Hypomorphic mutations in syndromic encephalocele genes are associated with Bardet-Biedl syndrome Carmen C Leitch 1 , Norann A Zaghloul 1 , Erica E Davis 1 , Corinne Stoetzel 2 , Anna Diaz-Font 3 , Suzanne Rix 3 , Majid Alfadhel 4 , Richard Alan Lewis 5 , Wafaa Eyaid 4 , Eyal Banin 6 , Helene Dollfus 2 , Philip L Beales 3 , Jose L Badano 1,7 & Nicholas Katsanis 1 Meckel-Gruber syndrome (MKS) is a genetically heterogeneous, neonatally lethal malformation and the most common form of syndromic neural tube defect (NTD). To date, several MKS- associated genes have been identified whose protein products affect ciliary function 1–5 . Here we show that mutations in MKS1, MKS3 and CEP290 (also known as NPHP6) either can cause Bardet-Biedl syndrome (BBS) or may have a potential epistatic effect on mutations in known BBS-associated loci. Five of six families with both MKS1 and BBS mutations manifested seizures, a feature that is not a typical component of either syndrome. Functional studies in zebrafish showed that mks1 is necessary for gastrulation movements and that it interacts genetically with known bbs genes. Similarly, we found two families with missense or splice mutations in MKS3, in one of which the affected individual also bears a homozygous nonsense mutation in CEP290 that is likely to truncate the C terminus of the protein. These data extend the genetic stratification of ciliopathies and suggest that BBS and MKS, although distinct clinically, are allelic forms of the same molecular spectrum. Defects of the primary cilium cause a number of human genetic disorders, collectively termed ciliopathies 6 because they are character- ized by an overlapping range of phenotypes. BBS represents a model ciliopathy for which 12 causal genes have been identified (ref. 7 and references within). In addition, heterozygous mutations in most BBS- related genes 8–11 , as well as in a transcript encoding a BBS-interacting protein 12 , can modulate the penetrance and the expressivity of the BBS phenotype. The study of ciliary and basal body proteins has highlighted an important role of vertebrate primary cilia in the transduction of major morphogenetic pathways that include Shh and Wnt signaling 13 . In BBS, disruption of the latter manifests as planar-cell polarity (PCP) defects that include failure of the neural tube to close in mice and perturbed gastrulation movements in zebrafish 14,15 . Although NTDs are infrequent in Bbs / mice and have never been reported in individuals with BBS, they are the cardinal manifestation of another ciliopathy, MKS, a syndrome of encephalocele, cystic kidneys, hepatic fibrosis and polydactyly. MKS has been attributed to ciliary dysfunc- tion, because of (i) the presence of disease-causing mutations in four genes 1,3–5,16 , each of which is present in ciliary proteomic collec- tions 17 , and (ii) the localization of all MKS-causing proteins to the basal body, primary cilium or both 2,3,18–20 . Mutations in at least three BBS genes cause MKS-like phenotypes but not encephalocele 21 . These observations, together with the presence of encephalocele in our Bbs4 mutant mice 14 , prompted us to hypothe- size that MKS might represent a more severe variant of BBS. If true, mutations in bona fide MKS genes should contribute causal and/or modifying mutations to BBS. To test this hypothesis, we first sequenced the coding exons of both MKS1 isoforms (see Methods for accession numbers; Supplementary Fig. 1 online) in 155 BBS-affected families, without preselection for mutational load in the known BBS loci. We identified potentially pathogenic mutations in six families, in all of which the diagnosis of MKS was excluded unambiguously (Table 1). In five families, we identified single heterozygous MKS1 mutations. One mutation, resulting in the amino acid substitution R123Q, was present in two unrelated families, a Lebanese pedigree in which the affected individual also bears a homozygous BBS10 mutation resulting in the amino acid change S73FsX91 and a Saudi pedigree with an affected member bearing a heterozygous BBS10 mutation resulting in the variant Q242FsX258 (ref. 11). In a third Middle Eastern family, we found an affected individual with both a heterozygous MKS1 mutation encoding the variant V339M and a homozygous BBS1 mutation encoding the variant R146X; in the fifth family, of Northern European descent, we found an MKS1 mutation resulting in an I450T change in the proband (Supplementary Fig. 2 online). Finally, in the Received 8 December 2007; accepted 23 January 2008; published online 9 March 2008; corrected after print 26 June 2008; doi:10.1038/ng.97 1 McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine, 733. N Broadway, Baltimore, Maryland 21205, USA. 2 Laboratoire de Ge ´ne ´tique Me ´dicale EA 3439 e ´ quipe Avenir/INSERM, Faculte ´ de Me ´ decine de Strasbourg, Universite ´ Louis Pasteur, 11 rue Humann, 67085 Strasbourg, France. 3 Molecular Medicine Unit, Institute of Child Health, University College London, 30 Guilford Street, London WC1N 1EH, UK. 4 Department of Pediatrics, King Fahad Hospital, P.O. Box 22490, Riyadh 11426, Saudi Arabia. 5 Departments of Ophthalmology, Molecular and Human Genetics, Pediatrics, and Medicine, Baylor College of Medicine, Smith Tower, 6550 Fannin, Suite 1501, Houston, Texas 77030, USA. 6 Department of Ophthalmology, Hadassah–Hebrew University Hospital, P.O. Box 12000, 91120 Jerusalem, Israel. 7 Institut Pasteur de Montevideo, Calle Mataojo 2020, CP11400 Montevideo, Uruguay. Correspondence should be addressed to N.K. (katsanis@jhmi.edu). NATURE GENETICS VOLUME 40 [ NUMBER 4 [ APRIL 2008 443 LETTERS © 2008 Nature Publishing Group http://www.nature.com/naturegenetics