Synergistic Genetic Interactions between Pkhd1 and Pkd1 Result in an ARPKD-Like Phenotype in Murine Models

Abstract

BackgroundAutosomal 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.MethodsTo 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.ResultsThe 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.ConclusionsThese 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.

The carboxy‐terminus of the human ARPKD protein fibrocystin can control STAT3 signalling by regulating SRC‐activation

“…4 Yet, the cytoplasmic tail is poorly conserved and Pkhd1-deficient mice do not develop a renal phenotype resembling human ARPKD. 6,8,14 Our genetic findings and the experimental cell culture data support the hypothesis that FC's cytoplasmic tail may indeed be functionally important in humans. The transmembrane domain and post-translational processing may be required for full functionality.…”

“…Our data have two important implications: Firstly, the pathogenic relevance of fibrocystin's cytoplasmic tail has been questioned as deletion of the murine FC C‐term does not result in the liver phenotype observed in other Pkhd1 ‐deficient models 4 . Yet, the cytoplasmic tail is poorly conserved and Pkhd1 ‐deficient mice do not develop a renal phenotype resembling human ARPKD 6,8,14 . Our genetic findings and the experimental cell culture data support the hypothesis that FC’s cytoplasmic tail may indeed be functionally important in humans.…”

“…The pathophysiology of cystogenesis in ARPKD and the molecular function of fibrocystin are not well understood. Overlapping functions with the ADPKD proteins have been suggested 6‐8 . We show that the C‐term of human FC can regulate SRC‐activation and that SRC and STAT3‐activation can be observed in ARPKD cyst‐lining epithelial cells.…”

The number of nephrons in different glomerular diseases

“…Data from living subjects are scanty, and direct comparisons among different glomerular diseases are lacking. The main result of the present study is that the change of eGFR in IgAN is due to a dysfunction of the glomeruli whereas in nephrotic/proteinuric syndromes it reflects the total number of glomeruli (Rauen et al, 2015, 2018). This is the first study in human subjects demonstrating this difference between the two syndromes.…”

Metabolic Changes in Polycystic Kidney Disease as a Potential Target for Systemic Treatment

“…Patient data and data from animal models of ADPKD and ARPKD furthermore suggest that there may be partial overlap in the cellular pathogenesis of the different types of PKD. Patients with variants in multiple PKD genes have been described that showed a severe phenotype [ 29 ] and crossings of a Pkd1 to a Pkhd1 mouse model resulted in a more severe phenotype [ 24 , 40 ]. A link between Fibrocystin and Polycystin-2 has also been described [ 41 ].…”