#1148 Polycystic kidney disease—beyond PKD1 and PKD2
Authors: de Faria Vitoria Paes , Dias Joana Pereira , Bessa Maria Beatriz , Martingo Maria Alexandra , Lopes Daniela , Almeida Clara , Gomes Ana Marta 
Published: May 2024

Abstract

Background and Aims
Autosomal dominant polycystic kidney disease (ADPKD) is a common form of hereditary kidney disease, with an estimated prevalence of 1 out of 1000 individuals worldwide. Although its key clinical manifestation is progressive formation and growth of renal cysts, extra-renal manifestations such as hypertension, cysts in other organs, cardiac valve disease and cerebral aneurysms reflect that it is a systemic disorder. Classical ADPKD is associated with heterozygous PKD1 and PKD2 pathogenic gene variants. Less common involved genes that may present as atypical cystic disease include GNAB, DNAJB11 and more recently ALG5/9. Genes associated to syndromic ciliopathies such IFT140 and OFD1 have also been related to atypical ADPKD. Literature reports that 10% patients with ADPKD phenotype will have de novo pathogenic variant. Around 7% of families have negative genetic testing despite clinical phenotype. Failure to consider broader genetic panels or exome sequencing if available may lessen diagnostic yield.

Method
We performed descriptive analysis on patients with no family history cystic disease and negative PKD1/PKD2 gene testing in our hereditary nephropathy clinic. Atypical cystic disease was defined as non-enlarged kidneys despite multiple cysts and asymmetrical kidney cyst affection in terms of size and quantity, determined through imaging studies.

Results
From January 2019 until December 2023, n = 39 genetic tests were performed in patients with cystic disease phenotype, n = 28 (13%) without family history. Twelve patients (31%) had positive PKD1/PKD2 variants analysed by PCR long range multiplex.
Fig. 1 represents the genetic test results in the remaining patients (n = 16) with cystic disease phenotype and no family history. Positive genetic testing was obtained in n = 7 (43%).
Ten patients (63%) presented features of atypical cystic disease. These had a median referral age of 48 years (IQR 34-58), n = 3 (30%) male. Three causal genetic variants were identified (1 with variants in IFT140 and 2 in OFD1 gene), the remaining with negative genetic test result.
Patients without atypical features had a median age of referral 55 years (IQR 38-58), n = 4 (60%) were male. Variants in GANAB, LRP5 and PKHD1 genes were identified in this subgroup.
Extra-renal manifestations were seen in in 70% and 50% of patients with and without atypical cystic disease, respectively. The main extra-renal manifestation encountered was hepatic cysts. One patient with a pathogenic OFD1 variant presented syndactyly.

Conclusion
In recent years, growing knowledge in the field and identification of genes involved in ciliopathies associated with atypical forms polycystic kidney disease have allowed for genetic diagnosis of non-PKD1/PKD2 forms. The identification of new genes associated with atypical forms of polycystic kidney disease has called attention to a potential overlap between polycystic and tubulointerstitial phenotypes.
With growing knowledge in genetics leading to improvement of cystic kidney disease panels and increased experience in exome sequencing, diagnostic yield is likely to increase in the coming years. Genetic basis of disease is important considering possibility of avoiding primary transmission through pre-implantatory genetic diagnosis but also in the management and diagnosis of gene-specific systemic manifestations.