Synergistic Genetic Interactions between Pkhd1 and Pkd1 Result in an ARPKD-Like Phenotype in Murine Models
Authors: Olson Rory J. , Hopp Katharina , Wells Harrison , Smith Jessica M. , Furtado Jessica , Constans Megan M. , Escobar Diana L. , Geurts Aron M. , Torres Vicente E. , Harris Peter C. 
Published: August 2019

Significance Statement
The lack of rapidly progressive murine models reflecting the more severe end of the spectrum of autosomal recessive polycystic kidney disease (ARPKD) inhibits progress to understanding ARPKD pathogenesis. Defects in primary cilia have been implicated in polycystic kidney disease, but their potential role is poorly understood. The authors generated and characterized new mouse and rat models of ARPKD and autosomal dominant polycystic kidney disease (ADPKD) and investigated the interaction between causative genes for these two conditions. Their digenic models demonstrated a synergistic interaction that better reflects the early-onset disease characteristic of ARPKD. Analysis of mRNA expression in the models highlighted different disrupted pathways, but with a commonality of dysregulated mechanisms associated with primary cilia. These models may improve understanding of ARPKD and preclinical testing for this disease.

Background
Autosomal recessive polycystic kidney disease (ARPKD) and autosomal dominant polycystic kidney disease (ADPKD) are genetically distinct, with ADPKD usually caused by the genes PKD1 or PKD2 (encoding polycystin-1 and polycystin-2, respectively) and ARPKD caused by PKHD1 (encoding fibrocystin/polyductin [FPC]). Primary cilia have been considered central to PKD pathogenesis due to protein localization and common cystic phenotypes in syndromic ciliopathies, but their relevance is questioned in the simple PKDs. ARPKD’s mild phenotype in murine models versus in humans has hampered investigating its pathogenesis.

Methods
To study the interaction between Pkhd1 and Pkd1, including dosage effects on the phenotype, we generated digenic mouse and rat models and characterized and compared digenic, monogenic, and wild-type phenotypes.

Results
The genetic interaction was synergistic in both species, with digenic animals exhibiting phenotypes of rapidly progressive PKD and early lethality resembling classic ARPKD. Genetic interaction between Pkhd1 and Pkd1 depended on dosage in the digenic murine models, with no significant enhancement of the monogenic phenotype until a threshold of reduced expression at the second locus was breached. Pkhd1 loss did not alter expression, maturation, or localization of the ADPKD polycystin proteins, with no interaction detected between the ARPKD FPC protein and polycystins. RNA-seq analysis in the digenic and monogenic mouse models highlighted the ciliary compartment as a common dysregulated target, with enhanced ciliary expression and length changes in the digenic models.

Conclusions
These data indicate that FPC and the polycystins work independently, with separate disease-causing thresholds; however, a combined protein threshold triggers the synergistic, cystogenic response because of enhanced dysregulation of primary cilia. These insights into pathogenesis highlight possible common therapeutic targets.