Methodological approaches, challenges, and opportunities in the application of Mendelian randomisation to lifecourse epidemiology: A systematic literature review

Diseases diagnosed in adulthood may have antecedents throughout (including prenatal) life. Gaining a better understanding of how exposures at different stages in the lifecourse influence health outcomes is key to elucidating the potential benefits of disease prevention strategies. Mendelian randomisation (MR) is increasingly used to estimate causal effects of exposures across the lifecourse on later life outcomes. This systematic literature review explores MR methods used to perform lifecourse investigations and reviews previous work that has utilised MR to elucidate the effects of factors acting at different stages of the lifecourse. We conducted searches in PubMed, Embase, Medline and MedRXiv databases. Thirteen methodological studies were identified. Four studies focused on the impact of time-varying exposures in the interpretation of "standard" MR techniques, five presented methods for repeat measures of the same exposure, and four described methodological approaches to handling multigenerational exposures. A further 127 studies presented the results of an applied research question. Over half of these estimated effects in a single generation and were largely confined to the exploration of questions regarding body composition. The remaining mostly estimated maternal effects. There is a growing body of research focused on the development and application of MR methods to address lifecourse research questions. The underlying assumptions require careful consideration and the interpretation of results rely on select conditions. Whilst we do not advocate for a particular strategy, we encourage practitioners to make informed decisions on how to approach a research question in this field with a solid understanding of the limitations present and how these may be affected by the research question, modelling approach, instrument selection, and data availability.

EPCO-29. KLF5 IS REQUIRED FOR THE SEX-DIFFERENCES ACROSS MULTIPLE TUMORIGENIC PHENOTYPES IN PEDIATRIC GLIOBLASTOMA (GBM)

High-grade glioblastoma (GBM) is a highly heterogeneous, malignant cancer with no cure to date. A sex-biased incidence and therapeutic response is reported (1) for GBM with a male to female incidence ratio of 1.6:1 and female patients having a 4-6 month survival advantage (2) compared to male patients. In our current work, we sought to determine the molecular basis for the sex-bias in incidence and therapeutic response using an isogenic murine model of GBM with dual loss of neurofibromin 1 (NF1) and P53 (DNP53) in astrocytes. Our model yielded GBM from the murine astrocytes in a sex-biased manner, with male GBM cells exhibiting a highly significant proliferative, migratory, in-vivo tumorigenicity, stem-cell enrichment rate and therapeutic resistance compared to female GBM cells. Using transposon calling cards, we mapped Brd4 enhancer usage across the genome in male, compared to female GBM cells and identified key transcription factors that differentially enriched at male-biased, compared to female-biased Brd4 bound enhancers. Interestingly, female-biased Brd4-bound enhancers were enriched for transcription factors with tumor suppressor roles including P53 and SMAD4, whereas male-biased Brd4-bound enhancers were enriched for transcription factors with oncogenic roles, including Oct4, MYC and KLF5. Genetic or chemical silencing of KLF5 reduced the tumorigenic potential in male GBM with rates of cellular proliferation, survival, migration and stem-cell frequency significantly reduced to levels that resemble female GBM. Using transposon calling cards, we mapped KLF5 genomic localization in male and female GBM cells and identified significant peaks that affiliated with differentially expressed genes between the sexes. Interestingly, genes induced by KLF5 in the male cells were affiliated with poor survival in patients with various cancers, whereas those induced by KLF5 in the female cells were affiliated with favorable prognosis. This signifies the need to explore molecular mechanisms by which KLF5 drives these sex differences in GBM.

HGG-25. Targeting KLF5 reduces tumorigenic phenotype in a murine glioblastoma (GBM) model

Sex differences are evident in the incidence, therapeutic response and survival of patients with various cancers including the most common, aggressive and incurable GBM. We generated a murine GBM model of male and female astrocytes with dual loss of NF1 and P53 that yielded a sex-biased transformation of the astrocytes with male cells being significantly more tumorigenic and therapeutically resistant compared to female cells. In our current work, we aimed to delineate the molecular mechanisms driving this sex-biased tumorigenic phenotype in order to deliver therapies that are more effective to patients with GBM. We examined the inhibition of KLF5 on tumorigenic phenotypes using cellular bio-functional assays. We used barcoded transposon calling cards to determine the genomic localization of KLF5 and the differentially induced gene expression in male versus female GBM cells. Chemical inhibition or shRNA knock-down of KLF5 significantly reduced proliferation, migration, clonogenic stem-cell frequency and survival, but increase apoptosis in male and female GBM cells. Interestingly, male, but not female, GBM cells exhibited an increased migratory phenotype after radiation that inhibition of KLF5 significantly reduced. Moreover, KLF5 inhibition significantly reduced the protein expression of tumorigenic PDGFRB, AKT, ERK and the stem marker Sox-2. Transposon calling cards mapped unique KLF5 genomic localization in male versus female GBM cells with significantly differential gene expression profiles between the two. The top genes induced by KLF5 in male cells were primarily affiliated with poorer prognosis and reduced survival, whereas in female cells they were affiliated with better prognosis and improved survival in patients with cancer. Our findings provide a promising exploratory avenue for KLF5 as a therapeutic target in GBM and warrants further investigation in order to delineate the precise molecular mechanisms driving this antitumor response so that targeted therapy would be more effective taking into consideration the sex-differences in patients with GBM.

586Effects of maternal circulating amino acids on offspring birthweight: a Mendelian randomisation analysis

Background

It is suggested amino acids are critical for fetal growth, but analyses assessing causality are lacking. Mendelian randomisation (MR) can be used to examine causal effects under instrumental variable (IV) assumptions.

Methods

We conducted a two-sample MR study utilizing summary data from genome-wide association studies (GWAS) of amino acids (sample 1, n = 86,507) and of offspring birthweight (sample 2, combined UK Biobank and Early Growth Genetics Consortium, n = 406,063). Seventy-five independent single nucleotide polymorphisms (SNPs) robustly associated with 18 amino acids (p < 4.9 × 10-10) were used as genetic instruments. Wald ratio and inverse variance weighted methods were used in MR main analysis. Sensitivity analyses were performed to explore IV assumption violations. To explore whether there was consistency between SNP-amino acid associations in pregnancy and in the GWAS, the latter were compared to associations in the Born in Bradford cohort.

Results

There was evidence of positive causal effects of maternal alanine (51.9 g birthweight increase per SD increase in amino acid level, 95% CI: 24.2, 79.5), glutamine (51.3 g, 95% CI: 33.5, 69.0), glycine (10.4 g, 95% CI: 1.3, 19.6) and serine (27.1 g, 95% CI: 11.2, 43.0) on birthweight and inverse causal effects of maternal isoleucine (-109.7 g, 95% CI: -194.6, -24.9) and histidine (-41.1 g, 95% CI: -78.5, -3.7) on birthweight. Sensitivity analyses to explore reverse causality and bias due to horizontal pleiotropy supported our findings.

Conclusions

Some maternal circulating amino acids have causal effects on birthweight.

Key messages

MR can be extended to probe effects of maternal nutrition on offspring development.

Genome-wide association study identifies 48 common genetic variants associated with handedness

Handedness has been extensively studied because of its relationship with language and the over-representation of left-handers in some neurodevelopmental disorders. Using data from the UK Biobank, 23andMe and the International Handedness Consortium, we conducted a genome-wide association meta-analysis of handedness (N = 1,766,671). We found 41 loci associated (P < 5 × 10-8) with left-handedness and 7 associated with ambidexterity. Tissue-enrichment analysis implicated the CNS in the aetiology of handedness. Pathways including regulation of microtubules and brain morphology were also highlighted. We found suggestive positive genetic correlations between left-handedness and neuropsychiatric traits, including schizophrenia and bipolar disorder. Furthermore, the genetic correlation between left-handedness and ambidexterity is low (rG = 0.26), which implies that these traits are largely influenced by different genetic mechanisms. Our findings suggest that handedness is highly polygenic and that the genetic variants that predispose to left-handedness may underlie part of the association with some psychiatric disorders.

CBIO-22. p53 GAIN-OF-FUNCTION MUTATIONS DRIVE SEX SPECIFIC EFFECTS ON GLIOMA TUMORIGENESIS

Sex differences in malignant brain tumors are well-established: Males exhibit greater incidence and poorer survival. Understanding the biology behind these sex differences requires investigation of the pathways known to drive gliomagenesis. The transcription factor TP53 (p53) is one of the most commonly mutated genes in glioblastoma. Most p53 mutations are missense mutations in the DNA-binding domain that lead to the expression of a full length mutant p53 protein. These mutations can endow p53 with oncogenic gains-of-function through aberrant DNA binding and regulation of noncanonical cancer-promoting target genes. Previously, we analyzed patient mutation data and identified six p53 mutations with sex differences in prevalence. In this study, we developed an in vitro mutant p53 glioma model to investigate the sex specific effects of three p53 point mutations: R175H, Y205C, and Y220C (Mm R172H, Y202C, and Y217C respectively). Male and female astrocytes isolated from p53flox/- mouse pups were transduced with a retrovirus expressing mutant p53, followed by a lentivirus expressing CRE recombinase to remove the endogenous WTp53. We then assessed cell proliferation, clonogenicity, and in vivo tumorigenesis in these cells. All three mutations assayed displayed sex differences in proliferation, with male cells overexpressing p53:Y202C and p53:Y217C growing faster than female cells, and female cells overexpressing p53:R172H growing faster than male cells. Male Y202C and Y217C expressing astrocytes also exhibited a trend toward greater clonogenicity compared to female astrocytes. This observation is supported by higher expression of the stem cell markers SOX2 and NESTIN in the male cells. We performed parallel flank injections of male and female astrocytes expressing each mutation or p53 KO. Only male astrocytes expressing p53:Y202C or p53:Y217C and female astrocytes expressing p53:R172H mutation were able to form tumors in vivo. Together, these data support a sex specific gain-of-function phenotype for three different p53 mutations observed in glioma.

TMIC-34. SENESCENT ASTROCYTE INDUCED INFLAMMATION MAY UNDERLIE SEX DIFFERENCES IN THE AGE-DEPENDENT RISE IN GLIOBLASTOMA INCIDENCE

Sex differences in the rates of glioblastoma increase with age. There are likely to be multiple biological mechanisms underlying these sex differences. Here, we focused on the changes that occur with aging in the secretomes of male and female astrocytes to determine whether sex differences in chemokine and cytokine secretion, and inflammatory cell recruitment, could be one of those mechanisms. We focused on glioblastoma (GBM) and the role that senescent astrocytes might play in the age-dependent widening of the gap between male and female GBM cases. We found that the senescence associated secretory phenotype of male and female astrocytes significantly differed, notably in the enrichment of Fractalkine (1.33:1 (F:M)), the primary chemoattractant for microglia, in female compared to male SASP. This was in contrast to the greater abundance of tumorigenic growth factors like bFGF (1.25:1 (M:F)), in the male SASP. Implantation of either male or female senescent astrocytes into the brains of mice resulted in the recruitment of microglia to the injection site. Regardless of the recipient mouse sex, female astrocyte implantation resulted in significantly larger microglial infiltrates (2-fold) and greater astrocyte activation, as compared to the injection of equal numbers of male senescent cells. We propose a model for how sex differences in astrocyte SASP could result in lower rates of GBM with age in females: greater microglial attraction to senescent cells results in their enhanced clearance and consequently a reduction in senescence-associated GBM promotion.

Using Mendelian randomization to determine causal effects of maternal pregnancy (intrauterine) exposures on offspring outcomes: Sources of bias and methods for assessing them

Mendelian randomization (MR), the use of genetic variants as instrumental variables (IVs) to test causal effects, is increasingly used in aetiological epidemiology. Few of the methodological developments in MR have considered the specific situation of using genetic IVs to test the causal effect of exposures in pregnant women on postnatal offspring outcomes. In this paper, we describe specific ways in which the IV assumptions might be violated when MR is used to test such intrauterine effects. We highlight the importance of considering the extent to which there is overlap between genetic variants in offspring that influence their outcome with genetic variants used as IVs in their mothers. Where there is overlap, and particularly if it generates a strong association of maternal genetic IVs with offspring outcome via the offspring genotype, the exclusion restriction assumption of IV analyses will be violated. We recommend a set of analyses that ought to be considered when MR is used to address research questions concerned with intrauterine effects on post-natal offspring outcomes, and provide details of how these can be undertaken and interpreted. These additional analyses include the use of genetic data from offspring and fathers, examining associations using maternal non-transmitted alleles, and using simulated data in sensitivity analyses (for which we provide code). We explore the extent to which new methods that have been developed for exploring violation of the exclusion restriction assumption in the two-sample setting (MR-Egger and median based methods) might be used when exploring intrauterine effects in one-sample MR. We provide a list of recommendations that researchers should use when applying MR to test the effects of intrauterine exposures on postnatal offspring outcomes and use an illustrative example with real data to demonstrate how our recommendations can be applied and subsequent results appropriately interpreted.

Brief Report: Intestinal Dysbiosis in Ankylosing Spondylitis

OBJECTIVE

Ankylosing spondylitis (AS) is a common, highly heritable immune-mediated arthropathy that occurs in genetically susceptible individuals exposed to an unknown but likely ubiquitous environmental trigger. There is a close relationship between the gut and spondyloarthritis, as exemplified in patients with reactive arthritis, in whom a typically self-limiting arthropathy follows either a gastrointestinal or urogenital infection. Microbial involvement in AS has been suggested; however, no definitive link has been established. The aim of this study was to determine whether the gut in patients with AS carries a distinct microbial signature compared with that in the gut of healthy control subjects.

METHODS

Microbial profiles for terminal ileum biopsy specimens obtained from patients with recent-onset tumor necrosis factor antagonist-naive AS and from healthy control subjects were generated using culture-independent 16S ribosomal RNA gene sequencing and analysis techniques.

RESULTS

Our results showed that the terminal ileum microbial communities in patients with AS differ significantly (P < 0.001) from those in healthy control subjects, driven by a higher abundance of 5 families of bacteria (Lachnospiraceae [P = 0.001], Ruminococcaceae [P = 0.012], Rikenellaceae [P = 0.004], Porphyromonadaceae [P = 0.001], and Bacteroidaceae [P = 0.001]) and a decrease in the abundance of 2 families of bacteria (Veillonellaceae [P = 0.01] and Prevotellaceae [P = 0.004]).

CONCLUSION

We show evidence for a discrete microbial signature in the terminal ileum of patients with AS compared with healthy control subjects. The microbial composition was demonstrated to correlate with disease status, and greater differences were observed between disease groups than within disease groups. These results are consistent with the hypothesis that genes associated with AS act, at least in part, through effects on the gut microbiome.

Measurement of the Bs(0) → Ds-Ds+ and Bs(0) → D-Ds+ effective lifetimes

The first measurement of the effective lifetime of the B(s)(0) meson in the decay B(s)(0) → Ds-Ds+ is reported using a proton-proton collision data set, corresponding to an integrated luminosity of 3 fb(-1), collected by the LHCb experiment. The measured value of the B(s)(0) → Ds-Ds+ effective lifetime is 1.379 ± 0.026 ± 0.017 ps, where the uncertainties are statistical and systematic, respectively. This lifetime translates into a measurement of the decay width of the light B(s)(0) mass eigenstate of ΓL = 0.725 ± 0.014 ± 0.009 ps(-1). The B(s)(0) lifetime is also measured using the flavor-specific B(s)(0)→ D-Ds+ decay to be 1.52 ± 0.15 ± 0.01 ps.

The impact of breastfeeding on FTO-related BMI growth trajectories: an application to the Raine pregnancy cohort study

INTRODUCTION

For years, body mass index (BMI) has been used by scientists to track weight problems and obesity in children and adults. Recent studies have implicated the fat mass and obesity gene (FTO) in the increase of BMI in young adults. A longer duration of breastfeeding is known to reduce the risk of being overweight later in life, but its ability to modify the effect because of FTO is not known.

METHODS

We studied 1096 children from the Western Australian Pregnancy (Raine) cohort who were followed up from birth to 14 years of age. Linear mixed-effects models were used to investigate BMI growth trajectories in boys and girls separately.

RESULTS

An association was found between BMI growth and the duration of exclusive breastfeeding (EXBF) among carriers of the risk allele of the FTO SNP rs9939609. In girls, EXBF interacts with the SNP at baseline and can reverse the increase in BMI because of SNP risk allele by age 14 years after 3 months of EXBF. In boys, EXBF reduces BMI both in carriers and non-carriers of the risk allele with an association found after 10 years of age. Six months of EXBF will put the boys' BMI growth curves back to the normal range.

CONCLUSIONS

Our study could have major health implications by providing new perspectives for the prevention of growth problems in children carrying risk alleles in the FTO gene.

Functional haplotypes in the PTGDR gene fail to associate with asthma in two Australian populations

BACKGROUND AND OBJECTIVE

Haplotypes in the promoter region of the prostanoid DP receptor (PTGDR) gene have been shown to functionally influence gene transcription and to be associated with asthma in two previous case-control studies in Caucasians. This study tested the association of PTGDR haplotypes with asthma phenotypes in two large Caucasian-Australian populations. These results were incorporated in a meta-analysis with previously published data to determine the overall role for these haplotypes in the risk of asthma.

METHODS

Three PTGDR promoter-region single nucleotide polymorphisms (SNP) were genotyped in 368 individuals from the Western Australian Twin Child Health study and 2988 individuals from the Busselton Health Study. Logistic regression and transition disequilibrium tests were used to assess whether SNP genotypes and three SNP haplotypes were associated with doctor-diagnosed asthma or intermediate quantitative traits. Longitudinal data from the Busselton Health Study were used to examine whether PTGDR influences changes in lung function over time. Meta-analysis incorporated the findings of this study with those of two previous studies in Caucasian populations.

RESULTS

Cross-sectional associations between PTGDR haplotypes and asthma phenotypes were non-significant (P > 0.05) in both populations. Longitudinal analyses of PTGDR and lung function were also non-significant. Meta-analysis, however, suggested that haplotype TCT was significantly associated with decreased risk of asthma (OR = 0.76; P = 0.02) while haplotype CCC was not significantly associated with asthma (OR = 1.30; P = 0.07).

CONCLUSIONS

These results suggest that despite the non-significant findings in the present study populations, PTGDR promoter haplotypes may account for a small but significant proportion of the risk of asthma in Caucasian populations.

Bayesian methods for meta-analysis of causal relationships estimated using genetic instrumental variables

Genetic markers can be used as instrumental variables, in an analogous way to randomization in a clinical trial, to estimate the causal relationship between a phenotype and an outcome variable. Our purpose is to extend the existing methods for such Mendelian randomization studies to the context of multiple genetic markers measured in multiple studies, based on the analysis of individual participant data. First, for a single genetic marker in one study, we show that the usual ratio of coefficients approach can be reformulated as a regression with heterogeneous error in the explanatory variable. This can be implemented using a Bayesian approach, which is next extended to include multiple genetic markers. We then propose a hierarchical model for undertaking a meta-analysis of multiple studies, in which it is not necessary that the same genetic markers are measured in each study. This provides an overall estimate of the causal relationship between the phenotype and the outcome, and an assessment of its heterogeneity across studies. As an example, we estimate the causal relationship of blood concentrations of C-reactive protein on fibrinogen levels using data from 11 studies. These methods provide a flexible framework for efficient estimation of causal relationships derived from multiple studies. Issues discussed include weak instrument bias, analysis of binary outcome data such as disease risk, missing genetic data, and the use of haplotypes.

Apolipoprotein E genotype is associated with serum C-reactive protein but not abdominal aortic aneurysm

OBJECTIVE

Apolipoprotein E (ApoE) genotype has been associated with systemic inflammation and athero-thrombosis however the association with abdominal aortic aneurysm (AAA) has not been previously examined. We assessed the association between ApoE genotype with AAA presence and growth, and serum C-reactive protein (CRP).

METHODS

Serum concentrations of CRP (in 1358 men) and 6 single nucleotide polymorphisms (SNPs) for ApoE (in 1711 men) were examined in subjects from the Health In Men Study. 640 men with small AAAs were followed by ultrasound surveillance for a mean of 4.1 years.

RESULTS

There was no association between ApoE genotype and AAA presence. Men heterozygote for the ApoE p.Arg176Cys polymorphism had slower AAA growth, odds ratio for AAA progression> or =median 0.41, 95% confidence intervals 0.21-0.80, p=0.01. Men heterozygote for the ApoE g.50093756A>G polymorphism had slightly more rapid AAA growth, odds ratio for AAA progression> or =median 1.48, 95% confidence intervals 1.02-2.14, p=0.04. None of the ApoE SNPs were associated with AAA growth however taking into account multiple testing. Two SNPs in ApoE were associated with serum CRP under a co-dominant model, ApoE p.Cys130Arg (SNP ID rs429358), p=0.00003 and ApoE g.50114786A>G (SNP ID rs4420638), p=0.00013. Adjusting for other risk factors plus serum creatinine the varepsilon4 allele was associated with lower serum CRP under a dominant model, coefficient 0.089, p=0.002.

CONCLUSION

We found no consistent association between ApoE genotype and AAA. We confirmed an association between ApoE genotype and serum CRP.

Analyses of associations with asthma in four asthma population samples from Canada and Australia

Asthma, atopy, and related phenotypes are heterogeneous complex traits, with both genetic and environmental risk factors. Extensive research has been conducted and over hundred genes have been associated with asthma and atopy phenotypes, but many of these findings have failed to replicate in subsequent studies. To separate true associations from false positives, candidate genes need to be examined in large well-characterized samples, using standardized designs (genotyping, phenotyping and analysis). In an attempt to replicate previous associations we amalgamated the power and resources of four studies and genotyped 5,565 individuals to conduct a genetic association study of 93 previously associated candidate genes for asthma and related phenotypes using the same set of 861 single-nucleotide polymorphisms (SNPs), a common genotyping platform, and relatively harmonized phenotypes. We tested for association between SNPs and the dichotomous outcomes of asthma, atopy, atopic asthma, and airway hyperresponsiveness using a general allelic likelihood ratio test. No SNP in any gene reached significance levels that survived correction for all tested SNPs, phenotypes, and genes. Even after relaxing the usual stringent multiple testing corrections by performing a gene-based analysis (one gene at a time as if no other genes were typed) and by stratifying SNPs based on their prior evidence of association, no genes gave strong evidence of replication. There was weak evidence to implicate the following: IL13, IFNGR2, EDN1, and VDR in asthma; IL18, TBXA2R, IFNGR2, and VDR in atopy; TLR9, TBXA2R, VDR, NOD2, and STAT6 in airway hyperresponsiveness; TLR10, IFNGR2, STAT6, VDR, and NPSR1 in atopic asthma. Additionally we found an excess of SNPs with small effect sizes (OR < 1.4). The low rate of replication may be due to small effect size, differences in phenotypic definition, differential environmental effects, and/or genetic heterogeneity. To aid in future replication studies of asthma genes a comprehensive database was compiled and is available to the scientific community at http://genapha.icapture.ubc.ca/.

Polymorphisms of the matrix metalloproteinase 9 gene and abdominal aortic aneurysm

BACKGROUND

Increased matrix metalloproteinase (MMP) 9 activity has been implicated in the formation of abdominal aortic aneurysm (AAA). The aim was to explore the association between potentially functional variants of the MMP-9 gene and AAA.

METHODS

The -1562C > T and -1811A > T variants of the MMP-9 gene were genotyped in 678 men with an AAA (at least 30 mm in diameter) and 659 control subjects (aortic diameter 19-22 mm) recruited from a population-based trial of screening for AAA. Levels of MMP-9 were measured in a random subset of 300 cases and 84 controls. The association between genetic variants (including haplotypes) and AAA was assessed by multivariable logistic regression.

RESULTS

There was no association between the MMP-9-1562C > T (odds ratio (OR) 0.70 (95 per cent confidence interval (c.i.) 0.27 to 1.82)) or -1811A > T (OR 0.71 (95 per cent c.i. 0.28 to 1.85)) genotypes, or the most common haplotype (OR 0.81 (95 per cent c.i. 0.62 to 1.05)) and AAA. The serum MMP-9 concentration was higher in cases than controls, and in minor allele carriers in cases and controls, although the differences were not statistically significant.

CONCLUSION

In this study, the genetic tendency to higher levels of circulating MMP-9 was not associated with AAA.

The association of C-reactive protein and CRP genotype with coronary heart disease: findings from five studies with 4,610 cases amongst 18,637 participants

Background

It is unclear whether C-reactive protein (CRP) is causally related to coronary heart disease (CHD). Genetic variants that are known to be associated with CRP levels can be used to provide causal inference of the effect of CRP on CHD. Our objective was to examine the association between CRP genetic variant +1444C>T (rs1130864) and CHD risk in the largest study to date of this association.

Methods and Results

We estimated the association of CRP genetic variant +1444C>T (rs1130864) with CRP levels and with CHD in five studies and then pooled these analyses (N = 18,637 participants amongst whom there were 4,610 cases). CRP was associated with potential confounding factors (socioeconomic position, physical activity, smoking and body mass) whereas genotype (rs1130864) was not associated with these confounders. The pooled odds ratio of CHD per doubling of circulating CRP level after adjustment for age and sex was 1.13 (95%CI: 1.06, 1.21), and after further adjustment for confounding factors it was 1.07 (95%CI: 1.02, 1.13). Genotype (rs1130864) was associated with circulating CRP; the pooled ratio of geometric means of CRP level among individuals with the TT genotype compared to those with the CT/CC genotype was 1.21 (95%CI: 1.15, 1.28) and the pooled ratio of geometric means of CRP level per additional T allele was 1.14 (95%CI: 1.11, 1.18), with no strong evidence in either analyses of between study heterogeneity (I² = 0%, p>0.9 for both analyses). There was no association of genotype (rs1130864) with CHD: pooled odds ratio 1.01 (95%CI: 0.88, 1.16) comparing individuals with TT genotype to those with CT/CC genotype and 0.96 (95%CI: 0.90, 1.03) per additional T allele (I²<7.5%, p>0.6 for both meta-analyses). An instrumental variables analysis (in which the proportion of CRP levels explained by rs1130864 was related to CHD) suggested that circulating CRP was not associated with CHD: the odds ratio for a doubling of CRP level was 1.04 (95%CI: 0.61, 1.80).

Conclusions

We found no association of a genetic variant, which is known to be related to CRP levels, (rs1130864) and having CHD. These findings do not support a causal association between circulating CRP and CHD risk, but very large, extended, genetic association studies would be required to rule this out.

Polymorphisms of the interleukin-6 gene promoter and abdominal aortic aneurysm

BACKGROUND

Elevated levels of circulating interleukin-6 (IL-6) have been reported in patients with abdominal aortic aneurysms (AAAs). Although this implicates inflammation as a cause of AAAs, there is also evidence that the aneurysmal aorta may secrete IL-6 into the circulation as a result of aortic proteolysis. Genetic association studies are one means of trying to clarify the role of specific mediators in the causal pathway. The aim of the present study was to examine the association between variants of the IL-6 gene and AAAs.

METHODS

An association study involving 677 men with screen-detected AAAs and 656 age-matched controls was performed. Three variants in the IL-6 promoter region were analysed: IL-6-174G>C (rs1800795), IL-6-572G>C (rs1800796) and IL-6-597G>A (rs1800797). Univariate regression of SNP genotype on AAA as a binary outcome was initially performed under a range of genetic models (additive, dominant and recessive). This was followed by multivariate analyses, testing the same models but including risk factors known to be associated with AAAs. All analyses and haplotype estimation were performed under a generalized linear model framework.

RESULTS

IL-6-572G>C polymorphism (frequency 1.5% in cases) was identified as an independent risk factor for AAA with an odds ratio (OR) of 6.00 (95%CI: 1.22, 29.41) when applied to the recessive model. No association was seen in the additive or dominant models. In a multivariate analysis using the most common haplotype (h.111, frequency 48.7%) as a reference, h.211 (frequency 4.4%) was an independent risk factor for AAA (OR 1.56, 95%CI: 1.02, 2.39).

CONCLUSION

The IL-6 572G>C polymorphism (and h.211 haplotype) is associated with AAA, however it is too rare to be an important cause of most AAAs. This does not support the concept that the elevated level of IL-6 reported in patients with AAAs is a primary cause of the aneurysmal process.