MO063DESCRIPTION OF GENETIC VARIANTS IN A COHORT OF TOLVAPTAN ADPKD PATIENTS
Authors: Corradi Valentina , Caprara Carlotta , Pegoraro Ofelia , Mancini Barbara , Giuliani Anna , Gastaldon Fiorella , Giavarina Davide , Ronco Claudio 
Published: May 2021

Abstract

Background and Aims
Autosomal Dominant Polycystic Kidney Disease (ADPKD) is the most common inherited renal cystic disease. It is genetically heterogeneous: 72-75% of ADPKD cases are related to mutations in the PKD1, 15-18 % to PKD2 and the remaining 7–10% of affected are genetically unresolved (GUR). Recent years, new drugs have emerged as promising agents that may retard the progression of ADPKD, such as Tolvaptan. In Italy Tolvaptan is available since 2016 and commonly used since 2017 in ADPKD patients, which fulfill the criteria of “rapid disease progression”, according to the European recommendations. High intra-interfamilial variability in pedigrees was observed, despite the same germ-line mutation. This could be explained by other clinical or genetic factors (environmental, modifier genes, etc), that may affect disease severity. The aim of the study is to describe the genetic variants in a cohort of Tolvaptan ADPKD patients (pts) referral to Renal Genetic Disease Ambulatory of Nephrology Department.

Method
Patients with ADPKD and in Tolvaptan treatment were enrolled. Diagnosis of ADPKD was made upon the revised Ravine’s criteria and Eligibility Criteria for Tolvaptan was made upon Italian indication for Tolvaptan prescription according to Italian Medicine Agency (AIFA) and European Medicines Agency (EMA). We performed genetic analysis (PKD1, PKD2 and PKHD1 genes) to identify mutations by NGS capture-based target enrichment kit (Sophia Genetic™), sequencing on Illumina MySeq Platform® and Sanger Sequencing on 3500 Series Genetic Analyzer (Applied Biosystems™).

Results
Eighteen pts [median age 46 (IQR 39-48) yrs ], 12 male, were included in the analysis. We manage to perform genetic analysis in all pts. Genetic analysis was essential in 4 patients without family history for Tolvaptan eligibility. Sixteen pts (88,9%) have mutations in PKD1, confirming what is already known from the literature for rapid progressor subjects; 2 pts are characterized by PKD2 mutations, both truncating. In only one pt, concomitant with a PKD1 mutation, also a PKHD1 mutation was found. In order to better characterize the cohort it was decided to subdivide the pts into 3 groups, by gene involvement and mutation type:
1st group: 12 Subjects with truncated PKD1 mutation (66.7%). In 7 pts (58,3%) the mutations are within exons (5 and from exon 11 to 15 inclusive) that encode for Immunoglobulin-like repeats or PKD domain of Polycystin 1 (PC1).
2nd group: 4 Subjects with non-truncated PKD1 mutations (22.2%). 3 pts (75.0%) are characterized by missense variants, as previous studies highlighted (a higher percentage of missense mutations in subjects with non-truncating mutations). In 2 pts (50%) the mutations are within exons (2 and 6) that encode for C-type lectin domain (CTLs) di PC1 and typical domain of extracellular protein.
3rd group: 2 Subjects with a PKD2 mutation (11.1%), both truncating. These data confirmed the lower mutation rate of PKD2 compared to PKD1 and highlighted an effective prevalence of truncation mutations in rapid disease progressors as previous reported.

Conclusion
Although our cohort of patients is small, we manage to perform genetic analysis in all pts reaching a detection rate of 100%. In 9 of 16 pts (56,3%) with PKD1 mutation the presence of mutations in exons coding PKD domain in PC1 or Immunoglobulin-like repeats or typical domain of extracellular protein allows us to hypothesize that the resulting alteration of the polycystin-mediated cell recognition and communication processes play a crucial role in the pathogenesis of ADPKD.