Identification and Dissection of Cardiovascular and Cerebrovascular Quantitative Trait Loci in the Stroke-Prone Spontaneously Hypertensive Rat

Human essential hypertension is a classic example of a complex, multifactorial, and polygenic trait. That a substantial fraction of the variation in blood pressure between individuals is genetically determined has been well established by classic epidemiologic, twin, and adoption studies, as well as the existence of rare Mendelian forms of hypertension. Similar evidence exists for a genetic basis to human stroke and left ventricular hypertrophy, unrelated to the effects of blood pressure. Given the heterogeneity of the human condition, however, little progress has been made towards the identification of the genes involved in these common disease states. Instead, a major strategy has been successfully developed using inbred animal models which results in the identification of a quantitative trait locus (QTL), a large chromosomal region which contains a gene or number of genes responsible for the quantitative trait under investigation. Known as a genome wide scan, this strategy involves the high fidelity phenotyping of a large segregating F2 population derived by crossing hypertensive and normotensive inbred rat strains, and the genotyping of a large panel of dimorphic microsatellite markers with a thorough coverage of the entire rat genome. The research described in this thesis incorporated the use of two genome wide scans to identify QTLs containing genetic determinants of hypertensive cardiovascular and cerebrovascular disease in the Glasgow SHRSP x WKY F2 cross. The first genome wide scan was performed in 140 male and female F2 hybrids (M:F = 65:75). Blood pressure at baseline and after 1% sodium chloride administration was measured by radio-telemetry; other phenotypes included heart rate, motor activity, and left ventricular hypertrophy. This was the first genome scan where physiological phenotypes were measured with a radio-telemetry system in an entire F2 cross. The second genome scan was performed in 59 male and female F2 hybrids (M;F 33;26) phenotyped by the volume of cerebral infarction following the experimental occlusion of their middle cerebral artery. QTLs affecting a given phenotype were mapped relative to micro satellite markers with the aid of the MAPMAKER/QTL 1.1 computer package. This resulted in the identification of three blood pressure QTLs; two on rat chromosome 2 and one on rat chromosome 3. The QTL close to the microsatellite markers D2Mghl2 (suggestive linkage with a maximal LOD score of 3.1) and D3Mghl6 (significant linkage with a maximal LOD score of 5.6) showed sex specificity being only present in the male F2 cohort. This was further confirmed by the likelihood ratio test for the difference in the locus effect between the sexes. This study was the first to show such sex specificity of autosomal QTLs for genetic hypertension. This genome scan also identified a new QTL for left ventricular hypertrophy on rat chromosome 14 (suggestive linkage, with a maximal LOD score of 3.1). The second genome scan identified a single highly significant QTL on rat chromosome 5 for the increased sensitivity to experimentally induced focal cerebral ischaemia, with a LOD score of 16.6. This QTL accounted for 67% of the phenotypic variance and was blood pressure independent. Sex also had a significant effect with male F2 hybrids having larger infarcts than females (p = 0.012), and explained 10% of the phenotypic variance. The microsatellite marker in the centre of this QTL was Anp, a marker within the gene encoding the atrial natriuretic peptide. DNA sequencing analysis of the coding regions of the Anp gene, and the gene for brain natriuretic peptide {Bnp) which is known to co- localise with the Anp gene, revealed no significant nucleotide differences between the SHRSP and the WKY strains that are potentially important to their function. Consequently the Anp and Bnp genes were not supported as candidates for the QTL for severity of cerebral ischaemia. The identification of the large QTLs is only the first step towards the ultimate goal of gene identification. The next step is the production of congenic strains and substrains containing progressively smaller chromosomal regions with the final task being positional cloning of the causal gene. (Abstract shortened by ProQuest.)