Investigation of the mechanisms mediating genetic associations with atrial fibrillation

PhD ThesisGenome wide association studies (GWAS) have identified multiple loci which are associated with increased risk of atrial fibrillation (AF). The mechanisms underlying these associations are not understood. None of the variants identified result in alteration of coding sequences; they are therefore likely to act by altering gene expression. Identification of intermediate gene expression phenotypes which are associated with the risk variants could provide important insights into disease pathogenesis which could in turn lead to development of therapeutic targets. One of the genetic variants identified by GWAS is situated within an intron of the gene HCN4, which encodes the major component of the If pacemaker current. This raised the question whether lower activity of HCN4 could be a risk factor for AF. By performing meta-analysis of randomised controlled trials of ivabradine, an If inhibitor, I demonstrated an increase in relative risk of incident AF of 15% in patients treated with ivabradine vs. controls, supporting the role of HCN4 in AF susceptibility. Analysis of total expression and allelic expression ratios of candidate genes in the GWAS hit regions in whole blood identified associations between AF risk variants and increased expression of KCNN3 and SYNE2 and decreased expression of CAV1. Analysis of total expression and allelic expression ratio in right atrial appendage tissue identified further associations between AF risk variants and increased expression of PITX2a/b and decreased expression of MYOZ1, CAV1, C9orf3 and FANCC in right atrial tissue. Furthermore, although the experiments were not designed to detect difference between AF cases and controls, I have shown that AF is associated with reduced expression of SYNE2, HCN4 and CAV1.British Heart Foundatio

Characterisation of a corona-stabilised switch in alternative gas mixtures

Sulphur hexafluoride (SF6) has traditionally been used as a switching medium within corona-stabilised switches (CSS). Due to its high global warming potential (GWP), however, other gases are under test in order to find a suitable alternative that can be used within CSS, without compromising on switching performance. Design changes may have to be made in order for the switch to remain at the high level of performance achieved when filled with SF6. This poster reports preliminary results obtained using a CSS operated with the refrigerant 1,3,3,3-tetrafluoropropene, known as HFO-1234ze as the basis of the operating gas. The electronegativity of HFO-1234ze makes it an attractive option to SF6 for switching applications. Additionally, the global warming potential (GWP) of this gas is 6 in a 100-year time horizon, compared to SF6 with a value of 23900. The performance of the switch has been characterized in terms of triggering range, delay time and jitter over a range of pressures when filled with air as a reference, as well as with HFO-1234ze in various mixtures with buffer gas nitrogen (N2) of the order of >80%. The results presented provide data on the feasibility of the approach of using HFO-1234ze as the operating gas in corona stabilised switches. It will also provide the initial basis for work refining the use of buffer gases and for the development of optimised switch configurations

Effect of surface deviation of solid insulation on impulsive flashover voltages under varying environmental conditions

In pulsed power engineering, solid spacers are used to insulate high voltage parts from extraneous metal parts. The applied voltage at which a discharge process initiates is important in the design process. In this paper, a method to potentially increase the failure voltage under multiple environmental conditions, without increasing the length of the solid spacer, was investigated. Three insulating materials: Delrin (Polyoxymethylene), Ultem (Polyetherimide) and HDPE (High Density Polyethylene), were tested under a 100/700 ns impulse voltage. Cylindrical spacers made of these materials were located in the center of a parallel-plane electrode setup in air, which provided a quasi-uniform field distribution. Breakdown tests published in this paper were performed in a sealed container at air pressures of −0.5, 0 and 0.5 bar gauge, with a constant relative humidity level of <10% RH. The materials were tested under both, negative and positive, polarity impulses. The surfaces of a set of solid spacers were subjected to a ‘knurled’ finish, where slight indentations are added to the surface of the materials, prior to testing, to allow comparison with the breakdown voltages for samples with ‘smooth’ (machined) surface finishes. For negative polarity impulses, the flashover voltage for smooth machined surfaces was generally higher than for knurled finishes. Under positive polarity impulses, the opposite effect was found to occur, where the flashover voltage of the samples with knurled surfaces was higher for all tests. The results will give designers, particularly within the pulsed power industry, information on flashover voltages of materials under an array of environmental conditions

Statistical analysis of impulsive flashover voltages across solid-air interfaces

Understanding of the influence of experimental conditions on the breakdown voltage of composite insulation is important to facilitate optimal design of high voltage systems. This includes ensuring that the statistical analysis performed on breakdown voltage data is relevant, in providing extra information on the failure voltages. Therefore, in order to investigate the applicability of statistical techniques in aiding to elucidate further detail about the breakdown process, multiple statistical methods were applied and analysed, in order to find the most suitable for the data found during a specific set of breakdown tests. Normal, lognormal, 2-parameter Weibull and 3-parameter Weibull fittings are discussed herein, as applied to the authors’ experimental data on the flashover voltages across solid-air interfaces, subjected to 100/700 ns impulse voltages. Negativebimpulse voltages were applied to composite insulation, consisting of one of three different solid materials in air - Delrin (Polyoxymethylene), Ultem (Polyetherimide) and HDPE (High Density Polyethylene). Samples of each material were machined to a smooth finish. The environmental conditions used in this paper were a fixed air pressure of −0.5 bar gauge, and relative humidity (RH) levels of 90% RH. The data points used in the statistical analyses were from the average flashover voltages found using the ASTM D3426-97 ‘step up’ testing procedure. Fitting the normal, log-normal, 2-parameter Weibull and 3-paramter Weibull distributions to breakdown voltage data obtained for each set of test conditions allowed for the relative quality of fit of each to be directly compared. The Kolmogorov-Smirnov (K-S) test was deployed in order to compare the maximum distance between the experimental data and the theoretical cumulative distribution function, thus verifying themost accurate method of statistical analysis for a given dataset. It was found that the overall best fit to the experimental data was given by the 3-parameter Weibull distribution

Impulsive flashover characteristics and Weibull statistical analysis of gas-solid interfaces with varying relative humidity

This paper informs on the flashover strength of three materials: Delrin (polyoxymethylene), HDPE (high-density polyethylene) and Ultem (polyetherimide) with smooth surface finishes, in zero-grade air at -0.5, 0 and 0.5 bar gauge, and at &#x003C;10%, &#x007E;50% and &#x003E;90% relative humidity (RH). Both negative and positive polarity impulse voltages were applied to investigate the potentially asymmetrical electrical performance of the geometrically-symmetrical electrode arrangement. In all tests, high voltage (HV) impulses with a nominal 100/700 ns wave-shape were applied. Each test conformed with the ASTM D3426-97 standard of &#x2018;step up&#x2019; testing, to find the average flashover voltage for each set of conditions. For negative polarity, each solid dielectric material demonstrated a decrease in flashover voltage as the RH was increased. For positive polarity, however, the flashover voltages were similar for all levels of RH, with the exemption of HDPE. A decrease in flashover voltage was found as the permittivity of the material increased for negative polarity, irrespective of humidity and pressure. Overall, the highest flashover voltage recorded for negative polarity was &#x007E;200 kV with an HDPE spacer, at 0.5 bar gauge and &#x003C;10% RH. The poorest performance was for a Delrin spacer in negative polarity, at -0.5 bar gauge and &#x003E;90% RH, at a value of &#x007E;53 kV. For positive polarity, the highest flashover voltage was for a Delrin spacer at &#x007E;50% RH and 0.5 bar gauge, with a voltage of &#x007E;180 kV; the lowest flashover voltage of &#x007E;60 kV was recorded with an HDPE spacer, at -0.5 bar gauge and &#x003E;90% RH

Soft windowing application to improve analysis of high-throughput phenotyping data.

MOTIVATION: High-throughput phenomic projects generate complex data from small treatment and large control groups that increase the power of the analyses but introduce variation over time. A method is needed to utlize a set of temporally local controls that maximizes analytic power while minimizing noise from unspecified environmental factors. RESULTS: Here we introduce \u27soft windowing\u27, a methodological approach that selects a window of time that includes the most appropriate controls for analysis. Using phenotype data from the International Mouse Phenotyping Consortium (IMPC), adaptive windows were applied such that control data collected proximally to mutants were assigned the maximal weight, while data collected earlier or later had less weight. We applied this method to IMPC data and compared the results with those obtained from a standard non-windowed approach. Validation was performed using a resampling approach in which we demonstrate a 10% reduction of false positives from 2.5 million analyses. We applied the method to our production analysis pipeline that establishes genotype-phenotype associations by comparing mutant versus control data. We report an increase of 30% in significant P-values, as well as linkage to 106 versus 99 disease models via phenotype overlap with the soft-windowed and non-windowed approaches, respectively, from a set of 2082 mutant mouse lines. Our method is generalizable and can benefit large-scale human phenomic projects such as the UK Biobank and the All of Us resources. AVAILABILITY AND IMPLEMENTATION: The method is freely available in the R package SmoothWin, available on CRAN http://CRAN.R-project.org/package=SmoothWin. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online

Chromosome 16q22 variants in a region associated with cardiovascular phenotypes correlate with ZFHX3 expression in a transcript-specific manner

Background: The ZFHX3 gene, located in Chromosome 16q22.3, codes for a transcription factor which is widely expressed in human tissues. Genome-wide studies have identified associations between variants within the gene and Kawasaki disease and atrial fibrillation. ZFHX3 has two main transcripts that utilise different transcription start sites. We examined the association between genetic variants in the 16q22.3 region and expression of ZFHX3 to identify variants that regulate gene expression. Results: We genotyped 65 single-nucleotide polymorphisms to tag genetic variation at the ZFHX3 locus in two cohorts, 451 British individuals recruited in the North East of England and 310 mixed-ancestry individuals recruited in South Africa. Allelic expression analysis revealed that the minor (A) allele of rs8060701, a variant in the first intron of ZFHX3, was associated with a 1.16-fold decrease in allelic expression of both transcripts together, (p = 4.87e-06). The minor (C) allele of a transcribed variant, rs10852515, in the second exon of ZFHX3 isoform A was independently associated with a 1.36-fold decrease in allelic expression of ZFHX3 A (p = 7.06e-31), but not overall ZFHX3 expression. However, analysis of total gene expression of ZFHX3 failed to detect an association with genotype at any variant. Differences in linkage disequilibrium between the two populations allowed fine-mapping of the locus to a 7 kb region overlapping exon 2 of ZFHX3 A. We did not find any association between ZFHX3 expression and any of the variants identified by genome wide association studies. Conclusions: ZFHX3 transcription is regulated in a transcript-specific fashion by independent cis-acting transcribed polymorphisms. Our results demonstrate the power of allelic expression analysis and trans-ethnic fine mapping to identify transcript-specific cis-acting regulatory elements