Therapeutic Target Found for Sickle
Studies in Human Cells Identify Regulator of Hemoglobin Switch
Sickle cell anemia and beta-thalassemia syndromes occupy a central
place in biomedical research. Most famously, sickle cell was the first inherited
disease found to be caused by a specific amino acid change. But this has
not meant an easy road to relief for patients: many basic science hurdles
have not budged for decades.
Following a lead from genomewide association studies, Vijay Sankaran (left),
Stuart Orkin, and colleagues have found evidence that the transcription factor
BCL11A directly regulates the long-studied fetal-to-adult hemoglobin switch
and thereby represents a new target for treating sickle cell anemia and beta-thalassemia.
New work from the labs of Stuart
Orkin and others indicates that this may
soon change. Led by Vijay Sankaran, an MD–PhD student in Orkin’s
lab, the team has found evidence that the zinc finger transcription factor
BCL11A downregulates expression of fetal hemoglobin (HbF). Keeping HbF expression “on” is
known to reduce disease severity, so the results point to BCL11A as a new
therapeutic target. The work was published in the Dec. 19 Science.
The researchers were led to BCL11A by genomewide association studies, which
aim to uncover disease-associated variants in a population of unrelated
individuals. “Hundreds of variants have been found to be associated
with various diseases,” said Alan Michelson, associate director for
basic research at the National Heart, Lung, and Blood Institute, who wrote
a commentary accompanying the paper. But rarely have such findings moved “from
hypothesis-generating to an actual pathway.” The current study
provides hope that this new generation of human genetic studies can yield
Sickle cell disease and beta-thalassemia arise from mutations in the beta
subunits of hemoglobin. In sickle cell, misshapen red blood cells tend to
clump and block blood vessels, causing severe pain crises and shortening
patients’ lives. The only FDA-approved drug for sickle cell, hydroxyurea,
exhibits considerable toxicity and is not universally effective. Beta-thalassemia
patients generally rely on lifelong blood transfusions. Some patients have
greatly reduced symptoms, however, because their bodies produce elevated
levels of HbF, which is built from gamma-globin, rather than switching to
the beta-containing, adult form.
“That switch had been known for years, and ever since the genes were
first cloned in the early 1980s, the major focus of the field has been to
try to understand how you regulate the switch,” said Orkin, who is
a Howard Hughes investigator and the David Nathan professor of pediatrics
at HMS and Children’s Hospital Boston; he is also the chair of pediatric
oncology at Dana–Farber Cancer Institute. Understanding the fetal-to-adult
switch could lead to strategies for reactivating HbF expression therapeutically.
The new study is a major advance for basic developmental biology and for
therapeutic aims, said Michelson, who was Orkin’s first MD–PhD
student and formerly an HMS faculty member at Brigham and Women’s Hospital.
It uncovers “an entirely new pathway that nobody knew about before.”
A Wide-angle Lens
Sankaran entered Orkin’s lab with his sights set on the hemoglobin
switch, but both of them were frustrated by the limitations of existing approaches.
As Orkin tells it, “Vijay came in and said he wanted to work on this
problem, and I said, ‘You’re out of your mind.’” But
Sankaran told him he wanted to take a human genetic approach, so Orkin said, “OK,
we’ll see how we do.”
Sankaran contacted Joel Hirschhorn, HMS associate professor of genetics
and the Broad Institute, about trying genomewide association studies. According
to Sankaran, they were lucky to get involved just when researchers were making
serious advances in designing these broad-based studies.
In other labs, several single-nucleotide polymorphisms (SNPs) had been found
to be associated with variations in HbF levels in normal subjects, and in two
studies published earlier this year, the Hirschhorn and Orkin labs found that
these variants were also associated with clinical outcomes in sickle cell and
beta-thalassemia patients. Most interesting was that variants in an intron of
BCL11A were associated with more than 10 percent of the variation in HbF levels
in the patients, who represented several different ethnic groups. This was an
unusually “common, widespread, and powerful association,” Orkin
“Hundreds of variants have been found to be associated with various
diseases.” But rarely have such findings moved “from hypothesis-generating
to an actual pathway.”
So Sankaran, along with colleagues in the Orkin lab and the lab of
Alan Cantor, an HMS assistant professor of pediatrics at Children’s, went to the bench
to figure out how BCL11A might be influencing HbF levels. In primary adult erythroblasts,
dialing down BCL11A using siRNA or shRNA increased gamma-globin expression and
HbF, supporting their hypothesis that BCL11A might be a negative regulator of
HbF. Cell morphology and microarray profiling indicated that knocking down BCL11A
increased HbF not through a general perturbation of cell state but rather through
a limited number of targets. In fact, chromatin immunoprecipitation (ChIP) showed
that BCL11A binds at several spots in the beta-globin gene cluster. “We
were a little surprised that it had such a direct effect,” Sankaran
The researchers found evidence that even though BCL11A acts in complex
with other proteins, it is the component regulated by the fetal-to-adult
switch. As a result, they believe that downregulating BCL11A in patients
could boost their HbF levels and ameliorate symptoms.
The next step is to see whether these results hold true in animals. “People
really want to know, will this work?” Sankaran said. “We feel pretty
confident, because in some ways nature has done an in vivo experiment for us.
We know that humans vary, and we know it affects clinical course.” In
vivo studies would also address concerns that knocking down BCL11A might
affect other cell lineages.
The researchers envision several approaches to therapy: identifying drugs
to lower expression or activity of BCL11A or knocking down BCL11A in bone
marrow by gene therapy and transplanting the marrow into patients.
Meanwhile, Philippe Leboulch, a visiting professor of medicine at Brigham
and Women’s, is pursuing the converse gene therapy strategy: adding
a “good” beta-globin
gene. Perhaps, Leboulch proposed, both strategies could be used together: “Combining
the two would not be a bad approach.”
In the United States, sickle cell afflicts an estimated 70,000 people,
mostly African Americans. Globally, the hemoglobinopathies are some of the
most common morbid genetic diseases, Orkin said, afflicting many different
the bulk are in underdeveloped countries, so they’re not so visible
to us.” But Maude Tessier of the Children’s intellectual property
office, who worked with the researchers to file a patent application, is
optimistic that companies might be able to take up the task of moving toward
treatments targeting BCL11A, both because of FDA incentives for addressing
diseases with “orphan” status
in the United States and because the need is so great internationally.
But for now, it’s back to the lab. The team does not yet understand, for
example, the roles of different versions of BCL11A and thus which ones would
be best to target. “The more we know about the mechanism by which BCL11A
and its variants control globin expression, the better able we’re going
to be to think about how that can be applied,” Orkin said.
Students may contact Stuart Orkin at Stuart_Orkin@dfci.harvard.edu for
more information on this or other lab projects.
Conflict Disclosure: The authors report no conflicts of interest.
Funding Sources: The National Institutes of Health, Howard Hughes Medical
Institute, Leukaemia Research Fund, and Kay Kendall Leukaemia Fund