Meet the PKD Research Team

Motivated by seeing patients struggle with high blood pressure and the complications of kidney failure, which require dialysis or kidney transplant, Mayo's PKD research group is investigating many ways to diagnose, understand and effectively cure PKD. The group includes Vicente Torres, M.D., Ph.D., a clinician and researcher who chairs the Division of Nephrology and Hypertension, molecular biologist Peter Harris, Ph.D., and Qi Qian, M.D., and Christopher Ward, Ph.D., who have recently launched independent careers in PKD research. The core group is supported by strong collaborations with investigators in many related areas, especially with Nicholas LaRusso, M.D., and David Nagorney, M.D., who have made significant contributions to the related polycystic liver disease.

Landmark Discoveries

In 2003, molecular biologist Peter Harris, Ph.D., was awarded the Lillian Jean Kaplan International Prize for Advancement in the Understanding of PKD. The award acknowledged Dr. Harris' dedication and extraordinary number of landmark discoveries in PKD research. Dr. Harris came to Mayo from Oxford University, England in 1999.

"Mayo offered me outstanding opportunities to study the interface between genetics and how the disease presents in patients," says Dr. Harris. "Mayo has a huge population of patients with a wide range of disease presentation that had been studied clinically but not genetically, which is my expertise. It also offered everything I needed in terms of core facilities, support, and depth of scientific and medical expertise. In particular, I have the opportunity to collaborate with Dr. Torres, who has earned international recognition both for his research and for his clinical expertise in PKD."

Dr. Harris' landmark discoveries include identification and complete cloning of one of two ADPKD genes (PKD1) and identification of the gene responsible for ARPKD, PKHD1. His characterization of the proteins encoded by these genes and documentation of mutations found in patients has revealed many more steps on the PKD stairway. Dr. Harris holds four patents for his genetic discoveries.

Giant Genes and Multiple Mutations

Landmark discoveries usually take years of collaboration and teamwork—and oftentimes, a little luck. Making good use of gene sequencing provided by the Human Genome Project, Dr. Harris' research team found the ARPKD gene (Nature Genetics, 2002) by analyzing the genetic linkage of an animal model.

Getting hold of an animal model usually means genetically engineering lab animals to produce the targeted disease—a difficult, time-consuming and expensive process. However, Dr. Harris was blessed with the kind of luck that favors the well-prepared when a report describing a rat that had spontaneously developed PKD, called the PCK rat model, was published in the Japanese literature. Dr. Torres learned about it, recognized its potential for accelerating the group's research, and wasted no time in bringing the model to Mayo Clinic.

"We used it to demonstrate how the gene manifests itself clinically," explains Dr. Torres. "That led to collaboration with Dr. Harris." Dr. Harris' research team analyzed the DNA links and discovered that the same genetic mutation that occurred by chance in the rats also caused the inherited recessive form of PKD in humans. They called the mutated gene that they identified PKHD1.

Dr. Ward was a key player in the discovery. He had played a significant role in the identification of the major disease-causing gene for the dominant form of PKD in Dr. Harris' Oxford lab and came with him to Mayo as a research associate. He is now independently funded by the National Institutes of Health (NIH) to develop new animal models of PKD. They will be instrumental in the quest to hammer in all of the missing steps on the PKD stairway—a huge challenge since the giant PKD genes cover more that ten times the number of DNA bases found in average genes. Scientists have already accrued a huge catalog of PKD mutations.

"There's a whole range of mutations in both of the diseases we deal with," says Dr. Harris. "In the dominant form, none of the single mutations is found in more than three percent of the population. In the recessive, the most common is only found in 15 percent so at least one of the mutations is likely to be new to us."

High Hopes for the First Potential Therapy

In the October 2003 issue of Nature Medicine, Dr. Torres and his collaborators at Indiana University reported a new treatment that stops the development of cysts and prevents loss of kidney function in rats and mice. The drug, OPC31260, is a vasopressin V2 receptor antagonist that inhibits the production of a chemical compound called cyclic AMP (cAMP).

The idea for the study came on the heels of a breakthrough by another group who showed that cAMP is largely responsible for the cell proliferation and fluid secretion that precedes the development of kidney cysts. Understanding that the vasopressin V2 receptor antagonists block the action of cAMP, Dr. Torres teamed up with Indiana University researchers to study OPC31260.

"It was an exciting moment when we first saw our data because the drug is relatively nontoxic so it is a good candidate for clinical trials in humans," says Dr. Torres. "We have now reproduced the results in four different PKD animal models."

Dr. Torres chairs an advisory committee for a pharmaceutical company that is currently conducting Phase II clinical trials on OPC31260. Dr. Torres looks forward to making the drug available to some of his patients in a multi-center clinical trial anticipated to begin in 2005.

HALT-PKD and CRISP

Knowing that hypertension is a common problem for people with PKD, Dr. Torres has conducted many studies on the renin-angiotensin system—hormones that regulate blood pressure. His studies suggested that some anti-hypertensive drugs slowed the progression of PKD and discovered that polycystic kidneys produce renin in an unlikely place—the epithelial cells. Recently, MR studies demonstrated that patients with PKD have reduced blood flow to their kidneys—potentially an important discovery.

"Reduced renal blood flow may be the most important determinant of disease progression," explains Dr. Torres. "In addition, if we can increase blood flow with antihypertensive drugs, we may be able to forestall the disease."

Dr. Torres' work, along with that of investigators at other medical centers, forms the basis for the Halt PKD study—a seven-year, multi-center clinical trial slated to begin recruiting patients in fall 2005. The main goals are to compare the effectiveness of two antihypertensive therapies (an ACE inhibitor alone, and an ACE inhibitor in combination with an angiotensin 2 receptor blocker), and two different levels of blood pressure control, on the progression of PKD. Another goal is to develop a specimen bank and information data system from patient samples.

Radiologists have been strong partners in helping relieve the burden of PKD. John Huston, M.D., collaborated with Dr. Torres to develop a standard procedure to detect intracranial aneurysms in patients with PKD using MR angiography. Bernard King, M.D., has played a key role in developing fast magnetic resonance (MR) imaging to measure the shape and volumes of the kidneys, kidney cysts, and liver cysts; and MR-derived renal blood flow.

Dr. King's collaboration with Dr. Torres is part of the Consortium of Renal Imaging to Study the Progression of PKD (CRISP). Although a genetic test is now available to test the presence of ADPKD before cysts develop, it cannot predict when or how badly the individual will get the disease. CRISP is a consortium of four centers formed by NIH to develop imaging techniques that can measure the progression of renal disease in patients with PKD. CRISP also measures glomerular filtration rates (GFRs)—the rate the kidneys filter the blood and renal blood flow. Mayo's Renal Function Lab measures and analyzes all GFRs for CRISP. The human studies are conducted in Mayo's CTSA Clinical Research Unit (CRU).

Dr. Torres is pleased with preliminary findings.

"Apart from indicating that reduced renal blood flow may be the most important determinant of disease progression, the MR studies allow us to determine how kidney function is affected by anatomical change," he explains. "We now know the average annual rate of growth of the kidneys and we have a better understanding of patient variability. That allows us to design better clinical trials and, we hope, will allow us to identify PKD and assess its early progression."

Genetic Diagnosis

In the meantime, Drs. Harris and Ward are working on a genetic diagnosis test for ARPKD.

"Developing a diagnostic test is complicated by the fact that every patient has a different combination of mutations," says Dr. Harris. "So you have to screen all 12,000 coding DNA bases of the gene." Despite the difficulty of this task, the Harris laboratory is working with Mayo's Department of Laboratory Medicine and Pathology to develop a clinical test for this disorder.

The scientists are hoping new research will also lead to breakthroughs in understanding the disease process. Following the discovery of the ARPKD gene, PKHD1, they discovered the gene codes for a large receptor-like protein called fibrocystin (Nature Genetics, 2002).

"The normal role of fibrocystin is unknown," says Dr. Ward. "We found that the basic defect causing the disease may be a failure of differentiation in the collecting ducts of the kidney and bile ducts of the liver. We hope further research on fibrocystin will yield a better understanding of PKD pathogenesis, perhaps lead to a novel diagnostic test and even a potential treatment for ARPKD."

Polycystins and Blood Vessels

In 1995, Drs. Harris and Ward isolated and described the first protein known to be produced by a PKD gene. The protein is now called polycystin-1. Its sister protein, polycystin-2 was isolated by a Yale team in 1996. Subsequently, this group, in collaboration with Dr. Torres, isolated genes causing a related disorder with cysts in the liver but no renal cysts. Understanding the function of these proteins is a focus of the present studies at Mayo.

More than 70 percent of people with ADPKD develop high blood pressure which, if left untreated, can cause further kidney damage and increase the risk of heart disease and stroke. Because cardiovascular complications are the leading cause of death in patients with PKD, Dr. Torres was interested in the how the polycystins might affect the smooth muscle of the blood vessels. His interest was picked up by Qi Qian, M.D., whom he mentored.

Dr. Qian is now an NIH-funded principal investigator researching the mechanisms of vascular complications in ADPKD. In collaboration with Gary Sieck, Ph.D., an expert on intracellular calcium regulation in smooth muscle cells, and Gianrico Farrugia, M.D., an expert in the smooth muscle of the digestive system, she has conducted intriguing studies on polycystin-2. Polycystin-2 appears to be a calcium channel that functions by creating a pore in the cell membrane through which calcium ions can flow into the cell.

In an oft-quoted paper published in Human Molecular Genetics in 2003, Dr. Qian reported that abnormal smooth muscle cells from a PKD mouse model produced about half the normal amount of polycystin-2.

"The mice had intracranial vascular abnormalities such as dilation, bleeding, and thickening and thinning of the vessel walls," says Dr. Qian. "When we dissociated the cells and stimulated them, the response was blunted, which is consistent with the disturbance in intracellular calcium regulation that we found."

Since calcium regulates contraction and other important cellular functions, Dr. Qian's study proposes a direct relationship between the decreased polycystin-2 level and vascular abnormalities that cause cardiovascular complications of PKD. She is now investigating the calcium defect's downstream effect. Her recent studies suggest a subsequent increase in a second messenger, cyclic AMP, the chemical compound that scientists recently found to be largely responsible for the cell proliferation and fluid secretion central to the development of kidney cysts. Dr. Qian's studies showed that cells that produced too little polycystin-2 and too much cAMP grow and multiply in an uncontrolled manner.

"When that happens in vessel walls, it causes them to thicken and stiffen, which could explain the early onset of hypertension in patients with PKD" says Dr. Qian. "Our goal is to figure out the signals that cause the cells to run amok and find a way to suppress them and calm them down."

Beyond the Field, Beyond the Present

Another of the Mayo PKD research group's discoveries is having a measurable impact outside the PKD field. In a collaborative effort they localized mutated proteins to tiny organelles that protrude from the surface of kidney tubule cells. These ‘primary cilia' appear to have a sensory function and contribute to the regulation of intracellular calcium.

"This research has inspired interest in the role of primary cilia in other diseases," says Dr. Torres. "And it has opened up an entirely new area of investigation for those of us trying to understand PKD."

As scientists reveal more and more missing steps on the PKD stairway, Dr. Harris is optimistic about the future.

"It's an exciting time for PKD," says Dr. Harris. "For the first time, we have an idea what the basic defects are. We know what gene is disrupted in both the recessive and the dominant form of PKD, and we're getting an idea of the role of the encoded proteins in the cell. We are exploring many avenues that may lead to therapies that slow the progression of the disease and preserve kidney function. Even if we cannot completely stop cyst formation, that would go a long way toward an effective cure."