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).

If I could turn back time

Ever since Yamanaka created induced pluripotent stem (iPS) cells, labs around the globe have been racing to find better ways to transform cells of one type into cells of another type. There are now several methods of doing so, going by names like transdifferentiation and re-specification (a technique developed by Boston Children’s George Daley, MD, PhD, and Sergei Doulatev, PhD).

Rossi calls what he’s done reprogramming, similar to what Yamanaka did in creating iPS cells but with a narrower scope and a directed goal: meeting the rising clinical demand for HSCs.

The key, he thinks, is to follow the natural developmental pathway for blood and immune cells and turn it around.

“Blood cell production invariably goes in one direction: from stem cells, to progenitors, to mature effector cells,” he says. “We wanted to reverse the process and derive HSCs from differentiated blood cells using transcription factors that we found were specific to HSCs.”

‘Control-Alt-Transcribe’

Derrick Rossi Program in Cellular and Molecular Medicine Boston Children's Hospital blood stem cells
Rossi explaining his iHSC study in his office. (Image courtesy B.D. Colen)
Rossi and his team started by screening 36 transcription factors that they found were expressed in mouse HSCs but not in cells derived from HSCs. Of these, they found that a cocktail of six—Hlf, Runx1t1, Pbx1, Lmo2, Zfp37 and Prdm5—plus an additional two—Mycn and Meis1—could reprogram both mid-level blood progenitor cells and fully mature myeloid cells from mice into iHSCs.

The secret, they found, was to use mice themselves as bioreactors. Rossi’s team first exposed differentiated mouse blood cells to viruses carrying genes for the eight transcription factors along with an additional gene that turned the whole cocktail on in the presence of the drug doxycycline. Then, rather than try to get the cells to reboot in a dish, they transplanted them directly into the mice and put the mice on the drug.

“In the blood research field, no one has the conditions to expand HSCs in the tissue culture dish,” explains Stuart Orkin, MD, one of the leaders of Dana-Farber/Boston Children’s Cancer and Blood Disorders Center and a co-author on the paper. “Instead, by letting the reprogramming occur in mice, Rossi takes advantage of the signaling and environmental cues HSCs would normally experience.”

Like natural HSCs, the resulting iHSCs could self-renew and give rise to all blood cell lineages, tests showed. The iHSCs also bore gene expression patterns nearly indistinguishable from those of normal iHSCs.

But even with those similarities, there’s a lot about the reprogramming process that still isn’t understood. What do each of the eight transcription factors in Rossi’s cocktail do? Can non-blood cells be similarly reprogrammed into iHSCs? Can we achieve the same results without using viruses as molecular shuttles? Will it work with human cells?

These and other questions need answers before iHSCs can serve as a clinically relevant source of transplantable HSCs. But even in their current form, iHSCs hold a lot of potential as tools for better understanding HSC biology and blood cell development.

“Our data show that the functional and molecular identity of HSCs can be tapped with relatively few factors using the paradigm of cellular reprogramming,” Rossi says, “in a manner similar to the generation of iPS cells.”