Autosomal recessive polycystic kidney disease in adulthood

Haplotype analysis improves molecular diagnostics of autosomal recessive polycystic kidney disease

BACKGROUND

Autosomal recessive polycystic kidney disease (ARPKD) is characterized by wide phenotypic variability, ranging from in utero detection with enlarged, echogenic kidneys to an adult presentation with congenital hepatic fibrosis. The ARPKD gene, PKHD1 , covers about 470 kb of DNA (67 exons), and mutation studies have found marked allelic heterogeneity with a high level of novel missense changes and neutral polymorphisms. To improve the prospects for molecular diagnostics and to study the origin of some relatively common mutations, the authors have developed a strategy for improved ARPKD haplotyping.

METHODS

A protocol of multiplex PCR and fluorescence genotyping in a single capillary has been developed to assay 7 highly informative simple sequence repeat (SSR) markers that are intragenic or closely flanking PKHD1.

RESULTS

Examples in which haplotype analysis, used in combination with mutation screening, improved the utility of molecular diagnostics, especially in families in which just a single PKHD1 mutation has been identified, are illustrated. The new markers also allow screening for larger DNA deletions, detecting unknown consanguinity and exploring the disease mechanism. Analysis of 8 recurring mutations has shown likely common haplotypes for each, and the divergence from the ancestral haplotype, by recombination, can be used to trace the history of the mutation. The common mutation, T36M, was found to have a single European origin, about 1,225 years ago.

CONCLUSION

Improved haplotype analysis of ARPKD complements mutation-based diagnostics and helps trace the history of common PKHD1 mutations.

Polycystic liver and kidney diseases

There have been remarkable advances in research on polycystic liver and kidney diseases recently, covering cloning of new genes, refining disease classifications, and advances in understanding more about the molecular pathology of these diseases. Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary disease affecting kidneys. It affects 1/400 to 1/1000 live births and accounts for 5% of the end stage renal disease in the United States and Europe, and is caused by gene defects in the PKD1 or PKD2 genes. Compared to ADPKD, polycystic liver disease (PCLD) is a milder disease and does not lower life expectancy. Both diseases are usually adult-onset diseases. Defects in genes, which code the hepatocystin and SEC63 proteins, have just recently been found to cause PCLD. It now seems that ADPKD is caused by malfunction of the primary cilia, a cell organ sensing fluid movement, and that PCLD is a sequel from defects in protein processing. Autosomal recessive polycystic kidney disease (ARPKD) belongs to a group of congenital hepatorenal fibrocystic syndromes. All ARPKD patients have a gene defect in a gene called PKHD1, the protein product of which localizes to primary cilia. We summarize the present clinical and molecular knowledge of these diseases in this review.

Molecular genetics of autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is a severe form of inherited childhood nephropathy ( approximately 1:20,000 live births) characterized by fusiform dilatation of collecting ducts and congenital hepatic fibrosis. Up to 30% die as neonates due to respiratory insufficiency and the majority of surviving infants develop hypertension. Progression to end stage renal disease occurs in 20-45% of cases within 15 years but a proportion maintain renal function into adulthood where complications of liver disease predominate. The ARPKD disease gene, PKHD1, has recently been identified through analysis of an orthologous animal model, the PCK rat. PKHD1 is a large gene ( approximately 470 kb) with 67 exons from which multiple transcripts may be generated by alternative splicing. It is highly expressed in kidney, with lower levels in liver and pancreas. The ARPKD protein, fibrocystin (4074 aa and 447 kDa), is predicted to be an integral membrane, receptor-like protein containing multiple copies of an Ig-like domain (TIG). Fibrocystin is localized to the branching ureteric bud, collecting and biliary ducts, consistent with the disease phenotype, and often absent from ARPKD tissue. In common with other PKD-related proteins, fibrocystin is localized to the primary cilia of renal epithelial cells, reinforcing the link between ciliary dysfunction and cyst development. Screens of PKHD1 have revealed 119 different mutations of various types spread throughout the gene. Several ancestral changes have been described, some localized to specific geographic populations. The majority of patients are compound heterozygotes and preliminary genotype/phenotype studies associate two truncating mutations with severe disease. The complexities of PKHD1, marked allelic heterogeneity and high level of missense changes complicate gene-based diagnostics.

[Pathology and genetic hereditary kidney cysts]

The classification of cystic kidney diseases according to the pathologic-anatomic potter classification may be difficult. New molecular genetic findings are important to understand the underlying pathogenesis, but less useful to classify the hereditary diseases. An exact classification of polycystic kidney disease in fetus and children is very important for the human genetic consultation. Therefore, the investigation of pathological anatomy of kidney and liver, as well as the evaluation of additional malformations and family history is necessary. For clinical use the mode of inheritance (autosomal dominant and autosomal recessive) is used to differentiate hereditary polycystic kidney diseases.

Zebrafish pronephros: A model for understanding cystic kidney disease

The embryonic kidney of the zebrafish is the pronephros. The ease of genetic analysis and experimentation in zebrafish, coupled with the simplicity of the pronephros, make the zebrafish an ideal model system for studying kidney development and function. Several mutations have been isolated in zebrafish genetic screens that result in cyst formation in the pronephros. Cloning and characterization of these mutations will provide insight into kidney development but may also provide understanding of the molecular basis of cystic kidney diseases. In this review, we focus on the zebrafish as a model for understanding cystic kidney disease and the links between cystic kidney disease and left-right patterning.

Survival of childhood polycystic kidney disease following renal transplantation: the impact of advanced hepatobiliary disease

Childhood PKD encompasses the diagnoses of AR and ADPKD, glomerulocystic disease, and syndromes such as tuberous sclerosis or Jeune's syndrome. Given the fact that a majority of PKD children with ESRD carry the diagnosis of ARPKD, natural history studies assessing the long-term prognosis of PKD patients following renal transplantation must focus on morbidity and mortality issues related to complications from congenital hepatic fibrosis. Using the NAPRTCS registry, we analyzed the patient and graft survival rates of 203 PKD patients and 7044 non-PKD patients undergoing renal transplantation between 1987 and 2001. Deceased PKD patients, all with a diagnosis of ARPKD, were further identified and characterized using a special questionnaire submitted to the principal investigators. Overall graft and patient survival rates were not significantly different between PKD and non-PKD patients. No differences in rates of acute rejection or time to first rejection were noted between PKD and non-PKD patients. The relative risk of living longer than 3 yr in the PKD patients was not significantly different from non-PKD patients (RR = 0.70, p = 0.28). Sepsis was identified as a likely factor in the cause of death in nine (64%) ARPKD patients and was comfirmed with a positive blood culture in four patients. Despite similar graft and patient survival rates among PKD and non-PKD children following renal transplantation, our results suggest that ARPKD transplant recipients appear to be at increased risk for sepsis that may be related to hepatic fibrosis and ascending cholangitis. The utility of early liver transplantation in ARPKD patients with significant hepatobiliary disease is discussed.

Milder presentation of recessive polycystic kidney disease requires presence of amino acid substitution mutations

Autosomal recessive polycystic kidney disease (ARPKD; MIM 263200) is a hereditary and severe form of polycystic disease affecting the kidneys and biliary tract with an estimated incidence of 1 in 20,000 live births. The clinical spectrum is widely variable: up to 50% of affected neonates die shortly after birth, whereas others survive to adulthood. Mutations at a single locus, polycystic kidney and hepatic disease 1 (PKHD1), are responsible for all typical forms of ARPKD. Mutation detection was performed in PKHD1 by DHPLC in 85 affected, unrelated individuals. Seventy-four amplicons were amplified and analyzed from the PKHD1 genomic locus. Sequence variants were considered pathogenic when they were not observed in 160 control individuals (320 chromosomes). For purposes of genotype-phenotype comparisons, families were stratified by clinical presentation into two groups: the severe perinatal group, in which at least one affected child presented with perinatal disease and neonatal demise, and the less severe, nonperinatal group, in which none of the affected children died in the neonatal period. Forty-one mutations were found in 55 affected disease chromosomes; 32 of these mutations have not been reported previously. Mutations were distributed throughout the portions of gene encoding the predicted extracellular portion of the protein product. The most commonly encountered mutation, T36M, was found in 8 of 55 disease chromosomes. Amino acid substitutions were found to be more commonly associated with a nonlethal presentation, whereas chain terminating mutations were more commonly associated with neonatal demise (chi(2) = 11.54, P = 0.003). All patients who survive the neonatal period have at least one amino acid substitution mutation, suggesting that such substitutions produce milder disease through production of partially functional protein products. The nature of the germline mutations in ARPKD plays a significant role in determining clinical outcome.

Autosomal recessive polycystic kidney disease: the clinical experience in North America

OBJECTIVE

We designed a longitudinal clinical database for autosomal recessive polycystic kidney disease (ARPKD), recruited patients from pediatric nephrology centers in the United States and Canada, and examined their clinical morbidities and survival characteristics. We initially targeted enrollment to children who were born and diagnosed after January 1, 1990, so as to capture a cohort that is representative of ARPKD patients born in the last decade. When a significant number of older ARPKD patients were also referred, we extended our database to include all patients who met our inclusion criteria, thereby allowing direct comparisons between a long-term survivor subset and a cohort that included both neonatal survivors and nonsurvivors.

DESIGN

Patient entry into our database required either compatible histopathology or ultrasonographic evidence of enlarged, echogenic kidneys and the presence of at least 1 of the following additional criteria: a) biopsy-proven ARPKD in a sibling; b) biliary fibrosis based on either clinical or histopathologic evidence; c) no sonographic evidence of renal cysts in the parents (parents must be >30 years of age); or d) parental consanguinity, eg, first-cousin marriage. Clinical questionnaires (primary data form and follow-up data form) were developed to collect initial patient data and follow-up data at yearly intervals.

RESULTS

Thirty-four centers provided clinical information for 254 patients and of these, 209 had sufficient data for analyses. When stratified by date of birth, 166 (79.4%) were born on or after January 1, 1990 (younger cohort) and 43 children (20.6%) were born before 1990 (older cohort). The gender distribution was equal in both cohorts. The median age at diagnosis was significantly later in the older cohort and no deaths were reported among these patients, suggesting that this group is biased toward long-term survivors. In the younger cohort, 74.7% of the patients are alive, with a median age of 5.4 years. In this group, 40.5% of patients required ventilation and 11.6% developed chronic lung disease. Hypertension was a common, but not universal finding in both cohorts. The relative risk for developing hypertension was higher in the older cohort, but the median age at diagnosis was significantly earlier in the younger cohort. Chronic renal insufficiency (CRI) was reported in approximately 40% of patients with no significant difference in the relative risk between age groups. However, in the younger cohort, the median age at diagnosis was significantly earlier and the age of diagnosis of CRI and hypertension were significantly correlated. Clinically significant morbidities related to periportal fibrosis were more common in the older cohort. There was a trend toward increasing frequency of portal hypertension with age in both cohorts. Portal hypertension was not significantly correlated with either systemic hypertension or CRI.

CONCLUSIONS

The ARPKD Clinical Database represents the largest single cohort of ARPKD patients collected to date. Our initial data analysis provides several new clinical insights. First, in our subset of long-term survivors, ARPKD has a slower rate of disease progression, as assessed by age of ARPKD diagnosis, as well as age of diagnosis of clinical morbidities. Second, neonatal ventilation was strongly predictive of mortality as well as an earlier age of diagnosis in those who developed hypertension or chronic renal insufficiency. However, for infants who survive the perinatal period, the long-term prognosis for patient survival is much better than generally perceived. Third, although systemic hypertension and CRI were significantly correlated with respect to age of diagnosis, similar relationships with portal hypertension were not evident, suggesting that disease progression may have organ-specific patterns. Fourth, only a subset of patients may be at risk for developing clinically significant manifestations of periportal fibrosis. Based on these observations, the next challenges will be to determine how various factors, such as specific mutations in the ARPKD gene, PKHD1(polycystic kidney and hepatic disease 1), variations in modifying gene loci, modulation by as yet unspecified environmental factors, and/or gene-environment interactions contribute to the marked variability in survival and disease expression observed among ARPKD patients.

A novel gene encoding a TIG multiple domain protein is a positional candidate for autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is a common hereditary renal cystic disease in infants and children. By genetic linkage analyses, the gene responsible for this disease, termed polycystic kidney and hepatic disease 1 (PKHD1), was mapped on human chromosome 6p21.1-p12, and has been further localized to a 1-cM genetic interval flanked by the D6S1714/D6S243 (telomeric) and D6S1024 (centromeric) markers. We recently identified a novel gene in this genetic interval from kidney cDNA, using cloning strategies. The gene PKHD1 (PKHD1-tentative) encodes a novel 3396-amino-acid protein with no apparent homology with any known proteins. We named its gene product "tigmin" because it contains multiple TIG domains, which usually are seen in proteins containing immunoglobulin-like folds. PKHD1 encodes an 11.6-kb transcript and is composed of 61 exons spanning an approximately 365-kb genomic region on chromosome 6p12-p11.2 adjacent to the marker D6S1714. Northern blot analyses demonstrated that the gene has discrete bands with one peak signal at approximately 11 kb, indicating that PKHD1 is likely to have multiple alternative transcripts. PKHD1 is highly expressed in adult and infant kidneys and weakly expressed in liver in northern blot analysis. This expression pattern parallels the tissue involvement observed in ARPKD. In situ hybridization analysis further revealed that the expression of PKHD1 in the kidney is mainly localized to the epithelial cells of the collecting duct, the specific tubular segment involved in cyst formation in ARPKD. These features of PKHD1 make it a strong positional candidate gene for ARPKD.