Introduction Increased left ventricular mass (LVM) is an independent risk factor for cardiovascular disease. Although haemodynamic factors regulating LVM have been extensively investigated, the genetic components of this clinically important phenotype remain unclear. LVM and cardiac gene expression are complex traits regulated by factors both intrinsic and extrinsic to the heart, and are well suited for study with specialised rodent genetic resources. Because gene expression is highly heritable and also because it lies between DNA sequence polymorphisms and physiological traits, genetic determinants of cardiac gene expression are good candidates for primary regulators of complex cardiac phenotypes.
Methods and Results To dissect the major determinants of LVM, LVM and cardiac gene expression were measured in the rat heart from a set of 29 recombinant inbred strains derived from the spontaneously hypertensive rat and the “normal” brown Norway strain. LVM was corrected for body mass, and cardiac gene expression was quantified with Affymetrix RAE 230 2.0 microarrays (four males/strain, 6–8 weeks old, 128 microarrays). Genome-wide linkage analysis was performed on LVM and cardiac gene expression using published approaches. A previously published control point for rat LVM on chromosome 17 was confirmed and refined (log of the odds, LOD, score 4.0), and was also shown to be independent of blood pressure in this model (fig 1). Osteoglycin (Ogn) was the gene within this region that had significant genetic control (genome-wide p<0.002), and was shown to be dynamically regulated in an in-vitro model of rat cardiac hypertrophy. Sequencing and luciferase assays showed a 47 base pair variant in the 3′UTR of Ogn that could account for the increased protein levels of Ogn seen in rat hypertrophy. For translational studies, myocardial gene expression was quantified with Affymetrix U133A microarrays in patients undergoing bypass surgery for angina (n = 7) or aortic valve replacement for aortic stenosis (n = 20). Correlation analysis of LVM with gene expression showed that out of all 22 284 transcripts, human OGN had the strongest association with LVM (r = 0.62, p = 0.0012). At the protein level, myocardial expression of OGN was increased in human hypertrophy (p = 0.019), as seen in the rat. Finally, in order to validate a functional role of Ogn in the in-vivo regulation of LVM, the Ogn knockout mouse was studied. Although there was no significant difference in LVM between wild-type and the knockout mouse at baseline, after stimulation with subcutaneous angiotensin II (1.5 μg/g per day) for 2 weeks, both the heterozygous and knockout mouse exhibited a resistance to left ventricular hypertrophy, which was shown to be independent of blood pressure (fig 2).
Conclusions Taken together, these data identify Ogn as a key regulator of LVM in rats, mice and in humans, and also that Ogn modifies the hypertrophic response to extrinsic factors such as hypertension and aortic stenosis.