Characterization of Novel Neurodegenerative Genes Implicated in Mitochondrial Dynamics

Mitochondria are dynamic organelles undergoing constant fusion, fission, and migration within cells. The mobile nature of the mitochondria is essential for nerve health, as mutations in two of the major mitochondrial fusion genes, MFN21 and OPA12,3, cause axonal peripheral neuropathy (Charcot-Marie-Tooth Type 2, CMT2), and dominant optic atrophy (DOA) respectively. Through collaborative exome sequencing and data sharing, we identified four families with recessive mutations in the nuclear encoded mitochondrial gene, SLC25A46 (Chapter 2). The patients in these families present a clinical spectrum of features ranging from optic atrophy, spasticity, peripheral neuropathy, and ataxia, to lethal infantile neurodegeneration. SLC25A46 is one of 53 members of the mitochondrial solute carrier family (SLC25)11, which typically transport metabolites across the inner mitochondrial membrane. Interestingly, SLC25A46 is similar to Ugo1, an essential component of the mitochondrial fusion mechanism in yeast. However, unlike Ugo1, SLC25A46 seems to play a greater role in mitochondrial fission in both cells and zebrafish models (Chapter 3). SLC25A46 strengthens the genetic overlap between optic atrophy and peripheral neuropathy, and is a novel mitochondrial dynamic factor. While mitochondrial dysfunctional plays a prominent role in nerve degeneration, abnormal protein aggregation is also another common feature. Here we describe a novel frame-shift mutation in NEFH associated with CMT2 in two families of dominant inheritance (Chapter 4). The frameshift mutations leads to the stop-loss and extended translation of 40 amino acids that would otherwise encode the 3’-UTR. Overexpression of this frameshift mutation in cultured cells results in prominent protein aggregation that is absent when wildtype NEFH is overexpressed. In vivo expression of the mutant protein in developing zebrafish larvae negatively affects the development of motor neurons in comparison to wildytpe NEFH overexpression. In conclusion, we have identified a novel mitochondrial gene associated with dynamics, characterized a novel aggregation mechanism in neurofilaments, and developed models to study neurodegenerative diseases genes in zebrafish