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Biomedical subjects

Roger M Harbord

Publications and source records attributed to Roger M Harbord.

6 recordsLinked to original sources
PubMed2006

Meta-analyses of observational and genetic association studies of folate intakes or levels and breast cancer risk.

BACKGROUND: Evidence from case-control studies suggests that increasing dietary folate intake is associated with a reduced risk of breast cancer. However, large cohort studies have found no such association, and animal studies suggest that folate supplementation may promote tumorigenesis. We conducted a meta-analysis to summarize the available evidence from observational studies on this issue and a meta-analysis of the association between a common polymorphism in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene, a key enzyme in folate metabolism, and breast cancer risk. METHODS: We searched Medline and ISI Web of Knowledge databases for relevant studies that were published through May 31, 2006. We used random-effects analysis to calculate odds ratios (ORs) for case-control studies or relative risks (RRs) for cohort studies for a 100-microg/d increase in folate intake. Unadjusted odds ratios were calculated for the studies of MTHFR genotype based on published genotype frequencies. RESULTS: A total of 13 case-control studies and nine cohort studies were included in the meta-analysis of folate intake and breast cancer risk. We found a summary OR of 0.91 (95% confidence interval [CI] = 0.87 to 0.96) from the case-control studies and a summary RR of 0.99 (95% CI = 0.98 to 1.01) from the cohort studies for a 100-microg/d increase in folate intake. We found evidence that the case-control studies may have suffered from substantial publication bias. The case-control and cohort studies may have been subject to measurement error, confounding, and possibly spurious associations arising from subgroup analyses; in addition, the case-control studies were potentially subject to recall bias and publication bias. Seventeen studies were included in the meta-analysis of MTHFR C677T genotype and breast cancer risk. We found no difference in breast cancer risk between MTHFR 677 TT homozygotes and CC homozygotes (OR = 1.05, 95% CI = 0.88 to 1.25), and there was no evidence of an interaction between folate intake and MTHFR genotype on breast cancer risk. CONCLUSION: A lack of dietary folate intake is not associated with the risk of breast cancer.

5,10-Methylenetetrahydrofolate Reductase (FADH2)↗
PubMed2006

A modified test for small-study effects in meta-analyses of controlled trials with binary endpoints.

Publication bias and related bias in meta-analysis is often examined by visually checking for asymmetry in funnel plots of treatment effect against its standard error. Formal statistical tests of funnel plot asymmetry have been proposed, but when applied to binary outcome data these can give false-positive rates that are higher than the nominal level in some situations (large treatment effects, or few events per trial, or all trials of similar sizes). We develop a modified linear regression test for funnel plot asymmetry based on the efficient score and its variance, Fisher's information. The performance of this test is compared to the other proposed tests in simulation analyses based on the characteristics of published controlled trials. When there is little or no between-trial heterogeneity, this modified test has a false-positive rate close to the nominal level while maintaining similar power to the original linear regression test ('Egger' test). When the degree of between-trial heterogeneity is large, none of the tests that have been proposed has uniformly good properties.

Endpoint Determination↗
PubMed2006

A unification of models for meta-analysis of diagnostic accuracy studies.

Studies of diagnostic accuracy require more sophisticated methods for their meta-analysis than studies of therapeutic interventions. A number of different, and apparently divergent, methods for meta-analysis of diagnostic studies have been proposed, including two alternative approaches that are statistically rigorous and allow for between-study variability: the hierarchical summary receiver operating characteristic (ROC) model (Rutter and Gatsonis, 2001) and bivariate random-effects meta-analysis (van Houwelingen and others, 1993), (van Houwelingen and others, 2002), (Reitsma and others, 2005). We show that these two models are very closely related, and define the circumstances in which they are identical. We discuss the different forms of summary model output suggested by the two approaches, including summary ROC curves, summary points, confidence regions, and prediction regions.

Diagnostic Tests, Routine↗
PubMed2005

C-reactive protein and its role in metabolic syndrome: mendelian randomisation study.

BACKGROUND: Circulating C-reactive protein (CRP) is associated with the metabolic syndrome and might be causally linked to it. Our aim was to generate estimates of the association between plasma CRP and metabolic syndrome phenotypes that were free from confounding and reverse causation, to assess the causal role of this protein. METHODS: We examined associations between serum CRP concentration and metabolic syndrome phenotypes in the British Women's Heart and Health Study. We then compared these estimates with those derived from a mendelian randomised framework with common CRP gene haplotypes to generate unconfounded and unbiased estimates of any causal associations. FINDINGS: In a sample of British women, body-mass index (BMI), systolic blood pressure, waist-to-hip ratio, serum concentrations of HDL cholesterol and triglycerides, and insulin resistance were all associated with plasma CRP concentration. CRP haplotypes were associated with plasma CRP concentration (p<0.0001). With instrumental variable analyses, there was no association between plasma CRP concentration and any of the metabolic syndrome phenotypes analysed. There was strong evidence that linear regression and mendelian randomisation based estimation gave conflicting results for the CRP-BMI association (p=0.0002), and some evidence of conflicting results for the association of CRP with the score for insulin resistance (p=0.0139), triglycerides (p=0.0313), and HDL cholesterol (p=0.0688). INTERPRETATION: Disparity between estimates of the association between plasma CRP and phenotypes comprising the metabolic syndrome derived from conventional analyses and those from a mendelian randomisation approach suggests that there is no causal association between CRP and the metabolic syndrome phenotypes.

Aged↗
PubMed2003

Likelihood-based estimation of microsatellite mutation rates.

Microsatellites are widely used in genetic analyses, many of which require reliable estimates of microsatellite mutation rates, yet the factors determining mutation rates are uncertain. The most straightforward and conclusive method by which to study mutation is direct observation of allele transmissions in parent-child pairs, and studies of this type suggest a positive, possibly exponential, relationship between mutation rate and allele size, together with a bias toward length increase. Except for microsatellites on the Y chromosome, however, previous analyses have not made full use of available data and may have introduced bias: mutations have been identified only where child genotypes could not be generated by transmission from parents' genotypes, so that the probability that a mutation is detected depends on the distribution of allele lengths and varies with allele length. We introduce a likelihood-based approach that has two key advantages over existing methods. First, we can make formal comparisons between competing models of microsatellite evolution; second, we obtain asymptotically unbiased and efficient parameter estimates. Application to data composed of 118,866 parent-offspring transmissions of AC microsatellites supports the hypothesis that mutation rate increases exponentially with microsatellite length, with a suggestion that contractions become more likely than expansions as length increases. This would lead to a stationary distribution for allele length maintained by mutational balance. There is no evidence that contractions and expansions differ in their step size distributions.

Alleles↗