Adapted from Brain Storms: The Race to Unlock the Mysteries of Parkinson's Disease, by Jon Palfreman, by arrangement with Scientific American/Farrar, Straus and Giroux, LLC (US), HarperCollins (Canada), Rider (UK), Uitgeverij Balans (Netherlands) and Beltz Verlagsgruppe (Germany). Copyright © 2015 by Jon Palfreman.
It all began with a routine office visit. In the spring of 1986 neurologist Larry Golbe conducted a clinical examination of a 48-year-old New Jersey fire chief named David. Golbe observed that the patient's movements were slow and restrained. During a finger-tapping exercise—a common way to detect abnormal movements—the chief quickly ran out of energy. When he stood up, this once athletic man now bent forward, with a stooped gait. When he walked, he didn't swing his arms but shuffled along with small steps. When Golbe tried to bend David's arms and legs at the elbow and the knee, he was met with resistance. David's face was expressionless, and he never blinked.
Golbe, who works at what is now the Rutgers Robert Wood Johnson Medical School, flipped through his patient's medical history. Ten years earlier David had been diagnosed with Parkinson's disease. Initially he was treated with Sinemet, a drug frequently prescribed to replace some of the dopamine depleted in the brain over the course of the illness. As Parkinson's progresses, patients lose the neurons that produce this critical neurotransmitter, notably in the substantia nigra—a tiny structure in the midbrain named for its dark pigment. The color there disappears as the dopamine cells die off. Less and less dopamine then travels to the neighboring striatum, where the elaborate orchestration between the brain and muscles takes place. And as this communication breaks down, it leads to the disease's classic motor symptoms—all of which David now exhibited.
For a while, the chief had responded to Sinemet. But over time the medicine became less effective, as is often the case. Golbe enrolled David in a study that he was conducting of selegiline, a newer medication that neuroscientists hoped would boost patients' dopamine levels by blocking the enzymes that break it down. But so far David had reported only minor benefits. Golbe—a dedicated, compassionate clinician whose own father had contracted Parkinson's—unfortunately had little else to offer. So he spent a few minutes counseling David and made a follow-up appointment for three months later. That was the last Golbe saw of him. A few weeks after their meeting, David tragically drowned in a swimming pool.
After the funeral, David's brother, Frank, came to see Golbe, concerned that he also might have Parkinson's. He did—and the diagnosis piqued Golbe's interest. He initiated a broad search for others in their family who might be affected and eventually unearthed a total of six relatives with typical parkinsonian symptoms. During his examinations of these family members, Golbe recalls that the patients told him “the family originated in Contursi, Italy.” He didn't know at the time if this was significant or not. But several months after David's death, Golbe got a visit from a Staten Island woman, named Joyce, with classic Parkinson's. She, too, was of Italian descent. Specifically, she told Golbe, she came from a small village in southern Italy: a village in the hills of Salerno province called Contursi.
As Louis Pasteur famously said, “Chance favors the prepared mind,” and Golbe immediately made the connection between David and Joyce. Although Parkinson's does not typically run in families, Golbe realized he might have stumbled on a rare exception: a family “kindred” that passed the illness from generation to generation. He called his boss and mentor Roger Duvoisin—a renowned neuroscientist who in the 1960s had helped pioneer the use of L-dopa, the first breakthrough drug for Parkinson's. Together they embarked on an international journey of medical detective work.
A lethal legacy
A year later Golbe was sitting in the small office of Salvatore La Sala in Contursi. La Sala, who had grown up in the village, was one of its only resident primary care physicians—and also served as its dentist. Golbe watched as another Italian collaborator, neurogeneticist Giuseppe Di Iorio, then at the University of Naples Federico II, conducted a clinical examination of a 40-year-old man named Mario. Periodically, La Sala spoke to clarify Di Iorio's requests and reassure his fellow Contursian. Golbe wished he understood more Italian. Di Iorio's English was also rudimentary, but somehow they managed to work together effectively, eventually cracking a genetic mystery.
After Golbe had plotted the American branches of the family tree, he suggested that Di Iorio visit the village church and examine Contursi baptismal and marriage records going back 12 generations. When they expanded the family tree on a huge chart, the multigenerational “pedigree” showed that Golbe's patients, David and Joyce, were seventh cousins—two of 574 descendants of a couple who married around 1700. Others now lived in Italy, Germany, Argentina, Canada and the U.S. The truly remarkable finding from the investigation was that 61 of the recent descendants had developed Parkinson's. The pedigree analysis showed that males and females were equally affected and that descendants had a 50 percent chance of contracting the bad gene and, along with it, Parkinson's.
Despite his limited Italian, Golbe followed Di Iorio's examination of Mario with little difficulty. Based on his clinical signs, Mario had inherited Parkinson's. La Sala, interestingly enough, was also a member of the family but hadn't inherited the mutation. He had played a key role mediating between the scientists and the family, explaining, for example, why the team needed to collect blood samples to take back to New Jersey for DNA analysis. Such molecular investigations might identify the specific genetic mutation responsible for the disease.
Meanwhile, back in New Jersey, other members of Duvoisin's department discovered a critical missing piece of the puzzle: they were able to confirm that the kindred members had genuine Parkinson's. Because other neurodegenerative diseases can produce tremors and gait problems that resemble it, a definitive diagnosis can be difficult to establish until after a patient's death, when pathologists look for curious blood cell–sized masses called Lewy bodies in brain tissue samples.
The New Jersey team had obtained and examined autopsy materials from two deceased family members—David, the fire chief, and his maternal uncle. Their brains showed extensive damage to the substantia nigra, and some of the surviving dopamine neurons contained the telltale Lewy bodies. As Duvoisin says, “It was classic Parkinson's pathology”—the first family kindred “where there was autopsy confirmation that it was Parkinson's.”
The next step was to find the mutant gene that caused one in two children in this family on average to contract Parkinson's because that gene might hold the key to the mystery of the disease. The New Jersey scientists searched unsuccessfully for more than seven years. Then, in 1995, Zach Hall, who at the time was director of the National Institute of Neurological Disorders and Stroke (NINDS), asked them to share their Contursi blood samples with other investigators, who might be in a better position to pull off the necessary feats of molecular wizardry. A collaboration was formed between the New Jersey researchers and two scientists then at the National Institutes of Health: Bob Nussbaum, a clinical geneticist with advanced molecular biology training, and Nussbaum's colleague Mihael Polymeropoulos.
As Hall had hoped, Nussbaum and Polymeropoulos quickly racked up some spectacular advances. Although the Contursi mutation could have been on any one of the 22 nonsex-linked chromosomes we humans possess, it would turn out to lie on chromosome 4. By sheer good fortune, Polymeropoulos was highly familiar with chromosome 4, having recently linked two other genetic disorders to it. This work had generated lots of biochemical markers along the chromosome, which guided the pair as they worked. Thus, within just nine days, they had narrowed their quest for the Contursi mutation to a short stretch of DNA along the so-called long arm of chromosome 4.
The aha! moment
It took another nine months of painstaking work before Nussbaum and Polymeropoulos sequenced what they thought was the actual mutated gene. Then, Nussbaum says, they got a very lucky break. They checked their sequence against GenBank, a giant open-access computerized database of gene and protein sequences run by the NIH, and got a hit: the mutated gene was a known entity, a gene called SNCA, which coded for a protein called alpha-synuclein.
According to Nussbaum and Polymeropoulos, the genetic story behind the Contursi kindred went roughly as follows. SNCA's normal role is to make a relatively obscure brain protein called alpha-synuclein. It is called synuclein, incidentally, to indicate that this protein can be found both in the synapses—the gaps across which neurons communicate—and in the nuclei of the neurons themselves. A single base change in the gene's million-letter genetic code, however, produced a mutant form of the protein, which caused affected members of the Contursi kindred to develop Parkinson's. On May 27, 1997, Nussbaum and Polymeropoulos submitted a paper to the journal Science, listing Duvoisin's team as co-authors, which linked a small mutation in a gene for alpha-synuclein with an aggressive form of Parkinson's. One month later—lightning fast for medical research articles—it appeared in print.
Nearly 20 years later it is clear that the discovery was transformative. The rare Contursi mutation does not show up in the DNA of regular Parkinson's patients, but the role of alpha-synuclein has proved to be a vital clue in the wider war on the disease.
It happened that at around the same time the Science paper appeared, Maria Grazia Spillantini, an Italian Alzheimer's researcher working at the University of Cambridge, had developed special staining techniques using antibodies to visualize alpha-synuclein in brain tissue. On a hunch, Spillantini decided to use the stain to search for alpha-synuclein in brain specimens of deceased patients with regular Parkinson's. And somewhat surprisingly, even though these individuals lacked the Contursi mutation, she found alpha-synuclein—lots of it. She found it in Lewy bodies.
As we have seen, Lewy bodies are found inside surviv-ing neurons of Parkinson's sufferers and used to confirm diagnosis after death. Remarkably, despite their pathological importance, in 1997 no one was sure what Lewy bodies were made of. Spillantini had found the answer: they were made up in part of alpha-synuclein. Researchers everywhere took note, realizing the finding might be extremely important. Even though the Contursi mutation does not account for the vast majority of Parkinson's cases, the fact that Lewy bodies, the marker of sick and dying neurons, were stuffed with alpha-synuclein implied that this protein might be a critical player in Parkinson's.
How Parkinson's progresses
In Germany the legendary neuroanatomist Heiko Braak, then at Goethe University Frankfurt, noticed Spillantini's August 1997 paper in Nature. Inspired by the discovery that Lewy bodies contained alpha-synuclein, he embarked on a massive Parkinson's project, examining the accumulated damage in patients who had survived for different lengths of time. Braak did full-body autopsies of 41 cases of Parkinson's, 69 cases with no Parkinson's and 58 age-related control subjects. He looked for Lewy bodies and Lewy neurites, deposits in the long axons that project to other nerve cells. He hunted not only in the brain but in the rest of the body as well. Using Spillantini's powerful new alpha-synuclein stain and a novel technique of examining under the microscope sections of especially thick neural tissue, Braak saw clearly what others throughout history had only suggested—that the distribution of Lewy bodies and Lewy neurites was not confined to a few areas of the midbrain.
He also discerned something much more profound: that the location of Lewy pathology appeared to change as the disease progressed. Mildly affected cases (people who had died with early-stage Parkinson's) showed Lewy pathology in the olfactory bulb of the nose, which transmits information about smells to the brain, and in part of the vagus nerve, a long projection that connects the gut to the brain. In more advanced cases, he found Lewy bodies and Lewy neurites in the brain stem as well. Still more advanced cases had them in the substantia nigra—marking damage to dopamine cells. The most advanced cases of all displayed Lewy pathology in the forebrain and the neocortex.
Braak argued this was compelling evidence that Parkinson's started perhaps decades before any tremor or rigidity appeared. He suggested that it began in the gut or nose—perhaps triggered by an infection—and then spread insidiously throughout the brain in six anatomical stages. Loss of smell and constipation might come in so-called Braak stage 1. REM sleep behavior disorder occurs in Braak stage 2. Classic Parkinson's—tremor, rigidity, slowness of movement—shows up in Braak stage 3 and loss of balance in Braak stage 4. In Braak stages 5 and 6, the pathology spreads to the forebrain and the neocortex, causing dementia. If Braak is right, then, according to British neuroscientist Christopher H. Hawkes, “by the time you go to see a neurologist, you're in Braak stage 3 to 4. And to put it crudely, the brain is well and truly pickled.”
Braak's theory, published in 2003, was initially met with skepticism. But the evidence for it and for the role of alpha-synuclein would grow. That same year a group of Mayo Clinic and NIH geneticists announced a landmark discovery in another family, the so-called Iowa kindred, that deepened the connection with alpha-synuclein. Over nearly a century, branches of the family had been studied by a series of Mayo Clinic physicians. Geneticist Katrina Gwinn, now at NINDS, had met one of these clinicians in the mid-1990s. She became fascinated with the kindred and had gotten to know some of them. Gwinn decided to track the genes behind Parkinson's in this family group, just as Nussbaum and his colleagues had done for the Contursi kindred. To begin, she recruited the help of two British geneticists: John Hardy, known for his Alzheimer's disease research, and his then postdoc, Matthew Farrer, both then working at Mayo's Florida campus.
More families, more proof
The team's first attempt to locate a mutation failed—perhaps because of a sample mix-up. So the researchers decided to start over. Because the process they used, called genetic linkage analysis, depends on having plenty of DNA samples, Farrer and Gwinn headed out into the field and asked kindred members for more blood. By 2001 the team had enough blood samples to redo the lab work. A new group member then at the Mayo Clinic in Florida, Andrew Singleton, took the lead. As he tracked the genetic markers using the new material, he realized the location appeared to include the alpha-synuclein gene found in the Contursi kindred. This was puzzling: previously they had tested the Iowa kindred for all known forms of the Contursi mutation and found nothing.
But as Singleton pressed harder, he noticed some very odd signals. And then he got very excited. As he recalls, “It suddenly occurred to me that what could be causing the disease were extra copies of one gene.” Pursuing this idea, Singleton went on to demonstrate that the Iowa kindred's Parkinson's wasn't caused by an error in the DNA sequence itself, as was the case for the Contursi clan. Instead affected members had what geneticists call copy number variation: Family members with Parkinson's had three copies of the normal alpha-synuclein gene—a triplication—on one copy of chromosome 4. On the other copy of chromosome 4, they had the usual single alpha-synuclein gene. Because they had a total of four copies of the gene, instead of the usual two, affected individuals had twice as much alpha-synuclein protein being pumped into their body.
Scientists realized just how significant this discovery was: there was a direct link between quantity of alpha-synuclein and disease. It showed that you didn't need a mutation to get Parkinson's, just too much alpha-synuclein. Hardy describes the news as “a beautiful surprise ... extremely unexpected. But once you get the result, it makes you understand everything.” Other researchers in Europe reported family pedigrees where affected members had both duplications and triplications. The people with the triplications had an earlier onset and much more aggressive illness than those with the duplications. This was also significant. Alpha-synuclein's toxicity depended on dose. The more alpha-synuclein, the worse the Parkinson's.
When I think about these breakthroughs now, it is strange to imagine that at one time neuroscientists dismissed the role of genetics in Parkinson's. Since the 1997 discovery of the alpha-synuclein mutation, some 18 potential genetic forms of Parkinson's have turned up, involving another 10 or so genes. Geneticists are confident that six of them are classically inherited either dominantly or recessively.
Of course, most people with Parkinson's don't have a known mutation. But buried in our genomes, there may be sequences that predispose us in some way to develop the disease. This heightened susceptibility may surface only after certain infections, as Braak proposes, or exposures to specific toxins, as indicated by some earlier research. Using special gene chips, geneticists have screened the DNA of people with Parkinson's and compared it with healthy controls. These genome-wide association studies have found a strong correlation with variations in the alpha-synuclein gene, as well as associations with variations in the gene encoding the tau protein, which is involved in Alzheimer's pathology. Understanding those sequences may ... well, there's no telling where that knowledge may lead.
see also:
This entry passed through the Full-Text RSS service - if this is your content and you're reading it on someone else's site, please read the FAQ at http://ift.tt/jcXqJW.
No comments:
Post a Comment