Contribution of HFE Polymorphisms to Pathogenetic Mechanisms of Amyotrophic Lateral Sclerosis
Open Access
- Author:
- Mitchell, Ryan Michael
- Graduate Program:
- Cell and Molecular Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 26, 2008
- Committee Members:
- James Robert Connor, Committee Chair/Co-Chair
Zachary Simmons, Committee Member
John Elliot Beard, Committee Member
Ian Alexander Simpson, Committee Member
Willard M Freeman, Ph D, Committee Member - Keywords:
- disease pathogenesis
ALS
HFE
iron
genetic association - Abstract:
- Relatively selective destruction of upper motor neurons in the motor cortex and lower motor neurons in the brainstem and spinal cord is the defining feature of amyotrophic lateral sclerosis (ALS). Motor neuron degeneration relentlessly progresses, with death occurring an average of three to five years after onset due to respiratory failure. Despite extensive research, definite causes of thev disease remain unknown for the vast majority of patients. Risk factors for the disease also remain mostly elusive; however, recently studies have identified the H63D HFE genetic variant, but not the C282Y HFE polymorphism, as a possible risk factor. I begin by extending the genetic association of the H63D HFE allele with sporadic ALS. Genotyping of a large number of ALS patients and control subjects from across the United States, and combining the results with those from populations around the world revealed a dose-dependent association of the H63D HFE allele with sporadic ALS. The association of this allele with other risk-associated alleles was explored, and suggests H63D HFE is unlikely to be merely tagging another allele. Previous functional analyses of HFE also suggest a potential contributory role of HFE variants to ALS pathogenesis. Wildtype HFE is critically positioned as a regulator of cellular iron acquisition. The potential cellular milieu resulting from dysfunctional HFE stands to contribute to pathogenesis of ALS by a variety of mechanisms. ALS pathogenesis is thought to involve a number of converging and cascading pathways including neuroinflammation, oxidative stress, glutamate excitotoxicity, and growth factor deficiency. Cellular iron dysregulation may potentiate each of these mechanisms either through generation of reactive oxygen species leading to oxidative stress or through other specific means. I assessed biomarkers associated with neuroinflammation and trophic factor signaling for the ability to distinguish ALS patients from control subjects and elucidate mechanisms of disease pathogenesis, and determine the impact of HFE polymorphisms on biomarker expression. In cerebrospinal fluid, expression levels of cytokines suggested an inflammatory profile associated with ALS, and demonstrated utility in classifying subjects by disease status. HFE polymorphisms were also associated with altered expression of several markers. In plasma samples, expression of the iron homeostasis proteins L-ferritin and transferrin was altered by disease status and presence of the H63D HFE allele. These two markers also suggested efficacy in classifying subjects by disease status. Additionally, the H63D HFE allele was associated with changes in inflammatory cytokines and trophic factors suggesting both potentially beneficial and detrimental effects in ALS pathogenesis. At the cellular level, our results suggest a contribution of the H63D HFE allele to glutamate excitotoxicity and inflammatory signaling. H63D HFE was associated with increased neuronal glutamate secretion as well as deficient cellular glutamate uptake, in contrast to C282Y HFE, which was associated with the opposite effects. Expression of H63D HFE, but not C282Y HFE, also resulted in increased cellular secretion of monocyte chemoattractant protein-1 (MCP-1), a potent recruiter and activator of microglia and macrophages. The effects of H63D HFE on glutamate regulation and secretion of MCP-1 could generally be mimicked by modulating cellular iron levels. Minocycline, a multifaceted antibiotic, has been explored for use in treating ALS patients due to its myriad effects on multiple pathways associated with disease pathogenesis. Our results suggest that response to this agent may be affected by HFE genotype. Collectively, our results establish the H63D HFE polymorphism as a risk factor for ALS and demonstrate cellular mechanisms by which it may contribute to pathogenesis of the disease.