Nephronophthisis: Disease Mechanisms of a Ciliopathy
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BRIEF REVIEW www.jasn.org Nephronophthisis: Disease Mechanisms of a Ciliopathy Friedhelm Hildebrandt,*†‡ Massimo Attanasio,* and Edgar Otto* Departments of *Pediatrics and †Human Genetics, University of Michigan, and ‡Howard Hughes Medical Institute, Ann Arbor, Michigan ABSTRACT Nephronophthisis (NPHP), a recessive cystic kidney disease, is the most frequent than 300 cases of NPHP have been pub- genetic cause of end-stage kidney disease in children and young adults. Positional lished in the literature.7 It has been re- cloning of nine genes (NPHP1 through 9) and functional characterization of their ported from virtually all regions of the encoded proteins (nephrocystins) have contributed to a unifying theory that defines world.14 The incidence of the disease is es- cystic kidney diseases as “ciliopathies.” The theory is based on the finding that all timated at nine patients per 8.3 million15 in proteins mutated in cystic kidney diseases of humans or animal models are expressed the United States or one in 50,000 live in primary cilia or centrosomes of renal epithelial cells. Primary cilia are sensory births in Canada.7,16 In the North Ameri- organelles that connect mechanosensory, visual, and other stimuli to mechanisms of can pediatric population with ESRD, epithelial cell polarity and cell-cycle control. Mutations in NPHP genes cause defects in pooled data indicate a prevalence of ap- signaling mechanisms that involve the noncanonical Wnt signaling pathway and the proximately 5% of all children with renal sonic hedgehog signaling pathway, resulting in defects of planar cell polarity and failure.17,18 tissue maintenance. The ciliary theory explains the multiple organ involvement in Positional cloning has identified nine NPHP, which includes retinal degeneration, cerebellar hypoplasia, liver fibrosis, situs genes causing nephronophthisis when inversus, and mental retardation. Positional cloning of dozens of unknown genes that mutated.13,19–29 These are monogenic re- cause NPHP will elucidate further signaling mechanisms involved. Nephrocystins are cessive genes, suggesting that mutations highly conserved in evolution, thereby allowing the use of animal models to develop in each single one is sufficient by itself to future therapeutic approaches. cause NPHP in a patient bearing muta- tions and indicating their gene products J Am Soc Nephrol 20: 23–35, 2009. doi: 10.1681/ASN.2008050456 are necessary for normal kidney func- tion. Positional cloning generated new insights into disease mechanisms of Nephronophthisis (NPHP) is an autoso- transplanted into patients with NPHP.9 NPHP by revealing the mechanism is re- mal recessive cystic kidney disease that NPHP is inherited in an autosomal reces- lated to the signaling pathways of pri- constitutes the most frequent genetic sive mode.4 It was first described by Smith mary cilia, centrosomes, and planar cell cause for ESRD in the first three decades and Graham in 19452 and by Fanconi et polarity.1,13,30,31 The demonstration that of life.1–4 Renal failure manifests at a me- al.,3 who introduced the term “familial ju- nephrocystin-1 and inversin/NPHP2 lo- dian age of 13 yr. Initial symptoms are rel- venile nephronophthisis.” In juvenile calize to primary cilia of renal tubular atively mild, start at approximately age 6 yr, NPHP, renal failure develops within the cells13 was among the first findings to and consist of polyuria, polydipsia, sec- first three decades of life.10–12 In contrast, support a new unifying theory of renal ondary enuresis, and anemia.5 Regular infantile NPHP, which is characterized by fluid intake at nighttime is a characteristic mutations in NPHP2/inversin, leads to re- 12,13 Published online ahead of print. Publication date feature of the patients’ history. Renal ultra- nal failure between birth and age 3 yr. available at www.jasn.org. sound reveals normal kidney size, in- In more than 10% of cases, NPHP is asso- Correspondence: Dr. Friedhelm Hildebrandt, creased echogenicity, and corticomedul- ciated with extrarenal involvement, pri- Howard Hughes Medical Institute, Departments of lary cysts (Figure 1A).6 Renal histology marily including retinal degeneration (Se- Pediatrics and of Human Genetics, University of exhibits a characteristic triad of cortico- nior-Løken syndrome), cerebellar vermis Michigan Health System, 8220C MSRB III, 1150 West Medical Center Drive, Ann Arbor, MI 48109- medullary cysts, tubular basement mem- aplasia (Joubert syndrome), liver fibrosis, 5646. Phone: 734-615-7895; Fax: 734-615-1386; brane disruption, and tubulointerstitial and cone-shaped epiphyses. These clinical E-mail: [email protected] 7,8 nephropathy (Figure 1B). Disease recur- features are discussed here in light of the Copyright ᮊ 2009 by the American Society of rence has never been reported in kidneys cilia/centrosome theory of NPHP. More Nephrology J Am Soc Nephrol 20: 23–35, 2009 ISSN : 1046-6673/2001-23 23 BRIEF REVIEW www.jasn.org cystogenesis.30 This theory states that the cystoproteins mutated32 in renal cystic disease in humans, mice, or zebrafish are expressed in primary cilia, basal bodies, or centrosomes.30,33 Basal bodies are the foundations from which cilia are assem- bled (Figure 2). After mitosis and cell di- vision are completed, basal bodies derive from the mother centriole of the centriole pair that had organized the mitotic spin- dle during cell division. As cilia are formed from the basal body, the daughter centriole is placed on the side of the nu- Figure 1. Morphology of nephronophthisis. (A) Renal ultrasound demonstrates in- creased echogenicity, loss of corticomedullary differentiation, and the presence of cor- cleus opposite to the basal body, thus ticomedullary cysts. In contrast to PKD, kidneys are not enlarged. (B) Renal histology in specifying cell polarity. The structure NPHP shows the characteristic triad of renal tubular cysts, tubular membrane disruption, and function of primary cilia and basal and tubulointerstitial cell infiltrates with interstitial fibrosis and periglomerular fibrosis (B bodies are delineated in Figure 2. is courtesy of D. Bockenhauer, London). Primary cilia are highly conserved structures that sense a wide variety of ex- POSITIONAL CLONING REVEALS NPHP9/NEK8,38 defining NPHP types 1 tracellular cues in a wide spectrum of ep- NEPHRONOPHTHISIS AS A through 9, respectively. This collection ithelial tissues. There is a broad range of “CILIOPATHY” of genes has made diagnostic testing pos- cues that can be received by specific cili- sible (http://www.renalgenes.org). Ho- ary receptors, including photosensation, By positional cloning, we and others mozygous deletions in the NPHP1 gene mechanosensation, osmosensation, and identified recessive mutations in nine account for approximately 21% of all olfactory sensation. In general, the novel genes as causing NPHP: NPHP1,19,20 NPHP cases, whereas the other genes pathogenesis of ciliopathies is based on NPHP2/inversin,13 NPHP3,22 NPHP4,21,27 contribute less than 3% each (Figure 3). an inability of epithelial cells to sense or NPHP5,23 NPHP6/CEP290,24,35 NPHP7/ As determined in more than 1000 fami- process extracellular cues.34 GLIS2,28 NPHP8/RPGRIP1L,29,36,37 and lies with NPHP, the causative genes are Figure 2. Cilia structure and intraflagellar transport. The cilium is a Tip hair-like structure that extends from the cell surface into the extra- complex cellular space. Virtually all vertebrate cell types can produce cilia. Eb1 Cilia consist of a microtubule-based axoneme covered by a special- ized plasma membrane. The axoneme has nine peripheral microtu- Ciliary Retrograde ϩ axoneme transport bule doublets. There may be two central microtubules (9 2 versus 9 ϩ 0 axoneme). 9 ϩ 2 cilia usually have dynein arms that link the microtubule doublets and are motile, whereas most 9 ϩ 0 cilia lack Ciliary membrane dynein arms and are nonmotile (“primary cilia”) with a few excep- tions. The ciliary axoneme is anchored in the basal body, a micro- tubule-organizing center derived from the mother centriole. The Anterograde transition zone at the junction of the basal body acts as a filter for transport the molecules that can pass into or out of the cilium. Nephrocystin-1 is localized at the transition zone of epithelial cells.47 During cilio- genesis, cilia elongate from the basal body by the addition of new Cell membrane axonemal subunits to the distal tip, the plus end of the microtu- IFT cargo Transition fibers bules. Axonemal and membrane components are transported in raft Dynein Pc1/Pc2 Basal complex body macromolecular particles (complex A and B) by so-called intraflagel- 129 B Kinesin-2 lar transport (IFT) along the axonemal doublet microtubules. A IFT Anterograde transport toward the tip is driven by heterotrimeric complexes Nucleus kinesin 2, which contains motor subunits Kif3a and Kif3b and a nonmotor subunit. Mutations of Kif3a cause renal cysts and cere- bellar vermis aplasia in mice.130 Retrograde transport back to the cell body occurs via the motor protein cytoplasmic dynein 1B.131 Centrosome Adapted from Bisgrove and Yost.132 24 Journal of the American Society of Nephrology J Am Soc Nephrol 20: 23–35, 2009 www.jasn.org BRIEF REVIEW still unknown in approximately 70% of main, localizes to adherens junctions and expressed in primary cilia, basal bodies, patients, indicating that additional genes focal adhesions of renal epithelial cells, or centrosomes.13,30 The interaction and are involved in the pathogenesis of and interacts with integral components co-localization to cilia and basal