Autosomal Dominant Polycystic Kidney Disease in Persian and Persian Related Cats
Leslie A. Lyons1, PhD; David S. Biller2, DVM, Diplomate American College of Veterinary Radiology
Polycystic kidney disease (PKD) is an inherited disease in Persian and Persian related cats. Polycystic kidney disease is not a new disease. This disease has been present and reported sporadically in the veterinary literature for approximately 30 years. Over the last 10 years however it has been characterized as a slowly progressive irreversible inherited renal disease. The inheritance in Persian and Persian related cats is as an autosomal dominant gene.
Autosomal dominant polycystic kidney disease is a late onset renal disease that causes enlarged kidneys with renal dysfunction occurring at 7 years of age on average. Multiple cysts are present bilaterally from birth, and slowly grow in size causing the kidney to enlarge and eventually reducing the kidneys function. Ultimately this can result in renal failure. Cysts may vary in size from less than 1 mm to greater than 1 cm in size, with older animals having larger and more numerous cysts. Clinical signs are nonspecific and include depression, anorexia or reduced appetite, polyuria, polydypsia, and weight loss. These are similar to signs of chronic renal failure of any etiology. Cysts have been documented in other organs in addition to the kidney including the liver and uterus. Due to the slow progression of the disease, not all cats with PKD will develop clinical renal disease and death. The rate of growth of renal cysts is quite variable, with growth rate varying between kidneys in the same cat, between affected parents and offspring, between siblings and between genetic lines.
The gene that causes feline PKD has not yet been identified but many clues can be obtained from other species that have similar disease presentations. Humans, mice, and dogs have well documented conditions with polycystic kidneys. The gene causing the canine disease has not been identified and many of the mouse models are induced by purposely disrupting particular genes. But, in humans, several genes have been identified that cause PKD conditions that are very similar to the feline disease. At least three of these genes produce clinical problems that are found in cats with PKD. Approximately 85% of human PKD is autosomal dominant, like what is found in the cat, and is caused by the gene termed "PKD1." The PKD2 gene causes autosomal recessive PKD in humans, but the earlier onset mimics more the feline condition. The mutations in these genes that lead to the cause of the disease are abundant. Since humans are a rather outbred population, most families with PKD have novel mutations within one of these two genes. Since cats are more inbred and only the Persian breed and not random bred cats have a high frequency for PKD, we suspect that only one mutation in one gene will cause feline PKD. Since the genetic maps of humans and cats are fairly well conserved, markers can be identified in cats that can help determine if one of the major human genes causes feline PKD. Large families segregating for the disease will help to resolve this question. Although other species provide good clues for PKD, the cats could still have a novel gene that causes the disease.
Neither physical findings nor routine laboratory data are sufficient to establish or exclude a definitive diagnosis of PKD early in the course of disease. Therefore, diagnosis of PKD early in life requires imaging. Early identification of PKD is beneficial because it would allow early recognition and treatment of animals with chronic renal failure. It would also allow the identification of asymptomatic affected cats before they are bred. In Persian cats, PKD is inherited as an autosomal dominant trait. Thus, if a PKD positive animal is bred, 50 % of the offspring will develop PKD. Polycystic kidney disease typically has late onset, and affected cats may live many years before developing clinical signs. Early screening followed by neutering of all affected cats could potentially eliminate this disease from the at-risk population.
The diagnosis of PKD may be difficult in its early stages because of variable severity. With minimal disease, the kidneys are normal in size, and their surfaces are smooth. As the size and number of cysts increase, the kidneys enlarge, but normal renal contour often is maintained. In more advanced disease, the cysts may project beyond the normal renal surface, causing it to become irregular. The renal collecting system may appear distorted grossly. Renal involvement is variably asymmetrical because of the progressive nature of the disease and the likelihood that only a small proportion of the nephrons are affected. With advanced disease, the renal parenchyma is almost completely replaced by numerous cysts.
Depending upon the age of the cat and extent of disease, the survey abdominal radiographs of cats with PKD can be unremarkable. As the disease progresses with age, symmetrical or asymmetrical renomegaly is apparent. Often it is not possible to identify the entire renal outline on radiographs, and renomegaly may only be appreciated by identifying displacement of perirenal structures. Initially, the enlarged kidneys may have smooth contours. With further progression, the margins of the kidney become more irregular or bosselated. Differential diagnoses for radiographically-evident renomegaly in cats should include hydronephrosis, granulomatous nephritis due to FIP (Feline Infectious Peritonitis), lymphoma, perinephric pseudocysts (fluid surrounding the kidneys), and PKD.
Ultrasonographyis a sensitive and noninvasive imaging technique for the diagnosis of renal cysts. Its accuracy and reliability however are dependent on the experience of the individuals performing and interpreting the examination. The sonographic characteristics of renal cysts include: 1) a spherical outline; 2) sharply marginated, smooth walls; 3) no internal echoes; and, 4) the presence of through-transmission (acoustic enhancement posterior to the lesion consistent with the size of the lesion). Through transmission may not be observed in cysts that are small.
Polycystic kidney disease is most easily diagnosed using ultrasound. We recommend using a high frequency transducer (7 MHz or higher) to better visualize smaller cysts allowing diagnosis of this disease earlier in life. PKD has been diagnosed as early as 4 weeks of age using ultrasound. With the proper equipment and experience this disease can be diagnosed with approximately 95% accuracy after 10 months of age. The older the cat, the larger the cysts allowing easier detection. All affected cats must have at least one parent that is affected (autosomal dominant trait).
No specifictherapy for PKD currently is available. Treatment of renal failure resulting from PKD is similar to that of chronic renal failure of any etiology and should be directed at slowing the progression and reversing the consequences of renal failure. This includes moderate dietary protein restriction with the use of high biologic value protein, fresh water available at all times, the use of phosphate binders, and treatment of anemia as necessary. No diet or regimen of phosphate restriction has been studied as to its effects on cystogenesis or on preservation of renal function or renal histology.
The Persian cat with PKD should be considered to have an inherited disease. If offspring or relatives are available for study, these animals should be examined by ultrasound. Owners should be informed that this is a heritable disease and that, if the animal is bred, 50 % of the offspring will be affected. It is important to emphasize to an owner that this is a late onset disease, the cat may live many years before demonstrating clinical disease, and breeding of affected cats has great potential to produce PKD-affected offspring. Confirming parentage has been an important step with predicting the disease status for certain offspring.
Elimination of polycystic kidney disease from Persian and Persian related breeds
The Persian cat with PKD should be considered to have an inherited disease. If offspring or relatives are available for study, these animals should be examined by ultrasound. Owners should be informed that this is a heritable disease and that, if the animal is bred, 50 % of the offspring will be affected. It is important to emphasize to an owner that this is a late onset disease, the cat may live many years before demonstrating clinical disease, and breeding of affected cats has great potential to produce PKD-affected offspring. Our definition for polycystic kidney disease is multiple cysts in both kidneys. We may have to alter this definition for ultrasound because we have recognized mature Persian cats ultrasonographically with a single cyst or multiple cysts in one kidney. These cats may have more cysts but the cysts could be smaller than the resolution ability of the equipment used. We suspect these cats are positive although they do not meet the above definition. We have recognized at least one cat with a single cyst that has produced positive offspring. Renal cysts in the general cat population are uncommon. A study is now being undertaken to evaluate non-Persian or Persian related cats that are renal healthy and determine the percentage of these cats that have cyst(s) within their kidneys. This may help us determine the significance of a single cyst or cysts in one or two of the kidneys of a Persian or Persian related cat. If a cat is found positive for cysts with ultrasound it's parents, siblings, and offspring should be checked. The quickest way to eliminate this heritable problem is to neuter or spay these individuals to remove them from the breeding population. If one of these individuals is an extremely valuable cat within the cattery then an alternative to consider is to breed that cat to a PKD negative cat and try to maintain the positive genetic traits that the breeder has worked so hard for, while eliminating the PKD gene through its offspring. It is important that breeders and veterinarians work together to recognize PKD, and develop PKD free breeding programs. This will improve the genetic health of the Persian and Persian related cat breeds.
Research to develop a DNA test or probe is now underway for PKD, but may take 3-5 years to develop. During that period of time the use of ultrasound which is noninvasive and highly accurate may help identify affected cats and reduce the incidence in the Persian population