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Synergistic Interaction cilia growth, maintenance, or morphology give rise to cysts,6 suggesting that both cilia formation and function are crucial between Ciliary Reflects for prevention of a polycystic kidney phenotype. NPHP and MKS belong to the growing number of ciliopa- the Importance of Mutational thies characterized by phenotypically distinct but partially Load in overlapping malformations. NPHP is a rare autosomal reces- sive renal disease with an incidence of approximately one in Sunjin Lee and Scott D. Weatherbee 100,000 live births and is the most frequent cause of ESRD in 7 Department of Genetics, Yale University School of Medicine, New children and young adults. The hallmarks of NPHP are cor- Haven, Connecticut ticomedullary cysts, tubulointerstitial nephropathy, and dis- ruption of the tubular basement membrane. Together, these J Am Soc Nephrol ●●: –, 2010. doi: 10.1681/ASN.2010030301 defects lead to renal failure, typically within the first three decades of life. To date, 10 genes encoding nephrocystins have been identified as being mutated in patients with NPHP, The formation of renal cysts occurs in roughly 10 to 15% of although disruptions in these genes account for only approx- 1 the population, and although simple cysts are largely asymp- imately 30% of NPHP cases, suggesting the presence of addi- tomatic, the formation of multiple cysts can be extremely tional genetic loci. detrimental. The two major multicystic syndromes, autoso- MKS is a rare autosomal recessive disorder first identified mal recessive polycystic kidney disease (PKD) and autosomal more than a century ago and has a current worldwide inci- dominant PKD, are primarily linked to disruptions in the dence of one in 140,000 live births.6 MKS is characterized by 2 pkhd1 and pkd1/pkd2 genes, respectively ; however, several postaxial , dysplasia of the hepatic duct, occipital additional syndromes are also characterized by kidney cyst meningoencephalocoele, and multicystic kidneys. In recent development, including nephronophthisis (NPHP), Joubert years, six genomic loci in humans have been linked to MKS syndrome, Bardet-Biedl syndrome, Orofaciodigital syn- and, with the exception of MKS2, have been mapped to indi- drome 1, and Meckel syndrome (MKS). During the past de- vidual genes. Nephrocystins and MKS primarily lo- cade, there has been a great upsurgence in identifying genes calize to cilia or basal bodies in renal epithelial cells, placing affected in these syndromes. One of the most striking discov- these syndromes within the growing list of ciliopathies. eries is that products of these genes localize to the Links between NPHP and MKS have begun to emerge in and/or the associated and affect ciliogen- recent years. The two syndromes share at least three under- 3 esis and cilia morphology or function when mutated, collec- lying genes. is mutated in MKS5/NPHP8, cep-290 in tively identifying these syndromes as ciliopathies. MKS4/NPHP6, and mks3 in MKS3/NPHP11. Mutations in Why do kidney cysts form? An increasing body of evi- these genes have also been found in Joubert syndrome, cer- dence indicates that mutations affecting ciliogenesis or cilia ebello-oculorenal syndromes, and Senior-Løken syndrome, function can give rise to cysts. Primary cilia extend from the suggesting they form a spectrum whereby the severity of phe- apical membrane of renal epithelial cells into the lumen, notype is a function of the strength of the mutation. This is where they act as mechanosensors. One model suggests that reflected by the fact that ciliopathies share an overlapping set fluid flow within the kidney bends the cilia and triggers intra- of clinical features, with MKS representing the severe end of cellular calcium signaling, which, in turn, regulates epithelial the phenotypic spectrum comprising defects in multiple or- 2 tubule morphology. and , the pro- gans and perinatal lethality. tein products of the pkd1 and pkd2 genes, respectively, local- In addition to the finding that the same genes can give rise 4 ize to renal cilia and may act as the mediators of mech- to different syndromes, one of the most exciting results from 5 anosensation in these structures. the field of research is that underlying mutations Mutations in pkd1 or pkd2 do not result in morphologic seem to act in a combinatorial manner to influence the phe- cilia defects but rather affect calcium influx, which leads to notype.8,9 This suggests that the mutational load within an cyst formation. In addition, mutations that result in aberrant individual can modulate the severity of the disease. Previous studies showed that MKS1 and MKS1-related proteins Published online ahead of print. Publication date available at www.jasn.org. (MKSR1/TZA2, MKSR2/TZA1) act in the same pathway and have no ciliary defects when mutated by themselves or to- Correspondence: Dr. Scott D. Weatherbee, Genetics Department, Yale Uni- 10,11 versity School of Medicine, P.O. Box 208005, New Haven, CT 06520. Phone: gether in Caenorhabditis elegans ; however, mks1; 203-737-1923; Fax: 203-785-4415; E-mail: [email protected] double mutants display ciliary defects including shortened Copyright ᮊ 2010 by the American Society of Nephrology cilia and aberrant ciliary positioning.

J Am Soc Nephrol ●●: , 2010 ISSN : 1046-6673/●●00- 1 EDITORIALS www.jasn.org

In this issue, Williams et al.12 build on these initial ob- phenotypic severity among MKS and NPHP is a conse- servations and provide data that corroborate a synergistic quence of mutational load, meaning that MKS and NPHP interaction between the ciliogenic MKS and NPHP path- lie within a phenotypic continuum rather than represent ways. To test the hypothesis that perturbation of either multiple distinct clinical entities and that the sum of muta- pathway alone is not sufficient to disrupt ciliogenesis but tions in ciliary genes define the severity of the phenotype. disruption of both together lead to a ciliary defect, the au- Further studies of MKS3 in conjunction with other ciliary thors analyzed mks3 mutants, mutants, and double proteins are now required to unravel how different muta- mutants to determine whether they would display defects in tional loads can lead to the clinical variability observed in ciliogenesis, ciliary positioning, and proper sensory func- patients with MKS as well as other ciliopathies and identify tion of cilia. further, second-site modifiers. The authors show that in C. elegans MKS3 localizes to the This study by Williams et al.12 offers a deeper understanding membrane of the dendritic tip of ciliated sensory neurons of the importance of mutational load on the presentation and and its cilium base and acts in the MKS1 pathway. This severity of ciliopathies and expands the understanding of the subcellular localization somewhat differs from MKS3 local- synergistic interactions between ciliopathy genes. Further ization in mammalian cells (IMCD3, HEK293, and BEC), analysis will hopefully allow targeted therapies to alleviate where it is localized mostly along the ciliary .13 the morbidity and mortality associated with these devastat- Using a putative mks3 null mutant to analyze its function in ing diseases. C. elegans, the authors found no obvious ciliogenesis defect (mild elongation), which is in contrast to mammalian mks3 null models in mouse (bpck) and rat (wpk), which exhibit DISCLOSURES 14,15 elongated cilia and kidney cysts. Interestingly, although None. mks3 mutants showed no defect in cilia morphology, mu- tants showed reduced chemotaxis, suggesting impaired sen- sory function. Interestingly, mks3;nphp4 double mutants exhibit defec- REFERENCES tive cilia positioning and shorter or absent cilia indicative of a defect in ciliogenesis and/or maintenance of the cilium. 1. Terada N, Ichioka K, Matsuta Y, Okubo K, Yoshimura K, Arai Y: The One interpretation of the data is that MKS3 may act as a natural history of simple renal cysts. J Urol 167: 21–23, 2002 2. Deane JA, Ricardo SD: Polycystic kidney disease and the renal cilium. potential regulator of cargo trafficking into the cilium by Nephrology (Carlton) 12: 559–564, 2007 tethering transition fibers to the membrane, thereby regu- 3. Fliegauf M, Benzing T, Omran H: When cilia go bad: Cilia defects and lating vesicle trafficking at the ciliary base or defective po- ciliopathies. Nat Rev Mol Cell Biol 8: 880–893, 2007 sitioning of the cargo in the cilium. A leaky transition zone 4. Yoder BK, Hou X, Guay-Woodford LM: The polycystic kidney disease could account for excess cargo import into the cilium and proteins, polycystin-1, polycystin-2, polaris, and cystin, are co-local- ized in renal cilia. J Am Soc Nephrol 13: 2508–2516, 2002 the elongated ciliary phenotype observed in mammals and 5. Nauli SM, Alenghat FJ, Luo Y, Williams E, Vassilev P, Li X, Elia AE, Lu in milder form in C. elegans. The addition of ciliary micro- W, Brown EM, Quinn SJ, Ingber DE, Zhou J: Polycystins 1 and 2 tubule defects of nphp4 mutants to the flawed ciliary mem- mediate mechanosensation in the primary cilium of kidney cells. 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Nat Genet 40: apical membrane in kidney tissues in vivo and in vitro, and 443–448, 2008 loss of MKS3 results in an increased number of cilia. The 9. Tayeh MK, Yen HJ, Beck JS, Searby CC, Westfall TA, Griesbach H, authors of this study do not comment on cilia numbers, Sheffield VC, Slusarski DC: Genetic interaction between Bardet-Biedl which may reflect a difference between a mammalian and syndrome genes and implications for limb patterning. Hum Mol Genet nematode role of MKS3. Along this line, MKS3 might play 17: 1956–1967, 2008 10. Bialas NJ, Inglis PN, Li C, Robinson JF, Parker JD, Healey MP, Davis additional roles in mammals versus nematodes, on the basis EE, Inglis CD, Toivonen T, Cottell DC, Blacque OE, Quarmby LM, of its differential localization. Elucidating the function of Katsanis N, Leroux MR: Functional interactions between the cili- MKS3 will also require investigating its potential genetic opathy-associated Meckel syndrome 1 (MKS1) protein and two interaction with other ciliopathy genes, which would give novel MKS1-related (MKSR) proteins. J Cell Sci 122: 611–624, 2009 important insights into the understanding of the pathogen- 11. Williams CL, Winkelbauer ME, Schafer JC, Michaud EJ, Yoder BK: Functional redundancy of the B9 proteins and nephrocystins in Cae- esis of the cystic phenotype. norhabditis elegans ciliogenesis. Mol Biol Cell 19: 2154–2168, 2008 Several reports recently provided data indicating that 12. Williams CL, Masyukova SV, Yoder BK: Normal ciliogenesis requires

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synergy between the cystic kidney disease genes MKS-3 and NPHP-4. 15. Tammachote R, Hommerding CJ, Sinders RM, Miller CA, Czarnecki J Am Soc Nephrol 21: 000–000, 2010 PG, Leightner AC, Salisbury JL, Ward CJ, Torres VE, Gattone VH 2nd, 13. Dawe HR, Smith UM, Cullinane AR, Gerrelli D, Cox P, Badano JL, Harris PC: Ciliary and centrosomal defects associated with mutation Blair-Reid S, Sriram N, Katsanis N, Attie-Bitach T, Afford SC, Copp and depletion of the Meckel syndrome genes MKS1 and MKS3. Hum AJ, Kelly DA, Gull K, Johnson CA: The Meckel-Gruber syndrome Mol Genet 18: 3311–3323, 2009 proteins MKS1 and meckelin interact and are required for primary cilium formation. Hum Mol Genet 16: 173–186, 2007 14. Cook SA, Collin GB, Bronson RT, Naggert JK, Liu DP, Akeson EC, Davisson MT: A mouse model for Meckel syndrome type 3. JAmSoc See related article, “Normal Ciliogenesis Requires Synergy between the Cystic Nephrol 20: 753–764, 2009 Kidney Disease Genes MKS-3 and NPHP-4,” on pages 000–000.

J Am Soc Nephrol ●●: , 2010 MKS and NPHP Redundancy 3