Published On September 22, 2009
THE SURGEON WHO REMOVED THE FIVE-YEAR-OLD BOY’S TONSILS in 1959 had never seen anything like them. Instead of pinkish lobes, the boy’s tonsils were huge and orange. Thinking that their extraordinary appearance might signal a rare malignancy, the surgeon sent the tonsils to the Armed Forces Pathology Institute in Washington, D.C. Though researchers there found no cancer, they did discover the reason for the tissue’s abnormal size and color: Its cells were bloated with cholesterol. That prompted a call to a leading cholesterol expert, the National Heart Institute’s Don Fredrickson, who packed his medicine bag and took a ferry to the boy’s home on Tangier Island, 12 miles off the coast of Virginia.
John Smith, of Jamestown fame, discovered the island in 1608, its stretches of pale sand reminding him of North Africa’s Tangier. To Fredrickson, 350 years later, the island seemed a likely place for a rare, inherited condition to become more prevalent than it could ever be in a more diverse, widely dispersed population. A handful of families had settled Tangier in 1686, and its inhabitants had remained physically and genetically isolated. After a cholera epidemic forced the island’s evacuation in 1866, many families never returned, and the gene pool became even smaller.
Looking down the throats of almost everyone on the island, Fredrickson found only one other set of big orange tonsils, in the boy’s sister. That told him this was probably a recessive genetic condition that followed the inheritance pattern first defined by Gregor Mendel in his famous pea experiments. In the case of Huntington’s disease and a few other Mendelian maladies, the defect is dominant, and children who inherit a copy of the affected gene from just one parent will develop the disease. In other conditions, including cystic fibrosis and sickle cell anemia, the gene is recessive: Both parents must pass it along for the child to get sick.
Tangier disease, as the boy’s condition came to be known, was indeed recessive. “The kids with two copies of the defective gene had no HDL [high-density lipids], the good form of cholesterol, in their blood,” says Mason Freeman, an expert on cholesterol metabolism at Massachusetts General Hospital. “The parents, with just one defective copy each, had some HDL, but much lower levels than the average person.” HDL normally helps cells get rid of their excess cholesterol; without HDL, too much cholesterol accumulates—leading, in this case, to swollen orange tonsils.
Fredrickson was intrigued because scientists already knew HDL was associated with heart disease risk—low levels were bad and high levels were protective—but didn’t understand why. If he could find additional cases like those of the boy and his sister, they might lead researchers to the gene responsible for the condition, and the chance to learn more about HDL. Fredrickson began a hunt that ended up taking 40 years, in part because it proved quite difficult to find other people with cholesterol-laden tonsils. To date, only about 100 people with Tangier disease have been identified worldwide.
The ultimate discovery of the gene, in 1999, filled in an essential chapter in the story of how the body processes cholesterol, says Freeman, who joined the search in the early 1990s. The first part of that tale, involving LDL, or low-density lipids—the so-called bad cholesterol—had been the study of another rare genetic disease, familial hypercholesterolemia, which causes dangerously high levels of LDL. Research into the gene responsible was crucial in understanding how an LDL-reducing statin worked—and how to make more effective statins. Now researchers studying the gene causing Tangier disease are developing therapies that could reduce cardiovascular risks by boosting HDL levels. And because the same gene also turns out to affect the risk of Alzheimer’s disease, advances in treating that scourge are also possible.
All of this research bolsters support for current diagnosis and treatment of heart disease. Doctors routinely rely on cholesterol readings and the ratio between HDL and LDL to gauge the health of patients’ hearts, and they prescribe exercise and dietary programs that can raise HDL readings, advice that may one day be supplemented by new medicines. Ironically, though, none of this can benefit people with Tangier disease, who have no HDL in the first place. Moreover, all the Tangier-related payoffs have come from a kind of genetic sleuthing that is largely considered old-fashioned compared with high-tech modern genetics. But so far, at least, those efforts have gone far beyond what newer methods have achieved.
ONE MAJOR BENEFIT OF STUDYING RARE GENETIC DISEASES IS that afflicted individuals can inform research in much the same way as “knockout” laboratory animals, which are bred without a particular gene in order to learn its function and gauge the impact of its absence. As naturally occurring test cases, these human knockouts often provide clues not only about their own conditions but also about many more common maladies—heart disease, diabetes, hypertension, arthritis, even aging.
“Tangier disease led us to the gene ABCA1, which is mutated in people with the disorder, and studying the gene has taught us a tremendous amount of what we know about HDL cholesterol metabolism,” says Freeman. ABCA1 manufactures a “transporter” protein, which moves excess cholesterol out of a cell that has more than it needs to maintain its oily membranes and produce hormones and other essential substances. Normally, this transporter sits on the cell membrane and ushers the cholesterol into another protein, a flat disk called apoA-1, which floats in the bloodstream. This disk latches onto the transporter when cholesterol is ready for loading. As the disk inflates with cholesterol, it balloons into the sphere known as the HDL molecule. HDL ferries its contents through the bloodstream to the liver, which breaks down the cholesterol and sends it to the bile for excretion in feces.
During that process, people with a high level of HDL are efficiently ridding their bodies of excess cholesterol, whereas those with low HDL levels might retain too much cholesterol in their cells. That can increase the risk of heart disease, because when the cells in arterial walls contain too much cholesterol, they attract inflammatory molecules that contribute to fatty plaques, which can burst and cause heart attacks.
In people with Tangier disease, the ABCA1 gene doesn’t function. This problem particularly affects immune cells known as macrophages, which converge on sites of infection and inflammation to devour invading microbes as well as dead or dying cells, taking in orange-hued cholesterol in the process. The tonsils’ job is to trap bacteria and viruses entering the throat, and they employ macrophages to aid in that effort. The macrophages become filled with cholesterol, and in the absence of the ABCA1 transporter, which would normally help rid them of some of that load, they make the tonsils swell.
BEYOND EXPLAINING THE TANGIER ISLAND BOY’S TONSILS, ABCA1 accounts for much of the genetic variation in HDL levels—and heart disease risks—in the general population. In most people, the gene isn’t completely on or off. Researchers have identified as many as 70 mutations in the ABCA1 gene that affect how well the transporter operates and thus how much HDL is in the bloodstream. Although such environmental factors as diet, exercise and smoking also have an impact on HDL levels, scientists think that genetics accounts for about a third of the variation in the general population. Much of the genetic variation, in turn, involves ABCA1.
Yet while inherited defects in that gene may limit someone’s HDL level, researchers are working on ways to right that wrong. “If we could make cholesterol transport more efficient, we could reduce the cholesterol burden in macrophages and raise HDL levels—and that would reduce heart disease risk for millions of people,” says John F. Oram, a medical researcher at the University of Washington in Seattle. Oram and others are using animal models to study ways of altering the transport system.
There are two main approaches. One is to increase the activity of ABCA1 itself; the other is to make the apoA-1 protein more effective. For the first, researchers are testing compounds that mimic the function of the liver X receptor (LXR), which regulates ABCA1’s function. Those compounds, called agonists, can rev up ABCA1 activity, and in animal models they prevent or even reverse atherosclerosis. But agonists also turn on other genes, some of which can cause an unacceptable risk in humans of fatty liver—a condition in which accumulating fat may scar liver tissue, causing inflammation and possibly liver failure. Wyeth Pharmaceuticals recently conducted a small Phase 1 safety trial of an LXR agonist, but researchers anticipate the need to tweak such compounds so they activate ABCA1 without leading to harmful side effects.
The second approach uses small pieces of proteins called peptides that are designed to improve apoA-1 function so that it transports cholesterol more efficiently. Several pharmaceutical companies are developing such peptides, which could have the added benefit of reducing the inflammation that underlies heart disease, diabetes and Alzheimer’s disease. Oram, one discoverer of the ABCA1 gene, has found that the protein it produces is unique among transporters in that it also functions as a receptor for molecules that may suppress an overly robust inflammatory response. “This is very exciting because it’s the first transporter with a direct biochemical link to both cholesterol and inflammation, two contributors to heart disease,” says Oram. “We know people with high HDL levels live longer than others do, and this may be why—their bodies suppress the inflammation involved in many chronic diseases.”
Oram is working with a synthetic peptide that mimics the anti-inflammatory effect of the ABCA1 protein. He suspects that such peptides could provide relief for many inflammatory processes, including those associated with arthritis and colitis, that are now treated with such immune-suppressing compounds as interleukin. The peptide works well in mice, but the research isn’t yet ready for human clinical studies.
RESEARCHERS HAVE ALSO BEGUN TO REALIZE THAT the importance of ABCA1 goes beyond the heart—to the brain, where it may affect the risk of Alzheimer’s disease. “Though the brain is the most cholesterol-rich organ in the body, there has been little research into cholesterol transport there,” says Cheryl Wellington, an Alzheimer’s researcher at the University of British Columbia, Vancouver. Wellington works with mice bred to lack a functioning ABCA1 gene.
According to Wellington, cells in the brain metabolize cholesterol differently from cells elsewhere in the body because cholesterol can’t pass through the blood-brain barrier, the tightly packed cells in the walls of capillaries in the brain. And though brain cells also use ABCA1 to transport cholesterol out of cells, the transporter protein doesn’t bind to apoA-1 but rather to a related protein, apoE, in the brain. In addition to clearing cholesterol from the bloodstream via the liver, as HDL does for cells outside the brain, apoE plays a role in clearing the proteins, called amyloid-beta (Abeta), that might otherwise form the amyloid plaques typical of Alzheimer’s disease. (The brain naturally carves Abeta out of a larger protein called amyloid precursor protein.) Normally, apoE clears Abeta as it’s produced, but in patients with Alzheimer’s, Abeta remains, accumulating as sticky amyloid deposits among neurons and leading to the cognitive problems of the disease.
Work in Wellington’s laboratory and elsewhere shows that ABCA1 is the main way the brain’s supply of apoE gets its lipids, and if ABCA1’s cholesterol transport is impaired—as it is in mice with Tangier disease—Alzheimer’s plaques grow much more readily. Boosting the function of ABCA1 sixfold prevents plaques from forming. Compounds developed for HDL therapy that mimic the liver X receptor’s ability to increase ABCA1 activity also work in protecting mice from Alzheimer’s.
“The key question is whether similar compounds will also work in people,” Wellington says. “If we could use a small molecule that could get past the blood-brain barrier, it could be a huge benefit to the aging population, in exactly the way that understanding HDL can help us design preventive strategies for cardiovascular disease. It has been tremendously valuable to the whole Alzheimer’s community to learn how apoE functions using the model of Tangier disease.”
CONSIDERING ALL IT HAS TAUGHT US, Tangier disease might seem a prime example of how genetic diseases are studied. Yet today, when genetic researchers want to understand conditions that affect millions, they typically take a different approach. Rather than search for a single gene with a major defect that has a profound effect on just a few people, scientists zip through the entire genomes of tens of thousands of people. Comparing those genetic maps, researchers look for minute differences in many genes that, taken together, may explain why some people are susceptible to widely prevalent conditions and why others are much less likely to get them.
The modern approach makes sense, at least in theory, because these conditions are caused by not just one gene but many, and telltale differences may be very small. Data-crunching resources are devoted to sorting out which combinations of genetic variations fit the profile of a person with a higher- or lower-than-average risk of a chronic disease. The goal is to find targets for preventing or treating the major ailments of today’s population.
At the time the Tangier Island boy’s case came to light, the ability to scan an entire genome was beyond imagining. Gene hunting was slow and tedious, and researchers originally had to scribble down observed sequences of DNA bases by hand. They took DNA samples from affected and unaffected family members to look for differences that signaled the presence of a disease gene. “It was like working out a jigsaw puzzle,” says Freeman of the MGH—and very different from modern genetics. Automated DNA sequencing is so fast and efficient that researchers can do in hours what once took months or years.
Yet the whole-genome approach is so new and the problems it is tackling are so complex that it may take some time to identify variations that provide as much clinically valuable information as research on single-gene diseases has. So far that hasn’t happened, and some researchers question the overall validity of the approach. But proponents point out that by culling massive databases, researchers were able to link high levels of LDL in the population to one already-known gene—HMG CoA reductase, which is the target of statins—so the approach could eventually find other similarly clinically important genes. “Still, we have not yet made leaps and bounds in understanding general physiology and identifying new therapeutic targets from whole-genome scans the way we have from studying rare Mendelian disorders, like Tangier disease,” says Freeman.
Daniel Rader, a cholesterol researcher at the University of Pennsylvania School of Medicine who has treated patients with Tangier disease, believes the two approaches to genetic research are converging. “There are many people with very low or very high HDL that can’t be accounted for by any known variation in the ABCA1 gene or any other gene,” Rader says. “I suspect the cause will turn out to be genes we haven’t been smart enough to find yet.” He says that whole-genome studies of large populations are identifying, for instance, previously unsuspected genes that are important for controlling the level of HDL and that harbor mutations in some people with very low or very high HDL. “These ‘new’ HDL genes will help us fill in some of the remaining gaps in our understanding of why some people seem to have higher or lower HDL levels—and a higher or lower risk of heart disease or other common disorders.”
To find such genes and their variations, Rader is always on the lookout for patients with unusual cholesterol metabolism who would be willing to take part in a genetic study. “I’m absolutely certain these modern genetic studies will identify new genetic variants and mutations that will turn out to be the causes of uncommon conditions”—ones that, like Tangier disease, will teach us more about ourselves.
“Tangier Disease: Still More Questions Than Answers,” by Jerzy-Roch Nofer and Alan T. Remaley,Cellular and Molecular Life Sciences, October 2005. A historical journey from the discovery of the ABCA1 gene, a mutation of which causes Tangier disease, to the growing understanding of its essential role in transporting cholesterol and its potential to reduce heart disease risk.
“Greasing the Wheels of Abeta Clearance in Alzheimer’s Disease: The Role of Lipids and Apolipoprotein E,” by Jianjia Fan, James Donkin and Cheryl Wellington, BioFactors, June 2009. A concise overview of the recently appreciated role of ABCA1 in forestalling Alzheimer’s disease.
“The Cell Cholesterol Exporter ABCA1 as a Protector From Cardiovascular Disease and Diabetes,” by Chongren Tang and John F. Oram, Biochimica et Biophysica Acta, July 2009. Oram, a discoverer of ABCA1, describes a crucial function of the gene: regulating the inflammatory process that can contribute to heart disease and Alzheimer’s, as well as diabetes and other disorders.
Stay on the frontiers of medicine