#1428 Rnd3, a novel gene involved in ADPKD
Authors: Modarage Kavindiya , Goggolidou Paraskevi 
Published: May 2024

Abstract

Background and Aims
Autosomal dominant polycystic kidney disease (ADPKD) is an adult-onset, multi-systemic disorder, that affects ∼12.5 million people worldwide. ADPKD is characterised by progressive kidney enlargement caused by uncontrollable growth of cysts, alongside extra-renal phenotypes such as cysts in the liver, pancreas, and hypertension. Truncated mutations in PKD1 and PKD2 that encode for PC1 and PC2, respectively, are responsible for 85% and 15% of reported cases. Mutations in PKD1 are associated with the more severe form of ADPKD and earlier mean age of onset. The phenotypic variability of ADPKD can be attributed to genic, allelic and gene modifier effects. Modifier genes influence the molecular and/or phenotypic expression of other genes, altering the range of symptoms and the severity of the disease. In mice with a single gene mutation, modifier genes reduce the penetrance, dominance modification, expressivity, and phenotypic pleiotropy. To understand the involvement of modifier genes in the ADPKD kidney phenotype, a novel mouse model Rnd3tm1b(EUCOMM)Hmgu was generated, and in adult and aged Rnd3tm1b/+ mice, a kidney phenotype characteristic of ADPKD was evident. Rnd3 is an atypical member of the Rnd subgroup of the Rho family of GTPases and has a role in actin cytoskeleton reorganisation, cell cycle progression, and cell migration.

Method
5 and 9-month-old kidneys from Rnd3tm1b/+ and age-matched wildtype mice were studied. In the Rnd3tm1b (EUCOMM)Hmgu mouse model, the TB1B mutation causes an inactivation of Rnd3 via the deletion of Exon 2 in C57BL/6N mice. RT-qPCR and SDS-Page Westerns were performed as well as staining for markers associated with Polycystic Kidney Disease.

Results
The onset of cyst formation in the cortex is apparent in 5-month-old Rnd3tm1b/+ kidneys compared to age-matched wildtype kidneys (Fig. 1a). A statistically significant 10% (p &lt; 0.0465) increase in the proliferative index was observed in 5-month-old Rnd3tm1b/+ mice (Fig. 1e,f). Vimentin expression was also increased in podocytes and confirmed by a statistically significant 25% (p &lt; 0.0461) increase in Rnd3tm1b/+ kidneys (Fig. 1g,h). Aquaporin-2 (Aqp2) expression was decreased in collecting ducts and verified by a statistically significant 0.4-fold-change (p &lt; 0.0451) decrease in protein level in kidneys of 5-month-old Rnd3tm1b/+ mice (Fig. 1i-k). A statistically significant 0.75-fold (p &lt; 0.0006) and 1.5-fold-increase (p &lt; 0.02) in Pkd1 and Pkd2 expression was observed in 5-month-old Rnd3tm1b/+ kidneys, respectively (Fig. 1c).
Kidneys of 9-month-old Rnd3tm1b/+mice presented with multiple cysts in the cortex, mimicking the ADPKD kidney phenotype (Fig. 1b). Vimentin staining in podocytes was increased and confirmed by a statistically significant 14% (p &lt; 0.0419) increase in Rnd3tm1b/+mice (Fig. 1l,m). Expression of Pkd1 and Pkd2 was downregulated by a statistically significant 0.39-fold (p &lt; 0.0001) and 0.2-fold-change (p &lt; 0.004) in Rnd3tm1b/+mice (Fig. 1d).

Conclusion
Together, these findings highlight the importance of Rnd3 as a potential modifier in the onset of ADPKD by mediating the expression of Pkd1 and Pkd2. Interestingly, there is a critical window where Pkd1 and Pkd2 expression is altered, exacerbating the phenotype between 5 and 9-month-old Rnd3tm1b/+ mice. In addition to this, by understanding how modifiers are involved in triggering the onset of phenotypes in ADPKD, potential diagnostic and therapeutic measures can be discovered to ultimately aid in diagnosing ADPKD at a much earlier stage.