS AND RESULTS: Three validating studies were performed in 25 patients undergoing routine coronary revascularization. Study 1: The influence of conductance catheter position in the right ventricle was examined in 10 patients. Insertion of the conductance catheter through the outflow tract was associated with a larger gain constant and a smaller parallel conductance compared with insertion through the tricuspid valve. Study 2: The reproducibility of contractility measurements with the use of a conductance catheter was examined in 7 additional patients. Removal and reinsertion of the conductance catheter was not associated with any significant difference in right ventricular volume or contractile function. Study 3: Right ventricular performance before and after cardiopulmonary bypass was compared in 8 additional patients. There was a fall in the slope of the right ventricular preload recruitable stroke work from 15.6 (3.8) to 11.0 (5.1) mm Hg (P=.01) and an increase in the slope of the end-diastolic pressure-volume relations from 0.05 (0.02) to 0.11 (0.05) mm Hg/mL (P=.001). CONCLUSIONS: The conductance technique can be used to study perioperative changes in right ventricular performance. Insertion of the conductance catheter through the outflow tract provides stable and reproducible data. There is significant impairment of right ventricular contractility in the early postoperative period.

Cardiac Catheterization↗
PubMed1997

Left ventricular parallel conductance during cardiac cycle in children with congenital heart disease.

This study examines the accuracy of the conductance catheter technique and, in particular, parallel conductance [expressed as offset volume (Vc)] changes during the cardiac cycle in the human left ventricle. Two groups of patients were assessed: group 1, with an open atrial septal defect, and group 2, with an interventricular communication. In a subgroup, pre- and postoperative data were compared to assess the possible impact of shunting or anatomic considerations on our measurements. Vc is normally obtained by a saline-dilution technique previously described by Baan et al. [Vc(Baan); J. Baan, E. T. Van der velde, H. G. Debruin, G. J. Smeenk, J. Koops, A. D. Van Dijk, D. Temmerman, P. J. Senden, and B. Buis. Circulation 70: 812-823, 1984]. This does not take into account potential changes during the cardiac cycle. Four cardiac cycles were taken from the hypertonic saline washin and were divided into six equal isochrones between the maximum and minimum first derivatives of left ventricular pressure (dP/dtmax and dP/dtmin, respectively). The apparent ventricular volume was regressed against stroke volume for the corresponding cardiac cycle. The volume at the gamma-intercept corresponds to the Vc at each time interval [Vc(t)]. In group 1, there was a variation in Vc(t) during systole, but the temporal changes were quite small, on the order of 4.28% (SD = 5.18%) of total corrected end-diastolic volume (mean maximal variation of 2.60 ml). Furthermore, the value of Vc obtained at dP/dtmax was not significantly different from that obtained at dP/dtmin. For group 2 as a whole, mean Vc(Baan) did not change significantly with ventricular septal defect closure (preoperative, 8.85 +/- 11.1 ml; postoperative, 9.82 +/- 11.84 ml). Group 2 children also exhibited a systolic cyclical variation in Vc(t) similar to group 1. Finally, Vc(t) as a percentage of end-diastolic volume was no different when group 1 and group 2 were compared. We conclude that in the left ventricle, even in the presence of a left-to-right shunt, there is a small but insignificant difference in parallel conductance during ventricular ejection. The magnitude of this cyclical change does not preclude ventricular volume measurement in congenital heart disease by the conductance catheter technique.

Child↗
PubMed1996

Does parallel conductance vary during systole in the human right ventricle?

OBJECTIVES: Right ventricular (RV) contractile performance remains poorly characterised, particularly in humans. Conductance catheter techniques have the potential to overcome the geometric difficulties in RV volume measurement that have hindered systematic studies of RV pressure volume relations. The present study examines changes in parallel conductance (Vc) that may occur during the cardiac cycle in the human right ventricle. METHODS: Using signals obtained from custom-built conductance catheters, six isochronal systolic values of Vc (Vc(t)) were measured during hypertonic saline wash-in. Studies were performed in nine patients undergoing right heart catheterisation. Their ages ranged from 7 to 39 years (median = 16) and their weights ranged from 20.3 to 84.7 kg (median = 50.0 kg). Measurements of mean Vc and isochronal Vc(t) and its variability during systole were assessed. Mean Vc was measured using the Baan technique (Vc(Baan)), Vc(t) was measured from six systolic isochrones obtained during the same period of hypertonic saline wash-in. RESULTS: The temporal changes in Vc(t) were small (mean 5.8%, median = 4.4%, range = 0.6-17.9%) of total corrected end-diastolic volume (mean maximal variation of 7.7 ml). The value of Vc(t) obtained at dp/dtmax (mean = 99.1 ml; median = 104.75 ml; range 20.15-196.7 ml) was not significantly different to that obtained at dp/dtmin (mean = 100.0 ml; median = 110.87 ml; range = 20.0-204.2 ml) (P > 0.05), but both were higher than the single Vc measurement (Vc(Baan)) obtained using the standard approach (P = 0.02). The correlation between Vc(Baan) and Vc(t) for group data; (Vc(Baan) = 89.69 ml, s.d. = 43.73 ml; Vc(t) = 98.16 ml, s.d. = 50.16 ml) produces a regression slope of 0.99 for all studies (P = 0.02). CONCLUSION: We conclude that parallel conductance does vary during systole in the human right ventricle of adults and older children after repair of congenital abnormalities but there is no significant difference in Vc(t) at dp/dtmin and dp/dtmax. However, there was a significant difference when the isochronal Vc(t) measurement is compared with the standard single value technique (Vc(Baan)) obtained using the hypertonic saline wash-in method. The excellent correlation between Vc(t) and Vc(Baan) suggests that the correction of Vc for the phase of the cardiac cycle is unnecessary for most purposes when studying the human right ventricle.

Adolescent↗
PubMed1995

The determination of volume of right ventricular casts using a conductance catheter.

Conductance catheters were used to assess the volume of right ventricular models made from post-mortem casts. Models were made from silastic rubber and filled with saline. The correlation between volume assessed by conductance catheter and real volume was investigated as saline was withdrawn from, and injected into, five models. The mean regression between real and conductance-derived volume was 1.05 (SE 0.04 mean R2 0.94 SE 0.02) with Y-intercept -0.25 ml SE 1.72. Parallel wall conductance was assessed through the tricuspid orifice sealed with human tricuspid tissue, or left open, and found to be negligible. There was no difference between volumes measured by a catheter through either the pulmonary or tricuspid orifice. We conclude that conductance catheters are suitable for the assessment of right ventricular volume, despite the complex geometry.

Adolescent↗
PubMed1991

Inherited platelet-storage pool deficiency associated with a high incidence of acute myeloid leukaemia.

A family with an inherited bleeding disorder extending over four generations, and multiple cases of myeloblastic and myelomonoblastic leukaemia was studied. Ten members of the family had, by history, a haemorrhagic diathesis. There were three documented cases of myeloblastic leukaemia, two documented cases of myelomonoblastic leukaemia and two more cases of leukaemia by history. In four of the cases the bleeding diathesis clearly antedated the leukaemia, in two by many years. The bleeding disorder is characterized by a long bleeding time, abnormal platelet aggregation, low platelet ADP and decreased numbers of platelet dense bodies consistent with a dense granule storage pool deficiency. The number of dense granules was decreased by immunofluorescence employing quinacrine or using an antibody to the dense granule membrane protein, granulophysin, confirming an absolute decrease in dense granule numbers rather than the presence of empty granule sacs. This congenital storage pool deficiency is associated with a high incidence of acute myeloid leukaemia in this family.

Acute Disease↗
PubMed1989

A reassessment of the bleeding time: association of age, hematocrit, platelet function, von Willebrand factor, and bleeding time thromboxane B2 with the length of the bleeding time.

In order to provide an overview of the relative contribution of platelet, von Willebrand factor, and other abnormalities to patients with clinical bleeding difficulties, we performed a retrospective survey of coagulation studies on 569 individuals referred to the University of Manitoba coagulation laboratory because they, or a closely related family member, showed clinical evidence of a bleeding disorder. There was a highly significant (p less than 0.001) negative correlation between the bleeding time and each of the following parameters: the platelet count; the hematocrit; the percent aggregation to collagen, epinephrine, ADP, and arachidonic acid; and the logarithm of von Willebrand factor antigen and a measure of its activity (ristocetin cofactor). A significant and independent inverse relationship between the length of the bleeding time and the extent of platelet adhesion to glass beads, patient age, and prothrombin consumption were also observed. Multivariate analysis of the ability of all parameters to predict the bleeding time showed an r2 of only 0.33. Bleeding time thromboxane B2, in a second smaller study of 70 patients, showed a negative correlation with the length of the bleeding time (p = 0.0001), and, when used together with the above parameters, significantly enhanced the ability to predict the length of the bleeding time (r2 = 0.55). Defects in platelet function, as measured in vitro, and significant enough to have an effect on the bleeding time, occurred with greater frequency than defects in von Willebrand factor in the Manitoba patients evaluated.

Adolescent↗
PubMed1988

Increased nausea and vomiting induced by naloxone in patients receiving cancer chemotherapy.

To evaluate the role of endogenous opiates in chemo-therapy-induced nausea and vomiting, the narcotic-antagonist naloxone was administered to six pediatric patients receiving cancer chemotherapy. Naloxone was administered by continuous intravenous (i.v.) infusion after randomized, double-blind controlled assignment at a dose of 0, 10 or 40 micrograms/kg/h for 12 h. Each patient was studied for four consecutive and identical courses of chemotherapy (eight courses for each naloxone dose or 24 courses in all). A dose-related increase in nausea (nausea score 2.5 +/- 2.24, 3.83 +/- 2.73, and 5.75 +/- 2.86/12 h, p = 0.003), vomiting (emetic events 6.0 +/- 7.50, 8.08 +/- 6.71, and 10.3 +/- 8.91/12 h, p = 0.035), and patient aversion (course preference rank 1.5 +/- 0.45, 2.83 +/- 1.17, and 3.25 +/- 0.42/4 courses, p = 0.014) was observed. The infusion of naloxone in the absence of chemotherapy was without effect. These results support a role for endogenous opiates in regulating chemotherapy-induced nausea and vomiting, and further suggest that narcotic agents may be effective antiemetics in this setting.

Adolescent↗
PubMed1985

Absent factor VIII response to synthetic vasopressin analogue (DDAVP) in nephrogenic diabetes insipidus.

To study the effect of 1-deamino-8D-arginine vasopressin (DDAVP) on the factor VIII response in nephrogenic diabetes insipidus (NDI), 0.30 microgram/kg DDAVP was given to 2 unrelated NDI patients, 3 obligate carriers, and 20 controls. Factor VIII coagulant activity (FVIIIC) and factor VIII related antigen (FVIIIR:Ag) responses were absent in both NDI patients and were decreased by approximately 50% in the carriers by comparison with controls. These results show that the vasopressin receptor defect in NDI is not confined to the kidney but is equally expressed in other tissues including the vascular endothelium and hepatic sinusoids, the respective sites of FVIIIR:Ag and FVIIIC production. A decreased factor VIII response may help in identifying carriers in families at risk.

Adolescent↗
PubMed1984

Shortening of bleeding time by 1-deamino-8-D-arginine vasopressin in various bleeding disorders.

To evaluate the effect of 1-deamino-8-D-arginine vasopressin (DDAVP) in various bleeding disorders, 10 micrograms/m2 DDAVP was administered to subjects with von Willebrand disease (13), platelet function defects (12), von Willebrand disease and platelet defects together (8), or isolated prolongation of the bleeding time (5). DDAVP shortened the bleeding time similarly in all patient groups. Shortening of the bleeding time was also observed in 2 patients with aspirin-induced platelet defects and in 2 normal subjects. DDAVP administration was associated with falls in the platelet count, mean platelet volume, and partial thromboplastin time, and rises in platelet adhesion, factor VIII coagulant activity, factor VIII related antigen, and von Willebrand factor activity. The basal bleeding time was the only predictor of the magnitude of the bleeding-time correction. Normal haemostatis was achieved with DDAVP plus epsilon-aminocaproic acid and no blood product support during operations in 18 patients with bleeding disorders.

Adolescent↗