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Polycystic kidney disease

Polycystic kidney disease (PKD) is a progressive, genetic disorder of the kidneys. It occurs in humans and other animals. PKD is characterized by the presence of multiple cysts (polycystic) in both kidneys. The disease can also damage the liver, pancreas and rarely the heart and brain. The two major forms of polycystic kidney disease are distinguished by their patterns of inheritance. more...

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Autosomal dominant polycystic kidney disease (ADPKD) is generally a late onset disorder characterized by progressive cyst development and bilaterally enlarged kidneys with multiple cysts. Kidney manifestations in this disorder include renal function abnormalities, hypertension, renal pain, and renal insufficiency. Approximately 50% of patients with ADPKD have end-stage renal disease (ESRD) by age 60 years. ADPKD is, however, a systemic disease with cysts in other organs such as the liver, seminal vesicles, pancreas, and arachnoid mater and non-cystic abnormalities such as intracranial aneurysms and dolichoectasias, dilatation of the aortic root and dissection of the thoracic aorta, mitral valve prolapse, and abdominal wall hernias.

Initial human symptoms are hypertension, fatigue and mild pain and urinary tract infections. The disease often leads to chronic renal failure and may result in total loss of kidney function, known as end stage renal disease (ESRD) which requires some form of renal replacement therapy (e.g. dialysis).

Autosomal recessive polycystic kidney disease (ARPKD) is much rarer that ADPKD and is often lethal. The signs and symptoms of the condition are usually apparent at birth or in early infancy.

Genetics

The disease exists both in an autosomal recessive and an autosomal dominant form. The autosomal dominant form, called ADPKD (autosomal dominant PKD or "Adult-onset PKD") is much more common but less severe. In 85% of patients, ADPKD is caused by mutations in the gene PKD1 (chromosomal locus 16p13.3-p13.1); in 15% of patients mutations in PKD2 (chromosomal locus 4q21-q23) are causative.

The recessive form, called ARPKD (autosomal recessive polycystic kidney disease) is the less common variant, mutations in the PKHD1 (chromosomal locus 6p12.2) cause ARPKD.

A very small number of families with polycystic kidney disease do not have apparent mutations in any of the three known genes. An unidentified gene or genes may also be responsible for this disease.

Polycystic kidney disease is one of the most common inherited disorders caused by mutations in a single gene. It affects about 500,000 people in the United States. The autosomal dominant form of the disease is much more common than the autosomal recessive form. Autosomal dominant polycystic kidney disease affects 1 in 400-1,000 people, while the autosomal recessive type is estimated to occur in 1 in 20,000-40,000 people.

Biology

Recent studies in fundamental cell biology of cilia/flagella using experimental model organisms like the green algae Chlamydomonas, the round worm Caenorhabditis elegans and the mouse Mus musculus have shed light on how PKD develops in patients. All cilia and flagella are constructed and maintained, including localizing of protiens inserted into ciliary and flagellar membranes, by the process of intraflagellar transport. Environmental sensing and cellular signaling pathways initiated from proteins inserted into ciliary/flagellar membranes are thought to be critical for normal renal cell development and functioning. Membrane protiens which function in developmental and physiological environmental sensing and intracellular signalling are sorted to and localized to the cilia in renal epithelial cells by intraflagellar transport. These epithelial cells line the lumen of the urinary collecting ducts and sense the flow of urine. Failure in flow-sensing signaling results in programed cell death or apoptosis of these renal epithelial cells producing the characteristc multiple cysts of PKD. PKD may result from mutations of signaling and environmantal sensing protiens, or failure in intraflagellar transport. Two PKD genes, PKD1 and PKD2, encode membrane proteins which localize to a non-motile cilium on the renal tube cell. Polycystin-2 encoded by PKD2 gene is a calcium channel which allows extracellular calcium ions to enter the cell. Polycystin-1, encoded by PKD1 gene, is thought to be associated with polycystin-2 protein and regulate its channel activity. The calcium ions are important cellular messengers which, in turn, trigger complicated biochemical pathways which lead to cell proliferation and differentiation. Malfunctions of polycystin-1 or polycystin-2 proteins, defects in the assembly of the cilium on the renal tube cell, failures in targeting these two proteins to the cilium, and deregulations of calcium signaling all likely cause the occurrence of PKD.

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Control of polycystic kidney disease: an overview - Tips from Other Journals
From American Family Physician, 10/1/02 by Anne D. Walling

Autosomal-dominant polycystic kidney disease (APKD) is estimated to occur in one person per 1,000. A review by Gibson and Watson stresses the importance of early detection of the condition and control of hypertension.

At least two forms of APKD are currently recognized. The more common PKD1 (approximately 86 percent of cases) is attributed to abnormalities on chromosome 16. Patients with PKD1 develop end-stage renal disease at a mean age of 57 (younger in men). PKD2, which is linked to abnormalities on chromosome 4, accounts for fewer than 14 percent of APKD cases. Patients with PKD2 develop end-stage renal disease at an average age of 69. The mechanism of cyst development is not completely understood but involves membrane proteins polycystin 1 and 2, which are important in epithelial-cell differentiation, cell adhesion, ion transport, and cell-to-cell signaling.

Cyst formation often begins in childhood, but APKD may be asymptomatic for many years. Patients may present when cysts distort surrounding tissues (causing pain) or become infected, after hemorrhage into cysts, or when stones form. Diagnosis is usually based on ultrasound scanning.

Approximately 2 percent of PKD1 patients younger than 40 years require renal transplantation, but the proportion rises to 75 percent among those older than 70 years. The rate of progression is variable. Factors that predict a need for early transplantation include male sex, young age at diagnosis, large kidneys, macroscopic hematuria, and hypertension. Blood pressure is the most significant factor in the progression of disease and should be maintained below 130/85 mm Hg. No specific antihypertensive agent is recommended for patients with APKD--the selection should be individualized, based on patient characteristics and response to treatment.

Patients with APKD also may develop abnormalities in other organs. Hepatic cysts may cause hepatomegaly or become infected. Intracranial aneurysms are twice as common in APKD patients than in comparable populations, possibly because of protein abnormalities in arterial walls. These aneurysms rupture at an earlier age than other intracranial vascular abnormalities, often with fatal consequences. Experts disagree on the benefit of screening for occult cranial aneurysms in patients with APKD.

COPYRIGHT 2002 American Academy of Family Physicians
COPYRIGHT 2002 Gale Group

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