Biallelic mutations in the TOGARAM1 gene cause a novel primary ciliopathy

Abstract

Background: Dysfunction in non-motile cilia is associated with a broad spectrum of developmental disorders characterised by clinical heterogeneity. While over 100 genes have been associated with primary ciliopathies, with wide phenotypic overlap, some patients still lack a molecular diagnosis.

Objective: To investigate and functionally characterise the molecular cause of a malformation disorder observed in two sibling fetuses characterised by microphthalmia, cleft lip and palate, and brain anomalies.

Methods: A trio-based whole exome sequencing (WES) strategy was used to identify candidate variants in the TOGARAM1 gene. In silico, in vitro and in vivo (Caenorhabditis elegans) studies were carried out to explore the impact of mutations on protein structure and function, and relevant biological processes.

Results: TOGARAM1 encodes a member of the Crescerin1 family of proteins regulating microtubule dynamics. Its orthologue in C. elegans, che-12, is expressed in a subset of sensory neurons and localises in the dendritic cilium where it is required for chemosensation. Nematode lines harbouring the corresponding missense variant in TOGARAM1 were generated by CRISPR/Cas9 technology. Although chemotaxis ability on a NaCl gradient was not affected, che-12 point mutants displayed impaired lipophilic dye uptake, with shorter and altered cilia in sensory neurons. Finally, in vitro analysis of microtubule polymerisation in the presence of wild-type or mutant TOG2 domain revealed a faster polymerisation associated with the mutant protein, suggesting aberrant tubulin binding.

Conclusions: Our data are in favour of a causative role of TOGARAM1 variants in the pathogenesis of this novel disorder, connecting this gene with primary ciliopathy.

Keywords: clinical genetics; developmental; molecular genetics.