Investigation of the role of a ciliopathy gene in familial idiopathic scoliosis: from zebrafish to human

Idiopathic Scoliosis (IS) is a human pathology characterised by a severe curvature of the axial skeleton without any obvious ossification defect, which affects 3% of the population, mainly girls, before puberty. Because its etiology remains mysterious, no genetic test or medication is available to either predict or stop its progression. Similar scoliotic phenotypes have been recently described in zebrafish mutant for a ciliary motility gene “Curly” and for the Wnt signalling co-receptor PTK7 suggesting links between cilia, the Wnt-pathway and scoliosis (1). Cilia are cellular antennae that support many functions: fluid propulsion and transduction of light, chemical and mechanical signals.

To achieve a better understanding of cilia function during morphogenesis, we produced a zebrafish mutant with a complete deletion of the Rpgrip1llocus. Rpgrip1l encodes a ciliary protein required for tissue-specific ciliogenesis and for primary cilia signalling. We have previously uncovered its crucial function in Hedgehog transduction in a murine model and its implication in Wnt-PCP establishment both in mice and zebrafish (2, 3). Mutations of the human RPGRIP1L gene lead to severe ciliopathies, Meckel and Joubert syndromes (4). Surprisingly, zebrafish rpgrip1l null mutants present only subtle defects in ciliary content and ciliary motility at larval stages and a high proportion of mutants develop until juvenile stages, displaying a severe scoliosis phenotype without presenting major defects in ossification or locomotion. 

Thus, zebrafish rpgrip1l mutant is a novel animal model to study the etiology of IS. We favor the hypothesis that this phenotype originates from a defect within the nervous system, ie from abnormal postural control or asymmetric locomotor comand, but it could also arise from non-isotropic growth of cartilage and tendons at puberty or intervertebral disk degeneration.To determine in which tissue(s) and at which stages Rpgrip1l and cilia are required between larval stage and puberty we plan to rescue the scoliosis phenotype of rpgrip1l mutants by reintroducing Rpgrip1l function by transgenesis using tissue-specific promoters. In parallel, we will analyse the presence and activity of motoneurons and ciliated (KA) interneurons in mutants before the appearance of scoliosis because their dysfunction could trigger asymmetric muscle contraction (5). As a non-biaised approach and more long-term project, we will prepare material for a transcriptomic analysis to compare juvenile trunks of wild-type animals versus mutants in order to identify the deregulated molecular pathways at the onset of the disease.

These studies will help uncover novel functions of cilia that directly or indirectly affect skeletal morphogenesis at puberty. They should help improve IS patients care on a long-term basis.

1: Boswell, TIGS, 2017. 2: Besse, Dvpt, 2011. 3: Mahuzier, JCB, 2012. 4: Delous, Nat. Gen, 2007. 5: Djenoune, Sci. Rep. 2017 6: Oliazadeh, Sci. Rep. 2017.