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Original Article

Exp Neurobiol 2018; 27(6): 550-563

Published online December 12, 2018

© The Korean Society for Brain and Neural Sciences

A De NovoRAPGEF2 Variant Identified in a Sporadic Amyotrophic Lateral Sclerosis Patient Impairs Microtubule Stability and Axonal Mitochondria Distribution

Keunjung Heo1,†, Su Min Lim2,†, Minyeop Nahm2,†, Young-Eun Kim3, Ki-Wook Oh2, Hwan Tae Park4, Chang-Seok Ki5*, Seung Hyun Kim2*, and Seungbok Lee1*

1Department of Brain and Cognitive Sciences and Dental Research Institute, Seoul National University, Seoul 08826, Korea.

2Department of Neurology, College of Medicine, Hanyang University, Seoul 04763, Korea.

3Department of Laboratory Medicine, College of Medicine, Hanyang University, Seoul 04763, Korea.

4Department of Molecular Neuroscience, College of Medicine, Dong-A University, Busan 49201, Korea.

5Green Cross Genome Corporation, Yongin 16924, Korea.

Correspondence to: *To whom correspondence should be addressed.
Chang-Seok Ki, TEL: 82-31-260-0601, FAX: 82-31-260-9087, e-mail:
Seung Hyun Kim, TEL: 82-2-2290-8371, FAX: 82-2-2296-8370, e-mail:
Seungbok Lee, TEL: 82-2-880-2330, FAX: 82-2-762-2583, e-mail:
These authors contributed equally to this work.

Received: October 31, 2018; Revised: November 24, 2018; Accepted: November 26, 2018

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that is frequently linked to microtubule abnormalities and mitochondrial trafficking defects. Whole exome sequencing (WES) of patient-parent trios has proven to be an efficient strategy for identifying rare de novo genetic variants responsible for sporadic ALS (sALS). Using a trio-WES approach, we identified a de novoRAPGEF2 variant (c.4069G>A, p.E1357K) in a patient with early-onset sALS. To assess the pathogenic effects of this variant, we have used patient-derived skin fibroblasts and motor neuron-specific overexpression of the RAPGEF2-E1357K mutant protein in Drosophila. Patient fibroblasts display reduced microtubule stability and defective microtubule network morphology. The intracellular distribution, ultrastructure, and function of mitochondria are also impaired in patient cells. Overexpression of the RAPGEF2 mutant in Drosophila motor neurons reduces the stability of axonal microtubules and disrupts the distribution of mitochondria to distal axons and neuromuscular junction (NMJ) synapses. We also show that the recruitment of the pro-apoptotic protein BCL2-associated X (BAX) to mitochondria is significantly increased in patient fibroblasts compared with control cells. Finally, increasing microtubule stability through pharmacological inhibition of histone deacetylase 6 (HDAC6) rescues defects in the intracellular distribution of mitochondria and BAX. Overall, our data suggest that the RAPGEF2 variant identified in this study can drive ALS-related pathogenic effects through microtubule dysregulation.

Graphical Abstract

Keywords: Amyotrophic lateral sclerosis, Whole exome sequencing, RAPGEF2, Missense mutation, Microtubules, Mitochondria