Of a large Italian kindred, numbering some 500 individuals over 12 generations, 60 family members are known to have had early onset familial (inherited) Parkinson's disease (PD).
Ten of them tested positive for carrying mutations of a little-known gene called alpha-synuclein. Three of five unrelated Greek families carried the same gene alteration, which conferred an ultra-rare, early onset form of the disorder. Fifty other Italians with sporadic PD showed no sign of the mutation. Discovery of that synuclein mutant gene's link to PD was reported in June 1997. (See BioWorld Today, June 27, 1997, and Dec. 4, 2001.)
Back around 1988 members of that Italian Contursi kindred - who had emigrated from Italy to the U.S. decades earlier - were genotyped at the NIH's Laboratory of Genetic Research Institute.
"For the vast majority of people with common, sporadic PD," observed clinical and research neurologist William Dauer, "we don't really know what the cause is. It's those rare families in which we've known since 1997 that single-gene synuclein mutations lead to dominantly or recessively inherited PD." The gene resides on the long arm of human chromosome 4.
Dauer, an assistant professor of neurology and pharmacology at Columbia University in New York, is lead author of a paper in the Proceedings of the National Academy of Science (PNAS) released online Oct. 8, 2002. Its title: "Resistance of a-synuclein null mice to the parkinsonian neurotoxin MPTP."
"Our central finding," Dauer told BioWorld Today, "is that an environmental toxin, MPTP - which is an experimental in vivo model for an environmental cause of PD - has been found to utilize a synuclein pathway, previously implicated in both genetic and sporadic forms of the disease."
People, not mice, were the first mammals to suffer PD signs and symptoms in reaction to MPTP. (That acronym stands for 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine.) It surfaced over 20 years ago as a neurotoxin that brought on parkinsonism. That was when a bunch of heroin addicts in California tried shooting up a new, synthetic designer narcotic they'd heard about. They ended up injecting MPTP instead. It sent four of them to the hospital with irreversible hallmarks of PD. The toxin has since become a favorite resource for creating mice that faithfully simulate the chronic pathology of the human motor disorder. MPTP selectively destroys dopamine neurons in the brain's substantia nigra. This triggers a PD-like syndrome in mice, monkeys and man.
"Conceptually," Dauer said, "it suggests that a variety of agents that may damage or kill dopamine neurons are of key importance to neurons that die of PD. Many of them may disturb the common molecular pathway involving synuclein. So that's our idea of linking up together some genetic and environmental causes of dopamine neuron degeneration. This is the first study to show in a model where the gene has been knocked out that it really has a dramatic effect on how this environmental toxin is able to kill neurons.
"There are only a few families in the world with these mutations," Dauer continued. "Those missense alterations are isolated instances of clear genetic, familial, PD versions, but now they've been linked to the common, sporadic form. And on the other end of the spectrum, in terms of environmental factors, similar to the mutant synuclein story in those few rare kindreds, there have been a very few patients exposed to MPTP, and they developed an acute-onset permanent parkinsonian condition."
Mice Sans Mutant Synuclein Gene Are Lucky
"The purpose of our in vivo experiments," Dauer recounted, "was to learn more about the normal role of synuclein by generating mice without it. That is, to explore the normal role of its protein, and its effect on the viability of dopaminergic neurons. One starts with a disease that's involved in neurodegeneration, and one doesn't know how that protein's mutation works. Whether it causes some novel gain or loss of function in the protein. So we began by removing the protein from mice.
"We generated those knockout animals, and interestingly - despite the fact that synuclein is a widespread protein in the brain, and quite abundant - the KO mice grew up looking quite normal. They didn't develop neurodegeneration. They ran around, were indistinguishable from their control littermates - their wild-type brothers and sisters. So then we explored the susceptibility to MPTP of the dopaminergic neurons in those synuclein-minus mice. Quite apart from laboratory studies, we knew that those California drug users who unfortunately took that toxin really did develop a pathology and a clinical syndrome quite similar to Parkinson's disease.
"We intoxicated our KO mice with a well-characterized MPTP regimen, then analyzed their brains to compare neurodegeneration in the wild-type animals and in the knockouts. We found that the KO mice, in the absence of synuclein, were almost completely protected from neurodegeneration. That result was the first demonstration that lacking the synuclein protein, dopamine neurons are strikingly resistant to MPTP, the prime neurotoxic model of PD. The fact that this is a prototype environmental toxin - and that this gene has also been implicated in genetic forms of Parkinson's disease - highlights the role of synuclein as a link between these various forms of PD."
Therapeutics? Not Now; Someday - Maybe
"That's still not entirely clear," Dauer pointed out, "but it does suggest that the normal function of synuclein may be important for the ability of dopaminergic neurons to survive various insults. Only the opposite sort of experiment had been done up to then - to overexpress various forms of synuclein-harboring neurons.
"I would say our results don't have any immediate implication for therapeutics," Dauer observed. "To the extent that they highlight the importance of that pathway, it would suggest that drug targets may come out that might yield new potential therapeutic agents." He added: "Since synuclein accumulates in PD abnormally, and now in our findings that when the protein is absent mice don't get neurodegeneration, one could take the general view that therapies inhibiting alpha-synuclein, blocking its function in some way, are conceivable as a strategy for treatment of Parkinson's disease some day.
"But at this point we have no idea how important the normal function of that gene is in the walking, talking human. Just because the mice looked OK," he concluded, "doesn't mean humans will look OK."