There are two major theories concerning the pathogenesis of ADPKD; the first and most accepted is that an individual is born with a defective allele and then the normal WT allele undergoes a somatic mutation that gives rise first to a tubular dilation and then a cyst, the `two hit' hypothesis. The second idea is that gene dosage is enough to cause disease, the level of the protein product of the PKD1 gene, polycystin-1 (PC1) decreases below a threshold and cysts appear in a stochastic manner. We wish to investigate these two ideas in a novel manner using the fact that PC1, PC2 and fibrocystin are all present on an exosome we term the exosome-like vesicle (ELV). We know that the level of PC1 is decreased in individuals with PKD1 mutations and that the amount of PC1 per ELV is inversely proportional to height adjust total kidney volume (HtTKV). Observing the total amount of PC1 in pooled exosomes tells us little about the pathogenesis of the disease. However, a new technology has become available which allows an investigator to determine the amount of PC1 on individual ELVs, NanoView. We know that ELVs contain CD133 (prominin) which colocalizes on exosomes with PC1. We predict that if the `two hit' hypothesis is true, then clones of PKD1 null cells in the kidney will produce ELVs that are CD133+ and PC1- and that the NanoView will see a bimodal distribution of PC1+ ELVs in ADPKD but not in normal urine. The ratio of CD133+/PC- to CD133+/PC1+ ELVs will correlate with HtTKV. If haploinsufficiency is the mechanism, then the mean intensity of PC1 per ELV will be inversely proportional to HtTKV. Thus, NanoView technology can distinguish between the `two hit' and haploinsufficiency models. To further dissect the mechanism of haploinsufficiency we will investigate two major ideas. The first is that the human PKD1 gene is unusual in that it has two long CT rich tracts in introns 21 and 22 which interfere with splicing and generate a smaller form of PC1, we term Trunc_PC1. We will investigate the possibility that splicing efficiency correlates inversely with HtTKV. The second idea is that there is person to person variation in PKD1 promoter strength and that the promoter attenuates with age. We will investigate this possibility using real time RT-PCR. We think that the different mechanisms have a profound effect on the way the disease may be treated. If haploinsufficiency is correct, then strategies designed to increase the amount of endogenous PKD1 mRNA and PC1 protein will be appropriate whereas in the `two hit' scenario these strategies will not work and may be deleterious as both alleles are defective.
Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a genetic disorder characterized by the growth of numerous cysts in the kidneys. The mechanism underlying ADPKD is obscure with two major theories, haploinsufficiency (where a simple decrease in protein level is enough to cause disease) and the `two hit' hypotheses (where an individual is born with a defective gene then acquires another mutation on the normal gene causing the disease). This application will use NanoView technology to measure the level of polycystin-1 on exosomes to distinguish these two ideas.
