Single nucleotide polymorphisms There are 3 \200\2235 million nucleotide positions in the human genome at which individuals may differ (single nucleotide polymorphisms SNPs) from ea ch other ( 0.1% of the genome). These SNPs can be detected by DNA microarray analysis. They are used for linkage analysis and are replacing microsatellite markers. This approach has already been very helpful for studies into polygenic diseases. Animal models of genetic renal disease Transgenic mouse models are very helpful for the stu dies of human genetic disorders, and are greatly facilitated by the mouse genome sequence. Mouse models of renal dis- ease have become available through identi \201 cation of dis- e ase phenotypes that are based on spontaneously arisen mutations. In addition, mouse models of human renal genetic disease have been generated by targeted disrup- tion of speci \201 c disease-causing genes. Known monogenic causes of renal disease In the past few years the genes responsible for man y monogenic renal disorders have been identi \201 ed by posi- tional cloning. There are re nal cystic diseases, such as autosomal dominant polycystic kidney disease (ADPKD), autosoma l recessive polycystic kidney disease (ARPKD), and nephronophthisis. There are monogenic gl omerular dis- eases, e.g. mutations in the podocin gene in steroid-resistant nephrotic synd rome, and mutations in speci \201 c genes have Zebra \201 sh As a lower vertebrate the zebra \201 sh (Danio rerio) has been a very usefu l model for the study of kidney organogenesis and human renal cystic diseases. CHAPTER 15.1 Investigation of inherited renal disease 583 antibody has been coupled to a color detection tool, e.g. by using horseradish peroxidase o r enhanced lumines- cence, which allows detection of the secondary antibody in its bound st ate to the primary antibody, and thereby detec- tion of the band representing the protein o f interest. If a suitable antibody is available, Western blotting thus allows detection of a protein of interest out of a complex protein mixture, such as total cell lysates from cel l culture. As an example, Western blotting can be employed for detection of proteins from c o-immunoprecipitation experiments (Fig 15.1.3). In very complex protein mixtures, size sepa ration by SDS \200\223PAGE alone may not yield suf \201 cient resolution. Under these circ umstances 2-dimensional gel electrophoresis may be used. In this method proteins are \201 rst separated by their characteristic isoelectric point through isoelectric focusing in a tube gel. Thereafter the linear tube gel is loaded onto an SDS \200\223PAGE gel and pro- te ins are size-separated in a direction perpendicular to the direction for isoelectric focusi ng. Thereby 2-dimensional size separation is achieved according to two independent characte ristics: isoelectric point and molecular weight. Flow of Genetic Information ( \200\235Central Dogma \200\235) Gene mRNA Protein Phenotype AAAA Monogenic Disease Gene defect Disease \200\234Pathogenesis \200\235 AAAA = Etiology Fig. 15.1.1 Central dogma of molecular biology and its relation to monogenic disease mechan isms. Protein \200\223protein interaction Co-immunoprecipitation Protein \200\223protein interact ions can be demonstrated by co-immunoprecipitation experiments. Both proteins or fragments thereof are coexpressed in a suitable cell line through transfection of this cell line with expression- plasmids containing inserts encoding these proteins. If no cognate antibody is available for these proteins, they can be expressed as fusion proteins containing a peptid e tag for which antibodies are available, such as myc-tags or FLAG-tags. After transfection and cell lysis the lysates are incubated with the antibody directed against one protein or its tag, antibodies with bound protein are precipitated and the precipitate isolated. The precipitate is then size- separated by SDS \200\223PAGE, and protein \200\223protein intera ction partners are detected by Western blot analysis of the precipitate (Fig. 15.1.3A). The results usually have to be corroborated in the reciprocal form, i.e. if protein \200\230A \200\231 precipitated with protein \200\230B \200\231, protein \200\230B \200\231 should also co-precipitate with protein \200\230A \200\231 (Fig. 15.1.3B). Fig. 15.1.2 The principle of haplotype analysis. Pull-down assays (in vitro assays) In so-called \200\230pull-down assays \200\231, in vitr o protein interaction is examined by adsorption of one protein-binding part- ner as fusion protein to a solid phase, e.g. in a column of this solid phase material. The column is then equili- brated with a lysate that contains the binding partner under high salt conditions, allowing the binding partner to interact. Other nonbinding proteins from the lysate are wa shed out at high salt. Finally the binding partner is eluted under low salt conditions and detected, e.g. by Western blot analysis. In this way interaction of the proteins poly- cyst in 1 and 2, the genes of which cause autosomal poly- cystic kidney disease types 1 and 2, r espectively, has been demonstrated. Caenorhabditis elegans Many genes which cause renal cystic disease (PKD1, PKD2, BBS, NPHP) show conservation of sequence homology and of function in the nematode Caenorhabditis elega ns (C. elegans). The genome has been fully sequenced and the cell lineage of every cell in development of C. elegans can be followed under a microscope using Nomarski optics. Therefo re, C. elegans is a very useful model for the study for physiologic and patho- physiologic mechanisms. Proteins Generation and use of antibodies as molecular probes Western blotting In Western b lotting proteins are \201 rst size-separated by sodium dodecyl sulfate \200\223polyacryla mide gel electro- phoresis (SDS \200\223PAGE), then electroblotted onto a suitable membrane to yield a spatial representation of the result from size separation. This representation can then be visu- alized by staining with amido black or Ponceau S. The membrane is overlai d with \200\230primary \200\231 antibody directed against the protein of interest. Unbound antibody is then washed away, and secondary antibodies recognizing the Fc fragments of the primary antibody are added. The secondary Mapping of protein \200\223protein interaction domains In vitro protein \200\223protein int eraction assays from in vitro interaction assays or from co-immunoprecipitation can be used to map protein \200\223protein interaction domains. For this purpose speci \201 c constru cts containing the full length of the protein and others that contain only fragments of the pro- tein are tested for interaction. Depending on the result, regions necessary for inter action can be delineated. F. HILDEBRANDT AND M. T. F. WOLF 584 (a) (b) Fig. 15.1.3 Co-immunoprecipitation of nephrocystin-1 and p130CAS. (a) Myc-tagged murine p13 0CAS and FLAG-tagged human nephrocystin were co-expressed in baby hamster kidney cells. Whe n nephrocystin was precipitated (IP) with the anti-FLAG- antibody, p130CAS was shown in a W estern blot to co-precipitate using an anti-myc-ab (right lane; CAS). Also shown is 1/20 of the cell lysate input as positive control (left lane), and IP with an irrelevant antibody (middle lane). Strong bands at 60 kDa are immunoglobulin heavy chains. (b) Vise versa, if p 130CAS was precipitated, nephrocystin-1 (NPHP1) co-precipitated, as detected by Western blo t using an anti- FLAG-tag antibody. Identi \201 cation of new protein \200\223protein interaction partners In order to identif y novel binding partners to a protein or peptide the yeast-2-hybrid system can be employed. In this way the protein p130CAS has been identi \201 ed as a binding partner to nephrocys tin, the product of the NPHP1 gene, which if defective gives rise to juvenile nephronophthi sis type 1 (Fig. 15.1.3). pathophysiological roles of newly discovered proteins. This is exempli \201 ed by subcellu lar localization studies of nephro- cystin-1 and inversin which are mutated in nephronoph- thisis type 1 and 2 respectively. Clinical phenotyping Correct de \201 nition of the clinical disease phenotype is of paramo unt importance for studies of positional cloning. Data on co-segregation of the affected st atus with poly- morphic markers can only be correctly interpreted if there is an a priori d e \201 nition of which member of a pedigree is affected. Proteomics The term \200\230proteomics \200\231 has been coined to represent the science o f studying the entirety of proteins translated within a certain cell type or organism. The proteome of an organism is more complex than the genome, since each gene may be transcribed into alternative splice forms, leading to distinct translation products. Furthermore, post -translational modi \201 cation, such as cleavage of a signal peptide or glycosylation, le ads to additional protein prod- ucts derived from the same gene. Modern techniques of prote in biochemistry, such as high resolution 2-dimensional electrophoresis and mass spectrometr y, allow rapid access to comparison of very complex protein expression patterns from whole cell extracts in a 2-dimensional pattern of protein \200\230spots \200\231. The high sensi tivity of mass spectrometry enables sequence-based identi \201 cation of these spots. Toge ther with sequence comparisons to genetic and protein databases, this provides a very stron g approach to studying differential protein expression. As an example the total protein exp ression pattern of primary cilia from human trachea has been resolved by mass spectrometry. This data set is very valuable for the study of renal cystic disease. Virtually all protei ns involved in the pathogenesis of renal cystic disease in humans and mice are expressed in primary cilia or basal bodies of renal epithelial cells. High- throughput protein analysis can also be performed with the help of microarray techniques. Genotype \200\223phenotype correlation Some genotype \200\223phenotype correlations are eme rging in monogenic renal diseases. They can be based on genetic locus heterogeneity, i.e. d ifferent genes cause a similar phe- notype (as in polycystic kidney disease types 1 and 2), allelic differences of the same gene, i.e. different mutations of the same gene decide on phenotypic differences (as in Alport syndrome) or on the effect of modi \201 er genes. The re are many more genes to be found for renal diseases. The rate-limiting step for disease g ene identi \201 cation is the availability of clinically well-characterized familial cases . Therefore, the three most important actions for nephrolo- gists to partake in the process of gene identi \201 cation of renal diseases are: 1. Take a family history and sketch a p edigree. 2. Check on the internet: is the gene for the disorder in question known? (http://www.ncbi.nlm.nih.gov/sites/ entrez? db=OMIM); and are molecular genetic diagnostics available? (www. Genetests.org). 3. If the gene has not been identi \201 ed and if a pedigree is informative: \200 autosomal recessive: \211 3 affected individuals of con- sanguineous parents; autosomal do minant: \211 10 affected individuals. Contact a molecular genetics laboratory to initiate the process of gene identi \201 cation of novel genes causing renal disease and its subseq uent functional evaluation. Phenotyping Cell biological phenotyping Immuno \202 uorescence studies Studies on subcellu lar localization of proteins can be very helpful to answer questions of the physiological a nd CHAPTER 15.1 Investigation of inherited renal disease 585 Further reading Burghes AH, Vessin HE, De La Chapelle A. Genetics. The land between Mendeli an and multifactorial inheritance. Science 2001; 293(5538): 2213 \200\2232214. Online Medelian Inheritance in Man (OMIM): h t t p : / / w w w . n c b i . n l m . n i h . g o v / s i t e s / entrez?db=OMIM Ethical debates and advice on genetic testing: Gabriel SB, Schaffner SF, Nguyen H, et al. The structure of haplo- type blocks in the human genome. Science 2002; 296(5576): 2225 \200\2232229. http://www.faseb.org/genetics/ashg/policy/ pol-00.htm Resource for available genetic diagno stic testing: Glazier AM, Nadeau JH, Aitman TJ. Finding genes that underlie com- plex traits. Science 2002; 298(5602): 2345 \200\2232349. http://www.GeneTests.org Mouse genome informatics & GeneTrap project: Hildebrandt F. Positional cloning and linkage analysis. In: Hildebrandt F, Igarashi P (eds) , Techniques in molecular medicine. Berlin: Springer; 1999. pp. 352 \200\223366. http://www.jax.org/resources/index.html Ostrowski LE, Blackburn K, Radde KM, et al. A proteomic analysis of human cilia: identifica tion of novel components. Mol Cell Proteomics 2002; 1: 451 \200\223465. http://tikus.gsf.de Genomic based drug discovery: Otto EA, Schermer S, Obara T, et al. Inversin mutations cause neph- ronophthisis type 2, li nking renal cystic disease to the function of primary cilia and left \200\223right axis det ermination. Nature Genet 2003; 34: 413 \200\223420. http://www.lexgen.com Caenorhabditis elegans genome informatics: http://www.wormbase.org Sayer JA, Otto EA, O \200\231Toole JF, et al. The centrosomal protein neph- rocystin-6 is m utated in Joubert syndrome and activates tran- scription factor ATF4. Nat Genet 2006; 38: 6 74-681. See also History and clinical examination of patients with renal disease, p. 2 Wolf MTF, Lee J, Panther F, et al. Expression and phenotype analysis of the nephrocystin-1 and nephrocystin-4 homologs in C. elegans. J Am Soc Nephrol 2005; 16: 676 \200\223687. Internet resources National Center for Biotechnology Information: http://www.ncbi.nlm.nih.gov C.L. CHOW AND A.C.M. ONG 586 Polycystic kidney disease in children Introduction Cystic disease in childhood is common and encompasses both genetic and nongene tic causes. This chapter concen- trates on autosomal recessive polycystic kidney disease (A RPKD), autosomal dominant polycystic kidney disease (ADPKD) and glomerulocystic disease. Ot her common causes of childhood cystic disease (cystic dysplasia, juvenile nephronopthisis, medullary sponge kidney and rare malformation syndromes) will be discussed in later section s. In ARPKD renal cysts originate from the renal collecting ducts while in ADPKD cysts aris e from any segment of the nephron. ADPKD generally presents in adult life while ARPKD prese nts in childhood. Glomerulocystic disease is typically associated with minimal tubular invo lvement and can result from several different causes. compression by the enlarged kidneys leads to severe pulmonary hypoplasia resulting in respi ratory distress. The majority of these patients die shortly after birth secondary to respir atory failure. Pneumomediastinum and recurrent pneumothorax are common problems. In milder renal disease, there is less reduction in amniotic \202 uid levels and perinatal respirat ory compromise is less. Kidney enlargement tends to be mild with variable degrees of renal impairment. This results in better survival in the perinatal period. These patients tend to present later with splenomegaly and hepatic complications. Periportal \201 brosis often leads to portal hypertension. Investigations The main differential diagnoses are: \200 \200 early onset ADPKD; glomerulocystic disease. Screening of the parents to determine the pattern of inher- itance is useful to distinguish between the two diseases- in ADPKD one would expect renal cysts in one of the par- ents. H owever, a negative USS in the parents does not exclude ADPKD due to spontaneous mutation, n onpaternity, or a false negative result (in a parent <30 years of age). The demonstration ( usually by USS) of polycystic kidneys and portal hypertension is diagnostic of ARPKD, when parental USS would be expected to be normal. Autosomal recessive polycystic kidney disease Epidemiology ARPKD is a rare condition with a n incidence of 1:20 000 live births and a carrier rate of 1:70. It is the most common inher ited renal cystic disease in infancy and childhood. Genetics Inheritance of ARPKD is autosomal recessive with a 25% risk of each offspring inhe riting the disease. All typical cases of ARPKD are linked to the polycystic kidney disease and hepatic disease 1 (PKHD1) locus on chromosome 6p21. PKHD1 has a complicated transcripti on pro \201 le that results in multiple variants of the integral membrane protein \201 b rocystin. The role of various splice forms is incompletely understood. Fibrocystin is found in the primary cilium and centrosome. PKHD1 gene products appear to be involved with regul ation of cell proliferation, cellular adhesion and repulsion. Allelic heterogeneity of a si ngle gene is thought to account for differences in phenotypic concordance within and betwee n families. Both genetic and environmental modify- ing factors may be present. Patients wit h two truncating mutations have the lethal phenotype. Missense mutations are more common in those with a moderate phenotype. Previously ARPKD was classi \201 ed into four groups acc ording to age of onset and was thought to be caused by different mutant genes (Blyth and Oc kenden, 1971). The four groups were perinatal, neonatal, infantile and juvenile. Earlier ag e of onset correlated with increased severity of renal disease and higher mortality. This c lassi \201 cation has fallen out of favor recently and ARPKD is thought to be a spectrum o f disease with a single gene defect. Prenatal diagnosis Typical features on USS include bilateral symmetrically enlarged hyperec hoic kidneys, oligohydramnios and absence of urine in the fetal bladder. These changes may not be seen until the third trimester so early prenatal USS cannot accurately exclude ARPKD . Fetal USS cannot reliably differentiate between ARPKD, ADPKD, glomerulocystic disease or Meckel syndrome. A thorough family history and parental screening should provide the diagno sis. Severely affected fetuses as a result of oligohydramnios may demonstrate Potter \200 \231s phenotype: low set \202 attened ears, deep eye creases, snubbed nose, micrognathia, pul- monary insuf \201 ciency, deformity of spine and limbs. High risk pregnancies can no w be screened as early as 13 weeks by chorionic villous sampling for haplotype based geneti c testing. This requires a comparison with affected sibling(s) DNA, parental DNA and the as sumption that the diagnosis is accurate in the index case. An alternative approach is mutat ional analysis of the PKHD1 gene. Clinical features The majority of patients are diagnosed in utero or at birth. There can be marked intra-familial phenotypic variability. All patients have renal and hepatic involvem ent to differing degrees. Renal disease usually begins in utero and hepatic \201 brosis i s evident by the time they reach adolescence. A distinction can be made between two groups of patients. The \201 rst group that presents in utero/perinatal with severe renal diseas e and high mortality and those that present later who have milder renal disease but more si gni \201 cant hepatic abnormalities. Severe renal disease in the fetus causes a reduction in fetal urine output leading to oligohydramnios. Thoracic cage Plain radiography In the neonate, chest X-ray may show a small thorax with pulmonary hypopl asia. Pneumothorax and pneumomedi- astinum are often present. Abdominal X-ray may demon- st rate bilateral flank masses with centrally displaced bowel. Ultrasound This is the modality of choice for diagnosis. In the neonate with signi \201 ca nt renal disease, USS shows symmetrically enlarged, diffusely hyperechoic kidneys maintaini ng their reniform shape. CHAPTER 15.2 Polycystic kidney disease in children 587 Hyponatremia This is a common feature of ARPKD in infants. It is a conse- quence of a defec t in free water excretion. Hyponatremia is augmented by the low sodium content of breast mi lk and formula. Due to the concentrating defect, ARPKD patients are at risk of signi \201 cant dehydration with intercurrent ill- ness. Careful \202 uid management is of great imp ortance dur- ing these episodes. Thiazide diuretics have been used in severely polyuric pat ients to reduce distal nephron solute and water delivery. Hyponatremia spontaneously improv es and is not seen in older children. In children presenting later with mild renal disease, kidneys may appear normal or show cha nges similar to the neonate. Renal size usually peaks at 1 \200\2232 years of age, then dec lines and stabilizes by 4 \200\2235 years. Focal dilatation of the collecting tubules may p roduce discrete cysts. Appearances of mac- rocysts and renal \201 brosis may mimic the ap pearances of ADPKD. Ultrasound of the liver may show biliary ectasia or increased liver ech ogenicity in the periportal region from bile duct proliferation and \201 brosis. In the o lder child there may be hepatosplenomegaly and evidence of portal hyper- tension depending on the severity of congenital hepatic \201 brosis. Doppler studies may show reversal of p ortal blood \202 ow. Prominent intrahepatic bile duct dilatation signi \201 es the prese nce of Caroli \200\231s disease Infection, hematuria and proteinuria Urinary tract infections occur in 30 \200\22343% of pa tients. Pyuria is a common \201 nding but does not always signify infection. Microscopic and gross hematuria have been reported but is uncommon. Proteinuria is frequently found but neph- rotic range proteinuria is rare. CT and MRI On unenhanced CT, the kidneys are smooth and enlarged with low attenuation re \202 ecting the large \202 uid volume in the dilated ducts. Nephrocalcinosis is a common feature. With intravenous contrast, a striated pattern is seen. If renal function is impair ed there may be poor opaci \201 cation and excretion giving poor images of the kidneys. Ot her CT \201 ndings include focal liver cysts, and enlarged portal and splenic veins in po rtal hypertension. MRI will show dilated ducts in a typical radial pattern. MRI is useful i n the evaluation of liver disease. Kidney Bilateral or unilateral nephrectomy is performed especially in neonates where there is signi \201 cant respiratory compro- mise or failure to thrive from malnutrition as a re sult of pressure effects. The severity of renal disease is variable. Severe renal dis- ease tends to present early in life with signi \201 cant mortality and morbidity. Renal replac ement therapy is indicated when patients are symptomatic or growth is compromised and refra ctory to medical management. Peritoneal dialysis is preferred in infants but in older child ren both peritoneal dialysis and hemodialysis can be used. Transplantation is the treatment of choice. To enable trans- plantation, patients may require splenectomy for signi \201 c ant hypersplenism and nephrectomy especially in the neonate to provide surgical space or to reduce the risk of respira- tory compromise. Pathology The cardinal features of this disease are renal collecting duct dilatation and bi liary duct ectasia. Ectasia and dilata- tion are present in 10 \200\22390% of renal collect ing tubules. Cysts are the result of circumferential epithelial prolifera- tion leading to tubular lengthening and fusiform dilatation of collecting ducts extending from the medulla to the cor- tex. This cystic epithelium also has abnormal function by paradoxically becomin g secretory in nature. Glomeruli are normal as is the collecting system with no component o f obstruction. The kidneys are enlarged but smooth and reniform. There is evidence of proli feration and ectasia of bile ducts which is a ductal plate malformation with periportal \201 bro- sis. The hepatic parenchyma is normal. Signi \201 cant \201 brosis is termed c ongenital hepatic \201 brosis (CHF) which can be found in other diseases and is not patho gnomonic for ARPKD. Widespread intrahepatic dilatation of bile ducts is referred to as Caro li \200\231s disease. This is present in 6 \200\22312% of patients. Congenital anomalies of other organs are rare. Liver The liver disease is referred to as congenital hepatic \201 brosis (CHF). Hepatic i nvolvement is variable in severity but is progressive and all patients develop hepatic \201 brosis eventually. Liver synthetic function is usually normal as the liver parenchyma is preserved. Hepatic encephalopathy is rare. However CHF is not exclusive to ARPKD and can be associated with other kidney diseases (including ADPKD) or systemic disorders. Portal h ypertension is the main complication. The underly- ing mechanism to its development is not clear. When portal hypertension increases in severity, patients present with splenomegaly, variceal bleeding and hypersplenism. The age of onset is variable but tends to be in older children. Variceal bleeding can be treated with sclerotherapy or banding. To reduce bleedin g and formation of new varices consider transjugular intrahepatic portosystemic shunt place ment. In a minority of cases surgical portocaval or splenorenal shunt insertion may be nece ssary. In anephric patients, portosystemic shunts have been noted to cause hyperammoniemia with fatal hepatic encephalop- athy. In these patients, combined liver kidney transplanta- tion should be considered. Hypersplenism may require splenectomy. CHF may cause ascending c holangitis. Patients present with abdominal pain, fever and elevated liver enzymes. Patient s have a higher risk if there are macroscopically dilated bile ducts. Cholangitis can lead to abscess formation, sepsis, Treatment Treatment is centered on symptom and complication management. There is no curativ e treatment. Hypertension Hypertension is present in 61 \200\22370% of patients. It is often severe and associated with poor renal survival. If not managed adequately, it may result in cardiac hy pertrophy, congestive cardiac failure and even death in infants. The exact mechanism is unk nown but, activation of the renin \200\223 angiotensin axis and expansion of the intravascu lar volume has been demonstrated. Treatment with angiotensin converting enzyme inhibitors i s recommended. Careful consideration and monitoring will be required in the context of exis ting renal impairment. C.L. CHOW AND A.C.M. ONG 588 fulminant hepatic failure and death. In severe cases surgical drainage or lobectomy may be required. In a small minority of cases, liver transplantation is consid- ered when severe h epatic dysfunction or chronic severe cholangitis is present. Genetics Inheritance is autosomal dominant. All early onset ADPKD is associated with PKD1. Recurrence rate of early onset ADPKD is 45% in affected siblings of early onset ADPKD patie nts. Bilateral enlarged kidneys are a common presen- tation. Glomerular cysts are prominent in very early onset ADPKD. Extrarenal cysts and intracranial aneurysms are rare in childho od. Respiratory Pulmonary hypoplasia is common in the affected fetus with corresponding respira tory failure. Bilateral nephrectomy and implementation of renal replacement therapy or cont inuous arteriovenous hemo \201 ltration to reduce \202 uid overload have been advocated to improve pulmonary prognosis. Pathology ADPKD is characterized by cystic dilatation of all parts of the nephron and gener ally presents in adulthood. Clinical features ADPKD is an uncommon cause of renal failure in childhood but is often sym ptomatic. The clinical spectrum is bimodal depending on the age of presentation \200\223 i t can range from asymptomatic patients (in childhood) to severe neph- ronomegaly, renal fai lure, hypertension and pulmonary hypoplasia (at birth). The latter patients are phenotypica lly similar to ARPKD. Children with ADPKD diagnosed in utero or within the \201 rst year of life have higher perinatal mortality, more severe renal disease and reduced renal surviv al. Children presenting after this period generally have normal renal function. Risk factor s for increased renal volume growth include: \200 \200 \200 Growth Growth retardation is correlated with the severity of renal impairment and is observ ed in 25% of patients. It is important to ensure good nutrition. Where required, patients should have supplements, and nasogastric or gastrotomy feeding to improve nutritional statu s. Good management of electrolyte imbalance, \202 uid status and acid \200\223base balanc e also play an important role. The use of recombinant growth hormone to improve growth shou ld be considered. Psycho-social In addition to the medical problems one must not underes- timate the psychoso cial stress on the patient and family of the diagnosis and its complications. Reassurance, support and counseling will be required for both the family and the patient. early renal enlargement; >10 cysts by age 12 years; hypertension. However these are not associated with a decline in renal function up to the age of 25 years . The impact on long term prognosis is unknown. Gross hematuria is found in 10% of children and proteinuria in 28%. Follow-up Frequency of follow-up will depend on the degree of renal impairment but should o ccur in specialist pediatric and adult renal units. Prognosis Within the first decade of life 50% of patients will require dialysis. Neonatal m ortality is high at 25 \200\22335%. The most common cause of death at this stage is pulmona ry insuf \201 ciency. The need for neonatal ventilation is a strong predictor of subsequen t development of renal failure and death. The majority of patients who survive the neonatal period survive to adulthood with survival rates reported at 67 \200\22379% at 15 years of age of these 86% of patients are alive and 72% have adequate renal function at 40 years of age. Extrarenal cysts These features are very rare in pediatric patients. There are rare reports of hepatic and pancreatic cysts in children. Intracranial aneurysms These are rarely found in pediatric patients. Cardiac abnormalities The incidence of mitral valve prolapse is 12% in children. ADPKD chil dren have a 5% incidence of congenital heart defects compared to 1% in unaffected family me mbers. Genetic counseling Parents of a child with ARPKD will both be carriers of the defective gen e. Each offspring will have a 25% chance of inheriting the disease and 50% risk of being a carrier. The offspring of an ARPKD patient with a nonaffected partner will have <1% risk of inheritance but all children will be carriers. Genetic counseling and preconception advise should be given to parents regarding subsequent pregnancies and to patients where appropri ate. Investigations In PKD1 families, 60% of affected children under 10 years will have cysts ri sing to 80% at age 18 years. In PKD2 fami- lies the reliability of diagnosis with USS is re duced with a 50% detection rate at age <15 years. Prenatal USS may show large hyperechoic k idneys but is not speci \201 c for ADPKD. In infancy kidneys can be normal or cystic. Cyst s are scattered throughout the medulla and cortex. Asymmetrical or single kidney involvemen t has been reported. The most crucial investigation in distinguishing early-onset ADPKD fro m ARPKD is for both parents to have a renal USS. If both parents (aged >30 years) have a no rmal renal USS the probability of ADPKD then reduces to that of a spontaneous mutation. Gen etic linkage studies and mutational analysis can be used but are not routine. The presence of unstable mutations and genetic imprinting have been discounted as underlying causes for early-onset presentations. Future prospects Vasopressin receptor-2 antagonists are effective in a rat model of ARPKD, the Pck rat. It is possible that these compounds may be similarly bene \201 cial in ARPKD. Autosomal dominant polycystic kidney disease Epidemiology With advances in imaging and gene tic testing, ADPKD is increasingly diagnosed in utero and in asymptomatic children. CHAPTER 15.2 Polycystic kidney disease in children 589 Further reading Bingham C, Bulman MP, Ellard S, et al. Mutations in the hepato- cyte nuclea r factor-1 gene are associated with familial hypoplas- tic glomerulocystic kidney disease . Am J Hum Genet 2001; 68: 219 \200\223224. Treatment Hypertension This is a common feature occurring in 33 \200\22347% of patients. Ac tivation of the renin angiotensin-aldosterone system has been documented and increased left ventricular mass has been documented in children. ACEIs and angiotensin receptor blockers (ARBs) are recommended. Fick-Brosnahan GM, Tran ZV, Johnson AM, et al. Progression of autosomal dominant polycystic kidney disease in children. Kidney Int 2001; 59: 1654 \200\2231662. Follow-up The frequency of follow-up will depend on the degree of renal impairment but shou ld occur in specialist pediatric and adult renal units. Guay-Woodford LM, Desmond RA. Autosomal recessive polycystic kidney disease: the clinical e xperience of North America. Pediatrics 2003; 111: 1072 \200\2231080. Lonergan GJ, Rice RR, Suarez ES. Autosomal recessive polycystic kidney disease: radiologic \200\223pathologic correlation. RadioGraphics 2000; 20: 837 \200\223855. Prognosis Prognosis is good if onset is in childhood, but poor if onset is within the \201 rst year of life. Peral B, Ong ACM, San Millan JL, et al. A stable nonsense mutation associated with a case o f infantile autosomal dominant polycystic kidney disease. Hum Mol Genet 1996; 5: 539 \200 \223542. Future prospects Potential drug treatments, including vasopressin type 2 (V2) receptor anta gonists, sirolimus and everolimus, are currently being tested in adult patients. Sweeney WE, Avner ED. Molecular and cellular pathophysiologyof autosomal recessive polycyst ic kidney disease (ARPKD). Cell Tissue Res 2006; 326: 671 \200\223685. Zerres K, Rudnik-Sch neborn S, Deger F. Childhood onset auto- somal dominant polycystic ki dney disease in sibs: clinical picture and recurrence risk. J Med Genet 1993; 30: 583 \200 \223588. Glomerulocystic kidney disease Glomerulocystic kidney disease (GCKD) is characterized by bi lateral cortical cysts from dilatation of Bowman \200\231s cap- sule and the proximal convo luted tubule. Glomerulocystic kidneys can be divided into three major categories: 1. GCKD c omprising both sporadic and nonsyndromal Zerres K, Rudnik-Sch neborn S, Steinkamm C, et al. Autosomal recessive polycystic kidney d isease. J Mol Med 1998; 76: 303 \200\223309. Internet resources The Polycystic Kidney Disease Charity: inherited forms. http://www.pkdcharity.co.uk/ National Kidney Federation: 2. Glomerulocystic kidneys as a feature of malformation syndromes (e.g. orofacial \200\223d igital syndrome type 1): cysts are variably expressed. http://www.kidney.org.uk/medical-info/kidney- disease/pckd.html NIH (NIDDK) Information Cle aring House: 3. Glomerular cysts in dysplastic kidneys: dysplastic fea- tures predominate. http://kidney.niddk.nih.gov/kudiseases/pubs/ polycystic/ Polycystic Kidney Disease Foundati on: A rare familial form of GCKD associated with hypoplastic kidneys has been documented. This autosomal dominant form is associated with maturity onset diabetes of the young (MODY) and is caused by mutations in hepatocyte nuclear factor (HNF)-1 . Dilatation of glomeruli is a lso seen in ADPKD and other multisystem diseases, e.g. tuberous sclerosis and Von Hippel \200\223Lindau disease. Glomerular cysts tend to be small and are located pre- dominantly i n the subcapsular region although the entire cortex can be involved. High resolution USS, C T or MRI is needed for the diagnosis. The clinical course is variable and the natural histo ry is not well documented. Infants and children present with bilater- ally enlarged kidneys with variable renal function. Adult patients may be asymptomatic or present with progressi ve renal failure. Early progression to renal failure with need for renal replacement therap y may occur. http://www.pkdcure.org See also Autosomal dominant polycystic kidney disease, p. 590 Nephronophthisis, p. 600 Tube rose sclerosis and von Hippel \200\223Lindau disease, p. 598 Rare syndromes with renal invo lvement, p. 640 Medullary sponge kidney, p. XXX Chronic kidney disease in children, p. 462 C.L. CHOW AND A.C.M. ONG 590 Autosomal dominant polycystic kidney disease Introduction Autosomal dominant polycystic kidney disease (ADPKD) is the most common geneti c cause of renal failure and accounts for 10% of patients on dialysis. Unlike other causes of chronic kidney disease, ADPKD is character- ized by progressive kidney enlargement. It i s also a sys- temic disorder characterized by cyst formation in other organs such as the li ver and the pancreas as well as noncystic complications such as arterial aneurysm formation . Female PKD2 patients have delayed ESRD compared to males. There is considerable intrafamili al phenotypic variability indicating the major modifying effects of nonallelic and environm ental factors on the phenotype compared to allelic factors. True genetic anticipation does not occur. Clinical features As most patients are asymptomatic, the commonest modes of presentation ar e with asymptomatic hypertension, inci- dental USS \201 ndings or a positive family histo ry. Epidemiology ADPKD has a prevalence of 1:400 to 1:1000 live births. The disease is usually evident by the 3rd and 4th decades of life but can present at any age. Features at presentation Flank, back and/or abdominal pain (60%). Urinary tract infection. Renal stones (20 \200\22330%). Hematuria (gross 30 \200\22350%, microscopic 25%). Hypertens ion (60%). Intracerebral aneurysm or bleed (6 \200\22316%). Symptoms of renal failure (usua lly in the 4th \200\2236th decade of life): \200 \200 \200 Genetics The inheritance pattern is autosomal dominant with 100% disease penetrance. Each o ffspring has therefore a 50% chance of inheriting the disease. There is a high rate of new mutation. Mutations in either of two genes can lead to ADPKD: \200 PKD1 (chromosome 16p13.3) accounts for 85 \200\22390% of cases. PKD1 encodes for the polycy stin-1 protein; PKD2 (chromosome 4q21) accounts for 10 \200\22315% of cases. PKD2 encodes f or the polycystin-2 protein. lethargy; poor appetite; \202 uid overload. Family history of: \200 \200 A small number of families unlinked to either gene could signify the potential existence of a third gene ( \200\230PKD3 \200\231). Polycystin-1 and polycystin-2 are integral membrane proteins. Polycystin-1 is likely to act as a receptor and is found in primary cilium, foca l adhesions, desmosomes, and adher- ens junctions. Polycystin-2 is a nonselective cation ch annel which is highly permeable to calcium ions and is found in the endoplasmic reticulum, centrosome and primary cilium. Both proteins are widely expressed in multiple tissues and c ell types including epithelial cells, vascular smooth muscle cells and cardiac myocytes. Th ey are thought to form a functional complex regulating key aspects of cellular behav- ior i ncluding proliferation, apoptosis, cell adhesion, mor- phogenesis and net transepithelial f luid secretion via multiple signal transduction pathways. Cysts form in <1% of nephrons alt hough all cells carry the germline mutation. This observation has led to the two-hit model of cystogenesis which proposes that a second muta- tion (second hit or somatic mutation) is required for cyst formation. Environmental factors and modifying genes may in \202 uence the rate of second hits (Fig. 15.3.1). A different mechanism underlying cystogenesis is the hap- loinsuf \201 ciency model where reduced expression of the normal allele below a thre shold level may predispose cells to genetic/environmental triggers for cyst formation. ADPKD; cerebrovascular accident/intracranial bleed. Cardiovascular (important for risk mana gement).