PETER A. DORIS, PH.D.

Current Projects

The systems biology of elevated blood pressure in the spontaneously hypertensive rat

High blood pressure is the major predisposing factor leading to cardiovascular disease in the population. Some of the susceptibility to high blood pressure (hypertension) is inherited. Because population genetics studies have achieved very little success in identifying the genes containing variation that contributes to susceptibility to hypertension, we lack knowledge of the genes involved, how they interact with other genes, how they interact with environmental risk factors and what pathogenetic processes they contribute that create hypertension. It is hard to enrich large-scale human population genetics studies with the additional power provided by biological information. By nature, they are largely observational, generally sparse in biological data and provide no opportunity for experimental investigation on a population scale.

Our studies are performed in a rat model of high blood pressure that offers several advantages. These include a robust hypertension phenotype attributable to polygenes in an inbred genetic background, investigation through forward and reverse genetics, hypothesis testing through experimentation, and availability of well-developed genomic resources for this species. While we don't argue that this model organism will provide information on the genes contributing to hypertension risk in humans, we do think that the model provides important opportunities to link the power of genetic approaches to cardiovascular disease with systems biology. The outcome of such work could include: identification of genes contributing through sequence variation to hypertension in this model, insight into the contribution of regulatory versus coding sequence polymorphism to complex disease, knowledge of if and how these genes interact (molecular pathways) to create disease, and knowledge of disease pathogenesis (how gene variation and interaction among variant genes affects normal physiology and organ function). Such information would provide the first comprehensive view of complex cardiovascular disease. It would illustrate how genetic variation becomes disease and it would provide a platform to assess how best to intervene to alter the consequences of genetic variation on cardiovascular disease risk. By revealing the broad picture of the links between gene variation, the variant genes and the disease process, new and important insights would be obtained that may speed progress to understanding the same process in human populations.

To illustrate some of our experimental approaches a few of our projects are described:

Genetics of gene expression in hypertension: Mapping of Mendelian diseases in humans has been very successful. Such diseases are most often the result of catastrophic mutations that affect the function of the encoded protein resulting in a disease phenotype that tends to be highly correlated among affected individuals. Complex diseases resulting from polygenic susceptibility do not appear to involve this type of mutation. Rather, phenotypes compound from underlying normal variation in multiple genes through interaction with environmental variables. This raises the question of what type of variation in genes contributes to complex disease. We have focused our studies on one major type of variation: variation in genes that alters expression of the variant gene. Heritable variation in expression is clearly a highly frequent allelic property of genes. An ongoing project in our laboratory is identifying which genes exist as allelic variants affecting gene expression in the spontaneously hypertensive rat (SHR). After identifying differentially expressed genes we seek variation in the sequence of these genes comparing SHR with its normotensive, genetically-related control strain. We then test whether allelic variation determines altered expression. Then we determine whether the identified SHR allele is common to all SHR substrains (these substrains are all similarly hypertensive and can be crossed without segregation of the blood pressure phenotype, indicating that the causative alleles are shared among SHR substrains). Finally, we assess whether there is a relationship between genes with heritable effects on expression (whose variant alleles occur in all SHR substrains) and segregation of these allelic variants with blood pressure in a cross between strains contrasting in blood pressure level. Our short-term goals are: to assess to what extent variation in blood pressure between the normotensive strain and SHR can be attributed to genes showing allelic expression and; to prove the effect of the allele on blood pressure using reverse genetic methods. In the long term we wish to understand if and how these allelic variants interact to produce the physiological trait of altered blood pressure regulation.

Redox stress, transcriptional control of gene expression and heritable susceptibility to hypertensive renal injury: Among the SHR sub-strains we study is one, SHR-A3, that uniquely experiences severe end organ damage as a result of hypertension. Our other SHR strains are remarkably resistant to these disease-producing consequences of hypertension, probably because, by chance during inbreeding, these other strains fixed natural alleles creating resistance. This provides a crisp illustration of a vitally important fact: the damage done by common cardiovascular diseases is a composite of heritable risks that influence predisposing factors such as blood pressure, in combination with heritable influences determining susceptibility to damage from these risk factors. The predisposing risks are commonly assessed directly or indirectly in the usual practice of medicine (e.g., blood pressure, serum lipid profile, insulin resistance, glucose levels, adiposity, etc) and therapies directed towards these risks are applied and assessed by measurement of their effect on these risks. However, we have no means to assess the likelihood of individuals to suffer harm from these pre-disease risks. Such assessments are done at the population, rather than individual level. We also have no means of mitigating the risk of harm in these individuals, except by addressing the predisposing risks. Disease prevention, the primary goal of our Institute's research, could be greatly advanced by understanding why individuals differ in their outcomes when equivalent predisposing risks exist. Such knowledge would arise from an understanding of how organ injury in cardiovascular disease occurs and how genes, such as those differing among our SHR sub-strains, create altered injury responses to the same level of hypertension. At present our work in this area is focused on hypertensive renal injury. We have developed evidence that injury is preceded by a major shift in the capacity of the kidney to detoxify reactive radicals. This can be readily seen as a decline in the total amount of glutathione, a major redox detoxification molecule, in the kidney of the injury-prone strain, However, we have learned that this decline is the outcome of a transcriptional program affecting the expression of many genes involved in radical detoxification (as well as many other genes not currently understood to function in this role). Using a bioinformatics approach to our gene expression data, we have identified the transcription factor responsible and have observed that one of the two splice variants of this transcription factor is completely absent in the injury-prone strain, but present in each of the 3 injury-resistant strains (as well as two related normotensive strains). Injury susceptibility appears to arise upstream of the transcription factor, as the isoform loss is post-transcriptional, and probably post-translational, and the gene encoding the transcription factor does not vary among strains. By identifying this pathway, as well as seeking the upstream elements responsible for altered information flow in this pathway we have created important new insights into the pathogenesis of hypertensive renal injury as well as identified new opportunities for therapy that targets the mechanism of injury.

Regulatory physiology of elevated blood pressure in a rat model of hypertension: Some of our efforts to understand the basis of hypertension and its end-organ injuries arise from new opportunities that have emerged recently in the field of genetics and genomics. However, we also study the disease process at the level of the cell and regulatory physiology where function is perturbed to produce the disease. In our rat model we know that hypertension occurs as a result of abnormal renal function. Specifically, there is a change in the renal handling of sodium, arising in the proximal tubules in immature animals and persisting until blood pressure plateaus at a sustained high level. By looking explicitly at gene expression in microscopic tissue samples (proximal tubules) from the kidney, we found that the hypertensive strain had an unexpected reduction in gene expression of a major sodium transport protein, sodium, potassium-ATPase, in spite of the fact that ion transport driven by this protein was increased in this strain. This lead to our hypothesis that a defect exists that affects the sub-cellular localization of this ion transporter. The ion transport function of the protein is regulated by its subcellular localization. When it is present in the basolateral membrane, it can act to reabsorb sodium. When reabsorption is reduced, the protein cycles back into intracellular endosomal membranes where it cannot act on the transepithelial reabsorption of sodium. We found that, in the hypertensive animal, there is a major shift out of the endosomal compartments into the basolateral compartment. We were unable to find any specific defect either in the gene sequence of the ion transporter in the hypertensive strain, or in its state of regulatory phosphorylation. This caused us to question whether there is a generalized defect in the distribution of membrane proteins that affects the sodium transport pathway. We have used proteomic methods to demonstrate a major shift in multiple membrane-inserted proteins during the development of hypertension. We are using quantitative proteomics to assess which proteins are affected and what properties affected proteins share. By this means we hope to climb up the defective regulatory pathway and eventually emerge with individual genes as targets for this transporter trafficking defect. This project illustrates how our work is directed to understand the mechanisms of disease by integrating the tools of modern genomics and proteomics with organ and cellular physiology to create a powerful systems biology approach.