Abstract
ARL13B encodes for the ADP-ribosylation factor-like 13B GTPase, which is required for normal cilia structure and Sonic hedgehog (Shh) signaling. Disruptions in cilia structure or function lead to a class of human disorders called ciliopathies. Joubert syndrome is characterized by a wide spectrum of symptoms, including a variable degree of intellectual disability, ataxia, and ocular abnormalities. Here we report a novel homozygous missense variant c.[223G>A] (p.(Gly75Arg) in the ARL13B gene, which was identified by whole-exome sequencing of a trio from a consanguineous family with multiple-affected individuals suffering from intellectual disability, ataxia, ocular defects, and epilepsy. The same variant was also identified in a second family. We saw a striking difference in the severity of ataxia between affected male and female individuals in both families. Both ARL13B and ARL13B-c.[223G>A] (p.(Gly75Arg) expression rescued the cilia length and Shh defects displayed by Arl13b hennin (null) cells, indicating that the variant did not disrupt either ARL13B function. In contrast, ARL13B-c.[223G>A] (p.(Gly75Arg) displayed a marked loss of ARL3 guanine nucleotide-exchange factor activity, with retention of its GTPase activities, highlighting the correlation between its loss of function as an ARL3 guanine nucleotide-exchange factor and Joubert syndrome.
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Acknowledgments
We thank the families for participating and supporting this study. We also thank Dr. Ute Hehr (Center for Human Genetics, Regensburg, Germany) for her opinion on the CT scans. We thank Cheryl Timms Strauss for editing of the manuscript. We also thank Christine Fischer for statistical advice.
Funding
This study was supported by the Medical Faculty of Heidelberg (R.R., S.B., and G.A.R.), the National Institutes of Health (GM110663 to T.C., A.B.L., A.A.I., and R.A.K.), and the Emory Integrated Genomics Core (EIGC), which is subsidized by the Emory University School of Medicine. R.R. was supported by a scholarship from the German Academic Exchange Service (DAAD; 91541533) and G.M. and R.C.W. gratefully acknowledge the support of the Klaus Tschira Foundation.
Author contributions
R.R. performed the genetic analysis, expression analysis in mice and SH-SY5Y cells, and data analysis. A.B.L. and T.C. performed and analyzed the data in Arl13b hnn MEFs. A.A.I. and R.A.K. performed and analyzed the GAP and GEF assays. H.A. evaluated the patients, clinical data, and CT scans. S.B. contributed to the data interpretation. N.P., M.S., and S.W. performed and analyzed WES data. G.M. and R.C.W. performed and analyzed the 3D structure modeling. E.B. interpreted the CT scans of the patients. G.A.R. initiated, supervised, and supported the project. R.R., S.B., R.K., T.C., and G.A.R. wrote the manuscript. All authors commented on the manuscript.
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This study was approved by the Institutional Ethical Review Committee, University of Health Sciences Lahore, Pakistan, and the Ethikkommission, Medical Faculty Heidelberg, Germany (S-035/2014).
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Rafiullah, R., Long, A.B., Ivanova, A.A. et al. A novel homozygous ARL13B variant in patients with Joubert syndrome impairs its guanine nucleotide-exchange factor activity. Eur J Hum Genet 25, 1324–1334 (2017). https://doi.org/10.1038/s41431-017-0031-0
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DOI: https://doi.org/10.1038/s41431-017-0031-0
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