By David N. Leff
A baby born into the world with sickle cell disease starts life with no symptoms.
"Nothing happens when the infant is born," explained molecular geneticist Chris Paszty, "because at birth it is still making high levels of fetal hemoglobin."
Adult hemoglobin, which ferries oxygen-rich blood around the body, consists of two proteins, alpha and beta. But during gestation, the fetus makes do entirely with gamma, or fetal, hemoglobin, which exerts potent anti-sickling effects.
"There's a switch that takes place between fetal and beta hemoglobin," Paszty pointed out, "and that happens during the first year after birth. So when they're born carrying the sickle cell gene, they're fine. The disease develops when they begin to grow older."
Then, as the fetal hemoglobin phases out, the sickling mutant kicks in, and with it the symptoms of the malady.
"What typically happens," Paszty went on, "is that their red blood cells become fairly fragile, so their rapid turnover means that people usually become anemic." Pasztyis a scientist at the University of California's Lawrence Berkeley National Laboratory.
Red blood cells are normally elliptical and supple. But in people with the disease, they become more rigid and distorted into a sickle or scimitar shape.
Paszty picks up the story: "Those sickled red cells are very sticky, and they have a tough time going through capillaries. So they cause reduced oxygen delivery to tissues around them, and get trapped in the liver and spleen. Over time, this damages organs, and if it gets bad enough, causes end-stage organ failure, and ultimately death."
But he added: "It's very variable; some people may die of a stroke in their teens; others do fine and live fairly healthy lives." Current life expectancy is 40 years of age, double what it was two decades ago.
The hereditary blood disorder affects almost 83,000 people in the U.S. Nearly all of them are African Americans, of whom one in 500 contract the disease. Elsewhere in the world, the sickle cell mutation is common in Africa and the Middle East.
That single-base-pair mutation arises from an adenine-to-thymine switch in the beta-globin gene. An individual who inherits two such mutant genes from his or her parents acquires sickle cell disease. But if only one parent carries the mutation, the offspring is a carrier of the sickle-cell trait, not the disease. In the U.S., one black in ten carries the sickle trait.
Although that mutant gene has been known for half a century, to date there is no cure for sickle cell disease. In children it threatens major childhood infections, which must be managed by antibiotics. In adults, the painful episodes of "sickling crisis" may require blood transfusions.
Anticancer Compound Restores Fetal Hemoglobin
Only one drug appears on the horizon to deal practically with the underlying disease, said Paszty. It is a 30-year-old anticancer agent called hydroxyurea, now completing clinical trials in children and adults as an anti-sickling therapeutic.
A co-author of these double-blind, controlled studies is research hematologist Elliott Vichinsky, at Oakland Children's Hospital, and the University of California. "It will probably get FDA approval," he told BioWorld Today.
Hydroxyurea works, Vichinsky explained, to reactivate fetal hemoglobin synthesis in sickle cell patients by preventing their stem cells from dying off. While many anticancer drugs have this effect, he observed, "hydroxyurea is easy to take in one-a-day pill form, and it's relatively non-toxic."
Numerous laboratories in many countries are researching possible therapeutic strategies for sickle cell disease, Paszty pointed out. "One of the goals," he said, "is to engineer human stem cells. That would be an attractive way of doing it, but technical difficulties make it impossible to do today."
He continued: "An alternate effort to cure the disease is bone marrow transplantation. But that's not something that can be applied on a wide scale."
The overriding obstacle that hinders such research is the lack of an animal model that faithfully reproduces the clinical course of sickle cell disease. After giving up on studying red blood cells from patients in test tubes, investigators tried constructing mouse models carrying the human mutant gene. What stymied them in this effort was the fact that the animals' own non-mutant globin genes rescued them from the human disease.
At this point, in the early 1990s, two widely separated molecular geneticists — Paszty at Berkeley and Tim Townes at the University of Alabama, Birmingham — decided unbeknownst to one another that it was high time to create true mouse mimics of the human condition.
When they finally met, midway through their projects, they found they were actually mimicking each other, and have since collaborated.
Their back-to-back papers in today's Science, dated Oct. 31, 1997, are titled, respectively: "Transgenic knockout mice with exclusively human sickle hemoglobin and sickle cell disease," first author, Paszty; and "Knockout-transgenic mouse model of sickle cell disease," senior author, Townes.
At Long Last, All-Sickle Mice
"We had to start from scratch," Paszty recalled, "and remake transgenic lines with fetal hemoglobin genes as well, and get rid of the mouse globin genes. Our group knocked out the alpha globin cluster, and Tim Townes' group managed to knock out the beta globin cluster. In addition, we put in normal human alpha globin and gamma and sickle beta globin."
The resulting mice, producing 100 percent human sickle hemoglobin, display virtually all the hallmarks and symptoms of the human disease — sickling of red blood cells, hemolytic anemia, tissue death from clogged blood vessels, organ damage. And they survive, so far up to nine months.
Paszty sees the new mouse colonies as being "probably most useful for blocking the stickiness of those red blood cells, for stem cell transplants and gene therapy."
Meanwhile, he observed, "A lot of labs have called to request that I send them these mice. I have to breed up the colony, and I seem to have a lot of paperwork these days. I'm very motivated to get the animals out to whomever wants them."
Vichinsky said of the new animal models, "I think they will enable us to rapidly accelerate more effective therapeutic regimens, without having to go through 20 years of patient testing." *