Dec. 6, 2010 Red blood cells (RBCs) have the very important role of carrying oxygen from the lungs to the rest of the body; therefore, disorders that affect RBCs can have a significant impact on quality of life. Sickle cell disease and malaria are two common RBC diseases that affect a significant portion of the U.S. and global populations. Sickle cell disease (SCD), a genetic blood disorder that causes deformed and dysfunctional red blood cells, affects an estimated 70,000 to 100,000 Americans, while malaria affects approximately 1,500 Americans annually and 190 to 311 million people worldwide. The red blood cells in sickle cell disease become rigid and form in a crescent "sickle" shape, which causes them to clump and become embedded in the blood vessels of organs, resulting in pain, infection, potential organ damage, and even death.
Research presented at the 52nd Annual Meeting of the American Society of Hematology highlights studies on transfusion therapy alternatives and genetic predictors for sickle cell-related stroke in children, the use of hydroxyurea therapy to reduce mortality in pediatric SCD patients, and a potential breakthrough that may inhibit the transmission of malaria-causing viruses.
"Research that looks for innovative ways to predict stroke risk and provides evidence for effective intervention for children affected with sickle cell disease will always be important, as this disease affects a significant number of children," said Alexis Thompson, MD, MPH, moderator of the press conference, Director of Hematology Services at Children's Memorial Hospital, and Associate Professor of Pediatrics at Northwestern University Feinberg School of Medicine, Chicago. "Furthermore, it is very exciting to know that we are closing in on the day when we will be able to control the transmission of malaria."
Validation of Genetic Predictors for Stroke in Children With Sickle Cell Anemia
Approximately 11 percent of patients with sickle cell anemia (SCA) suffer a stroke before the age of 20. Strokes in children not only cause physical disabilities, but can greatly impair cognitive function. The study investigators sought to identify genetic biomarkers, naturally occurring DNA variations, that could serve to predict which children with SCA are at the greatest risk of developing a stroke. Although genetic polymorphisms that may predict stroke risk have been previously identified, few have been validated, mainly because of a lack of a significant number of observable patients. This study aimed to validate previously described DNA biomarkers and identify new predictors with increased accuracy by studying a large patient population.
The investigators studied 130 pediatric patients from the Stroke With Transfusions Changing to Hydroxyurea (SWiTCH) study with SCA and documented primary stroke. They analyzed 103 pediatric SCA patients without previous clinical stroke prior to beginning hydroxyurea therapy from the Hydroxyurea Study of Long-Term Effects (HUSTLE) study as a control non-stroke group. To determine which biomarkers are associated with stroke risk, the investigators identified the genetic makeup of patiets in each group using 38 polymorpisms (DNA variations) with published associations for stroke risk, alpha-thalassemia trait (carriers for mutation in a subunit of the iron-containing hemoglobin protein in the blood that carries oxygen from the lungs to the tissues of the body), G6PD deficiency (an inherited condition in which the body does not produce the enzyme glucose-6-phosphate dehydrogenase, which helps RBCs function normally), and the beta-globin haplotype (a set of closely linked polymorphisms in the beta-globin gene that may contribute to the variable in severity of the condition).
Results from the study show that only five of the 38 DNA variations were significantly associated with stroke risk, and as shown in previous studies, inheritance of the alpha-thalassemia trait was also associated with stroke risk. In contrast, neither G6PD deficiency nor any of the classical beta-globin haplotypes were associated with stroke risk. Overall, these findings highlight that there is a genetic component to stroke development in SCA patients and emphasize the importance of validating which genetic polymorphisms have true associations with stroke risk.
"This is an important contribution to the research that surrounds pediatric sickle cell anemia," said Jonathan M. Flanagan, PhD, senior study author and Staff Scientist at the Department of Hematology at St. Jude Children's Research Hospital in Memphis, TN. "If we can predict which children are at greatest risk for developing stroke by analyzing the genes of our patients, it would dramatically improve our ability to be proactive and prevent stroke via clinical treatment."
Hydroxyurea Therapy Reduces Mortality Among Children With Sickle Cell Disease
Patients with sickle cell disease (SCD) often experience severe complications, including painful events caused by a lack of oxygen in the tissues due to blocked blood supply, severe anemia when the red blood count is extremely low, and acute chest syndrome (ACS), a life-threatening lung infection that can reduce the oxygen level in the blood. Although hydroxyurea was the first effective drug approved for adults with SCD-related complications and remains the standard of care for adult sickle cell patients, no data exist concerning its benefit in reducing mortality among children with SCD. Although death among this patient population is low in the United States, most developing countries still experience high mortality rates due to complications related to SCD.
Investigators from the Instituto Estadual de Hematologia Arthur de Siqueira Cavalcanti (HEMORIO) in Rio de Janeiro, Brazil, collected morbidity and mortality data for the first 10 years of the institution's hydroxyurea therapy program and compared clinical and survival outcomes among hydroxyurea-treated and untreated children. In Brazil, hydroxyurea is offered free of charge to patients with SCD who are three years of age or older and who have had two or more ACS events, three or more painful events in the previous year, or an overt stroke, and for whom transfusion therapy is not clinically recommended or is refused by the patient's family. Since 2000, the sickle cell program at HEMORIO has provided hydroxyurea to children with SCD who meet these criteria established by the Brazilian Ministry of Health.
Researchers at HEMORIO have followed 1,189 children with SCD between the ages of 3 and 18 years over the past 10 years. Of these, 224 patients met criteria and were treated with hydroxyurea. Among those treated with hydroxyurea, the incidence of complications from SCD significantly decreased in the first 12 months. Scientists observed a significant reduction in hospitalization (67.9 percent), ER visits (48.7 percent), and transfusions (36.3 percent) during hydroxyurea treatment. A total of 46 deaths occurred in this group: two among hydroxyurea-treated and 44 in non-hydroxyurea-treated children. The overall survival was 70.1 percent. At 10 years of age, the survival was 99.4 percent for treated children versus 97.4 percent for untreated children, and at 17.9 years of age, the difference in survival between the two groups increased and was 97.4 percent among treated versus 66.3 percent for untreated children. Untreated children were found to be 4.6 times more likely to die than those treated with hydroxyurea.
"These data demonstrate that hydroxyurea appears to be effective in reducing the incidence of acute events, such as hospitalization and transfusions, in children with sickle cell disease, but most importantly, it can prevent death," said Clarisse Lobo, MD, senior study author and Medical Director at the Instituto Estadual de Hematologia Arthur de Siqueira Cavalcanti. "Although these data were based on patients in Brazil, hematologists across the globe can use these results as they approach the treatment of their pediatric patients who suffer from sickle cell disease."
Stroke With Transfusions Changing to Hydroxyurea (SWiTCH): A Phase 3 Randomized Clinical Trial for Treatment of Children With Sickle Cell Anemia, Previous Stroke, and Iron Overload
Stroke occurs in 5-10 percent of children with sickle cell anemia (SCA), and has a 50-90 percent chance of recurrence without therapy. To prevent recurrent stroke, chronic monthly red blood cell transfusions are administered; however, their long-term use is problematic since they tend to cause serious side effects, such as alloimmunization and iron overload. Alloimmunization is a complication that occurs when the body's immune system attacks transfused blood because it sees it as foreign substance. Iron overload is a side effect of frequent blood transfusions in which the body accumulates abnormally high levels of iron, which can deposit in the liver and heart and cause life-threatening damage. Because of the risks of long-term blood transfusions, there is a need for an alternative form of therapy for the prevention of stroke, ideally one that improves the management of transfusional iron overload.
Stroke With Transfusions Changing to Hydroxyurea (SWiTCH) is a phase III multicenter study sponsored by the National Heart, Lung, and Blood Institute (NHLBI) that compared 30 months of alternative hydroxyurea therapy (a treatment to prevent recurrent stroke) and therapeutic phlebotomy -- (blood removal to treat iron overload) with standard treatment of transfusions and chelation, an oral therapy that removes excess iron from the body. The SWiTCH investigators predicted that a slightly increased number of recurrent stroke events would occur in the alternative treatment arm compared with the standard treatment group; however, this stroke risk would be balanced by an improved management of iron overload by repeated phlebotomy.
Between October 2006 and April 2009, 161 pediatric patients with SCA, documented stroke, and iron overload (average 15.5±10.1 mg Fe per gm dry weight liver at study entry), were enrolled in the SWiTCH study; 133 of these patients were randomized to receive treatment (67 received the alternative therapy and 66 received standard therapy). Before study enrollment, 12 percent of the patients had already suffered recurrent stroke. In May 2010, following a scheduled interim data analysis and review by the NHBLI-appointed Data and Safety Monitoring Board, the SWiTCH study was terminated by NHLBI for both safety and futility concerns, and specifically because the study's primary endpoints could not be reached. The liver iron concentration was not lower in the alternative treatment arm, hence no increased stroke risk between treatment arms could be justified and the potential risks of continuing the study treatments were no longer warranted. At the time of study termination, recurrent stroke events had occurred in seven patients in the alternative treatment arm. Transient ischemic attack events (brief stroke-like episodes) occurred in six of the alternative therapy patients and nine of the standard therapy patients; one death occurred in both groups.
"SWiTCH was the first study to directly compare transfusions with hydroxyurea for the treatment of sickle cell anemia," said lead study author Russell E. Ware, MD, PhD, Chair of the Department of Hematology at St. Jude Children's Research Hospital in Memphis, TN. "Although the study was terminated, the results will be helpful for other hematology researchers and clinicians who focus on this disease and should help the design of other clinical trials that include hydroxyurea for the treatment of cerebrovascular complications and stroke risk in children with SCA."
Anopheline Orthologs of the Human Erythroid Heme Exporter, FLVCR, Export Heme: Potential Targets to Inhibit Plasmodium Transmission
Heme, as part of hemoglobin, binds and transports oxygen in red blood cells. An excessive amount of free heme is toxic to cells by serving as a source of reactive oxygen species (ROS), which are molecules and ions of oxygen with unpaired electrons that are extremely reactive and cause oxidative damage to cell lipids, proteins, and DNA. Thus, tight regulation of cellular heme levels is necessary. Feline leukemia virus subgroup C receptor (FLVCR), a cell-surface transport protein that exports cellular heme, helps protect cells from heme toxicity.
In a single meal, a female mosquito ingests 2-3 times its weight in blood, resulting in large quantities of heme being released into the gut as the blood is broken down. Plasmodium, which is the parasite that causes malaria, will reproduce in the mosquito gut if ingested through infected blood. Fertilized parasitic egg cells (known as Plasmodium ookinetes) cross the lining of the mosquito gut to develop into Plasmodium cysts, or oocysts. After maturing for 1-3 weeks, these oocysts rupture, each releasing thousands of parasites that travel to the salivary gland, allowing the mosquito to transmit malaria when it bites another human. Studies demonstrate that mosquitoes with increased mid-gut oxidative stress, the oxidative damage caused by ROS, are resistant to Plasmodium transmission. Researchers from the University of Illinois at Chicago and Loyola University Chicago hypothesized that modulation of the heme export function of FLVCR would increase mosquito mid-gut oxidative stress, potentially impacting Plasmodium transmission at a crucial point in its life cycle.
The researchers isolated the genes encoding mosquito FLVCR proteins from two prevalent malaria-transmitting mosquitoes, Anopheles gambiae and Anopheles stephensi. They then verified that these proteins export heme and can protect cells from excess heme-induced oxidative stress. The expression of FLVCR in mosquitoes before and after a blood meal was also compared and, as expected, the heme export protein was highly expressed on the surface of gut cells following exposure to heme. Using gene silencing techniques, the researchers were able to significantly reduce mosquito gut FLVCR expression. In addition, they developed an antibody that may inhibit FLVCR heme export function. In future studies, they will test whether modulation of FLVCR function affects mosquito gut oxidative stress and most importantly, whether it impacts transmission of the malaria parasite. The research described is supported by a mentored career-development KO8 award from the National Heart, Lung, and Blood Institute of the National Institutes of Health.
"This study is still ongoing, but we are hopeful that inhibiting the function of this protein may have an impact on Plasmodium transmission without affecting mosquito survival. In that case, mosquito FLVCR proteins could serve as potential vaccine targets in the fight to control malaria," said John G. Quigley, MD, senior study author and Assistant Professor in the Department of Medicine, University of Illinois at Chicago.
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