The pain can begin suddenly. In your arms or legs, chest or stomach. Sharp or dull, throbbing or stabbing. It can last for days. Acetaminophen is not going to help; you’ll need to visit the ER.
This kind of pain crisis is common for the seventy-two thousand people in the United States — mainly African Americans — affected by sickle cell disease. Other symptoms include a shortage of red blood cells, serious infections, and damage to vital organs. At present there is no cure. Doctors know that a pain crisis happens when red blood cells become stuck in small blood vessels. But they don’t know what triggers it or how to prevent it.
That’s what Leslie Parise, professor and vice-chair of pharmacology, and her lab have spent the last fifteen years trying to understand. What they’ve found has changed the way doctors think about what causes a pain crisis, and has opened the way for some much-needed new therapies.
Back in 1989, Parise wasn’t researching sickle cell disease. Not because it wasn’t important, but her focus was cell adhesion — how cells stick — and her main passion was platelets. But two things happened that brought sickle cell research into her lab.
First, Chris Joneckis, a graduate student looking for a research project, approached Parise. “Chris was very interested in adhesion,” Parise says, “but not so interested in platelets.”
Secondly, Parise attended a conference of cell biologists and heard a talk on the potential of sickle cell adhesion. “It was an area that wasn’t very well understood,” she says.
Until then, sickle cell disease had been thought of as a mechanical problem. We knew that sickle cell was a genetic disease and that the problem lay in hemoglobin — the protein in red blood cells that carries oxygen around our bodies. People with the sickle gene have a structural defect in their hemoglobin, so the red blood cells are deformed and can’t carry oxygen efficiently. These cells are sickle shaped, more rigid, and, we believed, this caused them to get stuck more easily in small blood vessels.
Parise was intrigued by an idea that she had heard at the conference: sickle cells might be stickier than normal red blood cells. This theory suggested another way that cells might get stuck in capillaries, but more importantly, if true, it provided a target for drugs. So she returned to Chapel Hill and told Joneckis, who was excited by the idea and started to look at red blood cells’ adhesiveness.
Joneckis tested whether sickle cells had different adhesion molecules on their surface than did normal red blood cells. They did, and he published his findings in the December 1993 issue of Blood. For the first time, Parise’s group had shown that red blood cells don’t just happen to become lodged on blood vessel walls — they have the active ability to stick there.
Joneckis moved on and the research lay dormant until Sheritha Lee, a master’s student from North Carolina Central University, approached Parise. Lee was personally interested in sickle cell; as an African American her family had first-hand experience with the disease. “I was thrilled to find a project I could be passionate about,” Lee says, “to think that I could uncover information that might somehow be beneficial to members of my family.” Her research confirmed that sickle cells adhere to laminin, a blood vessel wall protein, and she narrowed down exactly where on the laminin molecule the sickle cells were sticking. Lee published her research in the October issue of Blood in 1998.
Then two new researchers joined the lab — Patrick Hines, an MD/PhD student, and Julie Brittain, a doctoral student. Both were interested in sickle cell research, particularly in what factors triggered a pain crisis.
Brittain’s research demonstrated that the sickle cells became stickier because of proteins that float around naturally in plasma. “This was a huge breakthrough in the sickle cell field because these cells had not been previously thought of as cells that could respond to become more adhesive,” Parise says. Brittain published her findings in the June 2001 issue of The Journal of Clinical Investigation.
After reading medical literature that noted a relationship between stress and the tendency for sickle cell patients to have a pain crisis, Hines took a slightly different approach. He knew that when stressed, our adrenaline or epinephrine levels rise, preparing us for fight or flight. So Hines investigated what epinephrine did to sickle cells. He found that it increased sickle cell activation of a protein called BCAM/Lu. It also increased BCAM/Lu adhesion to laminin, the adhesive protein Lee previously studied.
So the combination of stickier cells and stickier blood vessel walls explained why sickle cell patients were more likely to have red blood cells lodged in their vessels. When the researchers published their findings in the April 2003 issue of Blood, it was the first time anyone had biologically linked stress to a pain crisis.
Though Parise’s group may not have started out in sickle cell research, it is now a topic of major interest in the laboratory. Brittain, now a research assistant professor, describes herself as committed to sickle cell disease. “I can’t work on anything else,” Brittain says. “To know that these red blood cells are actually flowing through peoples’ veins breaks my heart and makes me angry.”
But Brittain’s research in sickle cell disease has also changed some fundamental beliefs about red blood cells. “For a century, everybody thought red blood cells were inert bags of hemoglobin that just circulate around. The notion that red blood cells could react was just not talked about,” she says.
Brittain’s continuing work in sickle cell disease has moved away from red blood cells and into white blood cells. “I’m a red blood cell researcher,” Brittain says. “Love ’em, can’t get enough of them. But there are other cells in blood.” White blood cells are part of the immune system, and there is no doubting the reactivity of these cells. In fact, it’s their job to react to infection.
Typically, sickle cell patients have increased white blood cell counts, and this is the single best indicator of disease severity and life span. “It’s not how many sickle cells you have, or your hemoglobin level; it’s how many white blood cells you have,” Brittain says.
Brittain’s research is now focused on one type of white blood cell called the monocyte. High monocyte counts seem to have the highest correlation with increased pain crisis. Most recently, Brittain has shown that red blood cells stick to monocytes, and that platelets stick as well. “So what you have is an aggregate of different cells that speeds through the blood of the sickle cell patient,” she says. These aggregates are another way that blood vessels clog.
It’s become increasingly clear to Brittain that sickle cell disease is a chronic state of inflammation and increased blood coagulation. “Sickle cell disease has gone from a hemoglobin problem, to a blood problem, to a blood vessel problem, and now we look at all those things,” says Brittain.
Sheritha Lee returned to Parise’s lab in 2001 as a doctoral student to investigate the role of inflammation in sickle cell disease. She was particularly interested in a protein called CD40L. “It’s been shown to cause chronic inflammation in many disease states,” Lee says. “So I measured CD40L in sickle cell plasma and found that patients had really high levels.” CD40L is normally found inside platelets, and Lee noted that in sickle cell patients the platelet CD40L level was half the level of normal. “It suggested that the protein was being relocated from its usual protected store in platelets,” Lee says. And once the CD40L got into the plasma, it could do real damage.
Apart from understanding the disease better, Brittain and Lee also want to be able to offer patients more treatment options.
Currently there is only one treatment for sickle cell disease — a drug called hydroxyurea, or HU. “HU was originally discovered as a cancer drug,” Brittain explains, “and was used for its bone marrow-destroying properties.” But an interesting side-effect of HU brought it to the attention of sickle cell physicians: HU causes fetal hemoglobin to be re-expressed in adults.
Fetal hemoglobin, a type of hemoglobin found only in fetuses, has a much higher affinity for oxygen. “It’s a neat evolutionary thing,” Brittain says, “that because of the fetal hemoglobin, the fetus is guaranteed to get oxygen before the mom.” Around six months after birth, the fetal hemoglobin shuts off and adult hemoglobin takes over. “It’s the adult hemoglobin that’s mutated in sickle cell disease; fetal hemoglobin is unaffected,” Brittain says.
Cancer research receives much more funding than sickle cell disease does. So the doctors “borrowed” HU from cancer studies, and it’s provided relief to a lot of sickle cell patients. Because HU causes fetal hemoglobin to be produced again in an adult, red blood cells are less prone to sickling. “But it’s less than perfect,” Brittain says. “It increases the risk of cancer and it causes fetal abnormalities, so if a woman gets pregnant, she can’t take the drug.”
Parise lab members are now investigating new treatments based on their research. But before they can try out drugs in people, they need a good model of the disease. There are mice that have been genetically engineered with a hemoglobin deficiency, and they develop many of the characteristics of sickle cell disease. Lee and others in the Parise lab intend to use the mice to develop new treatments. “We hope to treat the mice with antibodies or drugs to lessen the disease pathology,” Lee says, “and early results are promising.”
“It’s an exciting time to be involved in sickle cell research, because people are looking at the disease in a different way,” Brittain says. But for Brittain, and for the rest of the Parise lab, it’s more than a research project. Brittain feels a personal responsibility for the patients with the disease. “When you’re a sickle cell researcher, you can’t just go into the lab and work, work, work,” she says. “You must be an advocate for these patients and ensure that all of them have a voice.”