Joubert syndrome: insights into brain development, cilium biology, and complex disease

Abstract

Joubert syndrome (JS) is a primarily autosomal recessive condition characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability with a distinctive mid-hindbrain malformation (the "molar tooth sign"). Variable features include retinal dystrophy, cystic kidney disease, liver fibrosis and polydactyly. Recently, substantial progress has been made in our understanding of the genetic basis of JS, including identification of seven causal genes (NPHP1, AHI1, CEP290, RPGRIP1L, TMEM67/MKS3, ARL13B and CC2D2A). Despite this progress, the known genes account for <50% of cases and few strong genotype-phenotype correlations exist in JS; however, genetic testing can be prioritized based on clinical features. While all seven JS genes have been implicated in the function of the primary cilium/basal body organelle (PC/BB), little is known about how the PC/BB is required for brain, kidney, retina and liver development/function, nor how disruption of PC/BB function leads to diseases of these organs. Recent work on the function of the PC/BB indicates that the organelle is required for multiple signaling pathways including sonic hedgehog, WNT and platelet derived growth factor. Due to shared clinical features and underlying molecular pathophysiology, JS is included in the rapidly expanding group of disorders called ciliopathies. The ciliopathies are emerging as models for more complex diseases, where sequence variants in multiple genes contribute to the phenotype expressed in any given patient.

Figure 1. MRI findings in JS

(A–C) Classic Molar tooth sign (A) Axial T2-weighted image of vermis hypoplasia, long, thick superior cerebellar peduncles and deep interpedunclular fossa; (B) Sagittal T1-weighted image of vermis hypoplasia and elevation of the roof of the 4^th ventricle; (C) Coronal T2-weighted image of vermis hypoplasia and thick superior cerebellar peduncles; (D–F) “Mild” molar tooth sign (D) Axial T2-weighted image of mild vermis hypoplasia and long, mildly thick superior cerebellar peduncles; (E) Sagittal T1-weighted image of mild vermis hypoplasia and elevation of the roof of the 4^th ventricle; (F) Coronal T2-weighted image of vermis hypoplasia and thick superior cerebellar peduncles (G–I) Fetal MRI at 22 weeks gestation (G) Axial SSFSE image of vermis hypoplasia and large posterior fossa fluid collection; (H) Sagittal SSFSE image of vermis hypoplasia and abnormal configuration of the roof of the 4^th ventricle; (I) Axial SSFSE image of a small encephalocele (arrowhead); (J) Axial T2-weighted image of periventricular nodular heterotopias (black arrows), vermis hypoplasia and deep sulcus in the left temporal lobe; (K) Sagittal T1-weighted image of agenesis of the corpus callosum (asterisk) and elevation of the roof of the 4^th ventricle. Note that the vermis hypoplasia in B, D and K is somewhat obscured by the hemispheres impinging on the midline. SSFSE=Single Shot Fast Spin Echo

Figure 2. Clinical features in JS

(A) Pigmentary retinal dystrophy; (B) Chorioretinal coloboma (pale area); (C) Occipital encephalocele in a 19 week gestation fetus; (D) Nephronophthisis in a mature kidney: interstitial scarring with mild inflammation and tubular atrophy. Basement membranes (dark purple) around several tubules and Bowman’s capsule show marked redundancy with wrinkling and lamellation. Periodic Acid Schiff (PAS), 100X. (E) Cystic-dysplastic kidney in a 19 week gestation fetus: round, partially collapsed cysts are seen in all parts of the nephron as well as decreased numbers of glomeruli, excessive stroma and interruption of the nephrogenic zone (upper right, subcapsular). H&E, 40X; (F) Postaxial polydactyly in a 19 week gestation fetus; (G) Normal mature liver: the portal triad contains a branch of the portal vein (left), hepatic artery (middle) and similarly sized bile duct (right) embedded in a small amount of supportive collagen. Trichrome, 200X; (H) Ductal plate malformation in a 22 week gestation fetus: multiple large ducts that parallel the limiting plate and encircle hepatic artery branches are evidence of biliary dysgenesis. Trichrome, 200X; (I) Congenital hepatic fibrosis in a 9 year old: the portal triad has moderate fibrosis and an excess of bile ducts and hepatic artery branches with only minimal inflammation. Images courtesy of Meral Gunay-Aygun, Ekaterini Tsilou (A), Avery Weiss (B), Joseph Siebert (C, F), Laura Finn (D, E, G–I).

Figure 3. Prioritized genetic testing for JS based on current genotype-phenotype information

While each of the JS genes can cause a spectrum of phenotypes, patients with a mild MTS, particularly if they have nephronphthisis, should be tested for NPHP1 deletions and if negative, point mutations. Those with liver disease should be tested for MKS3/TMEM67 mutations, followed by CC2D2A and RPGRIP1L. Retinal plus renal disease should prompt testing for CEP290 mutations. Isolated retinal disease should prompt AHI1 testing followed by CEP290, while isolated renal disease should prompt RPGRIP1L testing. Patients without organ complications should be tested for AHI1, CC2D2A and CEP290 (particularly the G1890X mutation). If mutations are not found using this strategy, sequencing the remaining genes should be considered. The yield for the currently available DNA testing is 40–50%.

Figure 4. Primary cilium/basal body, JS gene products and the SHH, WNT and PDGFα signaling pathways

The known JS-associated proteins are depicted in red and have been shown to localize to the basal body and/or primary cilium in at least some contexts. ARL13B, RPGRIP1L and possibly MKS3, CEP290 and CC2D2A are required for primary cilium/basal body formation or maintenance ,,,,. Direct contact between proteins indicates in vivo or in vitro evidence for physical interactions that are likely cell type and cell cycle dependent. Activation of the SHH signaling pathway (orange) relieves the suppression of SMO activity by PTCH, allowing SMO to enter the cilium and alter the balance of GLI activator and repressor transcription factors to control cell fates in a wide variety of tissues (reviewed in Eggenschwiler and Anderson 2007; Wong and Reiter 2008)., arl13b and rpgrip1l mutations in mice disrupt SHH signaling and cause randomization of left-right asymmetry, neural tube defects and polydactyly., The primary cilium/basal body is required for PDGFα signaling pathway (blue) that is involved in regulation of the cell cycle, cytoskeletal organization and cell migration (reviewed in Andrae et al. 2008; Christensen et al. 2008)., Although the mechanism is unknown, the PC/BB also plays a role in regulating the balance between canonical (green) and non-canonical (black) WNT pathways (reviewed in Clevers 2006; Gerdes and Katsanis 2008).,