Cortisol responsive gene networks in cardiovascular disease

Abstract

Increased plasma cortisol levels are associated with Cardiovascular Disease (CVD) and CVD risk factors, however the tissue specific mechanisms underpinning this process are poorly understood. A genome wide meta-analysis by the CORtisol NETwork (CORNET) consortium identified genetic variants, spanning the SERPINA6/SERPINA1 locus on chromosome 14, associated with morning plasma cortisol and shown to be causal for ischaemic heart disease. SERPINA6 encodes Corticosteroid Binding Globulin (CBG), responsible for binding most cortisol in blood and putatively mediating delivery of cortisol to target tissues. This thesis addresses the hypothesis that genetic variants in SERPINA6 influence CBG expression in liver and cortisol delivery to extra-hepatic tissues, responsible for mediating cortisol-regulated gene expression.

The Stockholm Tartu Atherosclerosis Reverse Networks Engineering Task study (STARNET) provides RNA sequencing data for 7 vascular and metabolic tissues from 600 genotyped individuals (mean age 65.8, 70.3% male) undergoing coronary artery bypass grafting. We have identified 21 Single Nucleotide Polymorphisms (SNPs) associated with both variation for plasma cortisol at genome wide significance in CORNET (p ≤ 5×10-8) and with SERPINA6 gene expression in STARNET-liver (q ≤ 0.05) as cis-expression Quantitative Trait Loci (eQTLs). We go on to describe the extra-hepatic consequences of genetic variation for plasma cortisol by linking SNPs associated with plasma cortisol to genes expressed in trans across STARNET tissues, finding the highest representation of trans-genes in liver, subcutaneous and visceral abdominal adipose tissue (FDR = 15%). Through the use of published evidence, we then identify a sub-set of cortisol associated trans-genes that are putatively regulated by the glucocorticoid receptor, the primary transcription factor activated by cortisol.

Using causal methods, we have identified glucocorticoid regulated trans-genes that are responsible for the regulation of tissue specific gene networks. Cis-eQTLs were used as genetic instruments for the identification of pairwise causal relation- ships, from which cortisol associated gene networks could be reconstructed. Gene networks were identified in liver, subcutaneous fat and visceral abdominal fat, in- cluding a high confidence gene network specific to subcutaneous adipose (FDR < 10%) under the regulation of the interferon regulatory transcription factor, IRF2. Targets in this network include LDB2 and LIPA, both associated with coronary artery disease. Finally, we identify coordinated patterns of gene expression, representative of the STARNET gene networks, in gene expression data from the Metabolic Syndrome in Men (METSIM) and the Stockholm Atherosclerosis Gene Expression (STAGE) Study, both independent datasets from STARNET.

This thesis describes genetic variation at the SERPINA6/ SERPINA1 locus that is associated with changes in both morning plasma cortisol and SERPINA6 expression in liver, the gene that encodes CBG. Altered CBG levels in turn impact gene expression in extra-hepatic tissues through modulation of cortisol delivery. This supports a dynamic role for CBG in modulating cortisol delivery to tissues. The cortisol-responsive gene networks identified here represent candidate pathways to mediate cardiovascular risk attributable to elevated cortisol.