A Parkinson Disease Gene Discovered, an Oncogene Remembered

A BANDWAGON RUSH

Heutink's group made its discovery based on genetic mapping of two consanguineous but genetically isolated families from the Netherlands and Italy, and he says it was all based on a really good hunch. "We took a risky tactic to identify the gene. The region where the gene is located is pretty big, so we took a bit of an educated guess about what kind of mutation we'd find," says study coauthor Peter Heutink, now at VU University Medical Center in Amsterdam. "We made a bit of a gamble and it worked."

They did not, however, expect to find a gene already characterized. "Prior to our paper, there were maybe ten to fifteen papers on DJ-1," says Bonifati. Conditions were ripe for generating a lot of attention.

"The [research] community was slow to jump on the first two genes when they were cloned," says Ted Dawson, a neurologist and molecular biologist at the Johns Hopkins University School of Medicine, Baltimore. "There were already big players in the α-synuclein and parkin fields. I think people realized they had to jump on the bandwagon right away if they wanted to make an impact." Now, he says, "Everybody and their mother is working on DJ-1 ... because it's small, soluble, has antibodies [against it], and has been crystallized."

The discovery also altered the research direction of individuals who had been looking at DJ-1for years before the PD link was made. This study, says Oostra, "opened their eyes to a different function for the gene and its protein."

"We cloned the cDNA of DJ-1 and reported in 1997 that DJ-1 is a novel oncogene in collaboration with ras," says Ariga. Before the PD discovery, Ariga says their DJ-1 research focused on the roles in cancer and reproduction. In some instances of male infertility, Ariga explains, DJ-1 expression is reduced and the protein localizes incorrectly in sperm. A subsequent paper published by his team described a role for DJ-1 in male infertility.⁵ Conversely, he says, DJ-1 is over-expressed in cancer cells, including breast, lung, and prostate cancers.

The impact of Bonifati's discovery was felt before the paper's publication, however. Following a presentation of the Dutch group's work at a neuroscience meeting, Ariga recalls being contacted by groups from all over the world wanting to work on DJ-1. "Many E-mails from groups working on neurodegeneration ... came to me asking for DJ-1 cDNA and an anti-DJ-1 antibody," he recalls. "We were, of course, quite surprised at this discovery, but were glad that the importance of DJ-1 could be recognized, in addition to our findings regarding reproduction."

DJ-1 ALTERING DIRECTIONS

Source: Histogram produced using HistCite software.

The paper first describing DJ-1 was cited just a handful of times before researchers from The Netherlands linked the gene to Parkinson disease.

The DJ-1 mutation affects only a minority of individuals who have a sporadic form of Parkinson disease, but that hasn't lessened its impact. "Mutations in α-synuclein, the first Parkinson's disease gene, have obviously been discovered in only a very few patients as well," says Oliver Bandmann, a neurologist and molecular biologist at the University of Sheffield, UK. But the discovery of α-synuclein has greatly advanced the understanding of Parkinson disease; DJ-1 may be similarly revealing, he says. "Naturally occurring variants in DJ-1 may be a susceptibility factor for sporadic Parkinson's disease."

Indeed, the connection to PD triggered a rethinking of hypotheses proposed to explain disease pathogenesis and a redirecting of some research efforts. A number of studies had localized DJ-1 to the level of the mitochondria. Although it's not clear whether the protein strictly localizes at the mitochondria, such results excite Serge Przedborski, a cell biologist at Columbia University. "For many years in this field, there has been a lingering idea that mitochondrial dysfunction may play a part in the disease," he says.

Ariga's group in Japan was the first to show that DJ-1 exhibits some anti-oxidant properties, a finding that implies a role for oxidative stress in Parkinson disease, says Przedborski. Ariga says their recent work has identified roles for DJ-1 in transcriptional regulation, anti-oxidative stress, and mitochondrial function, and it also demonstrates protease activity.

"The function of DJ-1 is still mostly unknown, although several [proposed] functions have been put forward," says Bonifati. Preliminary evidence suggests a role for DJ-1 as a molecular chaperone, a mechanism implicated in other neurodegenerative diseases and currently a matter of particular focus, he explains. Another prevailing theory is a role for DJ-1 as an antioxidant, but that theory, he says, "is still quite controversial."

Until now, research on the mutant parkin and α-synuclein proteins pointed towards protein degradation as the main culprit in the pathogenesis of the disease, says Przedborski. "This was the first gene product telling us that maybe it wasn't just protein degradation. Ninety percent of research efforts deal with protein degradation." Neither the mutant parkin nor α-synuclein discoveries suggested oxidative stress or mitochondrial dysfunction, he explains. "Now that DJ-1 is coming into play, maybe those mechanisms are involved in the cascade."

"The protein degradation hypothesis had been buzzing around because of the parkin gene," says Heutink. The discovery of DJ-1 and what was known about the nature of the protein, "brings back the oxidative stress hypothesis that had been out of the spotlight for a while," he says. "It looks like it's involved in oxidative stress. That's the picture that's evolving."

Indeed, says Ariga, "The brain of some sporadic form of Parkinson's disease patients possessed abnormal forms of oxidized DJ-1." His group has recently found that oxidation of DJ-1 at three cysteine residues adversely affects protein function, with one residue in particular, cysteine 106, being particularly crucial. "There are two types of oxidation at C106 of DJ-1: sulfinic and sulfonic acids. It is, therefore, important to elucidate which oxidized form of DJ-1 is active or inactive, leading to the onset of Parkinson's disease."

The discovery this past year of two additional genes linked to PD – PINK⁶ and PARK-8⁷ – has done little to clarify the culprits. To date, the known genes affect only small patient subsets, yet Oostra believes that identifying still more genes and characterizing the function of their products is revealing, each adding its respective piece to the puzzle. "The challenge is to look for genes involved in Parkinson's disease at all the ages," says Oostra. "Hopefully at the end we'll understand the mechanism of developing Parkinson's disease."