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Biomedical subjects

L A Cox

Publications and source records attributed to L A Cox.

At least 19 recordsLinked to original sources
PubMed1998

Molecular basis of an apolipoprotein[a] null allele: a splice site mutation is associated with deletion of a single exon.

Apolipoprotein[a] (apo[a]), a unique component of atherogenic lipoprotein[a], is highly polymorphic in human and nonhuman primates. Null alleles, producing no detectable circulating Lp[a] or apo[a] isoforms, are found at high frequencies. The molecular basis of null alleles is not yet known. In baboons, approximately two-thirds of null alleles do not produce detectable hepatic transcripts (transcript negative nulls), and one-third of null alleles produce normal amounts of apo[a] transcripts (transcript positive nulls). We have cloned apo[a] cDNA from a baboon carrying a transcript positive null allele defective in secretion from primary hepatocytes. Compared with wild-type cDNA, the null allele contained an in-frame 47 amino acid deletion in the protease domain corresponding to one exon of the apo[a] gene. The null allele contains an A-->T substitution in the third nucleotide position of the intron downstream of the deleted exon which alters the donor splice site consensus sequence. Thus, this null is likely due to a mutation that prevents normal mRNA splicing, yielding a shortened protein that may be defective in intramolecular interactions required for normal processing and secretion of apo[a]. This is the first report of a molecular basis for apo[a] null alleles.

Alleles↗
PubMed1997

Does diesel exhaust cause human lung cancer?

Recent reviews of epidemiological evidence on the relation between exposure to diesel exhaust (DE) and lung cancer risk have reached conflicting conclusions, ranging from belief that there is sufficient evidence to conclude that DE is a human lung carcinogen (California EPA, 1994) to conclusions that there is inadequate evidence to support a causal association between DE and human lung cancer (Muscat and Wynder, 1995). Individual studies also conflict, with both increases and decreases in relative risks of lung cancer mortality being cited with 95% statistical confidence. On balance, reports of elevated risk outnumber reports of reduced risk. This paper reexamines the evidence linking DE exposures to lung cancer risk. After briefly reviewing animal data and biological mechanisms, it surveys the relevant epidemiological literature and examines possible explanations for the discrepancies. These explanations emphasize the distinction between statistical associations, which have been found in many studies, and causal associations, which appear not to have been established. Methodological threats to valid causal inference are identified and new approaches for controlling them are proposed using recent techniques from artificial intelligence (AI) and computational statistics. These threats have not been adequately controlled for in previous epidemiological studies. They provide plausible noncausal explanations for the reported increases in relative risks, making it impossible to infer causality between DE exposure and lung cancer risk from these studies. A key contribution is to show how recent techniques developed in the AI-and-statistics literature can help clarify the causal interpretation of complex multivariate data sets used in epidemiological risk assessments. Applied to the key study of Garshick et al. (1988), these methods show that DE concentration has no positive causal association with occupational lung cancer mortality risk.

Animals↗
PubMed1997

The biology of bone maturation and ageing.

The only indicator of development that is available from birth to maturity is skeletal age. This short review discusses how standard bone ages have been developed from assessment of radiographs, and describes the advantages and disadvantages of the 'atlas' approach as developed by Greulich and Pyle, and the bone by bone approach, as developed by Tanner. As the standards currently available are based mainly on historical series of radiographs from particular populations, it is stressed that national standards should be established and updated regularly if bone ages are to be used to assess development. The question of the clinical relevance of using bone age assessments of the hand and wrist to determine the state maturation of the whole skeleton and particularly, the growth potential is also discussed. It is concluded that, despite the difficulties of assessing bone age, and the assumptions on which the various methods are based, determination of skeletal development is clinically relevant in that it provides the only means of assessing rates of maturational change throughout the growing period.

Age Determination by Skeleton↗
PubMed1996

Chronic inhalation oncogenicity study of isoprene in B6C3F1 mice.

The oncogenic potential of isoprene as affected by concentration, length of daily exposure, and weeks of exposure over the life-span of the animal, as independent variables, was evaluated. Ten groups were exposed for 8 h/day, 5 days/week as follows (ppm-weeks): 0-80, 10-80, 70-40, 70-80, 140-40, 280-20, 280-80, 700-80, 2200-40, 2200-80. Two groups were exposed for 4 h/day: 2200-20, 2200-80. Groups were held until 96 or 105 weeks on study. The concentration x time (duration of exposure) values provided a series of theoretically equivalent exposure hazards. There was an exposure-related increased incidence of liver, lung, Harderian gland and forestomach tumors, and hemangiosarcomas and histiocytic sarcomas. The LOEL appeared to be 70 ppm. These results are similar to the profile of tumors seen in 1,3-butadiene (BD)-exposed mice without the early onset of T-cell lymphoma as seen with BD. Isoprene appears to be about one order of magnitude less potent than BD in mice. Statistical analyses indicated that the product of isoprene concentration, and length/duration of exposure was not a sufficient basis for predicting tumor risk at any site. Extrapolation of tumor probability between the high and low doses based on cumulative exposure was not appropriate and could not be justified by statistical models. A threshold effect level and strong nonlinearities with respect to concentration appeared to exist for tumor development in this study.

Administration, Inhalation↗
PubMed1996

Isoprene cancer risk and the time pattern of dose administration.

Most statistical risk assessment models assume that equal, measured on scale such as mg/kg/day, create equal tumor risks. This equivalent dose metric (EDM) hypothesis allows risks to be extrapolated from high concentrations to low-concentrations and from and species, sex, and strain to another, since it implies that all administered dose histories corresponding to the same total dose create the same risk. This paper tests the EDM hypothesis using data on tumor rates in B6C3F mice administered isoprene via inhalation. Its major conclusion is that the EDM hypothesis does not hold for isoprene. For example, it appears that exposure concentration has a greater impact on tumor rates than weeks of exposure. To predict tumor probabilities, the time pattern of dose administration must be considered. The asymmetric effects of concentration, hours-per-day, and number of days of exposure on tumor risks imply that complex dynamic risk models may be needed to accurately describe dose-time-response relations. The traditional concept of a dose-response relation as a static curve relating a numerical summary of dose to a numerical summary of response probability is probably not predictively useful for chemicals such as isoprene, and extrapolations of risk based on the EDM hypothesis could be misleading for such chemicals.

Animals↗
PubMed1996

Tanner-Whitehouse method of assessing skeletal maturity: problems and common errors.

The Tanner-Whitehouse method of assessing skeletal maturity has been in use for more than three decades. Problems in assigning bone ages arise from many sources. The most significant sources of error are poor positioning of the hand when the radiograph is taken, which alters the radiographic appearance of the epiphysis and makes interpretation unnecessarily difficult, and the lack of consistency in repeat ratings of the same film by one or more observers (intra- and inter-observer error). In addition, use of the system outside the limits of its design is a common error seen in clinical practice. Computer systems are now being developed with the aim of reducing many of the inconsistencies associated with radiographic investigations of normal children.

Age Determination by Skeleton↗
PubMed1996

Reassessing benzene risks using internal doses and Monte-Carlo uncertainty analysis.

Human cancer risks from benzene have been estimated from epidemiological data, with supporting evidence from animal bioassay data. This article reexamines the animal-based risk assessments using physiologically based pharmacokinetic (PBPK) models of benzene metabolism in animals and humans. Internal doses (total benzene metabolites) from oral gavage experiments in mice are well predicted by the PBPK model. Both the data and the PBPK model outputs are also well described by a simple nonlinear (Michaelis-Menten) regression model, as previously used by Bailer and Hoel [Metabolite-based internal doses used in risk assessment of benzene. Environ Health Perspect 82:177-184 (1989)]. Refitting the multistage model family to internal doses changes the maximum-likelihood estimate (MLE) dose-response curve for mice from linear-quadratic to purely cubic, so that low-dose risk estimates are smaller than in previous risk assessments. In contrast to Bailer and Hoel's findings using interspecies dose conversion, the use of internal dose estimates for humans from a PBPK model reduces estimated human risks at low doses. Sensitivity analyses suggest that the finding of a nonlinear MLE dose-response curve at low doses is robust to changes in internal dose definitions and more consistent with epidemiological data than earlier risk models. A Monte-Carlo uncertainty analysis based on maximum-entropy probabilities and Bayesian conditioning is used to develop an entire probability distribution for the true but unknown dose-response function. This allows the probability of a positive low-dose slope to be quantified: It is about 10%. An upper 95% confidence limit on the low-dose slope of excess risk is also obtained directly from the posterior distribution and is similar to previous q1* values. This approach suggests that the excess risk due to benzene exposure may be nonexistent (or even negative) at sufficiently low doses. Two types of biological information about benzene effects--pharmacokinetic and hematotoxic--are examined to test the plausibility of this finding. A framework for incorporating causally relevant biological information into benzene risk assessment is introduced, and it is shown that both pharmacokinetic and hematotoxic models appear to be consistent with the hypothesis that sufficiently low concentrations of inhaled benzene do not create and excess risk.

Administration, Oral↗
PubMed1995

The 5'-untranslated region of human transferrin mRNA, which contains a putative iron-regulatory element, is bound by purified iron-regulatory protein in a sequence-specific manner.

Human transferrin mRNA contains a 5'-untranslated region that (1) has homology to an iron responsive element and (2) is implicated in translational iron regulation of human transferrin transgenes in transgenic mice. Ferritin mRNA contains a 5'-untranslated region iron-responsive element, but iron regulation of ferritin differs from that of human transferrin transgenes in both magnitude and direction. Structural differences between the ferritin iron-responsive element and the human transferrin putative iron-responsive element may influence their iron-regulatory protein interactions and direct the differing translational responses. This study examines human transferrin RNA nucleotide sequence requirements for binding of cytoplasmic proteins and purified iron-regulatory protein. Mutations of the putative transferrin iron-responsive element similarly affected binding of purified iron-regulatory protein and liver cytoplasmic proteins, providing evidence that the IRP is one of the liver cytoplasmic proteins that binds the human transferrin iron-regulatory element and suggesting that it may be involved in iron-regulation of transferrin.

Animals↗
PubMed1995

Simple relations between administered and internal doses in compartmental flow models.

It is sometimes argued that the use of increasingly complex "biologically-based" risk assessment (BBRA) models to capture increasing mechanistic understanding of carcinogenic processes may run into a practical barrier that cannot be overcome in the near term: the need for unrealistically large amounts of data about pharmacokinetic and pharmacodynamic parameters. This paper shows that, for a class of dynamical models widely used in biologically-based risk assessments, it is unnecessary to estimate the values of the individual parameters. Instead, the input-output properties of such a model--specifically, the ratio of the area-under-curve (AUC) for any selected output to the AUC of the input--is determined by a single aggregate "reduced" constant, which can be estimated from measured input and output quantities. Uncertainties about the many individual parameter values of the model, and even uncertainties about its internal structure, are irrelevant for purposes of quantifying and extrapolating its input-output (e.g., dose-response) behavior. We prove that this is the case for the class of linear, constant-coefficient, globally stable compartmental flow systems used in many classical pharmacokinetic and low-dose PBPK models. Examples are cited that suggest that the value of the reduced parameter representing such a system's aggregate behavior may be relatively insensitive to changes in (and hence to uncertainties about) the values of individual parameters. The theory is illustrated with a model of pharmacokinetics and metabolism of cyclophosphamide (CP), a drug widely used in cancer chemotherapy and as an immunosuppressive agent.

Algorithms↗
PubMed1995

An exact analysis of the multistage model explaining dose-response concavity.

The traditional multistage (MS) model of carcinogenesis implies several empirically testable properties for dose-response functions. These include convex (linear or upward-curving) cumulative hazards as a function of dose; symmetric effects on lifetime tumor probability of transition rates at different stages; cumulative hazard functions that increase without bound as stage-specific transition rates increase without bound; and identical tumor probabilities for individuals with identical parameters and exposures. However, for at least some chemicals, cumulative hazards are not convex functions of dose. This paper shows that none of these predicted properties is implied by the mechanistic assumptions of the MS model itself. Instead, they arise from the simplifying "rare-tumor" approximations made in the usual mathematical analysis of the model. An alternative exact probabilistic analysis of the MS model with only two stages is presented, both for the usual case where a carcinogen acts on both stages simultaneously, and also for idealized initiation-promotion experiments in which one stage at a time is affected. The exact two-stage model successfully fits bioassay data for chemicals (e.g., 1,3-butadiene) with concave cumulative hazard functions that are not well-described by the traditional MS model. Qualitative properties of the exact two-stage model are described and illustrated by least-squares fits to several real datasets. The major contribution is to show that properties of the traditional MS model family that appear to be inconsistent with empirical data for some chemicals can be explained easily if an exact, rather than an approximate model, is used. This suggests that it may be worth using the exact model in cases where tumor rates are not negligible (e.g., in which they exceed 10%). This includes the majority of bioassay experiments currently being performed.

Algorithms↗
PubMed1994

Dual roles of the retinoblastoma protein in cell cycle regulation and neuron differentiation.

To assess the functions of the retinoblastoma protein (RB) during normal development, we have analyzed mouse embryos that lack a functional copy of the retinoblastoma gene (genotype: Rb-1 delta 20/Rb-1 delta 20). Our findings demonstrate that RB plays an important role in the regulation of the neuronal cell cycle. In mutant embryos, dividing cells are found well outside of the normal neurogenic regions in both the central and peripheral nervous systems. In addition to abnormal cell cycle regulation, however, the mutant embryos show two less expected phenotypes. First, many of the ectopically dividing cells die by apoptosis shortly after their entrance into S phase. In sensory ganglia, most nerve cells die by this process, beginning at about the same time as normal target-related neuronal death. Second, although the expression of certain differentiation markers such as N-CAM and Brn-3.0 appears to be near normal, nerve cells, especially in sensory ganglia, do not mature properly. Their morphology is stunted and expression of neuronal beta II tubulin is greatly reduced. Preferential reduction in the expression of TrkA, TrkB, and the low-affinity neurotrophin receptor p75LNGFR may be relevant to neuronal cell death and lack of neuronal differentiation seen in the mutant embryos. Primary cultures of dorsal root and trigeminal ganglion cells from later stage mutant embryos reveal a decrease in neuronal cell survival and in neurite outgrowth even in the presence of the appropriate neurotrophins. Taken together, these results suggest that the p110RB protein not only regulates progression through the cell cycle but is also important for cell survival and differentiation.

Animals↗
PubMed1994

Tumor suppressor genes and their roles in breast cancer.

Tumor suppressor genes have been identified by the occurrence of mutations in many families with hereditary forms of cancer, exposed during development of the tumor by loss of heterozygosity. They have a number of diverse functions. For example, both the RB gene of retinoblastoma and the p53 gene, which is commonly mutated in breast and colon cancer among others, produce proteins involved in distinct steps of cell cycle control, while the nm23 product prevents metastasis. Here we review the data developed until now on the possible presence and role of mutations in these and other tumor suppressor genes in breast cancer. A more complete understanding of the tumor suppressor genes could not only provide diagnostic information, but could lead to specific gene therapy to replace suppressor functions lost in individual tumors.

Animals↗
PubMed1994

Assessment of bone ages by the Tanner-Whitehouse method using a computer-aided system.

A computer-aided system to estimate bone age based on Fourier analysis was assessed by reference to the original radiographs used to produce the Tanner-Whitehouse 2 (TW2) standards for the radius, ulna and short finger bones. The computer-aided system involved matching a template of each bone to the scanned image of the radiograph. The computer then generated a stage of bone maturity, individual and total bone scores and a value for bone age. The bone ages assessed by the computer-aided system were no different from the original TW2 reference values, indicating the applicability of the system. The system was used to assess the bone ages of tall Dutch girls, and the results obtained were compared with more traditional assessments made by an experienced rater. For the radiographs from the tall girls, there was good agreement for individual bones between this method and the traditional assessment by the rater, but less agreement for the total 13-bone score and bone age.

Adolescent↗
PubMed1994

Preliminary report on the validation of a grammar-based computer system for assessing skeletal maturity with the Tanner-Whitehouse 2 method.

A series of hand and wrist radiographs was assessed manually by two individuals and by a fully automated computer system for determining bone age. Assessments were repeated after 1 month to determine variability between and within the methods of bone age assessment. There was slight intra-observer variation, but complete reproducibility when assessments were made by computer. The variation between the human assessors was less than that between human and computer assessments. The difference between overall maturity scores made by the human observer and the computer system was, however, acceptably small, and the majority of assessments were the same. It is concluded this computer system for assessing bone age in normal children is reliable and accurate, but that it needs to be validated against a much larger set of radiographs.

Adolescent↗
PubMed1993

Posttranscriptional regulation of chimeric human transferrin genes by iron.

Transferrin, the transferrin receptor, and ferritin are integral to the body's management of iron, an element required for life but highly toxic when present in excess. The transferrin receptor and ferritin are regulated posttranscriptionally by iron: the transferrin receptor by mRNA stability and ferritin by mRNA translation. Results described here indicate that transferrin, like ferritin, is regulated by iron at the level of translation. Chimeric genes introduced into the mouse genome were composed of the human transferrin 5' regulatory region fused to the chloramphenicol acetyl transferase (CAT) reporter gene. Iron administration to transgenic mice resulted in a significant decrease of transferrin-directed CAT enzyme activity and CAT protein in liver, but no significant decrease in human transferrin-CAT mRNA levels. Binding of specific RNA iron regulatory elements by proteins in cytoplasmic extracts have been shown to regulate ferritin and transferrin receptor synthesis. Similar results have been obtained with transferrin mRNA. A decreased binding of human transferrin 5'-untranslated region RNA by factors in cytoplasmic extracts of livers from mice receiving iron was found when compared to extracts from control mice. A human transferrin RNA-protein complex migrated electrophoretically with the same mobility as a ferritin iron responsive element RNA-iron responsive element binding protein complex. The ferritin iron responsive element RNA also competed with the human transferrin 5'-untranslated region RNA-protein complexes formed and vice versa. Therefore, iron modulation of human transferrin may share a factor common or similar to that observed in ferritin and transferrin receptor iron modulation.

Animals↗
PubMed1993

Expression of chimeric human transferrin-chloramphenicol acetyltransferase genes in liver and brain of transgenic mice during development.

Transferrin (TF) gene expression is tissue specific and is regulated during development. Transgenic mice have been developed which carry 1.2 or 0.67 kb of the TF 5' flanking region of the human TF gene fused to the bacterial chloramphenicol acetyltransferase (CAT) gene. The onset of expression of the chimeric human TF-CAT transgenes in liver and brain during development has been studied in these transgenic mice. In brain, the TF(0.67)CAT transgene began to express between 5 and 10 days after birth; in liver, the TF(0.67)CAT transgene was turned on between 10 and 20 days after birth. Endogenous mouse TF mRNA levels in liver and brain have also been measured during development by Northern analysis. In brain, the developmental expression pattern of the TF(0.67)CAT transgene is the same as the mouse endogenous TF gene; in liver, the transgene is turned on later than the endogenous mouse TF gene. DNA-protein mobility shift assays and DNase I footprinting analyses were conducted in the region of -621 to -409 bp of the human TF gene by using TF-CAT expressing liver nuclear extract from 27-day-old mice and nonexpressing liver nuclear extract from 7-day-old mice. The level of protein-DNA complex formation is several times higher in the expressing extracts, and the region from -481 to -463 bp of human TF gene is protected by the expressing extract but not the nonexpressing extracts. As demonstrated by this and other studies, the transgenic mouse model furnishes a unique opportunity to analyze developmental regulation of human transgenes.

Animals↗