A Darwinian view of AIDS

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A Darwinian view of AIDS

By | March 13, 2006

Rabbits were never meant to live in Australia. But a 19th-century game hunter

thought otherwise and imported the creatures. Over the decades, they did what

rabbits do best: multiplied and multiplied, until they became a major nuisance.

So, in the 1950s, Australian authorities introduced a viral disease called

myxomatosis in a bid to eradicate the rabbits. And it worked -- but only

briefly. In a matter of years, rabbits acquired the ability to resist the virus.

It is a classic tale of evolution -- a story that unspools again and again in

animals, people included. The march of human civilization, measured both by

survival and suffering, is framed in many respects by how well we compete with

microscopic organisms.

By exploring how humans evolve to win the battle with diseases and how they are

sometimes vanquished by those pathogens, scientists hope to find drugs that can

mimic successes and stave off the worst that viruses and bacteria can produce.

''It's a constant race between humans and their capacity to create new drugs and

pathogens and their ability to evolve, " said Dr. Cohen, an infectious

disease researcher at Fenway Community Health in Boston, which specializes in

AIDS treatment.

But before researchers can develop new drugs, they must understand the

evolutionary nature of individual diseases and how viruses and bacteria, in

turn, shape the evolution of humans and other animals.

Scientists have long known, for example, that in regions where malaria is

common, most notably Africa, humans have developed an intrinsic defense against

the parasitic illness. And that defense, which involves a change in red blood

cells, has proved to be a good thing for many Africans, making them less

susceptible to the disease. But the same trait has proved to be a bad thing for

their descendants in the United States and other malaria-free areas, because it

results in sickle cell disease, a condition that robs organs of oxygen, causing

debilitating bouts of pain and sometimes death.

There are vital history lessons, too, when it comes to HIV.

As researchers unlocked the secrets of HIV, they found a gene mutation they

suspect may protect against the virus that causes AIDS.

Human cells have locks on their surface -- scientists call them receptors -- and

a virus must insert its key into these locks to gain entry. One of those is

called CCR5, and HIV needs to unlock it to be able to infect cells. But

scientists in recent years discovered that 5 to 10 percent of people in northern

Europe don't have CCR5 receptors.

''And that's where the story gets interesting, " said Dr. Calvin Cohen, research

director for Community Research Initiative of New England, which conducts trials

of AIDS drugs.

In contrast, people in Africa and Asia universally possess CCR5. So researchers

theorized that lower HIV rates in northern Europe might be due in part to some

people lacking the cellular lock.

But why don't they have it? Right now, it's only an informed hunch, but

scientists suspect that the mutation exhibited by northern Europeans may be an

artifact of the bubonic plague. The theory goes like this: As the plague swarmed

Europe starting in the 14th century, it wiped out people who possessed CCR5 but

spared those who lacked it.

''What we're talking about is a Darwinian process, " Harmit Malik, who

specializes in the study of genetic conflict at the Fred Hutchinson Cancer

Research Center in Seattle. ''What was a really rare mutation was what survived.

Everyone else had fallen prey to this particular pathogen. "

And the thing is, people who lack CCR5 receptors appear not to suffer any

consequences.

''So we have an ideal combination, " Calvin Cohen said. ''HIV needs it, but we

don't. What an ideal target for drug development. "

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