Sci. Aging Knowl. Environ., 24 May 2006
Pushing the Envelope
Research blossoms on rare genetic disorders that might be linked to aging
A 50-year-old man who's winded after walking down the hall awaits a replacement for his swollen, stuttering heart. A 10-year-old girl with the rare genetic disorder Hutchinson-Gilford progeria syndrome (HGPS) stands only as tall as a 3-year-old and has developed diabetes and osteoporosis. Although these patients seem to have little in common, their suffering stems from defects in the same gene, LMNA. It encodes a pair of proteins called lamins that support the membrane sheathing the cell's nucleus. So far, researchers have tallied more than 200 LMNA mutations that--depending on who's counting--unleash at least a dozen diseases, known as laminopathies. The illnesses can short-circuit the heart, rearrange body fat, sap muscle strength, deafen cells to the hormone insulin, erode nerve cells, and spur numerous other problems.
The discovery of laminopathies has galvanized the once-sleepy specialty of lamin research. "The field is starting to explode," says molecular biologist Richard T. Lee of Harvard University. As geneticists collect mutant lamins, molecular biologists discover new jobs for the proteins that they once thought merely propped up the nucleus. Lamins take part in gene activity, DNA repair, cell division, and other processes, recent studies indicate. The surge in lamin studies could bring practical payoffs; scientists might soon launch the first clinical trials of an HGPS treatment. However, researchers are still groping to figure out how laminopathies incite such diverse symptoms.
The disorders enthrall researchers who study aging because two of them, HGPS and a form of Werner syndrome, resemble high-speed senescence (see "Of Hyperaging and Methuselah Genes"). In addition to illuminating these rare diseases, research on laminopathies might elucidate the mechanisms of normal aging because the disorders foster atherosclerosis, diabetes, arthritis, and other ailments that tarnish our golden years. Some studies posit an even tighter link between lamins and growing old by hinting that aging is a laminopathy, at least to some degree. Work published last month adds credence to the suggestion, revealing that cells from older folks and HGPS patients display many of the same lamin-related defects.
The Long and the Short of Lamin
LMNA works double time. Cells fashion two proteins--lamin A and lamin C--from the gene, using a process called alternative splicing to mint slightly different messenger RNA molecules. First, the cell manufactures an RNA copy of LMNA. Like a director editing a film, enzymes then chop out sections of this raw sequence and piece the remaining segments together, creating a messenger RNA molecule. The enzymes slice out fewer building blocks to make the lamin A-encoding messenger RNA, and the resulting protein is 74 amino acids longer than lamin C. A separate gene produces a third version of the protein, lamin B. It doesn't appear to cause disease, possibly because it's essential for early development, and embryos that lack it die before birth, says cell biologist Tom Misteli of the National Cancer Institute in Bethesda, Maryland.
A fresh strand of lamin C is ready for duty, but lamin A undergoes further primping. The enzyme farnesyltransferase hooks a farnesyl group, an intermediate in cholesterol metabolism, to one end of lamin A. After further modifications, another enzyme called Zmpste24 snips off lamin A's tail, farnesyl group and all. Why cells go through the rigmarole of adding and removing farnesyl isn't clear, says molecular biologist Brian Kennedy of the University of Washington, Seattle. According to the leading explanation, the attachment helps guide a newly formed lamin A molecule to the nuclear border.
The issue is crucial because most HGPS cases stem from the failure to jettison the farnesyl group, as researchers discovered 3 years ago (see "Lamin-tation"). The problem begins during RNA splicing. About 80% of HGPS patients carry a mutation that creates an abnormal splice site in the precursor messenger RNA molecule. Enzymes hew this molecule in the wrong spot, resulting in a shortened messenger RNA and a truncated version of lamin A. The missing segment contains the cutting site for Zmpste24. As a result, in cells from HGPS patients, farnesyl remains affixed to lamin A. If farnesyl does shepherd lamin A into position, the defect might explain why mutant lamin piles up at the edge of the nucleus--and stays there--in laminopathies.
Cell biologists nailed down the lamins' structural role in the 1970s and 1980s. The proteins link end to end and weave into a mesh that covers the inner surface of the nuclear membrane. It's strong, durable, and guaranteed for the life of the cell--or at least until the cell has to disassemble it to divide.
But after a string of discoveries, lamin research slumped, says cell biologist Susan Michaelis of the Johns Hopkins Medical Institutions in Baltimore, Maryland. Misteli remembers yawning through a conference session on the proteins in the late 1990s. "This field needs a bunch of diseases," he recalls telling a glassy-eyed colleague on the way out of the talk.
It got one in 1999, when molecular biologist Giséle Bonne of INSERM in Paris, France, and colleagues winkled out a LMNA mutation that caused one form of Emery-Dreifuss muscular dystrophy (1). Patients with the rare disorder lose muscle strength and joint flexibility and often die from an irregular heartbeat. Researchers tied HGPS to lamin A defects in 2003. Children with HGPS lose their hair, stop growing, and develop ailments that bedevil their grandparents, such as type 2 diabetes, atherosclerosis, and arthritis. They usually die from a heart attack or stroke before age 20 (see "Racing Against Time"). The adult "premature aging" disorder is Werner syndrome (see "Fry Review"). The WRN protein, which helps cells mend damaged DNA, is flawed in most Werner patients, but a few carry lamin defects (see "Atypical Situation").
Errors in proteins that interact with lamins can trigger similar disorders. For example, babies who lack Zmpste24 die shortly after birth from restrictive dermopathy, whose symptoms include tight skin, facial deformities, and weak bones; LMNA mutations can cause the same ruthless disease. Researchers continue to uncover LMNA glitches. For example, this year four new mutations turned up in a survey of Finnish heart-transplant recipients with the laminopathy dilated cardiomyopathy (2). Other lamin-linked disorders probably await discovery.
The observation that a single protein can trigger so many diseases "has brought a lot of interest back to lamins," says Michaelis. As researchers looked more closely at lamins, they had to revise the conclusion that the proteins are passive props for the cell membrane. Instead, the evidence suggests that the lamins actively participate in a system that senses when the cell is under mechanical stress--being squeezed or yanked--and adjusts gene activity accordingly. Lamins serve as a relay between the DNA and the cytoskeleton, the network of fibers that helps reinforce the cell.
Cells with faulty lamin react poorly to stress, Lee and colleagues found. The researchers put cells on a miniature version of the rack that stretched them equally in every direction (see "Tugging at Heartstrings"). "We're the guys who beat up on cells and see how tough they are," says Lee. Mouse connective tissue cells are wimpy if they carry defective lamin A. Pulling their nuclei out of shape is easier than in normal cells, and they are more likely to die when under pressure. Mechanical strain triggers normal cells to activate the stress-fighting protein NF-B, which flips on several genes. Gene activity doesn't increase as much in cells with mutant lamin A, the team determined.
Instead of huddling at the edge of the nucleus, recent work shows, normal lamins disperse throughout the organelle and cozy up to the chromosomes. Inside the nucleus, they can perform tasks besides providing support. For example, the proteins could influence gene activity, says Michaelis, because they hook up with molecules that switch genes on and off, known as transcription factors. Like a secretary who lets only certain visitors see the company president, lamins might determine which transcription factors reach the DNA. Lamins also glom onto the retinoblastoma protein, which helps control cell division, and protect it from being destroyed by the cell's recycling machinery.
Protecting DNA is part of the lamins' job description, researchers discovered last year (see "Falling Apart"). The team scrutinized cells from mice that lack Zmpste24 and amass abnormal lamin. Damaged and misshapen chromosomes littered the cells, whose DNA repair enzymes were sluggish. Moreover, Zmpste24-lacking animals were unusually susceptible to DNA-breaking radiation and chemicals.
Cells also require lamins to divide or grow up. In muscles, satellite cells pump out replacements for injured or worn-out cells. Without that ability, muscles wither. Earlier this year, Kennedy and colleagues showed that satellite cells from lamin-devoid mice divide slowly (3). Slothful satellite cells could explain why muscles deteriorate in some laminopathies. A new study by cell biologist and physician Howard Worman of Columbia University and colleagues suggests that lamin defects prevent young fat-storing cells from maturing (4). The cells' arrested development might account for the abnormal fat distribution in laminopathies such as Dunnigan-type familial partial lipodystrophy. Patients with this disorder lose fat on the trunk and legs but accumulate it on the face and neck.
This list of functions could grow as researchers probe lamins' activities, Worman says: "Lamins may be related to cellular processes we've never imagined they were linked to."
Although researchers have upgraded their lamin knowledge, they still can't explain how specific defects in the proteins lead to illness. The geneticists who pinpoint new mutations have outraced the molecular biologists who identify the source of symptoms such as atherosclerosis in HGPS and erratic heartbeat in Emery-Dreifuss muscular dystrophy. The reason, says Michaelis, is "we don't know enough about what lamins do." Worman puts it caustically. The field, he says, has produced "more review papers than data" about disease mechanisms.
There are two main hypotheses for the cause of laminopathies. The first blames health problems on damage to the nucleus. In diseases such as HGPS, the nucleus bulges and blisters. It can split open, spilling strands of DNA like string from a ripped baseball. Researchers have shown that defective lamin A spurs these deformities (5) and that trimming quantities of the protein can get the nucleus back in shape (6). The hypothesis could also clarify why several laminopathies result in muscle weakness or heart defects. A fragile nucleus would be more likely to break in organs that are continually contracting, the argument goes. However, Worman says he doubts that warped nuclei underlie most laminopathy symptoms. Cells in almost all laminopathies carry deformed nuclei, yet the symptoms of these diseases vary, he says.
A second possible explanation is that defective lamins bollix gene activity. They could alter which genes are on or off by tying up transcription factors. But lamins might also interfere with gene silencing. When cells want to mothball a stretch of DNA, they wrap it tightly around the protein spools called histones, forming so-called heterochromatin. Cell biologist Robert Goldman of Northwestern University Medical School in Chicago, Illinois, and colleagues showed that flawed lamin A spurs cells to lose heterochromatin (see "Loose Chromosomes Sink Cells"). Its disappearance might allow genes on those sections of the chromosomes to turn on when they aren't neededand when they might harm the cell.
Because researchers haven't uncovered the cause of laminopathies, they've been unable to craft drugs to battle the illnesses, which remain uncurable. But the news from the clinic isn't all grim. Doctors can alleviate some side effects of laminopathies, says Bonne. For example, her group has implanted defibrillators into laminopathy patients. The devices shock the heart back into rhythm when it begins to beat abnormally, and almost half of the patients needed the lifesaving jolts during the nearly 3 years of the study, she says. Of course, this measure doesn't correct the underlying problem.
Although scientists haven't designed HGPS treatments, they might be able to borrow some from the cancer ward. The drugs in question are farnesyltransferase inhibitors (FTIs), which block the addition of farnesyl groups to proteins. Last year, Michaelis's group and other research teams showed that the compounds corrected nuclear deformities in cells that made defective lamins (see "Turning Back the Clock"). In a subsequent study, Stephen Young of the University of California, Los Angeles, and colleagues reported that FTIs benefited mice that lack Zmpste24 (7). Untreated rodents lose muscle strength, often die young, and accumulate rib fractures faster than a rodeo rider. The drugs eased all three symptoms. Researchers are trying to organize trials of the drugs for HGPS patients. One advantage of these off-the-shelf compounds is that they have already gone through safety testing, some even in children, says Michaelis. However, clinical trials would face a huge logistical obstacle: There are only 50 or so people with HGPS in the world.
Misteli and his colleague Paola Scaffidi are investigating another strategy: stalling the production of faulty lamin A. They've devised a strand of artificial nucleotide bases, called a morpholino, that serves as a molecular Band-Aid, covering the abnormal splice site. The pair has shown that adding the strand to cells that make mutant lamin A repairs nuclear abnormalities (6). A new study (8) suggests that researchers might be able to go further and eliminate lamin A, probably through gene therapy, says Misteli. The paper reveals that genetically altered mice appeared healthy even if they made only lamin C.
Rejuvenating an Old Hypothesis
Whether HGPS and other laminopathies have anything to do with aging is controversial. Some evidence suggests a connection. For example, mice missing Zmpste24 fall to pieces--in stereotypically age-related ways--faster than do normal rodents (see "Nuclear Fallout"). However, most researchers are careful to distinguish between normal aging and the genetic diseases. A study published last month by Misteli and Scaffidi gives a dose of Geritol to the argument that HGPS is speedy aging (see "A Shared Splice Site?"). Cells even from young people occasionally employ the abnormal RNA splice site and produce defective lamin, the scientists found. But the defective proteins accumulated at the edge of the nucleus in elderly patients' cells, which were also more likely to show characteristic changes to the nucleus seen in HGPS. The alterations vanished after Misteli and Scaffidi engineered the older cells to produce the morpholino. The work indicates that lamin defects could underlie some of the problems that vex us in old age, Misteli concludes.
If further research confirms the link, we all suffer from laminopathies at some time. Treatments to keep nuclei in shape might help not just a child with HGPS but also a grandmother who can't make it to the mailbox without a walker.
May 24, 2006
A writer in Portland, Oregon, Mitch Leslie wonders how much it costs to mail a nuclear envelope.
Science of Aging Knowledge Environment. ISSN 1539-6150