Interpreting the pathogenicity of Joubert Syndrome missense variants inCaenorhabditis elegans

Ciliopathies are inherited disorders caused by cilia defects. Variants in ciliopathy genes are frequently pleiotropic and represent excellent case studies for interrogating genotype-phenotype correlation. We have employedCaenorhabditis elegansand gene editing to characterise two pathogenic biallelic missense variants (P74S, G155S) in B9D2/mksr-2associated with Joubert Syndrome (JBTS). B9D2 functions within the MKS module at the transition zone (TZ) ciliary subcompartment, and regulates the cilium’s molecular composition and signaling function. Quantitative assays of cilium/TZ structure and function, together with knock-in reporters, confirm both variant alleles are pathogenic. G155S causes a more severe overall phenotype and disrupts endogenous MKSR-2 organisation at the TZ. Recapitulation of the patient biallelic genotype shows that heterozygous worms phenocopy worms homozygous for P74S. This study also reveals a close functional association between the B9 complex and TMEM216/MKS-2. These data establishC. elegansas a paradigm for interpreting JBTS mutations, and provide insight into MKS module organisation.