Stories about: hematopoietic stem cells

Why blood stem cells are in our bones: Evolutionary observation may inform better bone marrow transplants

blood stem cells melanocytes hematopoietic stem cells
In normal zebrafish, blood stem cells in the kidney are protected from sunlight by melanocytes. When this layer is stripped away, stem cell numbers go down. (Image and video below courtesy of the Zon Laboratory and the Howard Hughes Medical Institute.)

Since the late 1970s, biologists have known that blood develops in a specific body location. But they’ve wondered why different creatures house their blood stem cells in different places. In humans and other mammals, they’re in the bone. In fish, they’re in the kidney. Why?

Strange as it seems, the two stem cell “niches” share something in common, say researchers led by Leonard Zon, MD, of Boston Children’s Stem Cell Program, the Harvard Department of Stem Cell and Regenerative Biology (HSCRB) and the Harvard Stem Cell Institute. Both protect blood stem cells from sunlight’s harmful ultraviolet rays. The findings, published today in Nature, may contain lessons for improving blood stem cell transplants for cancer, blood disorders and other conditions.

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A breakthrough in our understanding of how red blood cells develop

Artist's rendering of red blood cells
Red blood cells.

By taking a deep dive into the molecular underpinnings of Diamond-Blackfan anemia, scientists have made a new discovery about what drives the development of mature red blood cells from the earliest form of blood cells, called hematopoietic (blood-forming) stem cells.

For the first time, cellular machines called ribosomes — which create proteins in every cell of the body — have been linked to blood stem cell differentiation. The findings, published today in Cell, have revealed a potential new therapeutic pathway to treat Diamond-Blackfan anemia. They also cap off a research effort at Boston Children’s Hospital spanning nearly 80 years and several generations of scientists.

Diamond-Blackfan anemia — a severe, rare, congenital blood disorder — was first described in 1938 by Louis Diamond, MD, and Kenneth Blackfan, MD, of Boston Children’s. The disorder impairs red blood cell production, impacting delivery of oxygen throughout the body and causing anemia. Forty years ago, David Nathan, MD, of Boston Children’s determined that the disorder specifically affects the way blood stem cells become mature red blood cells.

Then, nearly 30 years ago, Stuart Orkin, MD, also of Boston Children’s, identified a protein called GATA1 as being a key factor in the production of hemoglobin, the essential protein in red blood cells that is responsible for transporting oxygen. Interestingly, in more recent years, genetic analysis has revealed that some patients with Diamond-Blackfan have mutations that block normal GATA1 production.

Now, the final pieces of the puzzle — what causes Diamond-Blackfan anemia on a molecular level and how exactly ribosomes and GATA1 are involved — have finally been solved by another member of the Boston Children’s scientific community, Vijay Sankaran, MD, PhD, senior author of the new Cell paper.

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More surprises about blood development — and a possible lead for making lymphocytes

blood development chart
Blood development in the embryo begins with cells that make myeloid and erythroid cells – but not lymphoid cells. Why? A partial answer is in today’s Nature.

Hematopoietic stem cells (HSCs) have long been regarded as the granddaddy of all blood cells. After we’re born, these multipotent cells give rise to all our cell lineages: lymphoid, myeloid and erythroid cells. Hematologists have long focused on capturing HSCs’ emergence in the embryo, hoping to recreate the process in the lab to provide a source of therapeutic blood cells.

But in the embryo, oddly enough, blood development unfolds differently. The first blood cells to show up are already partly differentiated. These so-called “committed progenitors” give rise only to erythroid and myeloid cells — not lymphoid cells like the immune system’s B and T lymphocytes.

Researchers in the lab of George Q. Daley, MD, PhD, part of Boston Children’s Hospital’s Stem Cell Research program, wanted to know why. Does nature deliberately suppress blood cell multipotency in early embryonic development? And could this offer clues about how to reinstate multipotency and more readily generate different blood cell types?

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Routing gene therapy directly into the brain

Image of mouse brain that received a transplantation of hematopoietic stem cells. The image shows the transplanted cells (green) rapidly engrafted and gave rise to new cells (also green) that have widely distributed throughout the entire brain. 
Image of a mouse brain that received a direct transplantation of hematopoietic stem cells. The image reveals the transplanted cells (green) rapidly engrafted and gave rise to new cells (also green) that have widely distributed throughout the entire brain.

A therapeutic technique to transplant blood-forming (hematopoietic) stem cells directly into the brain could herald a revolution in our approach to treating central nervous system diseases and neurodegenerative disorders.

The technique, which could be used to transplant donor-matched hematopoietic stem cells (HSCs) or a patient’s own genetically-engineered HSCs into the brain, was reported in Science Advances today by researchers from the Dana-Farber/Boston Children’s Cancer and Blood Disorders Center and the San Raffaele Telethon Institute for Gene Therapy.

In their study, the team tested the technique in a mouse model to treat lysosomal storage disorders, a group of severe metabolic disorders that affect the central nervous system.

The team’s findings are groundbreaking because, until now, it was thought that HSCs — from a healthy, matched donor or a patient’s own genetically-corrected cells — needed to be transplanted indirectly

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Live imaging captures how blood stem cells take root in the body

For years, the lab of Leonard Zon, MD, director of the Stem Cell Research Program at Boston Children’s Hospital, has sought ways to enhance bone marrow transplants for patients with cancer, serious immune deficiencies and blood disorders. Using zebrafish as a drug-screening platform, the lab has found a number of promising compounds, including one called ProHema that is now in clinical trials.

But truthfully, until now, Zon and his colleagues have largely been flying blind.

“Stem cell and bone marrow transplants are still very much a black box: cells are introduced into a patient and later on we can measure recovery of their blood system, but what happens in between can’t be seen,” says Owen Tamplin, PhD, in the Zon Lab. “Now we have a system where we can actually watch that middle step.”

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A first for CRISPR: Cutting genes in blood stem cells

CRISPR T-cells stem cells HIV gene editing
The CRISPR system (red) at work.

CRISPR—a gene editing technology that lets researchers make precise mutations, deletions and even replacements in genomic DNA—is all the rage among genomic researchers right now. First discovered as a kind of genomic immune memory in bacteria, labs around the world are trying to leverage the technology for diseases ranging from malaria to sickle cell disease to Duchenne muscular dystrophy.

In a paper published yesterday in Cell Stem Cell, a team led by Derrick Rossi, PhD, of Boston Children’s Hospital, and Chad Cowan, PhD, of Massachusetts General Hospital, report a first for CRISPR: efficiently and precisely editing clinically relevant genes out of cells collected directly from people. Specifically, they applied CRISPR to human hematopoietic stem and progenitor cells (HSPCs) and T-cells.

“CRISPR has been used a lot for almost two years, and report after report note high efficacy in various cell lines. Nobody had yet reported on the efficacy or utility of CRISPR in primary blood stem cells,” says Rossi, whose lab is in the hospital’s Program in Cellular and Molecular Medicine. “But most researchers would agree that blood will be the first tissue targeted for gene editing-based therapies. You can take blood or stem cells out of a patient, edit them and transplant them back.”

The study also gave the team an opportunity to see just how accurate CRISPR’s cuts are. Their conclusion: It may be closer to being clinic-ready than we thought.

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The costs of quiescence, for cars and blood cells

Old car aging blood cell hematopoietic stem cell blood disorder Derrick Rossi
Like an old, unused car, our aging blood stem cells can accumulate damage over time that they can't fully repair.

My first car was my grandfather’s 1980 Chevrolet Malibu. For about two years before my family gave it to me, it sat unused in Grandpa’s garage—just enough time for all of the belts and hoses to rot and the battery to trickle down to nothing.

Why am I telling this story? Because it’s much like what happens to the DNA in our blood-forming stem cells as we age.

Hematopoietic stem cells (HSCs) spend very little of their lives in an active, cycling state. Much of the time they’re quiescent or dormant, keeping their molecular and metabolic processes dialed down. These quiet periods allow the cells to conserve resources, but also give time an opportunity to wear away at their genes.

“DNA damage doesn’t just arise from mistakes during replication,” explains Derrick Rossi, PhD, a stem cell biology researcher with Boston Children’s Hospital’s Program in Cellular and Molecular Medicine. “There are many ways for damage to occur during periods of inactivity, such as reactions with byproducts of our oxidative metabolism.”

The canonical view has been that HSCs always keep one eye open for DNA damage and repair it, even when dormant. But in a study recently published in Cell Stem Cell, Rossi and his team found evidence to the contrary­­—which might tell us something about age-related blood cancers and blood disorders.

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Can blood cells be rebooted into blood stem cells?

Hematopoietic hierarchy blood development stem cells
The classic hematopoietic hierarchy. What if we could turn those arrows around?

Think, for a moment, of a cell as a computer, with its genome as its software, working to give cells particular functions. One set of genetic programs turns a cell into a heart cell, another set creates a neuron, still another a lymphocyte and so on.

The job of controlling which programs get booted up, and when, falls in part to transcription factors—genes that act like molecular switches to turn other genes on and off.

Derrick Rossi, PhD, spends a lot of his time thinking about transcription factors. A stem cell and blood development researcher in Boston Children’s Hospital’s Program in Cellular and Molecular Medicine, Rossi believes that transcription factors hold the power to achieve one of the most sought-after goals in regenerative medicine: producing, from other cell types, transplantable hematopoietic stem cells (HSCs).

“There are about 50,000 HSC transplants every year,” Rossi explains, noting that the success of a transplant is highly dependent on the number of cells a patient receives from her donor. “But HSCs only comprise about one in every 20,000 cells in the bone marrow.

“If we could generate autologous HSCs from a patient’s other cells,” he continues, “it could be transformative for transplant medicine and for our ability to model diseases of blood development.”

As they reported April 24 in Cell, Rossi and his collaborators have taken a significant step toward that goal: Using a cocktail of eight transcription factors, they reprogrammed mature mouse blood cells into what they have dubbed induced HSCs (iHSCs).

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New research on blood stem cells takes root

Word cloud of words associated with hematopoietic stem cells and blood development.
The demand for hematopoietic stem cell transplants is rising. But how can we get more cells? (Text from Bryder D, Rossi DJ and Weissman IL. Am J Pathol 2006; 169(2): 338–346.)
You need a lot of hematopoietic stem cells to carry out a hematopoietic stem cell transplant, or HSCT. But getting enough blood stem cells can be quite a challenge.

There are many HSCs in the bone marrow, but getting them out in sufficient numbers is laborious—and for the donor, can be a painful process. Small numbers of HSCs circulate within the blood stream, but not nearly enough. And while umbilical cord blood from newborn babies may present a relatively rare but promising source for HSCs, a single cord generally contains fewer cells than are necessary.

And here’s the rub: The demand for HSCs is only going to increase. Once a last resort treatment for aggressive blood cancers, HSCTs are being used for a growing list of conditions, including some solid tumor cancers, non-malignant blood disorders and even a number of metabolic disorders.

So how do we get more blood stem cells? Several laboratories at Boston Children’s Hospital and Dana-Farber/Boston Children’s Cancer and Blood Disorders Center are approaching that question from different directions. But all are converging on the same end result: making more HSCs available for patients needing HSCTs.

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