Gene-therapy hope for β-thalassaemia patients

A defective haemoglobin gene has been successfully replaced with a healthy copy.

Joseph Milton

Gene therapy for a form of β-thalassaemia, a genetic disorder whose sufferers require frequent blood transfusions because they cannot properly produce red blood cells, seems to have been successful in a patient who, three years after treatment, no longer requires transfusions¹. Doubts remain, however, over whether a set of lucky circumstances is behind the success.
Patients with β-thalassaemia carry faulty copies of the genes needed to produce the β-globin chain of haemoglobin, sometimes lacking the genes altogether. This leads to a shortage of red blood cells, the body's oxygen carriers.
Sufferers must have regular blood transfusions throughout their lives, an inconvenient and debilitating regime that ultimately shortens life expectancy. The only known cure is stem-cell transplantation, but few patients are able to find a suitable donor.
Because of the gruelling nature of this treatment, the development of gene therapies for β-thalassaemia is seen by many as an exciting prospect. The subject of the latest trial was an 18-year-old man with β^E/β⁰-thalassaemia — in this form of the disease, one copy of the β–globin gene produces unstable β-globin and the other copy is non-functional.
Around half of the patients with this form of β-thalassaemia are dependent on transfusions, and the patient concerned had received blood transfusions since the age of three.
Philippe Leboulch of Harvard Medical School, part of the team that carried out the study, described the treatment as "life-changing". "Before this treatment, the patient had to be transfused every month. Now he has a full-time job as a cook," he says.

Unrepeatable?

However, Michael Antoniou of King's College London, suggests that this case was "an extremely fortuitous event", and that the positive outcome seen is unlikely to be repeatable in other patients.
The procedure was carried out as follows. In 2007, an international team led by Marina Cavazzana-Calvo of University Paris-Descartes extracted haematopoietic stem cells (HSCs) from the patient's bone marrow. These cells give rise to all blood cell types, including the haemoglobin-containing red cells. The researchers cultured these cells, and mixed them with vectors based on the lentiviruses — a retrovirus subgroup with a long incubation period — into which a functional copy of the β-globin gene had been introduced. These vectors were shown in preclinical trials to be safer than those derived from the retroviruses — which are also replicated in a host cell — that have been used in previous gene-therapy procedures.
Chemotherapy was used to eliminate as many of the patient's faulty HSCs as possible, to prevent dilution of the genetically corrected cells, which were then transplanted. Levels of healthy red blood cells and normal β-globin in the subject's body gradually rose until, around a year after the treatment, he no longer required transfusions. After 33 months he remains mildly anaemic, but the fact that he remains transfusion-free has been hailed as a success.
However, that achievement is tempered by a cautionary note. The researchers have detected overexpression of a protein called HMGA2, which has been linked to cancers, in a high proportion of the genetically modified cells.
Overexpression occurred because the lentivirus vector can randomly integrate into chromosomes. By chance, one transplanted haematopoietic cell clone contains a vector insertion in the HMGA2 gene. A year after the transplant, the researchers noticed that the proportion of genetically modified cells that originated from this particular cell clone was rising until it reached a plateau at around 50%.
The reasons for the over-representation of that particular clone remain unclear, but that could be down to the fact that the patient's haematopoietic system was reconstituted from just a few modified HSCs. Luigi Naldini, a gene-therapy researcher at San Raffaele Telethon Institute for Gene Therapy in Milan, Italy, says that successfully grafting a larger initial population of modified HSCs could potentially prevent the problem from developing.
Looking at the haematopoietic system in its entirety, the researchers found that increased levels of HMGA2 were present in only about 5% of the patient's circulating cells, but overexpression of HMGA2 has led to enlargement of the patient's red blood cells. The researchers say that this enlargement caused by the overexpression of HMGA2 could be partly responsible for the therapeutic benefits, but it could also be a signal of future malignancies.
Antoniou suggests that the HMGA2 effect is "key" to the therapeutic effect, and that without the unintended insertion, combined with the patient's ability to produce some β-globin naturally, transfusions would probably still be required.
But Leboulch says that β-globin production from the modified cells was just as high before the cells containing the insertion reached the 50% mark, so that most of the therapeutic effect must be due to the implanted modified cells, rather than the expansion of the blood cells caused by the HMGA2 insertion. And Naldini says that the fact that β-globin expression by the implanted cells is being seen at all represents a major step forward.

Johns Hopkins Children's Center urges new screening program to improve sickle cell trait

The Johns Hopkins Children's Center top pediatrician is urging a "rethink" of a new sickle cell screening program, calling it an enlightened but somewhat rushed step toward improving the health of young people who carry the sickle cell mutation.
Beginning this fall, all Division I college athletes will undergo mandatory screening for the sickle cell trait. The program, rolled out by the National Collegiate Athletic Association (NCAA), is an attempt to prevent rare but often-lethal complications triggered by intense exercise in those who carry the genetic mutation yet don't have the disease.
Nationwide, newborns are screened for sickle cell disease, but carriers, or people with one mutant and one normal sickle cell gene, do not have symptoms of the disease and may be unaware that they are carriers.
While the program's goal is laudable, its implementation has been hasty and its consequences poorly thought out, warns Johns Hopkins Children's Center Director George Dover, M.D., in a Sept. 9 commentary for The New England Journal of Medicine.
The program is expected to affect nearly 170,000 college athletes and identify anywhere between 400 to 500 new cases each year. Carriers of the sickle cell trait are asymptomatic but are at higher risk for infarction of the spleen caused by lack of oxygen supply to the organ and exercise-induced rhabdomyolysis, a condition marked by the rapid breakdown of injured muscle followed by the release of proteins in the bloodstream that harm the kidneys and can lead to kidney failure. Research has shown that the risk of sudden death during exercise is between 10 and 30 percent higher among those who have the sickle cell trait than those without it. The program stems from the 2006 death of a 19-year-old freshman who died after football practice from exercise-induced rhabdomyolysis.
Dover and co-authors Vence Bonhaj, J.D., and Lawrence Brody, Ph.D., of the National Human Genome Research Institute, call the program "an enlightened first step by the NCAA toward improving the health of student athletes," but one rife with pitfalls and raising many questions. Such questions include: "Will any positive test results be followed by a second test to eliminate false positives?" and "Who is responsible for counseling students who test positive in order to explain the difference between actual disease and carrier status and the risks associated with each?"
Dover and his co-authors say that the following stipulations should be included in the program:
• Verifying test result accuracy by follow-up testing to eliminate false positives • Post-test counseling • Measures to prevent discrimination based on positive test results • Making athletic practice safer to reduce or eliminate the risk for death among carriers by instituting proper hydration and avoiding workouts during high humidity and peak heat
Students will be allowed to opt out of screening if they show proof of previous testing or sign a waiver releasing their college of any legal liability. These suggest that the program was designed primarily as a legal defense measure, but its medical, social and psychological consequences remain unaddressed, the authors say.
As the most extensive sickle cell screening program in the past 30 years, this initiative will likely pave the way for other mass screening programs among college athletes, including ones aimed at identifying the carriers of cardiac anomalies, the most common cause of sudden death in athletes.
"The precedent-setting nature of this screening program dictates that we proceed with caution because any subsequent genetic screening programs may be modeled after this prototype," says Dover, a pediatric hematologist and expert on sickle cell disease.
Some 100 million people worldwide and 2 million people in the United States are believed to be carriers of the sickle cell mutation (sickle cell trait) but do not have sickle cell anemia. Named for the unusually sickle-shaped red blood cells caused by an inherited abnormality, sickle cell anemia affects nearly 100,000 Americans, most of them African-American. In sickle cell anemia, the red blood cells become rigid, which reduces their oxygen delivery to vital organs and causes them to get stuck in the blood vessels, leading to severe pain and so-called "sickling crises," which require hospitalization.
Source : Johns Hopkins Children's Center

Promising results in mice could prevent fatal iron buildup in humans

A new study shows that a protein found in blood alleviates anemia, a condition in which the body's tissues don't get enough oxygen from the blood. In this animal study, injections of the protein, known as transferrin, also protected against potentially fatal iron overload in mice with thalassemia, a type of inherited anemia that affects millions of people worldwide.
Implications of the study, published in the January 24 online edition of Nature Medicine, could extend well beyond thalassemia to include other types of anemia including sickle cell anemia and myelodysplastic syndromes (bone marrow disorders that often precede leukemia) if proven in humans. The research was conducted by scientists at Albert Einstein College of Medicine of Yeshiva University.
"People who have thalassemia or other types of anemia need frequent blood transfusions over many years to correct the problem," says Mary E. Fabry, Ph.D., professor of medicine at Einstein and a study author. "But the human body has no way to get rid of the massive amount of iron in the transfused blood, and the resulting iron overload - especially its accumulation in the heart and liver - is often fatal. Our study suggests that treatment with transferrin could prevent this."
It's projected that over the next 20 years, more than 900,000 children with thalassemia will be born each year. Ninety-five percent of thalassemia births are in Asian, Indian, and Middle Eastern regions. However, the U.S. is seeing more cases due to a growing influx of immigrants.
In thalassemia, gene mutations lead to underproduction of the globin protein chains that form hemoglobin, the iron-containing, oxygen-carrying molecule in red blood cells. (Normal hemoglobin consists of four globin protein chains - two alpha chains and two beta chains.) Fewer globin chains mean a shortage of red blood cells, a shorter lifespan for red cells that are produced, and anemia.
Thalassemia is classified as alpha or beta thalassemia, depending on which of the globin protein chains are affected. In a 2009 study involving beta thalassemic mice at Einstein, Dr. Fabry and her colleagues made a paradoxical observation: Despite the rodents' anemia and iron overload, injecting them with more iron improved their anemia by increasing both hemoglobin and the number of red cells.
This finding indicated that "overload" iron wasn't accessible for use in making red cells. And it suggested to Yelena Z. Ginzburg, M.D., a postdoctoral research fellow in Dr. Fabry's lab at the time and a senior author of the present study, that transferrin might be able to tap into that stored iron.
Transferrin is a crucially important protein responsible for transporting iron in the bloodstream and delivering it to cells that need it - particularly the cells that develop into red blood cells. "Yelena [now a researcher at the New York Blood Center in New York City] hypothesized that too little transferrin in the circulation may account for the reduced red cell production and anemia observed in beta thalassemia," says Dr. Fabry. "So she decided to see if injections of transferring - obtainable as a byproduct of blood collection - could help in treating thalassemia."
In the present study, the researchers gave the beta thalassemia mice daily injections of human transferrin for 60 days. The results were impressive.
"The injected transferrin killed three birds with one stone," says Dr. Fabry. "It not only helped in depleting the iron overload that can be so toxic, but it recycled that iron into new red blood cells that ameliorated the anemia. Plus, those red cells survived for a longer time because they had fewer defects."
The Einstein researchers are cautiously optimistic that transferrin could have similar benefits for people.
"Before doing clinical trials, we need to work out a lot of details such as the proper dose of transferrin and the frequency of treatment," says Eric E. Bouhassira, Ph.D., another author of the study who is professor cell biology and of medicine and the Ingeborg and Ira Leon Rennert Professor of Stem Cell Biology and Regenerative Medicine at Einstein. "But transferrin's striking effectiveness in reducing iron overload makes me hopeful that people with anemia could really benefit from it."
The paper, "Transferrin therapy ameliorates disease in beta-thalassemic mice," appears in the January 24 online edition of Nature Medicine.
Source: Albert Einstein College of Medicine