Joubert syndrome (JS) and Meckel syndrome (MKS) are pleiotropic ciliopathies characterized by severe defects of the cerebellar vermis, ranging from hypoplasia to aplasia. Interestingly, ciliary conditional mutant mice have a hypoplastic cerebellum in which the proliferation of cerebellar granule cell progenitors (GCPs) in response to Sonic hedgehog (SHH) is severely reduced. This suggests that Shh signaling defects could contribute to the vermis hypoplasia observed in the human syndromes. As existing JS/MKS mutant mouse models suggest apparently contradictory hypotheses on JS/MKS etiology, we investigated Shh signaling directly on human fetal samples. First, in an examination of human cerebellar development, we linked the rates of GCP proliferation to the different levels and localizations of active Shh signaling and showed that the GCP possessed a primary cilium with CEP290 at its base. Second, we found that the proliferation of GCPs and their response to SHH were severely impaired in the cerebellum of subjects with JS/MKS and Jeune syndrome. Finally, we showed that the defect in GCP proliferation was similar in the cerebellar vermis and hemispheres in all patients with ciliopathy analyzed, suggesting that the specific cause of vermal hypo-/aplasia precedes this defect. Our results, obtained from the analysis of human samples, show that the hemispheres and the vermis are affected in JS/MKS and provide evidence of a defective cellular mechanism in these pathologic processes.cilia | granular neuron | hindbrain | developmental pathology J oubert syndrome (JS; Online Mendelian Inheritance in Man no. 213300) and Meckel syndrome (MKS; Online Mendelian Inheritance in Man no. 249000) are believed to represent the two extremes of the same multisystemic disorder. JS is characterized by a complex malformation of the cerebellum/brainstem, the most striking feature of which is hypoplasia or complete aplasia of the cerebellar vermis (1, 2). In MKS, which is lethal in utero, this defect is associated with occipital encephalocele or anencephaly, severely cystic kidneys, and abnormal liver and skeleton. To date, 16 genes responsible for JS (NPHP1, AHI1, ARL13B, INPP5E, KIF7, OFD1, TCTN1, and CEP41), MKS (MKS1, B9D1, and B9D2), or both (RPGRIP1L, CEP290, CC2D2A, TMEM216, TMEM67/MKS3, and TCTN2) have been identified (3-6). Although all JS/MKS genes encode a ciliary/basal body (BB) protein, which is compelling evidence that the primary cilium is involved in these pathologic conditions, the cellular mechanisms by which cilium dysfunction leads to severe brain malformations remain a mystery.Because the use of human subjects to study human pathologies is problematic, insight into diseases usually comes from the analysis of murine models. However, none of the currently available murine models of JS/MKS faithfully recapitulate the cerebellar phenotype seen in JS/MKS. Interestingly, in ciliary conditional mutant mice, the cerebellum is hypoplastic and Sonic hedgehog (Shh)-dependent proliferation of granule cell progenitors (GCPs) is sev...