From modeling to potential treatments of glutaric aciduria type I

Open Access
- Author:
- Zinnanti, William J
- Graduate Program:
- Neuroscience
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 30, 2006
- Committee Members:
- Keith C Cheng, Committee Chair/Co-Chair
Robert J Milner, Committee Chair/Co-Chair
James Robert Connor, Committee Member
Kathryn F Lanoue, Committee Member
Ian Alexander Simpson, Committee Member
David A Antonetti, Committee Member - Keywords:
- glutaric acid
striatal injury
neurometabolic disorders - Abstract:
- Glutaric aciduria type I (GA-1) is an autosomal recessive inherited disorder of glutaryl-CoA dehydrogenase (GCDH) deficiency. GCDH is required for complete oxidation of lysine and tryptophan within mitochondria. Affected individuals accumulate glutaric acid and 3-hydroxy-glutaric acid (3-OHGA) and often suffer acute striatal injury in the course of an encephalopathic crisis between 6 and 18 months of age. Current treatments for GA-1 include lysine restriction and low protein diets to limit glutaric acid producing substrates. Unfortunately, one-third of affected children do not respond to treatment and experience striatal injury despite early diagnosis and careful clinical management. An animal model is needed to better understand the events that lead up to striatal injury and to develop more effective treatments. Prior attempts to produce an animal model of GA-1 that presents with neuropathology similar to the human disease, including development of GCDH-deficient mice, have been unsuccessful. We hypothesized that neuropathology may be induced in GCDH-deficient mice by supplementation with increased dietary protein or lysine. Here, we show that high protein diets are lethal to 4-week-old and 8-week-old Gcdh-/- mice within 2-3 days and 7-8 days, respectively. High lysine alone resulted in vasogenic edema and blood brain barrier breakdown within the striatum, associated with serum and tissue glutaric acid accumulation, neuronal loss, hemorrhage, paralysis, seizures and death in 75% of 4-week-old Gcdh-/- mice after 3-12 days. In contrast, most 8-week-old Gcdh-/- mice survived the high lysine diet, but developed white matter lesions, reactive astrocytes and neuronal loss after 6 weeks. Thus, lysine supplementation in Gcdh-/- mice appears to be a useful model to study GA-1. Based on the neuropathology in this model including neuronal loss and blood-brain barrier breakdown, we hypothesized that GCDH expression may be limited to neurons and endothelial cells. Here we use double-label immunohistochemistry to identify the cell types expressing GCDH in the brain and electron microscopy to detect the earliest pathologic changes associated with protein or lysine diet exposure. Neuronal vacuolation with mitochondrial swelling and disruption are shown here as the earliest pathologic events consistent with neuronal localization of GCDH expression. In order to better understand the developmental differences associated with age-dependent susceptibility, we tested for changes in amino and organic acid accumulation between weanling and adult Gcdh-/- mice. Weanling but not adult Gcdh-/- mice on a high lysine diet had substantial brain lysine and glutaric acid accumulation with depletion of á-ketoglutarate, glutamate, glutamine, GABA, coenzyme-A, and ATP. Reduced brain lysine utilization with maturity may account for diminished susceptibility to encephalopathy in GA-1. Furthermore, we tested the hypothesis that controlling brain lysine uptake may be protective by supplementation with other basic amino acids known to compete with lysine. Based on the standard use of glucose therapy in human GA-1, we also tested whether glucose supplementation could increase the survival rate of lysine supplemented weanling Gcdh-/- mice. Here we show that both glucose and homoarginine therapy separately improved survival and reduced brain glutaric acid levels. However, combined glucose/homoarginine therapy provided 100% survival of lysine fed weanling Gcdh-/- mice and provided the greatest reduction in brain glutaric acid. We further studied the amino and organic acid changes with encephalopathy progression or intervention to detect the biochemical changes that may predict the difference between injury and survival. Our findings suggest that combined glucose/homoarginine treatment should be further tested and considered for treatment of GA-1.