Mitigating oxidative stress in childhood cerebral adrenoleukodystrophy -an investigation of N-acetylcysteine pharmacology

Abstract

Adrenoleukodystrophy (ALD) is an X-linked genetic disorder which affects the adrenal glands, peripheral neuronal system, the spinal cord and white matter of central nervous system (CNS). It is a progressive neurology disorder with incidence of 1 in 17,000 newborns. ALD is caused by mutations in the ABCD1 gene, which encodes the peroxisomal membrane transporter for transporting very long chain fatty acids (VLCFAs) into peroxisomes for degradation. As a result, VLCFAs accumulate in the plasma and tissues of ALD patients. Elevated VLCFAs along with ABCD1 gene mutations are used for the diagnosis of ALD. ALD has various clinical phenotypes. Childhood cerebral adrenoleukodystrophy (CCALD) is the cerebral form of ALD that affects young boys (4~10 years of age), causing progressive, debilitating effects on the CNS leading to death within a few years. The pathophysiology of CCALD is only partially understood, but it is known that VLCFAs accumulate in the plasma, brain and other tissues in CCALD patients, which can cause oxidative stress and downstream neurodegeneration. Recently, oxidative stress, the accumulation of free radicals (reactive molecules), has been shown to cause CNS neurodegeneration and play a major role in CCALD pathophysiology. Currently, the most successful treatment for CCALD is hematopoietic stem cell transplantation (HSCT), which halts disease progression and extends life when CCALD is treated early. But it is much less effective for late-stage CCALD. Based on evidence that oxidative stress plays a role in the disease, the Blood and Marrow transplantation group at University of Minnesota has utilized N-acetylcysteine (NAC) as adjunctive therapy together with HSCT in late-stage CCALD. This combinatorial approach has improved survival rate from 36% to 84% compared to HSCT only in a cohort study (Miller et al., 2011). However, NAC's mechanisms of action are still unclear in CCALD patients. As an FDA-approved drug, NAC is used as an antidote for acetaminophen overdose and as a mucolytic agent to reduce symptoms associated with cystic fibrosis. It has gained renewed attention as a potential therapy for a number of conditions including pulmonary, neurological, psychiatric, and cardiovascular diseases. With a long history of clinical use, several mechanisms including antioxidative and anti-inflammatory activities have been proposed as the basis for its therapeutic effects. However, the exact molecular mechanism by which NAC improves the survival rate of CCALD patients is still unclear. And this missing piece of information, which is the basis for my research work, is required to further optimize the therapy. In my thesis, four research projects were designed and implemented to address the pharmacology of NAC in CCALD related biological models. The first study was to investigate the downstream signaling molecules induced by NAC in the plasma of CCALD patients. Heme oxygenase-1 (HO-1) and ferritin were examined in CCALD patients before and after NAC exposure. Based on the clinical study results that the expression of HO-1 and downstream ferritin were induced by NAC, the second study was further designed in oligodendrocytes, which are CNS glial cells and closely related to demyelination and neurodegneration, to investigate the cytoprotective role of HO-1 induced by NAC. Moreover, we also tried to delineate the role of accumulation of VLCFAs in CCALD and its relationship with oxidative stress and mitochondria. The third study was designed in oligodendrocytes to investigate whether mitochondria and oxidative stress status are affected by pathophysiological concentrations of VLCFAs and if so, whether NAC could be used to reverse this condition. Finally, the fourth pharmacokinetic/pharmacodynamics study was designed and implemented in wild-type mice to address the relationship between NAC concentration and pharmacodynamic endpoints in vivo. This study is also critical to determine the biotransformation of NAC in vivo.The results from my studies indicate HO-1 as the newly discovered downstream mediators for NAC action. Studies also show for the first time that depletion of mitochondrial glutathione (mtGSH) is the pathological cause for CCALD, and that targeting mitochondrial dysfunction can be a potential effective intervention for CCALD patients. In addition, GSH levels, redox ratio, HO-1 and ferritin levels can serve as biomarkers or pharmacodynamic endpoints to evaluate NAC efficacy. In the long term, characterization of NAC mechanisms of action will help to optimize therapy in CCALD patients. In addition, the information generated from my studies on the efficacy of NAC in CCALD is also applicable to other neurodegenerative disorders sharing similar pathologies such as Gaucher's disease, multiple Sclerosis, Alzheimer's disease etc.