Tyrosine Nitration: Common Denominator in ALS?

ABSTRACT & COMMENTARY
Source: Beal MF, et al. Increased 3-nitrotyrosine in both sporadic and familial amyotrophic lateral sclerosis. Ann Neurol 1997;42:644-654.

Amyotrophic lateral sclerosis (als) is a relatively common neurodegenerative disorder characterized by progressive loss of motor neurons in the ventral spinal cord, brainstem, and motor cortex. With a mean time to paralysis and death of 3-5 years and no highly effective treatment, much effort has been focused upon fundamental mechanisms in this disorder, in the hope that better therapy may emerge. The vast majority (> 90%) of cases of ALS are sporadic; only 10% of patients show familial inheritance and have so-called familial ALS (FALS). From a clinical point of view, sporadic and familial ALS are not distinguishable.

A major breakthrough in ALS research came several years ago with the discovery that some 25% of patients with FALS harbored missense mutations in the gene encoding copper-zinc superoxide dismutase (SOD1); these mutations are not observed in patients with sporadic ALS (see Brown RH. Cell 1995;80:687-693 for a review). From a biochemical point of view, SOD1 has been intensively studied since its discovery in 1968 and is recognized as a major component in the cellular antioxidant network, being responsible for the disposal of superoxide. Many specific mutations have been found among the overall 2-3% of patients with ALS who do have SOD1 mutations. The common denominator of these mutations is that they do not affect the dismutase "active site" but, rather, attach to the metal binding site, specifically leading to the release of zinc from the enzyme. "Zincless" SOD1 acquires a new toxic gain of function that appears to be associated with motoneuron death in mutation-positive FALS patients. The toxic gain of function model is strongly supported by animal studies: mice with SOD1 "knockouts" do not develop motor neuron disease, but mice overexpressing a SOD1 bearing one of several (an example is G93A) mutations do develop a progressive and fatal motor neuron disease.

What might the new toxic gain of function be? "Zincless" SOD1, which still bears copper, is markedly enhanced with respect to peroxidase function. One result of this enhanced peroxidase function is the accelerated ability to catalyze the formation of nitronium cation from peroxynitrite, which results in nitration of free and protein-associated tyrosine, to form 3-nitrotyrosine. Peroxynitrite, in turn, is a highly reactive species formed from nitric oxide and superoxide; peroxynitrite is believed to play a major role in nitric oxide-mediated toxicity. One would predict from this that mutation-positive FALS patients should show increased levels of free and protein-associated tyrosine nitration.

This rather complex background is essential for understanding the importance of the focus article by Beal et al. In this study, autopsy material from patients with sporadic ALS, SOD1 mutation-positive FALS, and SOD1 mutation-negative FALS were examined and compared to both ischemic and non-ischemic controls. Levels of free 3-nitrotyrosine (3-NT) and a metabolite (3-nitro-4-hydroxyphenylacetic acid, 3-NHPEA) were determined in the spinal cord. Beal et al found approximately two-fold higher levels of 3-NT and four-fold higher levels of 3-NHPEA in the lumbar cord of patients with ALS compared to controls. Similar degrees of elevation were observed in sporadic ALS patients, mutation-positive FALS patients, or mutation-negative FALS patients. Furthermore, immunostaining with anti-nitrotyrosine antibodies revealed elevated protein-associated nitrotyrosine restricted to ventral-horn motorneurons in all categories of ALS patients.

COMMENTARY

This important study by Beal et al indicates that while only 2-3% of all ALS patients harbor SOD1 mutations, a final common pathophysiology is likely to affect all cases, which may be the excessive accumulation of protein nitrotyrosine in motorneurons. This idea would explain why "zincless" SOD1 is toxic but would imply that there must be other pathways that lead to excessive protein tyrosine nitration. Neurofilament assembly is known to be powerfully affected by tyrosine nitration, and this is known to be deranged in ALS motorneurons. Currently, the "nitrotyrosine accumulation hypothesis" would appear to be the leading theory of the cause of ALS.

These findings could also have practical significance for neurologists in the near future. Our laboratory (Ann Neurol 1997;42:442) has examined levels of protein tyrosine nitration in the cerebrospinal fluid of patients with ALS and other neurological conditions. Elevated CSF protein nitrotyrosine is reasonably specific for ALS and is detectable at first clinical presentation. This could result in a useful adjunctive diagnostic test for ALS in early cases where the diagnosis may be in doubt. —rt

About what percentage of ALS patients have SOD1 mutations?

a. 1-2%

b. 2-3%

c. 3-4%

d. 4-5%