Gene Editing Study Reveals Possible "Achilles Heel" of Sickle Cell Disease
September 16, 2015
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Children's Cancer and Blood Disorders Center have found that changes to a small
stretch of DNA may circumvent the genetic defect behind sickle cell disease
(SCD). The discovery, published in the journal Nature, creates a path
for developing gene editing approaches for treating SCD and other hemoglobin
disorders, such as thalassemia.
stretch of DNA, called an enhancer, controls the molecular switch BCL11A. This
switch, in turn, determines whether a red blood cell produces the adult form of
hemoglobin—which in SCD is mutated—or a fetal form that is unaffected by and
counteracts the effects of the sickle mutation. Other studies indicate that
sickle cell patients with elevated levels of fetal hemoglobin have a milder
form of the disease.
new Nature study—led by Stuart Orkin, MD, and Daniel Bauer, MD, PhD, of
Dana-Farber/Boston Children's, and Feng Zhang, PhD, of the Broad Institute of
MIT and Harvard—was spurred by the discovery that naturally occurring
beneficial variations in the DNA sequence in this enhancer dial down BCL11A
only in red blood cells.
mimic and improve upon the effects of these variations, the research team used
recently developed CRISPR-based gene editing tools to systematically cut out
tiny sections of DNA step-by-step along the entire length of the enhancer in
blood stem cells from human donors. They then allowed the cells to mature into
red blood cells and found that the amount of fetal hemoglobin the cells
produced had increased substantially. The team’s experiments revealed a
specific location in the enhancer that when cut leads to production of high
levels of fetal hemoglobin. Parallel experiments in an animal model revealed
that removal of this part of the enhancer affected BCL11A's expression only in
red blood cells, not in immune or brain cells, where BCL11A is also active.
These findings show that the effects are restricted to red blood cells, and
that other cell types are unaffected.
was no efficient way of conducting this kind of experiment until now,"
said Bauer, a pediatric hematologist/oncologist at Dana-Farber/Boston
Children's. "Our goal was to break the enhancer, rather than fix the
hemoglobin mutation, but to do so in very precise ways that are only possible
since gene editing technologies like CRISPR became available."
in exploring the potential clinical uses of the BCL11A switch has grown since
Orkin’s laboratory revealed its direct role in the transition from fetal to
adult hemoglobin in Nature in 2009. Another important step came in 2013,
when the journal Science published their report of the discovery of the
enhancer that directs expression of BCL11A only in red blood cells.
now targeted the modifier of the modifier of a disease-causing gene,"
explained Orkin, a leader of Dana-Farber/Boston Children's who serves as
chairman of pediatric oncology at Dana-Farber Cancer Institute and associate
chief of hematology/oncology at Boston Children's Hospital. "It's a very
different approach to treating disease.”
data provide proof of principle that targeted edits to BCL11A's enhancer in
blood stem cells could be an attractive approach for curing SCD and related
experiments may have revealed the genetic Achilles heel of sickle cell
disease," said Orkin. "Alterations to these specific portions of the
enhancer have the same effect as knocking the whole enhancer out altogether,
suggesting that this could be a promising strategy to translate into the
fixing the sickle mutation itself would seem the most straightforward approach,
it turns out that blood stem cells, the ultimate targets for this kind of
therapy, are much more resistant to genetic repair than to genetic
disruption," Bauer added. "Therefore, making a single DNA cut that
breaks the enhancer solely in blood stem cells could be a much more feasible
and Bauer are also affiliated with Harvard Medical School and the Harvard Stem
Cell Institute; Orkin is an investigator with the Howard Hughes Medical
Institute. Other members of the research team include lead authors Matthew C.
Canver, Elenoe C. Smith, Falak Sher, Luca Pinello and Neville E. Sanjana, as
well as co-authors Ophir Shalem, Diane D. Chen, Patrick G. Schupp, Divya S.
Vinjamur, Sara P. Garcia, Sidinh Luc, Ryo Kirita, Yukio Nakamura, Yuko
Fujiwara, Takahiro Maeda and Guo-Cheng Yuan.
This study was supported by the National
Institute of Diabetes and Digestive and Kidney Diseases (grant numbers
F30DK103359, R01DK097768, P30DK049216, K08DK093705,), the National Human Genome
Research Institute (grant numbers K99HG008399, K99HG008171), the National
Institute of Allergy and Infectious Diseases (grant number R01A1084905), the
National Heart, Lung and Blood Institute (grant number R01HL119099,
P01HL032262), the National Institute of Mental Health (grant number
DP1MH100706), the National Science Foundation, Jane Coffin Childs Memorial Fund
for Medical Research, the Doris Duke Charitable Foundation, the Charles H. Hood
Foundation, the Keck Foundation, the Klarman Family Foundation, the Leukemia
and Lymphoma Society, the Merkin Foundation, the McKnight Foundation, the Damon
Runyon Foundation, the Searle Scholars Foundation, the Simons Foundation, the
Vallee Foundation and Bob Metcalfe.