by Rob Dunn
The answer to Nissi Varki’s mystery came from her husband, Ajit Varki, with whom in recent years she had begun to collaborate. Ajit and Nissi Varki met at the Christian Medical College in Vellore, the preeminent medical college in India. There, they fell in love and, simultaneously, began to specialize professionally. Ajit was interested in internal medicine, hematology and oncology in particular, and Nissi in pathology. Nissi had already begun to study mice as models of humans to understand the biology of cancer and other diseases. Ajit researched practical treatments for common diseases. Neither of them studied the heart, nor did they imagine they would study the heart in the future. Ajit finished his degree before Nissi and went to the United States in search of new research skills. He took a job first at the University of Nebraska and then at Washington University in St. Louis. Nissi finished her degree and joined Ajit at Washington University, where she too found a position; eventually, they both moved to their current jobs at the University of California, San Diego.
Early in his career Ajit Varki found himself working on compounds called sialic acids. He started in 1984, accidentally. Ajit was treating a patient with a relatively rare blood disorder, aplastic anemia, by giving him a derivative of horse serum.8 But there was a problem: the patient’s immune system reacted to the serum, causing a rare serum sickness. Ajit determined that this particular serum sickness was due to a reaction of the human body to one of the forms of sialic acid in the horse serum.9 Sialic acids are sugars that coat the surfaces of cells, sometimes as densely as hundreds of millions per cell. The patient’s immune system was responding to the sialic acids in the horse serum as though they were dangerous, foreign entities. This was a very unusual reaction, “ridiculous” even, as Ajit would later say, because all mammals, all vertebrates in fact, were known to have the same exact forms of sialic acid on their blood cells, the two most common of which were Neu5Ac and Neu5Gc (note that the difference is just an A relative to a G, but much depends on this one-letter difference).10 Humans and horses should both have these same two sialic acids. The immune system of a human shouldn’t even have noticed that the horse serum was different. Some part of what was known about sialic acids, horses, and humans was wrong, but it wasn’t obvious just what. It was, as Ajit told me in an interview, “a kind of detective story.”
A decade passed, during which Ajit immersed himself in studying sialic acids and other sugars on the surfaces of cells; he became an internationally known expert on the topic (glycobiology), literally writing the book on these sugars.11 He and Nissi collaborated for the first time, briefly, to understand sialic acids in mice (lab animals could be studied much more easily than either horses or humans).12 Even though it was never the entirety of what Ajit worked on, the horse-and-human mystery stuck with him. He began to accumulate clues. The first clue came when he found that earlier studies had documented that humans seem to lack one of the two main kinds of sialic acid, Neu5Gc, found in other mammals. On its own, this was unusual. Then he found a study published in 1965 showing that some nonhuman apes had this normal sialic acid (Neu5Gc), an observation soon confirmed by Elaine Muchmore, also at the University of California, San Diego, with whom Ajit had begun to collaborate.
On the basis of these clues, Ajit Varki and Muchmore decided to compare the genes associated with the production of sialic acid in sixty humans and all of the apes. While it was known at the time that about 1.5 percent of the sequences of genes in humans and chimpanzees were different, none of those differences had yet been pinpointed. Varki and Muchmore would be the first to do it, and they were in for a surprise. In all of the apes (and the rest of the other mammals so far studied, including mice and horses), an enzyme modifies the basic sialic acid, Neu5Ac, by adding an oxygen atom to it, which converts it to Neu5Gc (N-glycolyl-neuraminic acid). The gene that produces this enzyme, CMAH, is broken in humans,13 missing ninety-two bits (nucleotides) of DNA. As a result, all sialic acid produced by humans is the Neu5Ac form and lacks the extra oxygen atom. Humans lost the ability to make Neu5Gc. Because sialic-acid sugars are found on all of the cells in the human body, this was enough of a difference to cause the human immune system to see the Neu5Gc in the horse serum as different.14 Every cell in a human is different from every cell in every other mammal. Humans are the odd species out.
Ajit and his colleagues had discovered the first genetic difference ever noted between humans and chimpanzees,15 a difference that, as he learned from the very beginning, made human immune systems different from those of chimpanzees but also nearly all other mammals, including the horse. We fixate on the conspicuous differences between chimps and us, but in a world filled with pathogens and the diseases they cause, the inconspicuous differences may be far more important. Sialic acid, Ajit was nearly sure, was an important piece of the story of human evolution and disease.
Ajit Varki wanted to pursue this issue further, but he needed to learn more about chimpanzees and other apes. The difference between chimpanzees and humans seemed fundamentally important, but it was unclear why it existed. He needed to know more about chimpanzees to make sense of the observation but also to understand what other differences he and everyone else might be missing.
Ajit took a sabbatical to spend time at the Yerkes primate research center, where Nissi would later make her observations about chimpanzee hearts. While there, Ajit did not focus on the heart. But he was able to document a list of diseases that seemed to differ between chimpanzees and humans, diseases that might somehow be associated with the change in sialic acid and recently evolved differences. The list included some cancers, AIDS, a range of infections, rheumatoid arthritis, and much more. And so, years later, when Nissi came home from Yerkes with her revelation about the differences between human and chimpanzee hearts, it was not a total surprise to either of them (though it would be to nearly everyone else). In thinking about the heart, Nissi and Ajit started to talk about sialic acids and Ajit’s earlier trip to Yerkes. It is ridiculous to imagine that the sugars that Ajit had spent decades studying would prove to be the answer to Nissi’s mystery about chimpanzee hearts, and yet that is precisely what happened.
In order to put these two stories together, what the Varkis needed was one final clue, and fortunately, it was one Ajit Varki had already turned up. When present in the human body, the normal mammalian sialic acid (the one with the extra oxygen) causes an immune response; the human body notices the extra oxygen and attacks. But something was odd. From what Ajit knew about sialic acids, it seemed that when humans consumed mammal meat, the sugars on the meat would get incorporated into human cells. If this was true, it would make some human cells, those with the sialic acid with an extra oxygen, look to the immune system as though they were foreign. The immune system might overreact and attack these cells, leading to all sorts of problems, including, and here was Ajit’s speculative jump, atherosclerosis. But Ajit did not have any proof of the theory that the sialic acids in human diets were getting incorporated into human cells, and so he wanted to do an experiment.
Ajit Varki wanted to feed humans mammal meat and see if the sialic acids from that meat (Neu5Gc) ended up on the cells of the humans. He couldn’t do the experiment on any other mammal because humans are the only ones who lack this particular mammalian sialic acid. He thought it would be easy enough to do on himself. But universities are very reluctant to allow scientists to do self-experiments (times have changed since the days of Forssmann). So before even proposing an experiment on humans, Ajit did one on human cells grown in petri dishes in the lab. When fed the normal mammal sialic acid (Neu5Gc), those cells incorporated it directly into their membranes! After seeing these data, the university review board allowed Varki to do an experiment on himself. Varki and one of his collaborators, Pascal Gagneux, then extracted sialic acid from pig salivary glands (sialic acids are very concentrated in spit). On the morning of February 16, 2001, Varki checked into a clinical research center at his university. Once there, Varki consumed an amount of sialic acid equivalent to eating “fo
urteen pork steaks,” 150 milligrams, in the form of a pig-spit Slurpee.
Over the next weeks, the levels of Neu5Gc sialic acid in Varki’s urine, saliva, and hair increased. This mammalian sialic acid was becoming part of his cells. The experiment was later repeated on Gagneux and Muchmore, with similar results.16 Like the old adage says, Varki became what he ate.
People who eat mammal meat incorporate the sugars from the meat into their cells. The body’s immune system sees the sialic acids on the tips of these sugars as foreign, and it attacks, a reaction that occurs throughout the body, including in the artery walls. The fact that this occurs is unassailable. This, for the Varkis, was the final clue related to Nissi Varki’s discovery of the differences between chimpanzee and human hearts.
Over dinners, lunches, and more formal meetings, the Varkis concluded that the differences in human sialic acids compared to those of other mammals combined with a diet rich in mammal meat contributed to the prevalence of atherosclerosis in humans. What is unclear is how much of human atherosclerosis is due to the loss of one type of sialic acid. The human immune system is unusual in its activity level, which is high, even without the influence of sialic acid from other mammals (vegetarians are not immune to atherosclerosis). Relative to other primates’, the human immune system is twitchy and overreactive.17 Together, these two factors—the lack of a particular sialic acid and a general twitchiness—may be at the heart of many diseases. It is this overreactivity that may cause humans to develop AIDS when infected with HIV (chimps contract HIV but do not develop the full-blown disease). It is this overreactivity that is associated with chronic hepatitis B and C, rheumatoid arthritis, asthma, type 1 diabetes, and other modern plagues. In other words, all humans appear to suffer more inflammation (in general, but particularly in the arteries) than chimpanzees do, but those who consume mammal meat may suffer it disproportionately.
All of this paints a compelling picture linking sialic acid to human immune response and diets. But it doesn’t answer one question: Why are we unusual in our sialic acid and immune reactivity in the first place? For this, too, the Varkis have an idea. Humanity’s most recent common ancestor, from whom all humans inherited at least some genes, lived at least a hundred thousand years ago. Any problems we all share, any uniquely human traits, are at least that old, and so if we are to discover what makes human hearts unique, we must go back that far. From the very beginning, it appears our ancestors were different from their close relatives in terms of something besides sialic acid, before the change in the sialic acid: our ancestors were plagued by disease.
Earlier work on which Nissi and Ajit collaborated showed that sialic acids played a key role in human interaction with many of the pathogens that cause disease. Pathogens such as the influenza virus use sialic acids to identify particular cells in the upper respiratory tract and latch onto them. Perhaps, the Varkis began to think, the change in sialic acid was an attempt by the cells within the bodies of our recent ancestors to escape some particular infectious disease.
The story of humans and disease is unlike that of any other mammal and disease. At some hard-to-define point very near to the origin of our kind, humans combined language skills, brainpower, and the beginnings of culture in such a way that allowed larger groups to live together than had ever lived together before. The largest known nonhuman primate group ever recorded contained about fifty individuals (evolutionary biologist Mark Moffett has even called this the fifty rule). Scholars aggressively debate exactly when this transition happened, why it happened, and how it changed humans, but what no one debates is that, as humans gathered in ever larger numbers, the risk posed by old and new pathogens increased. We are familiar with this phenomenon from schools. Put all the kids together, and pathogens and parasites—whether it’s the flu, norovirus, lice, mites,18 or even one of the viruses causing pinkeye—spread rapidly. Put our ancestors together, and the same thing happened. Most primates host about two hundred species of pathogens; humans now host more than two thousand species, two thousand different and potentially dangerous and deadly forms.
The greater density of human populations meant that pathogens could spread faster; it also meant that they could become more deadly. Pathogens do not generally evolve to be very deadly to their hosts because if they are too deadly, their host dies before they can get to another host, but there are a number of dissatisfying exceptions to this satisfying generality. One has to do with the density of the hosts. Once humans began to gather, the chances of a pathogen getting to another host improved and so the costs of deadliness declined. Perhaps some widespread and virulent pathogen, in these early moments of our human story, triggered evolutionary changes like those the Varkis found, changes in sugars throughout the body. If such a pathogen evolved, it might have killed so many of our ancestors that only those with versions of genes that allowed them to escape those pathogens would have survived. Maybe one of those protective genes was a broken version of the gene for Neu5Gc sialic acid.
Prior to the past few hundred years, our ancestors died of many things, but among the most common were diseases caused by a suite of species that lurked in the blood. It is these species that the Varkis think altered human bodies in ways that ultimately made heart disease more likely.
The idea that those pathogens that make their way to the blood might alter the evolution of the body is not far-fetched. Blood is the stuff that is simultaneously most precious to our bodies and most attractive to other species. Blood is the heart’s liquid, the ether through which the heart asserts its influence over the body. Blood is precious to our bodies’ cells. It has water where water is scarce, proteins, sugars, a reasonable pH, and a constant temperature. But the preciousness of the blood is also its weakness. Any parasite that gets access to the blood finds its own piece of an immense liquid paradise. Each adult human body holds about a gallon of blood. With about seven billion people on Earth, that is about seven billion gallons of blood. No wonder, then, that no fewer than twenty lineages of flies have independently evolved a fondness for blood, accompanied by massive changes in parts of their mouths that allow them to pierce skin, forcing blood to flow freely so they can do the job quickly enough to get away. Some bats feed on blood, as do leeches. One vampire finch even feeds on blood. A finch! Recently, a moth with a fondness for blood was found landing wantonly on salty arms in Siberia. But whereas these animals stopped at the skin—braced their feet and bit in—others, including the pathogens that began to latch onto human society as our groups grew in size, figured out ways to press their entire bodies through the flesh and into the stream of pulsing sustenance.
Nearly all of the species that make it into our blood influence both our blood and, via the blood, our hearts. The species whose impact we best understand is the malaria parasite, Plasmodium falciparum. Malaria kills more than a million people a year today, and that is many fewer than it killed before pesticides, mosquito nets, and prophylactic medication. A malaria parasite rides in a mosquito until the mosquito lands and punctures skin. When the mosquito jabs in, the parasite slides with the gush of saliva into the blood, where it rides to the liver, divides, and produces more parasites; these break free, travel back to the blood, and invade red blood cells. The body reacts with fever, attempting to kill the parasites with heat. But by the time the parasites have made it out of the liver and into the red blood cells in which they ride, again and again, through the heart, it is too late. When another mosquito comes along, one of the parasites, having helped consume its host into near disaster, catches a ride to the next body. In this way, malaria has moved from person to person around the world. In many human populations ten thousand years ago (even two hundred years ago), most people contracted malaria between birth and death. Perhaps one in ten of them died from it.
Chimpanzees and gorillas, like us, suffer from malaria. Nearly half of all chimps and gorillas are infected with malaria parasites at any given moment (bonobos, interestingly, appear to harbor none; why that is the case has not yet
been studied). But their malaria is different from ours, seemingly less deadly. Humans are known to have contracted one form of malaria by way of a gorilla about twelve thousand years ago, with the dawn of agriculture. A mosquito bit a gorilla with malaria; a malaria parasite from the gorilla hitched a ride on the mosquito, which then bit a human. The parasite arrived in the human and reproduced, and the species began to evolve in ways that allowed it to take better advantage of its new host. (The malaria parasite is not the only thing we have acquired from gorillas. Genital lice also appear to have made the jump, though that would have had to happen when a gorilla ancestor and a human ancestor were—ahem—touching.) Such host shifts are common, but the strange part of this story is why, prior to twelve thousand years ago, humans did not already have their own unique malaria parasite, why we ended up with a transmogrified and very deadly gorilla malaria. Chimps had chimp malaria parasites, gorillas had gorilla malaria parasites. Where was the human malaria parasite, the parasite that evolved to live in us before we picked up gorilla malaria? The Varkis posit that an ancient human malaria was missing because our ancestors had evolved a way to escape it. Perhaps that happened once human settlements became more dense and human malaria more deadly, and perhaps it happened because human malaria parasites (like chimp and gorilla malaria parasites) latched onto Neu5Gc, which was the form of sialic acid that our ancestors lost. If an absent or broken Neu5Gc made some of our ancestors immune to an ancient human malaria, the genes for this trait would have swept through human populations, potentially causing this parasite to go extinct. If that did occur, the reprieve in human malaria was, unfortunately, temporary, lasting only as long as it took our kind to be colonized by gorilla malaria.
This idea is plausible. We know that malaria can shape our genes; the colonization of humans twelve thousand or so years ago by the new malaria strain (which, in shifting to humans from gorillas, also had to shift the sialic acid to which it bound) clearly has. One of the changes that malaria caused in our genes was to favor versions of human genes that made it harder for malaria parasites to be able to persist and/or breed in our blood. These changes are primarily deformities of the hemoglobin in red blood cells. The deformities make the red blood cells less effective at holding on to oxygen and so tend to cause the heart to become more muscular and beat faster. These changes are actually negative changes, problems, but they are smaller problems than dying of malaria. It has been argued, quite plausibly, that the O blood type evolved as a response to malaria. Other mammals have A and B blood types, but not O. Individuals with O blood types lack certain sugars on their cells, sugars that make it easier for malaria parasites to find and enter the cells. As a result, individuals with the blood type O have a lower risk of dying of malaria. All of this together points to the possibility that malaria shaped human evolution in other ways as well, as any of the deadly pathogens of our history might have.
