Hospital Practice
ISSN: 2154-8331 (Print) 2377-1003 (Online) Journal homepage: http://www.tandfonline.com/loi/ihop20
Polycystic Kidney Disease: I. Etiology and Pathogenesis Jared J. Grantham To cite this article: Jared J. Grantham (1992) Polycystic Kidney Disease: I. Etiology and Pathogenesis, Hospital Practice, 27:3, 51-59, DOI: 10.1080/21548331.1992.11705379 To link to this article: http://dx.doi.org/10.1080/21548331.1992.11705379
Published online: 17 May 2016.
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Date: 18 July 2016, At: 23:38
Polycystic Kidney Disease: I. Etiology and Pathogenesis JARED J. GRANTHAM
UniversityojKansas
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Geneticists and cell biologists are simultaneously studying the disease's underlying mechanisms. Although a great many questions remain about cyst formation and its role in kidney failure, three fundamental constructs on its etiology and pathogenesis have been generated: renal tubular epithelial proliferation, fluid accumulation, and remodeling of the extracellular matrix. Polycystic kidney disease is one of the most common hereditary diseases in the United States, affecting more than 500,000 Americans. It is estimated that more than 5 million persons worldwide are at risk for the disease. PKD affects children, and also adults in the prime of life. It appears indiscriminately in men and women of all races and ethnic backgrounds, caustngrenal insufficiency in halfbefore age 70. Despite its high incidence and considerable impact on public health, PKD for many years was a relatively unknown disease. In popular accounts, cystic fibrosis (which affects approximately 30,000 Americans) is often identified as the most common hereditary disease in the United States. In fact, cystic fibrosis does not even come close to that status. After PKD, a 1-antitrypsin deficiency with its consequent emphysema affecting perhaps 120,000 Americans, is a distant second. Even if polycystic kidney disease has attracted little public attention, it is not being neglected scientifically. A number of geneticists are searching for PKD genes, and cell biologists are simultaneously studying the disease's underlying mechanisms. Although a great many questions remain about cyst formation and its role in kidney failure, the cell biology studies have coalesced into three fundamental constructs on the etiology and pathogenesis of PKD. At present, those three phenomena-epithelial proliferation, fluid accumulation, and matrix remodeling-offer the best targets for developing new pharmacologic approaches to treating PKD. This article will review the mechanisms underlying the etiology and pathogenesis of polycystic kidney disease and will close with a brief discussion of what the research implies about strategies for
treatment. In a subsequent article, William Bennett of the Oregon Health Sciences University will discuss the management ofPKD-especially the autosomal dominant version. Renal cysts are the most common abnormal structures found in the kidneys. Men and women over 50 years of age have a 50% chance of having one or more renal cysts. These common simple cysts are not the result of genetic influences, nor are they pathologic. In contrast, PKD involves at least dozens and often many thousands of renal cysts. Three distinct disorders account for most cases ofPKD (Table 1). Autosomal dominant polycystic kidney disease is characterized by a fairly slow progression from birth until 50 or 60 years of age, when the kidneys begin to malfunction. Autosomal dominant PKD has, however, a wide variation in expression; it has been observed in aborted fetuses or the fetuses of accident victims in the first trimester. Babies have been born with autosomally dominant polycystic kidney disease so advanced that Wilms' tumor was diagnosed. In other patients, the cysts are not discovered for 70 or 80 years, and even then only incidentally. Generally, dominant PKD is a slowly progressive disease that can begin in any section of the kidney tubule. Other sections-and in fact many nephrons-remain cyst free. Autosomal dominant PKD can be caused by at least two different mutations, a discovery that did
Dr. Grantham is Professor of Medicine and Director of Nephrology, Department of Medicine, University of Kansas School of Medicine, Kansas City. Hospital Practice March 15, 1992
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not surprise investigators who were struggling to understand the pathogenesis of renal cysts. The dominant variant is a systemic disease that by definition causes polycystic kidneys, and often (40% to 60% of the time) produces polycystic livers. Patients with autosomal dominant PKD also may have a polycystic pancreas, spleen, brain, or seminal vesicles, and often have mitral valve disorders and abdominal hernias. Because these other components appear in many but not all cases, the pathogenesis of the disease has been hard to understand. Autosomal recessive polycystic kidney disease tends to manifest earlier and is generally more severe. Patients with the recessive disease obviously have a double dose of a PKD gene (probably not the same gene that causes the dominant disease). Recessive disease begins very early in utero, and affected infants often die at birth or shortly thereafter of kidney failure and lung dysgenesis. In these cases, the kidneys may be so enlarged that they constitute 10% of the baby's total body weight. But again, there is variability in the expression of autosomal recessive PKD: Perhaps 25% of this patient population survives into infancy and adolescence, or even into adulthood. Cysts associated with autosomal recessive PKD begin in the collecting ducts far down in the nephron. Almost every collecting duct is affected, which explains why autosomal recessive PKD progresses rapidly and often lethally. The third important polycystic kidney disease has no hereditary basis. Acquired cystic kidney disease (ACKD) is often seen in patients with progressive renal failure developing from diabetes, glomerulonephritis, hy52
Hospital Practice March 15, 1992
pertension, or other noncystic diseases. These patients start to exhibit kidney cysts as they progress toward the end stage of their disease. This acquired form ofPKD is seen most commonly after dialysis. Initially, there was some thought that dialysis actually contributed to the formation of cysts, but it is now clear that dialysis merely extends the life of the patient long enough for the acquired cystic disease to appear. ACKD per se is not a major problem unless the cysts bleed, but these acquired kidney cysts are also associated with a high incidence of cancer. Patients with ACKD are 40 times more likely to have a kidney carcinoma than the population at large. As far as we know, neither of the genetic forms of polycystic kidney disease increases the risk of cancer. Cysts associated with ACKD can form in all segments of the nephron. It appears that they begin primarily in the proximal tubule, although they have been described in the distal parts of the tubule as well. There are many other polycystic kidney diseases, including some others associated with congenital disorders, but the bulk of the research effort currently focuses on these three forms ofPKD.
Pathogenesis Over the past decade, cell biologists have shown that epithelial proliferation, fluid accumulation, and matrix remodeling are the three pathogenetic forces involved in cyst formation (Figure 1). It is not yet clear which force or combination of forces is causative. My bias is for epithelial proliferation due to abnormalities in the regulation of cell growth. There are many candidate mechanisms for such abnormalities: a mutation in an
oncogene we have yet to discover, a defective suppressor gene, or a mutation that makes these cells more susceptible to normal growth factors (such as epidermal growth factor, nerve growth factor, or perhaps some unidentified growth factors). It does not appear that a circulating growth factor per se could account for PKD, because patients who get kidney transplants do not manifest the disease in the allograft. Therefore, something intrinsic to the kidney is probably responsible for cyst formation, at least in the dominantly inherited disease. Several years ago here at the University of Kansas, we found that cystic kidney epithelial cells in mice show an increased expression of the proto-oncogene c-myc. Marie Trudel, a Canadian researcher, engineered an activated c-myc oncogene into the mouse genome, and the transgenic mice developed PKD. Investigators have also made transgenic mice with another oncogene, SV-40 large T antigen, which also results in PKD mice. And a young investigator in our group, Vincent Gattone II, has published a preliminary report that TGF-a, a growth factor, induces epithelial cysts in transgenic animals. So we now know that a variety of factors can trigger abnormal growth in kidney epithelial cells. When the hyperplasia is triggered, the growing cells remain for a time in contact with the lumen of the tubule, urine fills the cavity made by the expanding cells, and a cyst is established. There is the alternative possibility that the trigger for cyst formation is actually an extracellular fluid matrix abnormality. Frank Carone at Northwestern University postulated an abnormality in heparan proteoglycan that results in a defect in the
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Table 1. Three Common Variants of Cystic Kidney Disease: Comparisons and Contrasts
Prevalence Age of onset Symptoms Kidney size Hypertension Hematuria Associated conditions Azotemia Liver disease Arterial aneurysm Differential diagnosis
Autosomal Dominant Polycystic Kidney Disease
Autosomal Recessive Polycystic Kidney Disease
1 :SOOto 1:1,000 In utero Common Enlarged Common Common
1:16,000 In utero Common Enlarged Common Occasional
500/o of dialysis patients Adulthood Common Variable Variable Common
Common 40% of patients
Common All patients
All patients No patients
10% to 40% of patients
No patients
No patients
Autosomal recessive polycystic kidney disease, tuberous sclerosis, multiple simple cysts
Autosomal dominant polycystic kidney disease, medullary sponge kidney
Autosomal dominant polycystic kidney disease, simple cysts, von Hippei-Lindau disease
feedback control of epithelial cell growth. A number of experiments in both acquired and hereditary models suggest that extracellular fluid and extracellular matrix molecules do play a role in cystogenesis. This hypothesis offers an economical explanation of the systemic nature of PKD: A defect in the basement membranes and other collagen products in the brain and the heart could explain the aneurysms and mitral valve prolapse frequently associated with PKD. The third key element is fluid accumulation. It does not seem to cause cyst formation. but it is a very important part of the process. Fluid accumulation is no mystery when the cystic tubule is
still connected to the glomerular ffiter. The fluid flows in a normal way from the ffiter into the tubule and the expanding cyst. But that makes sense only while the cyst is in its very early stages. When the cyst grows larger than 200 to 300 J.lm, it has been calculated that the glomerular filtrate should no longer be sufficient to keep the cyst full of fluid. In a large cyst, all the normal ffitrate should be reabsorbed by the epithelial cells. When the cyst is much larger than 200 J.lm, another process must result in the accumulation of fluid. Either the cells stop absorbing filtrate or a secretory process is involved. There is evidence for a secretory process. When cysts reach several millimeters. they separate from the tubule wall and are
Acquired Cystic Kidney Disease
sealed off. At this stage, there are no openings in the cyst and no conduit through which fluid could enter. When we took cells from the walls of these cysts and studied them in culture. we found unequivocal evidence that fluid moves in the secretory direction when the cell is stimulated with compounds that activate cyclic AMP (Figure 2). This was something of a surprise, since kidney epithelial cells were not supposed to be secretory. It turns out, however, that normal kidney cells probably do secrete fluid, although this process is usually hidden by the kidney's function. The human kidney normally filters about 150 liters of fluid a day. so if the kidney is also secreting 1 liter Hospital Practice March 15. 1992
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Figure 1. The pathogenesis of polycystic kidney disease as defined by In vivo and In vitro studies involves three major renal tubular events: epithelial proliferation (A), fluid accu-
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Hospital Practice March 15, 1992
mulation (B), and remodeling of the extracellular matrix (C). Both epithelial proliferation and matrix remodeling have been proposed as the initiating event In cyst formation.
Figure 2. Epithelial cells isolated from renal cysts are capable of secretion. When exposed to a cyclic AMP stimulant, renal epithelial cells are induced to move chloride (1), sodium (2), and ter (3) into the luminal compartment. Chloride Is mobilized from within the cells and Is directly affected by cAMP stimulation. Sodium is secondarily af. fected, moving for the most part between cells, aithough some intracellular sodium may also be pumped into the cyst cavity. Water is presumably secreted into the cystic lumen in response to the sodium-chloride gradient that has been established.
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w•
per day, that secretion would be reabsorbed along with the normal filtrate-and the kidney's secretory function would escape notice. But when kidney tubules are isolated from their filters, their secretory capability then is apparent. We think that kidney epithelium can secrete fluid in almost all segments of the nephron. We know that, for a fact, in the proximal tubules. We think that it is true in thick ascending limbs, and that it is probably also true in the collecting duct. We are currently studying kidney cell secretion of fluid. As in other secretory epithelium. the process is activated by cAMP and makes use of a sodium pump to set up a gradient for sodium. Chloride comes into the cell from the interstitium, borrowing the energy of the sodium gradient. Potassium comes in, too, but sodium drives the process. Chloride leaves the cell on the luminal surface by going down an electrochemical gradient into the lumen.· Patricia Wilson at Johns Hopkins University and Ellis Avner at the University of Washington have suggested that cyst cells may also mislocate some of their sodium pumps (Na-K-ATPase) from the basolateral to the Hospital Practice March 15. 1992
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apical surface. This would enhance the net movement ofNaCl toward the secretory direction. The fluid secretion associated with cyst formation calls to mind the defect in cystic fibrosis, which involves a deficiency in the chloride transport process. To a certain extent, cystic fibrosis and PKD are mirror images. In cystic fibrosis, cells do not secrete enough fluid. In polycystic kidneys, cells appear to be making too much fluid. This is not to suggest that PKD involves a defect in the cystic fibrosis transmembrane conductance regulator gene, but that the two pathologic processes may involve defects of the same secretory pathway. Again, there is no evidence that fluid accumulation initiates cystogenesis. Nevertheless, fluid accumulation is an essential component of cyst formation. The mutation or mutations that trigger cell proliferation and matrix remodeling must preserve (if not enhance) the machinery for fluid secretion if the growing mass is to form a cyst rather than a tumor (Figure 3). In fact, some transgenic models that apparently are heterogeneous develop a mixture of solid tumors and cysts, as though some ofthe nephron segments are expressing the chloride transporter and others are not. The acquired form of PKD is triggered by a different factor or factors. Evidence is accumulating to implicate something in the blood-an increase in a growthpromoting factor that apparently affects kidney epithelial cells. The interesting experiment here is done inadvertently with transplanted kidneys: If you follow a patient with ACKD who receives a successful kidney transplant, acquired cystic disease disap56
Hospital Practice March 15. 1992
pears in the native kidney. If you give a patient with autosomal dominant PKD a kidney from someone who is unrelated, cysts do not develop in the transplanted kidney, nor do cysts disappear in the native kidney. Acquired cystic kidney disease usually manifests in kidneys that are already functionally impaired. The acquired disease seems to be a consequence of a normal growth control factor signaling surviving nephrons to grow, presumably to compensate for the functional loss of a large percentage of nephrons. The problem is that the surviving nephrons grow so much that they become cysts, and then for reasons that are not at all clear, something in the uremic environment causes a certain fraction of those cysts to become tumors, which in turn may become carcinomas. In the genetic forms of PKD, the cysts almost invariably remain benign, floppy sacks of fluid.
Etiology ofPKD and the Search for PKD Genes Despite the progress we have made in understanding epithelial proliferation, matrix remodeling, and fluid accumulation, it is likely that we will not know for certain which is the primary problem in autosomal dominant PKD and autosomal recessive PKD until the genes for each are cloned. The search for the PKD gene has been complicated by its location in a gene-rich segment of DNA. Since 1985, it has been known that the gene responsible for autosomal dominant PKD is located somewhere near the tip of the short arm of chromosome 16 (Figure 4). S. T. Reeders and his group at Yale University cloned the segment of chromosome 16 that contains
the PKD gene, which turns out to contain more than a dozen candidate genes. Workers are now sequencing these genes, making probes to see if they localize in the kidney, and looking in genetic libraries or gene data bases for homologies with other known genes. Another complication in the gene search has been the continuing uncertainty about the disease's primary defect. It is one thing to clone and sequence genes, but how will molecular geneticists know when they have the right gene? The PKD gene could have a peculiar, very fine regulatory function at some level in genetic transcription. Or perhaps it is a gene for regulating cell proliferation and chloride transport. In contrast, investigators who were searching for the cystic fibrosis gene had a solid lead from pathogenetic studies: It had already been shown that sweat ducts from patients with cystic fibrosis were relatively impermeable to chloride. In the search for the PKD gene, we cannot tell investigators what to look for because we do not know which defect is primary. We know that we are looking for something that affects growth regulation, and something that affects matrix remodeling, but we do not know at all what parts of those processes are abnormal. So the gene search is proceeding, somewhat blindly, even as the cell biology search proceeds, also somewhat blindly, with the hope that somewhere the two approaches will meet. That could happen almost any time now, or both searches could go on for years. There is an element of serendipity in the process, which is illustrated by a recent success in which biochemist
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Figure 3. Although fluid accumulation probably does not initiate renal cyst formation, it is viewed as integral to the process. In conjunction with epithelial proliferation and ma-
trix remodeling, fluid accumulation would be expected to lead to formation of a cyst (left). In the absence of fluid accumulation, the expected result would be a tumor (right). Hospital Practice March 15, 1992
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Figure 4. The genes responsible for polycystic kidney disease remain to be precisely localized and characterized. However, it has been determined that the gene responsible for autosomal dominant PKD is located near the tip of the short arm of chromosome 16.
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Billy Hudson, here at the University of Kansas, had sequenced a segment of a glomerular basement membrane protein that he suspected was important in kidney disease. Independently, Reeders had fished a gene of unknown function out of a eDNA library. The gene turned out to code for a basement membrane protein, and the two discoveries revealed that the previously unknown protein plays an important role in Goodpasture syndrome, a rare immune complex disease that includes a progressive glomerulonephritis, and in Alport syndrome, a hereditary nephropathy. Unfortunately, no such connection has yet been made between cell biology and DNA studies ofPKD. An additional complication is the fact that more than one gene is responsible for PKD. The gene that was localized in 1985 to the short arm of chromosome 16 turns out to be responsible for approximately 90% of the 500,000 cases of the autosomal dominant disease in the United States (in Australia it seems to account for 80% ofthe total, in other places as much as 95%). The 10% of U.S. autosomal dominant PKD cases not linked to this PKD-1 type have not yet been linked to any other genetic marker. Interestingly, these PKD-2 patients seem to have a milder form of the disease than do the PKD-1 types. PKD-2 patients do, however, have all the secondary features, such as liver cysts and aneurysms. The variable expression of the disease may result not only from the fact that more than one gene can cause PKD. There may also be more than one mutation within the PKD-1 gene, which would account for some of the subtle differences in how the disease is
expressed. Alternatively, there must be cofactors other than the genetic mutation. Why. for instance, does one infant die of autosomal dominant PKD while someone else in the family-presumably with the same genetic defect-survives to age 80 with no ill effects? The heterogeneity and variable expression of the disease indicate that there are many important cofactors; at the same time, these findings provide a reason for optimism. If we identify the cofactors that result in the early demise of one PKD patient and the long, relatively trouble-free life of another, we will have identified targets for treating the disease.
Implications for Developing Treatment Both cell biology and molecular genetics promise insight into strategies for treating PKD. Studies aimed at understanding the myriad events involved in cystogenesis can be expected to identify where to intervene, just as understanding the renin-angiotensin system has resulted in targeted therapy for the hypertension associated with PKD (and, of course, for other forms of hypertension). Other treatment approaches for the underlying defect in polycystic kidney disease will be developed as the molecular events in cystogenesis are revealedand new approaches are very much needed; current therapy is directed only at alleviating symptoms of the disease. Cystreduction surgery offers some relief in advanced cases ofPKD, but it does not offer a long-term solution. Cancer chemotherapy models are of no use in treating kidney cysts because they tend to grow very slowly over many
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decades. Cancer chemotherapy is successful only when a rapidly growing tumor picks up the drug, incorporates it, and kills itself. Chemotherapy that would kill a slow-growing cyst would kill virtually everything else along with it. Certain kinds of tumors are hormone responsive. and drugs (tamoxifen, for example) that interfere with various hormone receptors are being identified. Areceptor-blocking strategy may also work for cysts, if they do indeed have abnormal receptors for known substances, such as epidermal growth factor or nerve growth factor. It is already known that drugs that activate cAMP in experimental models also activate and accelerate the formation of cysts. This is work we have done in an in vitro model with agents such as the antidiuretic hormone arginine vasopressin and prostaglandins. We are now studying whether this also happens in animals. Slowing fluid accumulation is another promising strategy that relies on gaining an understanding of the events at the molecular level. The proliferating epithelial cells are not malignant. They do not account for most of the mass in the large cysts that compress and distort normal parenchyma and lead to renal insufficiency. By themselves, the superfluous epithelial cells might not disrupt kidney function seriously. Thus, a treatment that prevents fluid accumulation would not cure the disease but would control its effects. Presumably. this would be a pharmacologic approach similar to diuresis, a simple enough strategy in which the reabsorption of fluid in the tubule is blocked. To prevent fluid accumulation in PKD, we will need drugs that block secretion rath-
er than absorption. And there already are some candidate drugs that appear to have the potential of blocking chloride transport in epithelial cells. Growing understanding of the cellular and molecular biology of cystogenesis offers reason for optimism about developing new treatments for PKDeven before we understand what the PKD genes are doing. Cloning the genes for PKD and identifying their defective protein
products will no doubt lead the way to additional treatment, but this advance is farther off. Gene therapy is not a realistic hope before the turn of the millennium. This is not to diminish the importance and the promise of finding the PKD genes: Having the gene product in hand and knowing its function will be a great help in defining the events involved in cystogenesis. Both DNA and cell biology studies are needed to develop new therapy for polycystic kidney disease. D
Selected Reading Gardner KD Jr. Bernstein J: The Cystic Kidney. Kluwer Academic, Hingham, Mass, 1990 Ishikawa 1: Acquired cystic disease: Mechanisms and manifestations. Semin Nephrolll :671. 1991 Grantham JJ: Acquired cystic kidney disease. Kidney lnt 40: 143, 1991 Mangoo-Karim R et al: Renal epithelial cyst formation and enlargement in vitro: Dependence on cAMP. Proc Natl Acad Sci 86: 6007. 1989 Wilson PD et al: Reversed polarity of Na-K-ATPase: Mislocation to apical plasma membranes in polycystic kidney disease epithelia. Am J Physiol 260:F420, 1991 Wilson PD: Cell biology of human autosomal dominant polycystic disease. Semin Nephrol11:607, 1991 Grantham JJ: Polycystic kidney disease: Neoplasia 1n disguise. Am J Kidney Dis 15:110, 1990 Trudel M, D'Agati V. Costantlnl F: c-myc as an indicator of polycystic kidney disease in transgenic mice. Kidney lnt 39:665, 1991 Cowley BD et al: Elevated proto-oncogene expression in polycystic kidneys of the C57BL/6J (cpkl mouse. JAm Soc Nephrol2:1048, 1991 Reeders ST et al: A hl.ghly polymorphic DNA marker linked to adult polycystic kidney disease on chromosome 16. Nature 317 : 542, 1985 Kimberling WJ et al: Linkage heterogeneity of autosomal dominant polycystic kidney disease. N Engl J Med 319:913, 1988 Carone FA et al: Basement membrane antigens in renal polycystic kidney disease. Am J Pathol130:466, 1988 Grantham JJ, Gabow PA: Polycystic kidney disease. In Diseases of the Kidney. 4th ed, Schrier RW, Gottschalk CW (Eds). Lit1le, Brown, Boston, 1988, pp 583-615
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ISSN: 2154-8331 (Print) 2377-1003 (Online) Journal homepage: http://www.tandfonline.com/loi/ihop20
Polycystic Kidney Disease: I. Etiology and Pathogenesis Jared J. Grantham To cite this article: Jared J. Grantham (1992) Polycystic Kidney Disease: I. Etiology and Pathogenesis, Hospital Practice, 27:3, 51-59, DOI: 10.1080/21548331.1992.11705379 To link to this article: http://dx.doi.org/10.1080/21548331.1992.11705379
Published online: 17 May 2016.
Submit your article to this journal
View related articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ihop20 Download by: [RMIT University Library]
Date: 18 July 2016, At: 23:38
Polycystic Kidney Disease: I. Etiology and Pathogenesis JARED J. GRANTHAM
UniversityojKansas
Downloaded by [RMIT University Library] at 23:38 18 July 2016
Geneticists and cell biologists are simultaneously studying the disease's underlying mechanisms. Although a great many questions remain about cyst formation and its role in kidney failure, three fundamental constructs on its etiology and pathogenesis have been generated: renal tubular epithelial proliferation, fluid accumulation, and remodeling of the extracellular matrix. Polycystic kidney disease is one of the most common hereditary diseases in the United States, affecting more than 500,000 Americans. It is estimated that more than 5 million persons worldwide are at risk for the disease. PKD affects children, and also adults in the prime of life. It appears indiscriminately in men and women of all races and ethnic backgrounds, caustngrenal insufficiency in halfbefore age 70. Despite its high incidence and considerable impact on public health, PKD for many years was a relatively unknown disease. In popular accounts, cystic fibrosis (which affects approximately 30,000 Americans) is often identified as the most common hereditary disease in the United States. In fact, cystic fibrosis does not even come close to that status. After PKD, a 1-antitrypsin deficiency with its consequent emphysema affecting perhaps 120,000 Americans, is a distant second. Even if polycystic kidney disease has attracted little public attention, it is not being neglected scientifically. A number of geneticists are searching for PKD genes, and cell biologists are simultaneously studying the disease's underlying mechanisms. Although a great many questions remain about cyst formation and its role in kidney failure, the cell biology studies have coalesced into three fundamental constructs on the etiology and pathogenesis of PKD. At present, those three phenomena-epithelial proliferation, fluid accumulation, and matrix remodeling-offer the best targets for developing new pharmacologic approaches to treating PKD. This article will review the mechanisms underlying the etiology and pathogenesis of polycystic kidney disease and will close with a brief discussion of what the research implies about strategies for
treatment. In a subsequent article, William Bennett of the Oregon Health Sciences University will discuss the management ofPKD-especially the autosomal dominant version. Renal cysts are the most common abnormal structures found in the kidneys. Men and women over 50 years of age have a 50% chance of having one or more renal cysts. These common simple cysts are not the result of genetic influences, nor are they pathologic. In contrast, PKD involves at least dozens and often many thousands of renal cysts. Three distinct disorders account for most cases ofPKD (Table 1). Autosomal dominant polycystic kidney disease is characterized by a fairly slow progression from birth until 50 or 60 years of age, when the kidneys begin to malfunction. Autosomal dominant PKD has, however, a wide variation in expression; it has been observed in aborted fetuses or the fetuses of accident victims in the first trimester. Babies have been born with autosomally dominant polycystic kidney disease so advanced that Wilms' tumor was diagnosed. In other patients, the cysts are not discovered for 70 or 80 years, and even then only incidentally. Generally, dominant PKD is a slowly progressive disease that can begin in any section of the kidney tubule. Other sections-and in fact many nephrons-remain cyst free. Autosomal dominant PKD can be caused by at least two different mutations, a discovery that did
Dr. Grantham is Professor of Medicine and Director of Nephrology, Department of Medicine, University of Kansas School of Medicine, Kansas City. Hospital Practice March 15, 1992
51
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not surprise investigators who were struggling to understand the pathogenesis of renal cysts. The dominant variant is a systemic disease that by definition causes polycystic kidneys, and often (40% to 60% of the time) produces polycystic livers. Patients with autosomal dominant PKD also may have a polycystic pancreas, spleen, brain, or seminal vesicles, and often have mitral valve disorders and abdominal hernias. Because these other components appear in many but not all cases, the pathogenesis of the disease has been hard to understand. Autosomal recessive polycystic kidney disease tends to manifest earlier and is generally more severe. Patients with the recessive disease obviously have a double dose of a PKD gene (probably not the same gene that causes the dominant disease). Recessive disease begins very early in utero, and affected infants often die at birth or shortly thereafter of kidney failure and lung dysgenesis. In these cases, the kidneys may be so enlarged that they constitute 10% of the baby's total body weight. But again, there is variability in the expression of autosomal recessive PKD: Perhaps 25% of this patient population survives into infancy and adolescence, or even into adulthood. Cysts associated with autosomal recessive PKD begin in the collecting ducts far down in the nephron. Almost every collecting duct is affected, which explains why autosomal recessive PKD progresses rapidly and often lethally. The third important polycystic kidney disease has no hereditary basis. Acquired cystic kidney disease (ACKD) is often seen in patients with progressive renal failure developing from diabetes, glomerulonephritis, hy52
Hospital Practice March 15, 1992
pertension, or other noncystic diseases. These patients start to exhibit kidney cysts as they progress toward the end stage of their disease. This acquired form ofPKD is seen most commonly after dialysis. Initially, there was some thought that dialysis actually contributed to the formation of cysts, but it is now clear that dialysis merely extends the life of the patient long enough for the acquired cystic disease to appear. ACKD per se is not a major problem unless the cysts bleed, but these acquired kidney cysts are also associated with a high incidence of cancer. Patients with ACKD are 40 times more likely to have a kidney carcinoma than the population at large. As far as we know, neither of the genetic forms of polycystic kidney disease increases the risk of cancer. Cysts associated with ACKD can form in all segments of the nephron. It appears that they begin primarily in the proximal tubule, although they have been described in the distal parts of the tubule as well. There are many other polycystic kidney diseases, including some others associated with congenital disorders, but the bulk of the research effort currently focuses on these three forms ofPKD.
Pathogenesis Over the past decade, cell biologists have shown that epithelial proliferation, fluid accumulation, and matrix remodeling are the three pathogenetic forces involved in cyst formation (Figure 1). It is not yet clear which force or combination of forces is causative. My bias is for epithelial proliferation due to abnormalities in the regulation of cell growth. There are many candidate mechanisms for such abnormalities: a mutation in an
oncogene we have yet to discover, a defective suppressor gene, or a mutation that makes these cells more susceptible to normal growth factors (such as epidermal growth factor, nerve growth factor, or perhaps some unidentified growth factors). It does not appear that a circulating growth factor per se could account for PKD, because patients who get kidney transplants do not manifest the disease in the allograft. Therefore, something intrinsic to the kidney is probably responsible for cyst formation, at least in the dominantly inherited disease. Several years ago here at the University of Kansas, we found that cystic kidney epithelial cells in mice show an increased expression of the proto-oncogene c-myc. Marie Trudel, a Canadian researcher, engineered an activated c-myc oncogene into the mouse genome, and the transgenic mice developed PKD. Investigators have also made transgenic mice with another oncogene, SV-40 large T antigen, which also results in PKD mice. And a young investigator in our group, Vincent Gattone II, has published a preliminary report that TGF-a, a growth factor, induces epithelial cysts in transgenic animals. So we now know that a variety of factors can trigger abnormal growth in kidney epithelial cells. When the hyperplasia is triggered, the growing cells remain for a time in contact with the lumen of the tubule, urine fills the cavity made by the expanding cells, and a cyst is established. There is the alternative possibility that the trigger for cyst formation is actually an extracellular fluid matrix abnormality. Frank Carone at Northwestern University postulated an abnormality in heparan proteoglycan that results in a defect in the
Downloaded by [RMIT University Library] at 23:38 18 July 2016
Table 1. Three Common Variants of Cystic Kidney Disease: Comparisons and Contrasts
Prevalence Age of onset Symptoms Kidney size Hypertension Hematuria Associated conditions Azotemia Liver disease Arterial aneurysm Differential diagnosis
Autosomal Dominant Polycystic Kidney Disease
Autosomal Recessive Polycystic Kidney Disease
1 :SOOto 1:1,000 In utero Common Enlarged Common Common
1:16,000 In utero Common Enlarged Common Occasional
500/o of dialysis patients Adulthood Common Variable Variable Common
Common 40% of patients
Common All patients
All patients No patients
10% to 40% of patients
No patients
No patients
Autosomal recessive polycystic kidney disease, tuberous sclerosis, multiple simple cysts
Autosomal dominant polycystic kidney disease, medullary sponge kidney
Autosomal dominant polycystic kidney disease, simple cysts, von Hippei-Lindau disease
feedback control of epithelial cell growth. A number of experiments in both acquired and hereditary models suggest that extracellular fluid and extracellular matrix molecules do play a role in cystogenesis. This hypothesis offers an economical explanation of the systemic nature of PKD: A defect in the basement membranes and other collagen products in the brain and the heart could explain the aneurysms and mitral valve prolapse frequently associated with PKD. The third key element is fluid accumulation. It does not seem to cause cyst formation. but it is a very important part of the process. Fluid accumulation is no mystery when the cystic tubule is
still connected to the glomerular ffiter. The fluid flows in a normal way from the ffiter into the tubule and the expanding cyst. But that makes sense only while the cyst is in its very early stages. When the cyst grows larger than 200 to 300 J.lm, it has been calculated that the glomerular filtrate should no longer be sufficient to keep the cyst full of fluid. In a large cyst, all the normal ffitrate should be reabsorbed by the epithelial cells. When the cyst is much larger than 200 J.lm, another process must result in the accumulation of fluid. Either the cells stop absorbing filtrate or a secretory process is involved. There is evidence for a secretory process. When cysts reach several millimeters. they separate from the tubule wall and are
Acquired Cystic Kidney Disease
sealed off. At this stage, there are no openings in the cyst and no conduit through which fluid could enter. When we took cells from the walls of these cysts and studied them in culture. we found unequivocal evidence that fluid moves in the secretory direction when the cell is stimulated with compounds that activate cyclic AMP (Figure 2). This was something of a surprise, since kidney epithelial cells were not supposed to be secretory. It turns out, however, that normal kidney cells probably do secrete fluid, although this process is usually hidden by the kidney's function. The human kidney normally filters about 150 liters of fluid a day. so if the kidney is also secreting 1 liter Hospital Practice March 15. 1992
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Figure 1. The pathogenesis of polycystic kidney disease as defined by In vivo and In vitro studies involves three major renal tubular events: epithelial proliferation (A), fluid accu-
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mulation (B), and remodeling of the extracellular matrix (C). Both epithelial proliferation and matrix remodeling have been proposed as the initiating event In cyst formation.
Figure 2. Epithelial cells isolated from renal cysts are capable of secretion. When exposed to a cyclic AMP stimulant, renal epithelial cells are induced to move chloride (1), sodium (2), and ter (3) into the luminal compartment. Chloride Is mobilized from within the cells and Is directly affected by cAMP stimulation. Sodium is secondarily af. fected, moving for the most part between cells, aithough some intracellular sodium may also be pumped into the cyst cavity. Water is presumably secreted into the cystic lumen in response to the sodium-chloride gradient that has been established.
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per day, that secretion would be reabsorbed along with the normal filtrate-and the kidney's secretory function would escape notice. But when kidney tubules are isolated from their filters, their secretory capability then is apparent. We think that kidney epithelium can secrete fluid in almost all segments of the nephron. We know that, for a fact, in the proximal tubules. We think that it is true in thick ascending limbs, and that it is probably also true in the collecting duct. We are currently studying kidney cell secretion of fluid. As in other secretory epithelium. the process is activated by cAMP and makes use of a sodium pump to set up a gradient for sodium. Chloride comes into the cell from the interstitium, borrowing the energy of the sodium gradient. Potassium comes in, too, but sodium drives the process. Chloride leaves the cell on the luminal surface by going down an electrochemical gradient into the lumen.· Patricia Wilson at Johns Hopkins University and Ellis Avner at the University of Washington have suggested that cyst cells may also mislocate some of their sodium pumps (Na-K-ATPase) from the basolateral to the Hospital Practice March 15. 1992
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apical surface. This would enhance the net movement ofNaCl toward the secretory direction. The fluid secretion associated with cyst formation calls to mind the defect in cystic fibrosis, which involves a deficiency in the chloride transport process. To a certain extent, cystic fibrosis and PKD are mirror images. In cystic fibrosis, cells do not secrete enough fluid. In polycystic kidneys, cells appear to be making too much fluid. This is not to suggest that PKD involves a defect in the cystic fibrosis transmembrane conductance regulator gene, but that the two pathologic processes may involve defects of the same secretory pathway. Again, there is no evidence that fluid accumulation initiates cystogenesis. Nevertheless, fluid accumulation is an essential component of cyst formation. The mutation or mutations that trigger cell proliferation and matrix remodeling must preserve (if not enhance) the machinery for fluid secretion if the growing mass is to form a cyst rather than a tumor (Figure 3). In fact, some transgenic models that apparently are heterogeneous develop a mixture of solid tumors and cysts, as though some ofthe nephron segments are expressing the chloride transporter and others are not. The acquired form of PKD is triggered by a different factor or factors. Evidence is accumulating to implicate something in the blood-an increase in a growthpromoting factor that apparently affects kidney epithelial cells. The interesting experiment here is done inadvertently with transplanted kidneys: If you follow a patient with ACKD who receives a successful kidney transplant, acquired cystic disease disap56
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pears in the native kidney. If you give a patient with autosomal dominant PKD a kidney from someone who is unrelated, cysts do not develop in the transplanted kidney, nor do cysts disappear in the native kidney. Acquired cystic kidney disease usually manifests in kidneys that are already functionally impaired. The acquired disease seems to be a consequence of a normal growth control factor signaling surviving nephrons to grow, presumably to compensate for the functional loss of a large percentage of nephrons. The problem is that the surviving nephrons grow so much that they become cysts, and then for reasons that are not at all clear, something in the uremic environment causes a certain fraction of those cysts to become tumors, which in turn may become carcinomas. In the genetic forms of PKD, the cysts almost invariably remain benign, floppy sacks of fluid.
Etiology ofPKD and the Search for PKD Genes Despite the progress we have made in understanding epithelial proliferation, matrix remodeling, and fluid accumulation, it is likely that we will not know for certain which is the primary problem in autosomal dominant PKD and autosomal recessive PKD until the genes for each are cloned. The search for the PKD gene has been complicated by its location in a gene-rich segment of DNA. Since 1985, it has been known that the gene responsible for autosomal dominant PKD is located somewhere near the tip of the short arm of chromosome 16 (Figure 4). S. T. Reeders and his group at Yale University cloned the segment of chromosome 16 that contains
the PKD gene, which turns out to contain more than a dozen candidate genes. Workers are now sequencing these genes, making probes to see if they localize in the kidney, and looking in genetic libraries or gene data bases for homologies with other known genes. Another complication in the gene search has been the continuing uncertainty about the disease's primary defect. It is one thing to clone and sequence genes, but how will molecular geneticists know when they have the right gene? The PKD gene could have a peculiar, very fine regulatory function at some level in genetic transcription. Or perhaps it is a gene for regulating cell proliferation and chloride transport. In contrast, investigators who were searching for the cystic fibrosis gene had a solid lead from pathogenetic studies: It had already been shown that sweat ducts from patients with cystic fibrosis were relatively impermeable to chloride. In the search for the PKD gene, we cannot tell investigators what to look for because we do not know which defect is primary. We know that we are looking for something that affects growth regulation, and something that affects matrix remodeling, but we do not know at all what parts of those processes are abnormal. So the gene search is proceeding, somewhat blindly, even as the cell biology search proceeds, also somewhat blindly, with the hope that somewhere the two approaches will meet. That could happen almost any time now, or both searches could go on for years. There is an element of serendipity in the process, which is illustrated by a recent success in which biochemist
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Figure 3. Although fluid accumulation probably does not initiate renal cyst formation, it is viewed as integral to the process. In conjunction with epithelial proliferation and ma-
trix remodeling, fluid accumulation would be expected to lead to formation of a cyst (left). In the absence of fluid accumulation, the expected result would be a tumor (right). Hospital Practice March 15, 1992
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Figure 4. The genes responsible for polycystic kidney disease remain to be precisely localized and characterized. However, it has been determined that the gene responsible for autosomal dominant PKD is located near the tip of the short arm of chromosome 16.
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Billy Hudson, here at the University of Kansas, had sequenced a segment of a glomerular basement membrane protein that he suspected was important in kidney disease. Independently, Reeders had fished a gene of unknown function out of a eDNA library. The gene turned out to code for a basement membrane protein, and the two discoveries revealed that the previously unknown protein plays an important role in Goodpasture syndrome, a rare immune complex disease that includes a progressive glomerulonephritis, and in Alport syndrome, a hereditary nephropathy. Unfortunately, no such connection has yet been made between cell biology and DNA studies ofPKD. An additional complication is the fact that more than one gene is responsible for PKD. The gene that was localized in 1985 to the short arm of chromosome 16 turns out to be responsible for approximately 90% of the 500,000 cases of the autosomal dominant disease in the United States (in Australia it seems to account for 80% ofthe total, in other places as much as 95%). The 10% of U.S. autosomal dominant PKD cases not linked to this PKD-1 type have not yet been linked to any other genetic marker. Interestingly, these PKD-2 patients seem to have a milder form of the disease than do the PKD-1 types. PKD-2 patients do, however, have all the secondary features, such as liver cysts and aneurysms. The variable expression of the disease may result not only from the fact that more than one gene can cause PKD. There may also be more than one mutation within the PKD-1 gene, which would account for some of the subtle differences in how the disease is
expressed. Alternatively, there must be cofactors other than the genetic mutation. Why. for instance, does one infant die of autosomal dominant PKD while someone else in the family-presumably with the same genetic defect-survives to age 80 with no ill effects? The heterogeneity and variable expression of the disease indicate that there are many important cofactors; at the same time, these findings provide a reason for optimism. If we identify the cofactors that result in the early demise of one PKD patient and the long, relatively trouble-free life of another, we will have identified targets for treating the disease.
Implications for Developing Treatment Both cell biology and molecular genetics promise insight into strategies for treating PKD. Studies aimed at understanding the myriad events involved in cystogenesis can be expected to identify where to intervene, just as understanding the renin-angiotensin system has resulted in targeted therapy for the hypertension associated with PKD (and, of course, for other forms of hypertension). Other treatment approaches for the underlying defect in polycystic kidney disease will be developed as the molecular events in cystogenesis are revealedand new approaches are very much needed; current therapy is directed only at alleviating symptoms of the disease. Cystreduction surgery offers some relief in advanced cases ofPKD, but it does not offer a long-term solution. Cancer chemotherapy models are of no use in treating kidney cysts because they tend to grow very slowly over many
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decades. Cancer chemotherapy is successful only when a rapidly growing tumor picks up the drug, incorporates it, and kills itself. Chemotherapy that would kill a slow-growing cyst would kill virtually everything else along with it. Certain kinds of tumors are hormone responsive. and drugs (tamoxifen, for example) that interfere with various hormone receptors are being identified. Areceptor-blocking strategy may also work for cysts, if they do indeed have abnormal receptors for known substances, such as epidermal growth factor or nerve growth factor. It is already known that drugs that activate cAMP in experimental models also activate and accelerate the formation of cysts. This is work we have done in an in vitro model with agents such as the antidiuretic hormone arginine vasopressin and prostaglandins. We are now studying whether this also happens in animals. Slowing fluid accumulation is another promising strategy that relies on gaining an understanding of the events at the molecular level. The proliferating epithelial cells are not malignant. They do not account for most of the mass in the large cysts that compress and distort normal parenchyma and lead to renal insufficiency. By themselves, the superfluous epithelial cells might not disrupt kidney function seriously. Thus, a treatment that prevents fluid accumulation would not cure the disease but would control its effects. Presumably. this would be a pharmacologic approach similar to diuresis, a simple enough strategy in which the reabsorption of fluid in the tubule is blocked. To prevent fluid accumulation in PKD, we will need drugs that block secretion rath-
er than absorption. And there already are some candidate drugs that appear to have the potential of blocking chloride transport in epithelial cells. Growing understanding of the cellular and molecular biology of cystogenesis offers reason for optimism about developing new treatments for PKDeven before we understand what the PKD genes are doing. Cloning the genes for PKD and identifying their defective protein
products will no doubt lead the way to additional treatment, but this advance is farther off. Gene therapy is not a realistic hope before the turn of the millennium. This is not to diminish the importance and the promise of finding the PKD genes: Having the gene product in hand and knowing its function will be a great help in defining the events involved in cystogenesis. Both DNA and cell biology studies are needed to develop new therapy for polycystic kidney disease. D
Selected Reading Gardner KD Jr. Bernstein J: The Cystic Kidney. Kluwer Academic, Hingham, Mass, 1990 Ishikawa 1: Acquired cystic disease: Mechanisms and manifestations. Semin Nephrolll :671. 1991 Grantham JJ: Acquired cystic kidney disease. Kidney lnt 40: 143, 1991 Mangoo-Karim R et al: Renal epithelial cyst formation and enlargement in vitro: Dependence on cAMP. Proc Natl Acad Sci 86: 6007. 1989 Wilson PD et al: Reversed polarity of Na-K-ATPase: Mislocation to apical plasma membranes in polycystic kidney disease epithelia. Am J Physiol 260:F420, 1991 Wilson PD: Cell biology of human autosomal dominant polycystic disease. Semin Nephrol11:607, 1991 Grantham JJ: Polycystic kidney disease: Neoplasia 1n disguise. Am J Kidney Dis 15:110, 1990 Trudel M, D'Agati V. Costantlnl F: c-myc as an indicator of polycystic kidney disease in transgenic mice. Kidney lnt 39:665, 1991 Cowley BD et al: Elevated proto-oncogene expression in polycystic kidneys of the C57BL/6J (cpkl mouse. JAm Soc Nephrol2:1048, 1991 Reeders ST et al: A hl.ghly polymorphic DNA marker linked to adult polycystic kidney disease on chromosome 16. Nature 317 : 542, 1985 Kimberling WJ et al: Linkage heterogeneity of autosomal dominant polycystic kidney disease. N Engl J Med 319:913, 1988 Carone FA et al: Basement membrane antigens in renal polycystic kidney disease. Am J Pathol130:466, 1988 Grantham JJ, Gabow PA: Polycystic kidney disease. In Diseases of the Kidney. 4th ed, Schrier RW, Gottschalk CW (Eds). Lit1le, Brown, Boston, 1988, pp 583-615
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