Polycystin expression in PKD-1, infantile PKD-1 and TSC-2/PKD-1 cystic kidney: Evidence against a two-hit disease mechanism in cyst initiation.
ASN 30th Annual Meeting, San Antonio
J Am Soc Nephrol (Sep) 8:378A 1997
In the past year, two papers have been published supporting the "two-hit hypothesis" as the mechanism of cyst formation in ADPKD1 (see Germino, G.G., Hospital Practice, 3/15/97). For all genes on somatic chromosomes, each of the two chromosomes carries a version (an allele) of the gene. The hypothesis holds that ADPKD patients inherit one mutated PKD1 allele from the affected parent (thus present in all cells), but that the second normal PKD1 allele produces sufficient Polycystin 1 (Pcys1) protein to support normal cellular function and normal kidney development. Because of an intrinsically high mutation rate in the normal PKD1 gene, a random second mutation in the remaining normal allele ("somatic" mutation) then abolishes Pcys function in the cell so affected. Typically occurring after birth, this leads to a functionally altered renal tubular cell, which then proliferates and gives rise to a single cyst. Multiple unique second-hit mutations lead to multiple unique cysts. This explains some of the observed variability of the disease among affected relatives despite identical mutations in PKD1. This hypothesis predicts that all cells of a given cyst will contain similarly mutated allele pairs, one reflecting the inherited germline mutation (common to all cells) and the other an acquired mutation unique to a specific cyst.
To test this, Ong et al collected ADPKD1 tissues from 13 cases in which the germline mutation was shown to involve loss of the C-terminal end of the Pcys1 protein. Using monoclonal antibodies which recognized respectively the N- and the C-terminal ends of the Pcys1 molecule, the authors reasoned that positive immunoreactivity for the C terminus indicates expression of the "normal" (non-germline, somatic mutant) allele.
The results indicate presence of immunoreactive Pcys1/C terminus (thus expression of the "normal" allele) in most cysts. A few cysts exhibited absence of (immunoreactive) Pcys1 protein, most commonly in very large cysts. Authors concluded that continued presence of the "normal" allele in most cyst epithelial cells speaks against the two-hit hypothesis.
Comment: Although attempting to draw major conclusions from abstracted material is hazardous, the data as presented raise important questions. First, the antibody method employed detects immunoreactivity, not functional activity of Pcys 1 protein. Thus, the persistently expressed protein from the "normal" allele could in fact be mutated at a site in the molecule such that functional activity is lost while immunoreactivity is retained. Persistence of immunologically detectable but nonfunctional protein would not then negate the presence of a second unique mutation in the previously normal allele.
It would therefore seem reasonable to withhold judgement until the functional integrity of the expressed protein is ascertained. The finding of "absent Pcys" in larger, thus likely older, cyst walls may indicate mutational deletion of both alleles; alternatively, it could indicate further phenotypic changes in cyst epithelial cells which have undergone continuous proliferative cycles. On balance, in my view, the second-hit hypothesis still offers the best overall explanation for both clinical and molecular findings but still lacks definitive documentation. (Susan P. Bagby, M.D., Oregon Health Sciences University and VA Portland Medical Center, Portland, OR)
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ASN 30th Annual Meeting, San Antonio
Cystic disease : Hereditary polycystic disease