Joubert syndrome (JS) is a recessive neurodevelopmental disorder characterized by hypotonia, ataxia, abnormal eye movements, and variable cognitive impairment. It is defined by a distinctive brain malformation known as the “molar tooth sign” on axial MRI. Subsets of affected individuals have malformations such as coloboma, polydactyly, and encephalocele, as well as progressive retinal dystrophy, fibrocystic kidney disease, and liver fibrosis. More than 35 genes have been associated with JS, but in a subset of families the genetic cause remains unknown. All of the gene products localize in and around the primary cilium, making JS a canonical ciliopathy. Ciliopathies are unified by their overlapping clinical features and underlying mechanisms involving ciliary dysfunction. In this work, we identify biallelic rare, predicted-deleterious ARMC9 variants (stop-gain, missense, splice-site, and single-exon deletion) in 11 individuals with JS from 8 families, accounting for approximately 1% of the disorder. The associated phenotypes range from isolated neurological involvement to JS with retinal dystrophy, additional brain abnormalities (e.g., heterotopia, Dandy-Walker malformation), pituitary insufficiency, and/or synpolydactyly. We show that ARMC9 localizes to the basal body of the cilium and is upregulated during ciliogenesis. Typical ciliopathy phenotypes (curved body shape, retinal dystrophy, coloboma, and decreased cilia) in a CRISPR/Cas9-engineered zebrafish mutant model provide additional support for ARMC9 as a ciliopathy-associated gene. Identifying ARMC9 mutations as a cause of JS takes us one step closer to a full genetic understanding of this important disorder and enables future functional work to define the central biological mechanisms underlying JS and other ciliopathies.
Bibliographical noteFunding Information:
We deeply thank the families who participated in this study. We also thank the Joubert syndrome and Related Disorder Foundation for referring families, Heather Mefford and her laboratory for help with arrays, the Center for Microscopy and Image Analysis, University of Zurich (ZMB), for their assistance with Scanning Electron Microscopy, in particular Gery Barmettler, the Genotyping and Sequencing Core Facilities at King Faisal Specialist Hospital & Research Centre for their technical help, and Darren Gilmour for the Cas9 protein. This work was supported by the following: NIH Eunice Kennedy Shriver National Institute of Child Health and Human Development U54HD083091 (Genetics Core and Sub-project 6849 ) to D.D., NIH National Institute of Neurological Diseases and Stroke ( R01NS064077 to D.D. and R01NS050375 to W.B.D.), private donations from families to D.D., the Swiss National Science Foundation ( PZ00P3_163979 ) to R.B.-G., the Netherlands Organization for Scientific Research (NWO Vici-865.12.005 ) to R.R., the King Salman Center for Disability Research to F.S.A., Dandy-Walker Alliance to W.B.D., and NIH National Human Genome Research Institute ( U54HG006493 ) to M.B. and D.N. We acknowledge the support of the Saudi Human Genome Program (F.S.A.).
- Joubert syndrome
- basal body
- primary cilium