Unraveling The Pathogenic Molecular Mechanisms Of Morc2 Mutations Causing Charcot-Marie-Tooth Type 2Z

Abstract

Charcot-Marie-Tooth (CMT) neuropathy is a group of clinically and genetically heterogeneous diseases characterized by slow progressive length-dependent degeneration of motor and sensory peripheral nerves. A new form of axonal CMT (CMT2Z) was recently identified and is caused by mutations in the Microrchidia 2 (MORC2) gene. It is currently not known how mutations in the MORC2 gene cause axonal degeneration. The MORC2 protein is a chromatin modifier involved in the epigenetic regulation of gene transcription and DNA repair. MORC2 also regulates the activity of ATP-citrate lyase in the cytoplasm, which catalyzes the formation of acetyl-CoA. The missense mutation (c.568C>T, R190W) has been identified in seven unrelated families and is the most common mutation to date. To improve the understanding of the genetic and functional mechanisms by which MORC2 mutations lead to disease the following aims were undertaken (1) to assess the genetic mechanism of the recurrent MORC2 R190W mutation, (2) to conduct functional evaluation of the disease-associated MORC2 mutations (E174G/R190W) and (3) to establish a C. elegans model for CMT2Z. Bisulfite sequencing showed the c.568C nucleotide, which resides within a CpG dinucleotide, is prone to methylation suggesting that methylation-mediated deamination of the CpG dinucleotide may give rise to the recurrent MORC2 R190W mutation. Cell models have shown MORC2 mutations (E174G/R190W) do not affect expression or subcellular localization. ATP-citrate lyase activity was also normal in fibroblast cells from a patient with the MORC2 R190W mutation. This suggests the mechanism through which the MORC2 mutations cause disease involves impaired DNA repair and/or altered gene expression or an unknown function. C. elegans strains overexpressing human MORC2 wild type (WT) or mutant (R190W) protein specifically in the GABAergic motor neurons were also generated. Human WT and mutant MORC2 caused neuron morphological defects. Therefore, both WT and mutant MORC2 protein appears to be toxic to C. elegans GABAergic neurons. This thesis has addressed fundamental questions regarding the pathogenicity of MORC2 mutations and has provided direction for future experiments.