On gross pathological examination, the polycystic kidney is impressive. The profound morphological disorganization of a tissue that is normally an exquisite exemplar of elegant design speaks to the magnitude of the cellular de-differentiation and dysregulation that can occur as a consequence of relatively small genetic alterations. Autosomal dominant polycystic kidney disease is fairly common, affecting between 1 in 500 to 1 in 1,000 people. The disease is characterized by the slow development, over decades, of large fluid filled cysts in the kidneys. These cysts dramatically enlarge the kidneys and, more importantly, severely compromise the functional integrity of the remaining normal parenchyma. Clinically significant impairments of renal function will usually occur by late middle age. Roughly 50% of ADPKD patients will progress to end stage renal disease, requiring transplant or dialysis.[
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3] There is substantial variability in disease presentation, even within families. Although this heterogeneity can be explained by the variable nature of disease causing somatic mutations, modifier genes may also be inherited independently of the PKD mutation. These modifiers may include the angiotensin-1-converting enzyme (ACE) gene, the CFTR gene or the Tuberous Sclerosis Complex-2 gene, all of which may affect disease severity.[
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9] Although poor prognostic factors such as hypertension, early onset, male gender, increased kidney size and rate of growth, and microalbuminuria have been identified, the reasons for some of these apparent correlations are less well understood.[
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Most patients present with flank pain, hypertension, hematuria, renal insufficiency, and/or proteinuria. The flank pain may be secondary to calculi, renal hemorrhage, or be due to a urinary tract infection.[
2] Renal function begins to decline in the fourth decade of life with the glomerular filtration rate (GFR) decreasing by 4.4 to 5.9 mL/min per year.[
1] The decrease in GFR is inversely proportional to kidney size and cyst volume as assessed by the Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP).[
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20] Because ultrasound measurements cannot discern changes in kidney size over short time intervals, magnetic resonance imaging (MRI) with or without gadolinium has become the gold standard for assessing changes in kidney volume and thereby prognosis.[
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22]
As there is no specific or targeted clinically approved therapy, current practice focuses on strict blood pressure control with ACE inhibition and the use of statins to reduce the associated cardiac mortality that coincides with chronic kidney disease.[
23] Some patients experience abdominal and back pain from the enlarged kidneys and in these cases relief may be obtained via partial or total nephrectomy, or sclerosis of the cysts.[
24] These procedures may also be required to accommodate an allograft. For those patients who progress to end stage renal disease, the options are limited to dialysis or renal transplantation. Hemodialysis is often preferred for technical ease in the setting of enlarged kidneys, and the outcomes of patients on dialysis are comparable to or better than those of non-ADPKD patients.[
25] Mortality in patients with ADPKD is most often attributable to cardiac disease, infection, intracranial aneurysm, and hypertensive intracerebral hemorrhages.[
26]
Amongst ADPKD patients, 85-90% of cases result from mutations in PKD1, while another 10-15% of cases are accounted for by mutations in PKD2.[
27] There are also a small number of families with classic presentations of PKD whose members appear to have mutations at loci distinct from those of PKD1 and PKD2, suggesting that a third locus may be involved.[
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30] Patients with mutations in PKD1 experience a more severe phenotype, with one study reporting a median age of death or onset of end-stage renal disease of 54.3 years versus 74.0 years for patients with PKD2 mutations.[
31] This discrepancy is hypothesized to result from increased cyst number, not size.[
32] At present, over 300 truncating mutations of PKD1 and 91 mutations of PKD2 have been identified in patients with ADPKD. There are approximately another 100 disease-causing mutations which are missense.[
1] Given this large database, molecular diagnostics are now becoming an option for situations when imaging studies are inconclusive, for when patients wish to know their genetic predisposition, or when the question of organ donation arises.[
33] Interestingly, greater than 10% of patients come from families in which neither parent is affected.[
1]
PKD1 (polycystic kidney disease 1, ch16p13.3, 46 exons) encodes polycystin-1 (PC-1), a 462 kD, 4303 amino acid integral membrane protein with 11 transmembrane domains, a long extracellular N-terminus with multiple binding domains and a short cytoplasmic C-terminus that interacts with multiple proteins, including the protein product of PKD2 (polycystic kidney disease 2, ch4q21, 15 exons), polycystin-2.[
3] Polycystin-2 (PC-2) is a significantly smaller 110 kD protein with six transmembrane domains.
Both of the polycystin proteins exhibit complex subcellular localizations. Polycystin-1 is found in the basolateral plasma membrane domain of polarized epithelial cells, where it participates both in intercellular adherence junctions and in focal adhesion complexes with the underlying basement membrane.[
34] In addition, a cleavage product of PC-1 that includes the C-terminal tail can translocate to the nucleus to regulate gene transcription.[
35,
36] Most of the polycystin-2 protein is concentrated in intracellular compartments, where it appears to play a role in regulating the release of calcium from intracellular stores. Its role as a cation channel is consistent with the fact that it is a member of the TRP family of ion channels.[
37] PC-2 may also play a role in cellular proliferation and differentiation by controlling cell cycle regulation.[
38] Both PC-1 and PC-2 are localized to the primary cilium that graces the apical surfaces of most polarized epithelial cell types. This non-motile, chemo- and mechano-sensory structure seems to be critically intertwined in the pathophysiology of renal cystic disease. Mutations in many genes that encode proteins involved in ciliary function lead to some manner of cystic disease.[
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40]
While the exact physiological and pathological roles of these two proteins are still debated, it is clear that renal cystogenesis occurs when both copies of one or the other polycystin gene are either mutated or knocked out.[
41,
42] In mice, homozygous mutations of PKD1 and PKD2 result in embryonic lethality at E12.6-16.5.[
42] Heterozygous mice appear essentially phenotypically normal, occasionally developing a few hepatic and renal cysts later in life.[
43] In addition, decreasing PKD1 expression is sufficient to cause cystic disease in mice[
44] while overexpression of polycystin-1 in transgenic mice also results in renal cyst formation.[
45] A study by Piontek et al. revealed that inactivation of PKD1 prior to postnatal day 13 in conditional knockout mice results in an extremely rapid disease course of cyst development, while inactivation after this developmental time point results in much milder disease progression.[
46,
47] These findings suggest that the polycystin proteins may function as important “brakes” on cell growth and division during renal development and that rapid proliferation of renal epithelial cells, such as occurs during renal development, may create an environment that facilitates the cellular consequences of polycystin mutation to become manifest. There is also evidence for a
trans-heterozygous model of ADPKD, consistent with a two-hit hypothesis for disease initiation.[
48] A third-hit model has also been proposed, in which renal injury stimulates the rapid cellular proliferation that, as noted above, may be a prerequisite for the cystic changes to occur after somatic mutagenesis or in association with reduced polycystin expression. This model may explain, at least in part, why disease initiation occurs so long after the initial genetic insult.[
49]