Pathogenic missense mutations result in defective trafficking of polycystin-1 to cilia

A new study suggests that missense variants in PKD1 contribute to the pathogenesis of autosomal dominant polycystic kidney disease (ADPKD) through altered G-protein site (GPS) cleavage and aberrant cilial expression of its protein product polycystin-1. The way in which these missense mutations promote disease was previously unclear.

Yiqiang Cai and colleagues used a systematic approach to characterize the molecular effects of various mutant forms of polycystin-1 in vitro and analysed the pathogenic consequences in vivo. They developed NH₂-terminal and COOH-terminal epitope-tagged cDNA constructs of wild type and variant forms of polycystin-1 to enable differential detection of the protein fragments that result from GPS-mediated cleavage. “As anti-polycystin-1 antibodies have limited application, these molecular tools significantly advanced our study,” says Cai.

...GPS cleavage of polycystin-1 was necessary for normal trafficking...

Biochemical studies indicated that a subset of missense mutations perturbed trafficking of polycystin-1 to the cilia. Although GPS cleavage of polycystin-1 was necessary for normal trafficking, some variants failed to localize to cilia despite undergoing GPS cleavage. An interaction between polycystin-1 and polycystin-2 was not required for trafficking to cilia, but was necessary to maintain steady-state expression levels of the COOH-terminal fragment of polycystin-1.

The researchers used bacterial artificial chromosome (BAC)-transgenic mouse models to validate their discoveries in vivo. Expression of a wild-type Pkd1 BAC transgene rescued the Pkd1^−/− lethal phenotype; transgenic mutations that prevented GPS cleavage of polycystin-1 could not rescue the null mutation.

“We now have evidence suggesting that different mutations in PKD1 and PKD2 might cause various molecular and/or cellular defects that ultimately result in a common phenotype—cyst formation,” says Cai. “Understanding early defects is important for a better understanding of ADPKD pathogenesis, and could lead to identification of a drug target to control the progress of cyst development.”

The researchers hope to develop a high throughput cell-based system to identify agents that can improve the ciliary expression of polycystin-1 variants. They suggest that agents that assist protein folding may be candidate therapeutics to target cilia-trafficking defects.