Cystin is a novel cilia-associated
protein that is disrupted in the cpk mouse, a well-characterized mouse model of
autosomal recessive polycystic kidney disease (
ARPKD). Interestingly, overexpression of the Myc gene is evident in animal models of
ARPKD and is thought to contribute to the renal cystic phenotype. Using a yeast two-hybrid approach, the growth suppressor
protein necdin, known to modulate Myc expression, was found as an interacting partner of cystin. Deletion mapping demonstrated that the C-terminus of cystin and both termini of
necdin are required for their mutual interaction. Speculating that these two
proteins may function to regulate gene expression, we developed a
luciferase reporter assay and observed that
necdin strongly activated the Myc P1 promoter, and cystin did so more modestly. Interestingly, the
necdin effect was significantly abrogated when cystin was co-transfected.
Chromatin immunoprecipitation and electrophoretic mobility shift assays revealed a physical interaction with both
necdin and cystin and the Myc P1 promoter, as well as between these
proteins. The data suggest that these
proteins likely function in a regulatory complex. Thus, we speculate that Myc overexpression in the cpk kidney results from the dysregulation of the cystin-
necdin regulatory complex and c-Myc, in turn, contributes to cystogenesis in the cpk mouse.