Whole Exome Sequencing Identified Novel ARMC9 Variations in Two Cases With Joubert Syndrome

Background: Biallelic variations in the armadillo repeat-containing 9 (ARMC9) gene were recently defined to cause Joubert syndrome (JS) type thirty. In this study, two unrelated families with probands displaying typical indications of JS were enrolled and underwent a series of clinical and genetic investigations. Methods: Routine evaluation including magnetic resonance imaging (MRI) was carried out. Whole-exome sequencing (WES) was performed on the probands to detect causative variants. Next, in silico structural and molecular dynamic (MD) analysis was conducted on the missense variant for analyzing its intramolecular impact. Meanwhile, an in vitro study with the minigene system was performed to explore the specific impact on mRNA splicing of another variant. Results: Two unrelated patients from two different families came to our hospital exhibiting typical JS presentations, such as the "molar tooth sign." Using WES, we identified that both probands carried the compound heterogeneous variants in ARMC9 (NM_025139.6), with c.1878+1G > A and c.895C > T (p.Arg299Ter) in family 1 and c.1878+1G > A and c.1027C > T (p.Arg343Cys) in family 2. These variants were inherited from their unaffected parents by Sanger sequencing, respectively, and ARMC9 c.895C > T (p.Arg299Ter) and c.1878+1G > A were novel variants. In silico analysis indicated the c.1027C > T (p.Arg343Cys) would likely affect the secondary structure of the ARMC9 protein. The minigene study demonstrated that the splice site variant c.1878+1G > A abolished the canonical donor site, resulting in an 18bp intronic retention of intron 20. Conclusion: The findings in this study expanded the mutation spectrum of ARMC9-associated JS, and we suggested that the function of ARMC9 in the pathogenesis of JS might involve the development of primary cilia, after discussing the function of the ARMC9 protein.