Translational research approaches to study pediatric polycystic kidney disease

As ARegPKD has already been running for a few years, the first publications have emerged and can demonstrate the strength and the potential of the chosen international approach. Clinical research in the ARPKD field has been facing the typical problems associated with rare and variable disorders. ARPKD is a very rare disease with an estimated incidence of 1:20.000 [1]. Despite very important work by various groups [8‐10] it has been a challenge to gather detailed longitudinal clinical data on large cohorts. This has impeded the definition of primary endpoints for clinical trials. A very clearly-defined hard primary endpoint is needed for a clinical trial. To allow studying in a cohort in a clinical trial such an end point would need to be reached by a substantial percentage of a study cohort in a reasonable period of time. As numbers in a potential trial on a rare disease like ARPKD will remain limited it appears even more important to clearly define comparable subcohorts that would be at a high risk to reach such an endpoint. Thus, in a first step it is crucial to describe the clinical course of the disease under standard of care conditions and to identify potential clinical, genetic or biochemical risk factors for severe disease progression, which may either affect the kidneys, the liver or both organs. ARegPKD analyses have described the ARPKD phenotype in adults and studied the potential consequences of very early bilateral nephrectomy in ARPKD patients [20, 21]. To identify potential risk markers, genetics is an obvious candidate for a recessive disorder. Genotype-phenotype correlations have for a long time been limited to the type of variant in ARPKD with biallelic PKHD1 variants being associated with severe phenotypes [1]. Indeed, surviving ARPKD patients with biallelic PKHD1 variants have only recently been described [22, 23]. ARegPKD data has recently highlighted the importance of the localization of the variant in addition to its type in patients with missense variants [24]. To study potential genotype-phenotype associations, the gene was categorized into four sections. Patients with either two missense variants in one section or a missense variant and a null variant were analyzed together under the assumption that the weaker variant would define the phenotype in a recessive disorder. The data show that patients with variants affecting the amino acids 709-1837 of the ARPKD protein fibrocystin showed better renal outcome, whereas patients with variants affecting amino acids 2625-4074 showed poorer hepatic outcome. The data may explain some of the variability between the kidney and the liver phenotypes that has been described for ARPKD. The findings may also be a helpful contributor to a risk stratification for ARPKD patients and will open various research fields to study the cellular protein function of fibrocystin, e.g. in preclinical mouse models.

In addition to genetics easy-to-obtain clinical data are highly important for stratification of patients into risk groups. A recent study on 385 ARegPKD patients identified prenatal sonographic identification of enlarged kidneys, kidney cysts as well as documentation of oligo-/anhydramnios as candidates for prenatal markers to predict early dialysis dependency in ARPKD patients [25]. Thirty-six patients with the need for dialysis in the first year of life were compared to 349 patients that did not require dialysis in this time frame. Various markers showed an increased hazard ratio for early dialysis dependency in a multivariate Cox regression analysis with documentation of oligohydramnios or anhydramnios, and the need for postnatal respiratory support being most-powerful markers. Interestingly, a predictive model derived from the dataset could identify a gradual increase of probability of early postnatal dialysis dependency according to the isolated or combined documentation of antenatal detection of enlarged kidneys, kidney cysts and documentation of oligo-/anhydramnios [25]. Thus, the clinical data define a first high-risk profile for severe kidney disease in ARPKD and have served as a basis for the establishment of two first clinical phase 3 trials that are aiming to study the safety of Tolvaptan and its effects on the need of kidney replacement therapy in ARPKD patients (NCT04782258, NCT04786574).

Thus, clinical and genetic markers have been identified in ARegPKD to categorize kidney disease progression in ARPKD. Such markers could in the future be supported and reinforced by radiological findings or biochemical markers. Most recently, we identified early childhood htTKV as a potential risk marker for kidney survival in ARPKD [26]. Yet, our understanding of the ARPKD pathophysiology and of the functional role of the ARPKD protein fibrocystin remains limited, hampering the development of novel therapeutic concepts for ARPKD. Work in mouse models that would complement clinical research has for a long time been hampered by the fact that orthologous ARPKD mouse models did not recapitulate the kidney phenotype of ARPKD. More recently, two novel paths have opened up for cellular ARPKD research. The ability to obtain, culture and reprogram epithelial cells from patients’ urine may become an important tool especially for recessive disorders like ARPKD. Furthermore, a novel digenic mouse model that was obtained by crossing an known orthologous ARPKD mouse model with a well-established orthologous ADPKD model shows a kidney phenotype that resembles human ARPKD [27]. This model may become helpful for functional preclinical studies or screening of potential therapeutic approaches. The data again point to a potential overlap between ARPKD and ADPKD in a spectrum of diseases.

While ADPKD typically becomes clinically symptomatic in adulthood, cystogenesis starts in childhood or even antenatally. Major clinical variability has been described in ADPKD that can only partially be explained by the underlying genetics. It was for a long time believed that children of patients with ADPKD should not be examined but a “wind of change” has recently been noted in this field [28]. More attention has been given to a concept of prevention of disease progression by early modification of ADPKD risk factors. This includes initiating the treatment of modifiable risk factors already in children. The challenges for treating ADPKD in childhood and adolescence have recently been summarized and first specific recommendations on the diagnosis and management of ADPKD in children and young people have been published [4, 16]. Increased htTKV, hypertension and proteinuria are widely-accepted risk markers in adults with ADPKD with more recent emerging comparable data for children with ADPKD. Ambulatory blood pressure measurements and detection of proteinuria in childhood are highly valid examinations and there is a well-established link between hypertension and proteinuria as a consequence of pediatric ADPKD. Furthermore, hypertension and proteinuria are known pediatric risk markers for the progression of CKD. Thus, hypertension and proteinuria may also become promising candidates for surrogate endpoints in clinical trials bridging early disease progression to findings in adults. While it is generally accepted that strict antihypertensive therapy is required and helpful already in pediatric ADPKD in specific, there is a lack of knowledge on the overall clinical development of ADPKD throughout childhood and adolescence. As previously described for ARPKD this knowledge gap is a major challenge to identify and study risk patterns and novel markers in pediatric ADPKD. For adult ADPKD additional urinary, plasma or radiological markers have been described and risk scores like the PROPKD score or the Mayo-TKV-Score are well-established and some of these markers may become interesting candidates for evaluation in childhood and adolescence [1]. Very briefly summarized the PROPKD score is based on four variables: gender, type of genetic variant (truncating PKD1 variant, non-truncating PKD1 variant, PKD2 variant), hypertension before the age of 35 years, first urologic event before the age of 35 years. These variables are weighed differently resulting in a final point score between 0 and 9 points that subsequently allows the classification of patients in three risk categories (low risk, intermediate risk, high risk). A score of ≤3 has a negative predictive value of 81.4% for CKF before 60 years of age, whereas a score >6 shows a positive predictive value of 90.9% for chronic kidney failure before 60 years of age. The Mayo-TKV-Score classifies patients into typical and atypical radiological presentation, with five age-adjusted htTKV subclasses in the typical group (1A-1E in increasing order). Patients in higher classes show more rapid decline of kidney function and thus have a higher risk to develop CKF after 10 years.

For progress in pediatric ADPKD research we will need the association of biobanking and deeply phenotyped patients. The samples obtained during a clinical trial on the efficacy of pravastatin in childhood were for example recently used for metabolic profiling of children and young adults with ADPKD [29]. An additional recent example in the pediatric field includes the evaluation of plasma copeptin, urinary epidermal growth factor (EGF) and urinary MCP-1 as potential early markers in a cross-sectional study of 53 genotyped ADPKD patients with a mean age of 10.4 years. As expected, kidney function was very good in this cohort with a mean eGFR of 122.7 ml/min/1.73m². Patient samples were compared to samples from age-, sex- and BMI-matched healthy controls. While plasma copeptin and urinary EGF did not show major differences, urinary MCP-1 was significantly higher in ADPKD patients compared to controls. This finding was driven by patients with PKD1 variants independent of their underlying genotype. A group of patients with very early onset ADPKD or early symptomatic ADPKD showed higher urinary MCP-1 levels than asymptomatic patients. Human fetal ADPKD kidneys displayed prominent MCP-1 staining and M2 macrophage infiltration and cellular models with PKD1 haploinsufficiency exhibited increased MCP-1 secretion. Thus, urinary MCP-1 may become an easily-obtainable marker of disease severity for subgroups of pediatric ADPKD patients [30]. It may in the future be complemented by radiological findings obtained by both novel MR techniques or 3D-ultrasound [28, 30].