Publication Date

2017-07-13

Availability

Open access

Embargo Period

2017-07-27

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Human Genetics and Genomics (Medicine)

Date of Defense

2017-05-30

First Committee Member

Derek M. Dykxhoorn

Second Committee Member

Juan I. Young

Third Committee Member

Jeffery M. Vance

Fourth Committee Member

Gary W. Beecham

Fifth Committee Member

Juan Dominguez-Bendala

Abstract

Parkinson disease (PD) is a neurodegenerative disease characterized by the loss of dopaminergic neurons of the substantia nigra. Parkinson disease related variants have been identified in many different genes, and the products of these genes are involved in autophagy, mitochondria and cell stability pathways. However, these identified variants are responsible for only 15% of PD cases, with the bulk of these cases being idiopathic PD. While genes do play a role, the most important risk factor in PD is age. Though the average age of onset for PD is 62, early onset forms of PD have been diagnosed in patients even prior to 40 years of age. This poses the question, how does genetics play a role in PD dysfunction, and is the dysfunction that causes PD present throughout life, or does it only manifest in later stages. The advent of patient-specific induced pluripotent stem cell technology (iPSCs) allows for a unique model system for disease study. Though useful in furthering our understanding of the disease, previous model systems lacked the human genetic background relevant to PD. iPSCs derived from patients with confirmed disease status, and in some cases PD-associated variants can be differentiated into cell types of interest for in-vitro characterization. Identification of commonalities between the many forms of PD could be valuable in identifying targets which will aid in the development of novel therapeutic strategies. Patient-specific iPSC lines were derived from individuals with PD. These lines were subsequently differentiated into midbrain dopaminergic neurons. In the first study, individuals carrying variants in GBA (beta-glucocerebrosidase) exhibited an early temporal phenotype. GBA-variant lines induced from neuronal aggregates consistently displayed deficits in neurite outgrowth after five days of differentiation. When compared to control lines these cells exhibited migration from the tight aggregates and shorter neurite growth from aggregates. This phenotype is less prevalent as cells were passaged from aggregate culture, and in non-aggregate based differentiation scheme this phenotype was not observed. The findings of our transcriptomic and pathway analysis indicate that patient derived PD lines showed altered expression in genes involved in extracellular matrix receptor interaction, focal adhesion, and p53 signaling, when compared to age-matched controls. Furthermore, this is consistent with transcriptomic studies utilizing post-mortem PD patient brains. In total, our data indicates that in the case of GBA-variant lines the dysfunction that leads to PD may be present early on. This could account for the fact that many GBA carriers with PD suffer from early onset PD, and in some cases, a more severe form of the disease. The transcriptomic study highlighted differential expression in genes related to focal adhesion, and extracellular matrix receptor interaction dysfunction in these gene sets has been previously demonstrated in other systems. Additionally, these pathways have been linked to neuronal cellular instability and subsequent cell death seen in PD.

Keywords

Parkinson's; Neurodegeneration; Stem Cells; Genetics; GBA; dopaminergic neurons

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