Recurrent fetal loss associated with bilineal inheritance of type 1 autosomal dominant polycystic kidney disease

Co-Inheritance of Pathogenic Variants in PKD1 and PKD2 Genes Determined by Parental Segregation and De Novo Origin: A Case Report

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disease, and it is typically caused by PKD1 and PKD2 heterozygous variants. Nonetheless, the extensive phenotypic variability observed among affected individuals, even within the same family, suggests a more complex pattern of inheritance. We describe an ADPKD family in which the proband presented with an earlier and more severe renal phenotype (clinical diagnosis at the age of 14 and end-stage renal disease aged 24), compared to the other affected family members. Next-generation sequencing (NGS)-based analysis of polycystic kidney disease (PKD)-associated genes in the proband revealed the presence of a pathogenic PKD2 variant and a likely pathogenic variant in PKD1, according to the American College of Medical Genetics and Genomics (ACMG) criteria. The PKD2 nonsense p.Arg872Ter variant was segregated from the proband's father, with a mild phenotype. A similar mild disease presentation was found in the proband's aunts and uncle (the father's siblings). The frameshift p.Asp3832ProfsTer128 novel variant within PKD1 carried by the proband in addition to the pathogenic PKD2 variant was not found in either parent. This report highlights that the co-inheritance of two or more PKD genes or alleles may explain the extensive phenotypic variability among affected family members, thus emphasizing the importance of NGS-based techniques in the definition of the prognostic course.

Cystic Kidney Diseases in Children and Adults: Differences and Gaps in Clinical Management

Cystic kidney diseases, when broadly defined, have a wide differential diagnosis extending from recessive diseases with a prenatal or pediatric diagnosis, to the most common autosomal-dominant polycystic kidney disease primarily affecting adults, and several other genetic or acquired etiologies that can manifest with kidney cysts. The most likely diagnoses to consider when assessing a patient with cystic kidney disease differ depending on family history, age stratum, radiologic characteristics, and extrarenal features. Accurate identification of the underlying condition is crucial to estimate the prognosis and initiate the appropriate management, identification of extrarenal manifestations, and counseling on recurrence risk in future pregnancies. There are significant differences in the clinical approach to investigating and managing kidney cysts in children compared with adults. Next-generation sequencing has revolutionized the diagnosis of inherited disorders of the kidney, despite limitations in access and challenges in interpreting the data. Disease-modifying treatments are lacking in the majority of kidney cystic diseases. For adults with rapid progressive autosomal-dominant polycystic kidney disease, tolvaptan (V2-receptor antagonist) has been approved to slow the rate of decline in kidney function. In this article, we examine the differences in the differential diagnosis and clinical management of cystic kidney disease in children versus adults, and we highlight the progress in molecular diagnostics and therapeutics, as well as some of the gaps meriting further attention.

Hypomorphic PKD1 Alleles Impact Disease Variability in Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) manifesting earlier than expected on the basis of family history can identify clinically tolerant PKD1 alleles with reduced expression. Hypomorphic PKD1 alleles can cause mild kidney disease or liver cysts in the absence of clinically manifest kidney involvement. The presented data highlight pleiotropic ADPKD clinical presentations and varying severity of kidney disease from PKD1 allele combinations.

TRPP2 ion channels: The roles in various subcellular locations

TRPP2 (PC2, PKD2 or Polycytin-2), encoded by PKD2 gene, belongs to the nonselective cation channel TRP family. Recently, the three-dimensional structure of TRPP2 was constructed. TRPP2 mainly functions in three subcellular compartments: endoplasmic reticulum, plasma membrane and primary cilia. TRPP2 can act as a calcium-activated intracellular calcium release channel on the endoplasmic reticulum. TRPP2 also interacts with other Ca²⁺ release channels to regulate calcium release, like IP3R and RyR2. TRPP2 acts as an ion channel regulated by epidermal growth factor through activation of downstream factors in the plasma membrane. TRPP2 binding to TRPC1 in the plasma membrane or endoplasmic reticulum is associated with mechanosensitivity. In cilium, TRPP2 was found to combine with PKD1 and TRPV4 to form a complex related to mechanosensitivity. Because TRPP2 is involved in regulating intracellular ion concentration, TRPP2 mutations often lead to autosomal dominant polycystic kidney disease, which may also be associated with cardiovascular disease. In this paper, we review the molecular structure of TRPP2, the subcellular localization of TRPP2, the related functions and mechanisms of TRPP2 at different sites, and the diseases related to TRPP2.

The wind of change in the management of autosomal dominant polycystic kidney disease in childhood

Significant progress has been made in understanding the genetic basis of autosomal dominant polycystic kidney disease (ADPKD), quantifying disease manifestations in children, exploring very-early onset ADPKD as well as pharmacological delay of disease progression in adults. At least 20% of children with ADPKD have relevant, yet mainly asymptomatic disease manifestations such as hypertension or proteinuria (in line with findings in adults with ADPKD, where hypertension and cardiovascular damage precede decline in kidney function). We propose an algorithm for work-up and management based on current recommendations that integrates the need to screen regularly for hypertension and proteinuria in offspring of affected parents with different options regarding diagnostic testing, which need to be discussed with the family with regard to ethical and practical aspects. Indications and scope of genetic testing are discussed. Pharmacological management includes renin-angiotensin system blockade as first-line therapy for hypertension and proteinuria. The vasopressin receptor antagonist tolvaptan is licensed for delaying disease progression in adults with ADPKD who are likely to experience kidney failure. A clinical trial in children is currently ongoing; however, valid prediction models to identify children likely to suffer kidney failure are lacking. Non-pharmacological interventions in this population also deserve further study.

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

BACKGROUND

Autosomal 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.

METHODS

To 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.

RESULTS

The 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.

CONCLUSIONS

These 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.

Prevalence Estimates of Polycystic Kidney and Liver Disease by Population Sequencing

BACKGROUND

Estimating the prevalence of autosomal dominant polycystic kidney disease (ADPKD) is challenging because of age-dependent penetrance and incomplete clinical ascertainment. Early studies estimated the lifetime risk of ADPKD to be about one per 1000 in the general population, whereas recent epidemiologic studies report a point prevalence of three to five cases per 10,000 in the general population.

METHODS

To measure the frequency of high-confidence mutations presumed to be causative in ADPKD and autosomal dominant polycystic liver disease (ADPLD) and estimate lifetime ADPKD prevalence, we used two large, population sequencing databases, gnomAD (15,496 whole-genome sequences; 123,136 exome sequences) and BRAVO (62,784 whole-genome sequences). We used stringent criteria for defining rare variants in genes involved in ADPKD (PKD1, PKD2), ADPLD (PRKCSH, SEC63, GANAB, ALG8, SEC61B, LRP5), and potential cystic disease modifiers; evaluated variants for quality and annotation; compared variants with data from an ADPKD mutation database; and used bioinformatic tools to predict pathogenicity.

RESULTS

Identification of high-confidence pathogenic mutations in whole-genome sequencing provided a lower boundary for lifetime ADPKD prevalence of 9.3 cases per 10,000 sequenced. Estimates from whole-genome and exome data were similar. Truncating mutations in ADPLD genes and genes of potential relevance as cyst modifiers were found in 20.2 cases and 103.9 cases per 10,000 sequenced, respectively.

CONCLUSIONS

Population whole-genome sequencing suggests a higher than expected prevalence of ADPKD-associated mutations. Loss-of-function mutations in ADPLD genes are also more common than expected, suggesting the possibility of unrecognized cases and incomplete penetrance. Substantial rare variation exists in genes with potential for phenotype modification in ADPKD.

Establishing life is a calcium-dependent TRiP: Transient receptor potential channels in reproduction

Calcium plays a key role in many different steps of the reproduction process, from germ cell maturation to placental development. However, the exact function and regulation of calcium throughout subsequent reproductive events remains rather enigmatic. Successful pregnancy requires the establishment of a complex dialogue between the implanting embryo and the endometrium. On the one hand, endometrial cell will undergo massive changes to support an implanting embryo, including stromal cell decidualization. On the other hand, trophoblast cells from the trophectoderm surrounding the inner cell mass will differentiate and acquire new functions such as hormone secretion, invasion and migration. The need for calcium in the different gestational processes implicates the presence of specialized ion channels to regulate calcium homeostasis. The superfamily of transient receptor potential (TRP) channels is a class of calcium permeable ion channels that is involved in the transformation of extracellular stimuli into the influx of calcium, inducing and coordinating underlying signaling pathways. Although the necessity of calcium throughout reproduction cannot be negated, the expression and functionality of TRP channels throughout gestation remains elusive. This review provides an overview of the current evidence regarding the expression and function of TRP channels in reproduction.

TRPP2 ion channels: Critical regulators of organ morphogenesis in health and disease

Ion channels control the membrane potential and mediate transport of ions across membranes. Archetypical physiological functions of ion channels include processes such as regulation of neuronal excitability, muscle contraction, or transepithelial ion transport. In that regard, transient receptor potential ion channel polycystin 2 (TRPP2) is remarkable, because it controls complex morphogenetic processes such as the establishment of properly shaped epithelial tubules and left-right-asymmetry of organs. The fascinating question of how an ion channel regulates morphogenesis has since captivated the attention of scientists in different disciplines. Four loosely connected key insights on different levels of biological complexity ranging from protein to whole organism have framed our understanding of TRPP2 physiology: 1) TRPP2 is a non-selective cation channel; 2) TRPP2 is part of a receptor-ion channel complex; 3) TRPP2 localizes to primary cilia; and 4) TRPP2 is required for organ morphogenesis. In this review, we will discuss the current knowledge in these key areas and highlight some of the challenges ahead.

Molecular pathways and therapies in autosomal-dominant polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is the most prevalent inherited renal disease, characterized by multiple cysts that can eventually lead to kidney failure. Studies investigating the role of primary cilia and polycystins have significantly advanced our understanding of the pathogenesis of PKD. This review will present clinical and basic aspects of ADPKD, review current concepts of PKD pathogenesis, evaluate potential therapeutic targets, and highlight challenges for future clinical studies.

Genetics and pathogenesis of autosomal dominant polycystic kidney disease: 20 years on

Autosomal dominant polycystic kidney disease (ADPKD), the most common inherited kidney disorder, is characterized by the progressive development and expansion of bilateral fluid-filled cysts derived from the renal tubule epithelial cells. Although typically leading to end-stage renal disease in late middle age, ADPKD represents a continuum, from neonates with hugely enlarged cystic kidneys to cases with adequate kidney function into old age. Since the identification of the first causative gene (i.e., PKD1, encoding polycystin 1) 20 years ago, genetic studies have uncovered a large part of the key factors that underlie the phenotype variability. Here, we provide a comprehensive review of these significant advances as well as those related to disease pathogenesis models, including mutation analysis of PKD1 and PKD2 (encoding polycystin 2), current mutation detection rate, allelic heterogeneity, genotype and phenotype relationships (in terms of three different inheritance patterns: classical autosomal dominant inheritance, complex inheritance, and somatic and germline mosaicism), modifier genes, the role of second somatic mutation hit in renal cystogenesis, and findings from mouse models of polycystic kidney disease. Based upon a combined consideration of the current knowledge, we attempted to propose a unifying framework for explaining the phenotype variability in ADPKD.

The TRPP subfamily and polycystin-1 proteins

It has been exciting times since the identification of polycystic kidney disease 1 (PKD1) and PKD2 as the genes mutated in autosomal dominant polycystic kidney disease (ADPKD). Biological roles of the encoded proteins polycystin-1 and TRPP2 have been deduced from phenotypes in ADPKD patients, but recent insights from vertebrate and invertebrate model organisms have significantly expanded our understanding of the physiological functions of these proteins. The identification of additional TRPP (TRPP3 and TRPP5) and polycystin-1-like proteins (PKD1L1, PKD1L2, PKD1L3, and PKDREJ) has added yet another layer of complexity to these fascinating cellular signalling units. TRPP proteins assemble with polycystin-1 family members to form receptor-channel complexes. These protein modules have important biological roles ranging from tubular morphogenesis to determination of left-right asymmetry. The founding members of the polycystin family, TRPP2 and polycystin-1, are a prime example of how studying human disease genes can provide insights into fundamental biological mechanisms using a so-called "reverse translational" approach (from bedside to bench). Here, we discuss the current literature on TRPP ion channels and polycystin-1 family proteins including expression, structure, physical interactions, physiology, and lessons from animal model systems and human disease.

Experimental therapies and ongoing clinical trials to slow down progression of ADPKD

The improvement of imaging techniques over the years has contributed to the understanding of the natural history of autosomal dominant polycystic kidney disease, and facilitated the observation of its structural progression. Advances in molecular biology and genetics have made possible a greater understanding of the genetics, molecular, and cellular pathophysiologic mechanisms responsible for its development and have laid the foundation for the development of potential new therapies. Therapies targeting genetic mechanisms in ADPKD have inherent limitations. As a result, most experimental therapies at the present time are aimed at delaying the growth of the cysts and associated interstitial inflammation and fibrosis by targeting tubular epithelial cell proliferation and fluid secretion by the cystic epithelium. Several interventions affecting many of the signaling pathways disrupted in ADPKD have been effective in animal models and some are currently being tested in clinical trials.

Functional polycystin-1 dosage governs autosomal dominant polycystic kidney disease severity

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations to PKD1 or PKD2, triggering progressive cystogenesis and typically leading to end-stage renal disease in midlife. The phenotypic spectrum, however, ranges from in utero onset to adequate renal function at old age. Recent patient data suggest that the disease is dosage dependent, where incompletely penetrant alleles influence disease severity. Here, we have developed a knockin mouse model matching a likely disease variant, PKD1 p.R3277C (RC), and have proved that its functionally hypomorphic nature modifies the ADPKD phenotype. While Pkd1+/null mice are normal, Pkd1RC/null mice have rapidly progressive disease, and Pkd1RC/RC animals develop gradual cystogenesis. These models effectively mimic the pathophysiological features of in utero-onset and typical ADPKD, respectively, correlating the level of functional Pkd1 product with disease severity, highlighting the dosage dependence of cystogenesis. Additionally, molecular analyses identified p.R3277C as a temperature-sensitive folding/trafficking mutant, and length defects in collecting duct primary cilia, the organelle central to PKD pathogenesis, were clearly detected for the first time to our knowledge in PKD1. Altogether, this study highlights the role that in trans variants at the disease locus can play in phenotypic modification of dominant diseases and provides a truly orthologous PKD1 model, optimal for therapeutic testing.

Conditional deletion of Pkd1 in osteocytes disrupts skeletal mechanosensing in mice

We investigated whether polycystin-1 is a bone mechanosensor. We conditionally deleted Pkd1 in mature osteoblasts/osteocytes by crossing Dmp1-Cre with Pkd1(flox/m1Bei) mice, in which the m1Bei allele is nonfunctional. We assessed in wild-type and Pkd1-deficient mice the response to mechanical loading in vivo by ulna loading and ex vivo by measuring the response of isolated osteoblasts to fluid shear stress. We found that conditional Pkd1 heterozygotes (Dmp1-Cre;Pkd1(flox/+)) and null mice (Pkd1(Dmp1-cKO)) exhibited a ∼ 40 and ∼ 90% decrease, respectively, in functional Pkd1 transcripts in bone. Femoral bone mineral density (12 vs. 27%), trabecular bone volume (32 vs. 48%), and cortical thickness (6 vs. 17%) were reduced proportionate to the reduction of Pkd1 gene dose, as were mineral apposition rate (MAR) and expression of Runx2-II, Osteocalcin, Dmp1, and Phex. Anabolic load-induced periosteal lamellar MAR (0.58 ± 0.14; Pkd1(Dmp1-cKO) vs. 1.68 ± 0.34 μm/d; control) and increases in Cox-2, c-Jun, Wnt10b, Axin2, and Runx2-II gene expression were significantly attenuated in Pkd1(Dmp1-cKO) mice compared with controls. Application of fluid shear stress to immortalized osteoblasts from Pkd1(null/null) and Pkd1(m1Bei/m1Bei)-derived osteoblasts failed to elicit the increments in cytosolic calcium observed in wild-type controls. These data indicate that polycystin-1 is essential for the anabolic response to skeletal loading in osteoblasts/osteocytes.

TRPP channels and polycystins

The founding member of the TRPP family, TRPP2, was identified as one of the disease genes causing autosomal dominant polycystic kidney disease (ADPKD). ADPKD is the most prevalent, potentially lethal, monogenic disorder in humans, with an average incidence of one in 400 to one in 1,000 individuals worldwide. Here we give an overview of TRPP ion channels and Polycystin-1 receptor proteins focusing on more recent studies. We include the Polycystin-1 family since these proteins are functionally linked to TRPP channels.

Pkd1 and Pkd2 are required for normal placental development

Background

Autosomal dominant polycystic kidney disease (ADPKD) is a common cause of inherited renal failure that results from mutations in PKD1 and PKD2. The disorder is characterized by focal cyst formation that involves somatic mutation of the wild type allele in a large fraction of cysts. Consistent with a two-hit mechanism, mice that are homozygous for inactivating mutations of either Pkd1 or Pkd2 develop cystic kidneys, edema and hemorrhage and typically die in midgestation. Cystic kidney disease is unlikely to be the cause of fetal loss since renal function is not required to complete gestation. One hypothesis is that embryonic demise is due to leaky vessels or cardiac pathology.

Methodology/Principal Findings

In these studies we used a series of genetically modified Pkd1 and Pkd2 murine models to investigate the cause of embryonic lethality in mutant embryos. Since placental defects are a frequent cause of fetal loss, we conducted histopathologic analyses of placentas from Pkd1 null mice and detected abnormalities of the labyrinth layer beginning at E12.5. We performed placental rescue experiments using tetraploid aggregation and conditional inactivation of Pkd1 with the Meox2 Cre recombinase. We found that both strategies improved the viability of Pkd1 null embryos. Selective inactivation of Pkd1 and Pkd2 in endothelial cells resulted in polyhydramnios and abnormalities similar to those observed in Pkd1^−/− placentas. However, endothelial cell specific deletion of Pkd1 or Pkd2 did not yield the dramatic vascular phenotypes observed in null animals.

Conclusions/Significance

Placental abnormalities contribute to the fetal demise of Pkd^−/− embryos. Endothelial cell specific deletion of Pkd1 or Pkd2 recapitulates a subset of findings seen in Pkd null animals. Our studies reveal a complex role for polycystins in maintaining vascular integrity.

Polycystic kidney disease: inheritance, pathophysiology, prognosis, and treatment

Both autosomal dominant and recessive polycystic kidney disease are conditions with severe associated morbidity and mortality. Recent advances in the understanding of the genetic and molecular pathogenesis of both ADPKD and ARPKD have resulted in new, targeted therapies designed to disrupt cell signaling pathways responsible for the abnormal cell proliferation, dedifferentiation, apoptosis, and fluid secretion characteristic of the disease. Herein we review the current understanding of the pathophysiology of these conditions, as well as the current treatments derived from our understanding of the mechanisms of these diseases.

Molecular advances in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic disease (ADPKD) is the most common form of inherited kidney disease that results in renal failure. The understanding of the pathogenesis of ADPKD has advanced significantly since the discovery of the 2 causative genes, PKD1 and PKD2. Dominantly inherited gene mutations followed by somatic second-hit mutations inactivating the normal copy of the respective gene result in renal tubular cyst formation that deforms the kidney and eventually impairs its function. The respective gene products, polycystin-1 and polycystin-2, work together in a common cellular pathway. Polycystin-1, a large receptor molecule, forms a receptor-channel complex with polycystin-2, which is a cation channel belonging to the TRP family. Both polycystin proteins have been localized to the primary cilium, a nonmotile microtubule-based structure that extends from the apical membrane of tubular cells into the lumen. Here we discuss recent insights in the pathogenesis of ADPKD including the genetics of ADPKD, the properties of the respective polycystin proteins, the role of cilia, and some cell-signaling pathways that have been implicated in the pathways related to PKD1 and PKD2.

Bilineal inheritance of type 1 autosomal dominant polycystic kidney disease (ADPKD) and recurrent fetal loss

We report for the first time a family with type 1 ADPKD in which the marriage between affected non-consanguinous individuals resulted in two live-born heterozygous offspring and two fetuses lost in mid-pregnancy. Given a 25% chance for mutant compound heterozygosity in the offspring of this family, our findings suggest that compound heterozygosity of PKD1 mutations in humans may be embryonically lethal.

Autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease is the most prevalent, potentially lethal, monogenic disorder. It is associated with large interfamilial and intrafamilial variability, which can be explained to a large extent by its genetic heterogeneity and modifier genes. An increased understanding of the disorder's underlying genetic, molecular, and cellular mechanisms and a better appreciation of its progression and systemic manifestations have laid out the foundation for the development of clinical trials and potentially effective treatments.

Phenotypic heterogeneity in pediatric autosomal dominant polycystic kidney disease at first presentation: a single-center, 20-year review

BACKGROUND

The presentation of autosomal dominant polycystic kidney disease (ADPKD) in childhood provides an insight into comorbidities and potential areas for interventions and investigation.

METHODS

Phenotypic heterogeneity at the time of first presentation was studied with respect to age of diagnosis, mode of presentation, parental inheritance pattern, renal function, associated hypertension, and hyperlipidemia. Fifty-five children (median age of presentation, 8.7 years; 27% < 1 year) with ADPKD from 44 families followed up between March 1983 and March 2003 were reviewed. The diagnosis was based on family history and ultrasound confirmation of cysts. Progression of renal disease was followed over the study period (mean duration of follow-up, 4.9 years).

RESULTS

A family history of ADPKD was known at presentation in 89%, which precipitated the screening diagnostic imaging in 59% of these children. Maternal inheritance was displayed in 51%, whereas 5% had no known family history of ADPKD. Bilateral renal findings were present in 78%. Hypertension (>95(th) percentile for age) was present in 22%, and hyperlipidemia was present in 54%. Renal function was not significantly diminished in 98% of patients with creatinine clearance > or =3rd percentile for age, and 7% had persistent proteinuria (>150 mg/d). No subjects had hepatic, splenic, or pancreatic cysts on ultrasound scan. A subpopulation of 10 patients had features of ADPKD dating back to prenatal ultrasound scans. All prenatal cases were characterized by bilateral renal findings, 90% had a known family history of ADPKD at the time of presentation, and 89% of these patients displayed maternal inheritance. Follow-up studies showed a persistence of hyperlipidemia despite pharmacotherapeutic treatment of hypertension, infrequent proteinuria, and sustained renal function in most patients.

CONCLUSION

The results of this study show that many children at the time of first presentation have a significant prevalence of modifiable risk factors: hypertension, proteinuria, and hyperlipidemia, in the face of normal renal function. The results also show a unique presentation existing in prenatal subjects.