1. RATIONALE

The proposed work is aimed at identifying genes contributing to blood pressure (BP) elevation related to excess body-fat in adolescence (i.e. preclinical obesity-induced hypertension). It builds on our previous research and is motivated by the following three overarching themes:

1.1 Adolescence is a period of preclinical disease: Adolescence is a period of human development when adult body composition and physiology develop and initial stages of cardiovascular disease, including hypertension, emerge1. Importantly for public health, this early preclinical stage may also be a period when disease trajectories can still be altered and full-blown diseases prevented.

1.2 Obesity is the leading risk factor for hypertension: Obesity has become a major health problem worldwide due to its growing prevalence and causal relationship to common chronic diseases, such as hypertension. It is spreading across all age groups, including children and adolescents. Epidemiological studies suggest that 65-78% of hypertension is due to obesity and that obesity-induced hypertension, as compared with obesity-unrelated hypertension, is associated with a greater risk for cardiovascular disease6. Despite the high public-health importance, our understanding of how excess body fat increases BP is still limited. We do know that the underlying mechanisms relate to not only excess but also intra-abdominal deposition of adipose tissue, and they commonly involve central sympathetic activation, insulin resistance, oxidative stress and augmented activity of the renin-angiotensin-aldosterone system.

1.3 Identification of genetic risk factors is a tool to uncover mechanisms of obesity-induced hypertension: The causal role of environment in the current epidemic of obesity and obesity-induced co-morbidities is undeniable. Genes, however, play a critical role in that they modify individuals' susceptibility to these disorders by regulating underlying mechanistic pathways. Therefore, identifying these genes may help us to uncover the pathways and thus open new avenues for prevention and treatment.

2. AIMS

Recent advances in high-through-put genotyping made significant inroads vis-a-vis the genetic architecture of hypertension, but preclinical stages and obesity-related hypertension have not been studied. To fill this gap, we propose to identify genes (and the underlying mechanisms) of preclinical stages of obesity-induced hypertension and to examine them across different ages. We propose to achieve this overarching goal by realizing Aims 1 and 2:

Aim 1: To identify genes and underlying mechanisms of adiposity and related BP variations in adolescence, we will perform genome-wide association studies (GWAS) with the Illumina Human610W-Quad BeadChip containing probes for greater than 550K single nucleotide polymorphisms (SNPs) in the Saguenay Youth Study (SYS). The SYS is an ongoing population-based investigation of 1,000 adolescents recruited from a French-Canadian founder population who undergo detailed cardiovascular, metabolic and behavioral phenotyping (15-hour protocol) . The SYS is ideally suited for the proposed studies for the following reasons: (1) it is an adolescent cohort allowing us to investigate preclinical stages of obesity-induced hypertension; (2) all adolescents are recruited from a genetic founder population where power of genetic analyses is expected to be higher (compared with regular outbred populations) due to more homogenous genetic background (and environment) and, hence, fewer genes contributing to the expression of complex traits, such as adiposity and BP; and (3) all adolescents undergo extensive high-fidelity phenotyping not only of adiposity and BP but also of variables that may provide additional mechanistic insight (e.g. sympathetic modulation of vasomotor tone) and characterize environment (diet, physical activity and psychosocial environment). To our knowledge, the SYS is currently the largest sample of adolescents (n=1,000) in whom intra-abdominal, visceral fat is measured directly with magnetic resonance imaging (MRI) and in whom sympathetic modulation of vasomotor tone is assessed with power spectral analysis of beat-to-beat BP. Furthermore, as we study predominantly disease-free adolescents (not yet hypertensive and still free of co-morbidities), this high-fidelity phenotyping is not confounded by e.g. BP-lowering medication. To date, we have phenotyped 750 adolescents and genotyped (610K SNPs) 600 of them, and we have published 19 peer-reviewed papers (with 2 additional being currently under review). We have also obtained encouraging results in our initial GWAS. The funds requested to meet Aim 1 are to support phenotyping of the remaining 250 adolescents, genotyping of the remaining 400 adolescents, as well as data management and performance of statistical analyses in all 1,000 adolescents. Note that we request funding for only cardiovascular and metabolic phenotyping of the remaining 250 adolescents, as the cost of their recruitment and both MRI and behavioral phenotyping is already covered by another CIHR grant (see Letter of Collaboration from Dr. Tomas Paus).

Aim 2: To replicate genotype-phenotype associations identified in Aim 1 across different ages (n=13,244 individuals). To achieve this aim we will test the associations in the SYS in the following replication cohorts: (1) the Avon Longitudinal Study of Parents and Children study (ALSPAC; 6,863 adolescents), which is a longitudinal birth cohort established in England (see Letter of Collaboration from Dr. George Davey-Smith); (2) the 1966 Northern Finland Birth Cohort (NFBC1966; 4,936 young adults), which is also a longitudinal birth cohort established in a Finish founder population (see Letter of Collaboration from Dr. Jarvelin Marjo-Riitta), and (3) the Quebec Hypertensive Family Study (QHFS; 897 older adults), which is a cross-sectional sample of adults recruited as members of hypertensive families from the same French-Canadian founder population as the SYS adolescents. All individuals to be analyzed in replication studies have already been phenotyped (BP and adiposity) and most of them were also genome-wide genotyped (317K SNPs in ALSPAC; 370K SNPs in NFBC1966). Therefore, the funds requested for Aim 2 are mainly to support data manipulation and management, and genetic analyses.