A founder variant in tubulin binding co-factor B is associated with complex hereditary spastic paraparesis
2Ruth and Bruce Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Israel
3Sackler Faculty of Medicine, Tel Aviv University, Israel
4Paris Brain Institute (ICM Institut du Cerveau), Sorbonne Université, INSERM, CNRS, Assistance Publique-Hôpitaux de Paris, Pitié-Salpêtrière University Hospital, France
5UF de Génomique du Développement Département de Génétique Médicale Laboratoire de Médecine Génomique, Assistance Publique-Hôpitaux de Paris, Sorbonne Université, France
6Genetic Institute, Samson Assuta Ashdod University Medical Center, Israel
7Genetic Institute, Rambam Health Care Campus, Israel
8The Genetics Institute, Tel Aviv Sourasky Medical Center, Israel
9Department of Molecular Microbiology, B. Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Israel
10Department of Genetics and Developmental Biology, The Rappaport Faculty of Medicine and Research Institute, Technion - Israel Institute of Technology, Israel
Hereditary spastic paraparesis (HSP) is a group of Mendelian disorders related to length dependent axonal degeneration. Microtubule (MT) dysfunction is one of the molecular mechanisms for HSP. MT are dynamic structures controlling axonal outgrowth and retraction, their structural diversity is attributed to variability in its subunit, the αβ-tubulin heterodimer. TBCB encodes a tubulin binding co-factor, not previously associated with disease, that regulates α-tubulin dissociation from the MT thus negatively regulating axonal growth. We performed exome sequencing on two siblings with early onset global developmental delay and progressive spasticity appearing in late childhood. Both siblings had a homozygous variant in TBCB, c.589T>A; p.(Tyr197Asn) located in a highly conserved residue in the CAP-Gly domain and predicted as damaging. Interestingly, carrier rate in Ashkenazi descent is 1/200. Automated data sharing and gene matching tools led to the identification of 5 additional patients from 3 unrelated families carrying the same homozygous variant. We demonstrate the homologous amino acid substitution in Saccharomyces cerevisiae orthologue, Alf1, increases Benomyl sensitivity similar to Alf1 knock down. In Drosophila melanogaster dTBCB RNAi resulted in lower survival rates and decreased climbing ability. Using CRISPR/Cas9 we generated an in vivo model for the Tyr197Asn mutation in Drosophila. The mutation is semi-lethal as indicated by reduced number of viable homozygotes, and appears to cause neurologic phenotype. Altogether, we show TBCB has an important role in CNS development and potentially in axonal function in drosophila and humans. Furthermore, our findings confirm the Tyr197Asn variant in TBCB results in a novel recessive HSP disorder.