ABSTRACT
Phosphotidylinositol (PtdIns) signaling is tightly regulated both spatially and temporally by subcellularly localized PtdIns kinases and phosphatases that dynamically alter downstream signaling events. Joubert syndrome is characterized by a specific midbrain-hindbrain malformation ('molar tooth sign'), variably associated retinal dystrophy, nephronophthisis, liver fibrosis and polydactyly and is included in the newly emerging group of 'ciliopathies'. In individuals with Joubert disease genetically linked to JBTS1, we identified mutations in the INPP5E gene, encoding inositol polyphosphate-5-phosphatase E, which hydrolyzes the 5-phosphate of PtdIns(3,4,5)P3 and PtdIns(4,5)P2. Mutations clustered in the phosphatase domain and impaired 5-phosphatase activity, resulting in altered cellular PtdIns ratios. INPP5E localized to cilia in major organs affected by Joubert syndrome, and mutations promoted premature destabilization of cilia in response to stimulation. These data link PtdIns signaling to the primary cilium, a cellular structure that is becoming increasingly recognized for its role in mediating cell signals and neuronal function.
Subject(s)
Cilia/pathology , Mutation , Phosphatidylinositols/genetics , Phosphoric Monoester Hydrolases/genetics , Signal Transduction/genetics , Acetylation , Amino Acid Substitution , Animals , Base Sequence , Brain/diagnostic imaging , Case-Control Studies , Catalytic Domain , Cell Line , Chromosomes, Human, Pair 9 , Cilia/enzymology , Consanguinity , Culture Media, Serum-Free , Fibroblasts/metabolism , Fibroblasts/ultrastructure , Genetic Linkage , Green Fluorescent Proteins/metabolism , Haplotypes , Homozygote , Humans , Hydrolysis , Mice , Mice, Transgenic , Molecular Sequence Data , Mutation, Missense , Phosphatidylinositol 4,5-Diphosphate/genetics , Phosphatidylinositol Phosphates/genetics , Phosphoric Monoester Hydrolases/chemistry , Phosphoric Monoester Hydrolases/metabolism , Physical Chromosome Mapping , Pigment Epithelium of Eye/cytology , Polymorphism, Single Nucleotide , Protein Structure, Tertiary , Radiography , Serum/metabolism , Tubulin/metabolismABSTRACT
Joubert syndrome (JS) is an autosomal recessive disorder marked by agenesis of the cerebellar vermis, ataxia, hypotonia, oculomotor apraxia, neonatal breathing abnormalities, and mental retardation. Despite the fact that this condition was described >30 years ago, the molecular basis has remained poorly understood. Here, we identify two frameshift mutations and one missense mutation in the AHI1 gene in three consanguineous families with JS, some with cortical polymicrogyria. AHI1, encoding the Jouberin protein, is an alternatively spliced signaling molecule that contains seven Trp-Asp (WD) repeats, an SH3 domain, and numerous SH3-binding sites. The gene is expressed strongly in embryonic hindbrain and forebrain, and our data suggest that AHI1 is required for both cerebellar and cortical development in humans. The recently described mutations in NPHP1, encoding a protein containing an SH3 domain, in a subset of patients with JS plus nephronophthisis, suggest a shared pathway.