Molecular and functional investigation of disease-associated cytoskeletal proteins protrudin and MYO1H

Abstract

Hereditary Spastic Paraplegia (HSP) is a rare, inherited and progressive neurodegenerative disorder characterized by muscle weakness and spasticity, especially in the lower extremities. In general, HSP pathology is defined as degeneration of axons in the corticospinal pathway starting from the distal end and progressing towards the cell body. HSP can be inherited as autosomal recessive, autosomal dominant, X chromosome-linked or mitochondrial. HSP is classified into two subgroups as pure and complex according to clinical findings. In pure HSP, symptoms are limited to weakness and spasticity in the lower extremities, bladder-related disorders, and deep sensory loss. In complex HSP, neurological disorders such as seizures, dementia, cerebellar ataxia, optic atrophy, etc. may be observed in addition to the symptoms of pure form. For these reasons, HSP is both clinically and genetically heterogeneous. Moreover, many parameters such as age of onset, symptoms, severity or progression of the disease may vary due to these heterogeneities. Identification of the causative gene has great importance for understanding the mechanism of the disease or providing a target molecule to be used for its treatment. Although there are 85 genes or loci associated with HSP so far, mutations in SPAST (SPG4) and ATL (SPG3) genes account for 50% of all cases. When the cellular roles of proteins encoded by other genes whose mutations have been found to cause HSP are analyzed, it is seen that most of them are involved in the regulation of intracellular transport and in the morphogenesis of the endoplasmic reticulum (ER). The remaining HSP proteins have roles in pathways such as mitochondrial regulation, myelination, lipid biosynthesis and nucleotide metabolism. Within the scope of this thesis, identification of the candidate causative gene and mutation by whole exome sequencing method in a Turkish family diagnosed with pure HSP and identification of the possible cellular effects of the mutation to be identified were aimed to be investigated. This study consists of two parts: The whole exome sequencing data were first screened for 80 loci or genes HSP-related so far. Among these genes, the c.244G>A (p.V82I) variation in the ZFYVE27 gene was detected in all patients but not in healthy individuals. ZFYVE27 gene encodes a protein called Protrudin which is localized in the ER membrane. Protrudin is involved in cellular mechanisms such as the regulation of bidirectional membrane trafficking and recycling of endosomes, formation of cellular extensions, regulation of tubular structure formation for ER morphogenesis, neuronal elongation, and neurite growth. In the first report in the literature associating the ZFYVE27 gene with HSP, the ZFYVE27 gene product Protrudin was identified as an interaction partner of Spastin protein encoded by the SPAST gene. In the same study, it was shown that a different mutation from what I detected in the family involved in this thesis disrupted this interaction between Protrudin and Spastin protein.