bone marrow transplant
Posted on by Dr. Francis Collins
Forty-five years ago, when I was a first-year medical student, a lecturer introduced me to a young man with sickle cell disease (SCD). Sickle cell disease is the first “molecular disease”, with its cause having been identified decades ago. That helped me see the connection between the abstract concepts of molecular genetics and their real-world human consequences in a way no textbook could. In fact, it inspired some of my earliest research on human hemoglobin disorders, which I conducted as a postdoctoral fellow.
Today, I’m heartened to report that, thanks to decades of biomedical advances, we stand on the verge of a cure for SCD. While at the American Society of Hematology meeting in San Diego last week, I was excited to be part of a discussion about how the tools and technologies arising from the Human Genome Project are accelerating the quest for cures.
The good news at the meeting included some promising, early results from human clinical trials of SCD gene therapies, including new data from the NIH Clinical Center. Researchers also presented very encouraging pre-clinical work on how gene-editing technologies, such as CRISPR, can be used in ways that may open the door to curing everyone with SCD. In fact, just days before the meeting, the first clinical trial for a CRISPR approach to SCD opened.
One important note: the gene editing research aimed at curing SCD is being done in non-reproductive (somatic) cells. The NIH does not support the use of gene editing technologies in human embryos (germline). I recently reiterated our opposition to germline gene editing, in response to an unethical experiment by a researcher in China who claims to have used CRISPR editing on embryos to produce twin girls resistant to HIV.
SCD affects approximately 100,000 people in the United States, and another 20 million worldwide, mostly in developing nations. This inherited, potentially life-threatening disorder is caused by a specific point mutation in a gene that codes for the beta chain of hemoglobin, a molecule found in red blood cells that deliver oxygen throughout the body. In people with SCD, the mutant hemoglobin forms insoluble aggregates when de-oxygenated. As a result the red cells assume a sickle shape, rather than the usual donut shape. These sickled cells clump together and stick in small blood vessels, resulting in severe pain, blood cell destruction, anemia, stroke, pulmonary hypertension, organ failure, and much too often, early death.
The need for a widespread cure for SCD is great. Since 1998, doctors have used a drug called hydroxyurea to reduce symptoms, but it can cause serious side effects and increase the risk of certain cancers. Blood transfusions can also ease symptoms in certain instances, but they too come with risks and complications. At the present time, the only way to cure SCD is a bone marrow transplant. However, transplants are not an option for many patients due to lack of matched marrow donors.
The good news is that novel genetic approaches have raised hopes of a widespread cure for SCD, possibly even within five to 10 years. So, in September, NIH’s National Heart, Lung, and Blood Institute launched the Cure Sickle Cell Initiative to accelerate development of the most promising of these next generation of therapies
At the ASH meeting, that first wave of this progress was evident. A team led by NHLBI’s John Tisdale, in collaboration with Bluebird bio, Cambridge, MA, was among the groups that presented impressive early results from human clinical trials testing novel gene replacement therapies for SCD. In the NIH trial, researchers removed blood precursor cells, called hematopoietic stem cells (HSCs), from a patient’s own bone marrow or bloodstream and used a harmless virus to insert a sickle-resistant hemoglobin gene. Then, after a chemotherapy infusion to condition the patient’s existing bone marrow, they returned the corrected cells to the patient.
So far, nine SCD patients have received the most advanced form of the experimental gene therapy, and Tisdale presented data on those who were farthest out from treatment [1,2]. His team found that in the four patients who were at least six months out, levels of gene therapy-derived hemoglobin were found to equal or exceed their levels of SCD hemoglobin.
Very cool science, but what does this mean for SCD patients’ health and well-being? Well, none of the gene therapy trial participants have required a blood transfusion during the follow-up period. In addition, improvements were seen in their hemoglobin levels and key markers of blood-cell destruction (total bilirubin concentration, lactate dehydrogenase, and reticulocyte counts) compared to baseline. Most importantly, in the years leading up to the clinical trial, all of the participants had experienced frequent painful sickle crises, in which sickled cells blocked their blood vessels. No such episodes were reported among the participants in the months after they received the gene therapy.
Researchers did report that one patient receiving this form of gene therapy developed myelodysplastic syndrome (MDS), a serious condition in which the blood-forming cells in the bone marrow become abnormal. However, there is no indication that the gene replacement technology itself caused the problem, and MDS has previously been linked to the chemotherapy drugs used in conditioning regimens before bone marrow transplants.
The NIH trial is just one of several clinical trials for SCD that are using viral vectors to deliver a variety of genes with therapeutic potential. Other trials actively recruiting are led by researchers at Boston Children’s Hospital, Cincinnati Children’s Hospital, and the University of California, Los Angeles.
While it’s hoped that genes inserted by viral vectors will provide long-lasting or curative treatment, other researchers are betting that new gene-editing technologies, such as CRISPR, will offer the best chance for developing a widespread cure for SCD. One strategy being eyed by these “gene editors” is to correct the SCD mutation, replacing it with a normal gene. Another strategy involves knocking out certain DNA sequences to reactivate production of fetal hemoglobin (HbF).
The HbF protein is produced in the developing fetus to give it better access to oxygen from the mother’s bloodstream. But shortly after birth, the production of fetal hemoglobin shuts down, and the adult form kicks in. Adults normally have very low levels of fetal hemoglobin, which makes sense. However, from genome-wide association studies of human genetic variation, we know that that actual levels of HbF are under genetic control.
A major factor has been mapped to the BCL11A gene, which has subsequently been found to be a master mediator for the fetal to adult hemoglobin switch. Specifically, variations in a red cell specific enhancer of BCL11A affect an adult’s level of HbF— levels of BCL11A protein lead to higher amounts of fetal hemoglobin. Furthermore, it’s been known for some time that rare individuals keep on producing relatively high levels of hemoglobin into adulthood. If people with SCD happen to have a rare mutation that keeps fetal hemoglobin production active in adulthood (the first of these was found as part of my postdoctoral research), their SCD symptoms are much less severe.
Currently, two groups—CRISPR Therapeutics/Vertex Pharmaceuticals and Sangamo Therapeutics/Bioverativ—are gearing up to begin the first U.S. human clinical trials of gene-editing for SCD within the next few months. While they employ different technologies, both approaches involve removing a patient’s HSCs, using gene editing to knock out the BCL11A red cell enhancer, and then returning the gene-edited cells to the patient. The hope is that the gene-edited cells will greatly boost fetal hemoglobin production, thereby offsetting the effects of SCD.
All of this is exciting news for the 100,000 people living in the United States who have SCD. But what about the 300,000 babies born with SCD every year in other parts of the world, mostly in low- and middle-income countries?
The complicated, high-tech procedures that I just described may not be practical for a very long time in places like sub-Saharan Africa. That’s one reason why NIH recently launched a new effort to speed the development of safe, effective genome-editing approaches that could be delivered directly into a patient’s body (in vivo), perhaps by infusion of the CRISPR gene editing apparatus. Recent preclinical experiments demonstrating the promise of in vivo gene editing for Duchenne muscular dystrophy make me optimistic that NIH’s Somatic Cell Genome Editing Program, which is hosting its first gathering of investigators this week, will be able to develop similar approaches for SCD and many other conditions.
While moving forward in this fast-paced field, it is important that we remain ethical, but also remain bold on behalf of the millions of patients with genetic diseases who are still waiting for a cure. We must continue to assess and address the very serious ethical concerns raised by germline gene editing of human embryos, which will irreversibly alter the DNA instruction book of future children and affect future generations. I continue to argue that we are not ready to undertake such experiments.
But the use of gene editing to treat, perhaps even to cure, children and adults with genetic diseases, by correcting the mutation in their relevant tissues (so-called somatic cell gene editing), without risk of passing those changes on to a future generation, holds enormous promise. Somatic cell gene editing is associated with ethical issues that are much more in line with decades of deep thinking about benefits and risks of therapeutic trials.
Finally, we must recognize that somatic cell gene editing is a profoundly promising approach not only for people with SCD, but for all who are struggling with the thousands of diseases that still have no treatments or cures. Real hope for cures has never been greater.
 NIH researcher presents encouraging results for gene therapy for severe sickle cell disease. NIH News Release. December 4, 2018
Sickle Cell Disease (National Heart, Lung, and Blood Institute/NIH)
Cure Sickle Cell Initiative (NHLBI)
John Tisdale (NHLBI)
Somatic Cell Genome Editing Program (Common Fund/NIH)
NIH Support: National Heart, Lung, and Blood Institute; Common Fund
Posted on by Dr. Francis Collins
Each time your cells divide, telomeres—complexes of specialized DNA sequences, RNA, and protein that protect the tips of your chromosomes—shorten just a bit. And, as the video shows, that shortening renders the genomic information on your chromosomes more vulnerable to changes that can drive cancer and other diseases of aging.
Consequently, over the last few decades, much research has focused on efforts to understand telomerase, a naturally occurring enzyme that helps to replace the bits of telomere lost during cell division. But there’s been a major hitch: until recently, scientists hadn’t been able to determine telomerase’s molecular structure in detail—a key step in figuring out exactly how the enzyme works. Now, thanks to better purification methods and an exciting technology called cryo-electron microscopy (cryo-EM), NIH-funded researchers and their colleagues have risen to the challenge to produce the most detailed view yet of human telomerase in its active form .
This structural biology advance is a critical step toward learning more about the role of telomerase in cancers, as well as genetic conditions linked to telomerase deficiencies. It’s also an important milestone in the quest for drugs targeting telomerase in different ways, perhaps to slow the growth of cancerous cells or to boost the proliferative capacity of life-giving adult stem cells.
One reason telomerase has been so difficult to study in humans is that the enzyme isn’t produced at detectable levels in the vast majority of our cells. To get around this problem, the team led by Eva Nogales and Kathleen Collins at the University of California, Berkeley, first coaxed human cells in the lab to produce larger quantities of active telomerase. They then used fluorescent microscopy, along with extensive knowledge of the enzyme’s biochemistry, to develop a multi-step purification process that yielded relatively homogenous samples of active telomerase.
The new study is also yet another remarkable example of how cryo-EM microscopy has opened up new realms of scientific possibility. That’s because, in comparison to other methods, cryo-EM enables researchers to solve complex macromolecular structures even when only tiny amounts of material are available. It can also produce detailed images of molecules, like telomerase, that are extremely flexible and hard to keep still while taking a picture of their structure.
As described in Nature, the researchers used cryo-EM to capture the structure of human telomerase in unprecedented detail. Their images reveal two lobes, held together by a flexible RNA tether. One of those lobes contains the highly specialized core enzyme. It uses an internal RNA template as a guide to make the repetitive, telomeric DNA that’s added at the tips of chromosomes. The second lobe, consisting of a complex of RNA and RNA-binding proteins, plays important roles in keeping the complex stable and properly in place.
This new, more-detailed view helps to explain how mutations in particular genes may lead to telomerase-related health conditions, including bone marrow failure, as well as certain forms of anemia and pulmonary fibrosis. For example, it reveals that a genetic defect known to cause bone marrow failure affects an essential protein in a spot that’s especially critical for telomerase’s proper conformation and function.
This advance will also be a big help for designing therapies that encourage telomerase activity. For example, it could help to boost the success of bone marrow transplants by rejuvenating adult stem cells. It might also be possible to reinforce the immune systems of people with HIV infections. While telomerase-targeted treatments surely won’t stop people from growing old, new insights into this important enzyme will help to understand aging better, including why some people appear to age faster than others.
As remarkable as these new images are, the researchers aren’t yet satisfied. They’ll continue to refine them down to the minutest structural details. They say they’d also like to use cryo-EM to understand better how the complex attaches to chromosomes to extend telomeres. Each new advance in the level of atomic detail will not only make for amazing new videos, it will help to advance understanding of human biology in health, aging, and disease.
 Cryo-EM structure of substrate-bound human telomerase holoenzyme. Nguyen THD, Tam J, Wu RA, Greber BJ, Toso D, Nogales E, Collins K. Nature. 2018 April 25. [Epub ahead of publication]
High Resolution Electron Microscopy (National Cancer Institute/NIH)
Nogales Lab (University of California, Berkeley)
Collins Lab (University of California, Berkeley)
NIH Support: National Institute of General Medical Sciences
Posted on by Dr. Francis Collins
In certain people with cancer or other serious diseases, transplants of healthy adult stem cells can be lifesaving. But donating blood-forming stem cells is a bit more complicated than giving blood. For example, stem-cell donors most often undergo five days of injections to build up enough of those vital cells in the blood for donation.
Wouldn’t it be great if we could find a way to make the donation process easier? Such improvements are now on the horizon.NIH-funded researchers recently found that, at least in mice, a single injection of two complementary treatments can generate enough stem cells in 15 minutes . What’s more, stem cells harvested in this way have qualities that appear to increase the odds of transplant success.
Posted on by Dr. Francis Collins
As a practicing dermatologist, Sherrie Divito sees lots of patients each week at Brigham and Women’s Hospital, Boston. She also sees lots of research opportunities. One that grabbed her attention is graft-versus-host disease (GvHD), which can arise after a bone-marrow transplant for leukemia, lymphoma, or various other diseases. What happens is immune cells in the donated marrow recognize a transplant patient’s body as “foreign” and launch an attack. Skin is often attacked first, producing a severe rash that is a harbinger of complications to come in other parts of the body.
But Divito saw something else: it’s virtually impossible to distinguish between an acute GvHD-caused rash and a severe skin reaction to drugs, from amoxicillin to carbamazepine. In her GvHD studies, Divito had been researching a recently identified class of immune cell called tissue-resident memory T (Trm) cells. They remain in skin rather than circulating in the bloodstream. The clinical similarities made Divito wonder whether Trm cells may also help to drive severe skin allergies to drugs.
Divito has received a 2016 NIH Director’s Early Independence Award to find out. If correct, Divito will help not only to improve the lives of thousands of people with GvHD, but potentially benefit the millions of other folks who experience adverse reactions to drug.
Posted on by Dr. Francis Collins
Bone marrow transplants offer a way to cure leukemia, sickle cell disease, and a variety of other life-threatening blood disorders.There are two major problems, however: One is many patients don’t have a well-matched donor to provide the marrow needed to reconstitute their blood with healthy cells. Another is even with a well-matched donor, rejection or graft versus host disease can occur, and lifelong immunosuppression may be needed.
A much more powerful option would be to develop a means for every patient to serve as their own bone marrow donor. To address this challenge, researchers have been trying to develop reliable, lab-based methods for making the vital, blood-producing component of bone marrow: hematopoietic stem cells (HSCs).
Two new studies by NIH-funded research teams bring us closer to achieving this feat. In the first study, researchers developed a biochemical “recipe” to produce HSC-like cells from human induced pluripotent stem cells (iPSCs), which were derived from mature skin cells. In the second, researchers employed another approach to convert mature mouse endothelial cells, which line the inside of blood vessels, directly into self-renewing HSCs. When these HSCs were transplanted into mice, they fully reconstituted the animals’ blood systems with healthy red and white blood cells.