Scientists at Weill Cornell Medical College believe they have solved how to expand adult hematopoietic stem cells (HSCs) outside the human body for medical use in bone marrow transplantation – a vital step towards being able to produce enough stem cells required to re-establish a healthy blood system.
The researchers, working together with scientists from Memorial-Sloan Kettering Cancer Center, described in the journal Blood how they created a protein to expand adult HSCs when they were taken from the bone marrow of a donor. The protein keeps the expanded HSCs in a stem-like state, preventing them from differentiating into specialized blood cells before they are transplanted into the recipient patient’s bone marrow.
Seeking out compatible done marrow donors is challenging, and even when a “match” is found, harvesting enough bone marrow cells required for a transplant using current methods, usually means additional rounds of harvesting are needed to mobilize blood stem cells.
By expanding healthy HSCs in the laboratory not so many stem cells need to be harvested from donors. Also, adult blood stem cells could be frozen and stored for future expansion and usage – something that cannot be done with current technology.
Dr. Pengbo Zhou, professor of pathology and laboratory medicine at Weill Cornell, said:
“Our work demonstrates that we can overcome a major technical hurdle in the expansion of adult blood stem cells, making it possible, for the first time, to produce them on an industrial scale.”
Dr. Zhou says that bone marrow banks may one day be placed next to blood banks, if this latest technology passes future tests.
Dr. Zhou says “The immediate goal is for us to see if we can take fewer blood stem cells from a donor and expand them for transplant. That way more people may be more likely to donate. If many people donate, then we can type the cells before we freeze and bank them, so that we will know all the immune characteristics. The hope is that when a patient needs a bone marrow transplant to treat cancer or another disease, we can find the cells that match, expand them and use them.”
The scientists believe that more people may eventually choose to store their own marrow for potential future use. A person’s own blood stem cells are the best treatment for most blood cancers; they can also be used to slow down aging, and some other useful purposes.
Hematopoietic stem cells (HSCs) that reside in bone marrow can become any type of blood cell, including every type of immune cell.
For patients with blood cancers caused by abnormal blood cells, a treatment option includes removing the unhealthy marrow and transplanting healthy blood stem cells from a donor. If anti-cancer therapies damage a cancer patient’s blood, they may need a bone marrow transplant. Immune deficiency disorders and other illnesses can also be treated with bone marrow transplantations.
Donating bone marrow is sometimes an arduous and painful process. The marrow is extracted with a needle from a large bone under general anesthesia. Providing enough stem cells for the recipients may also require several sessions of bone marrow extraction.
Scientists have been trying to expand HSCs for a few years. They have focused on the transcription factor HOXB4, which encourages HSCs to make copies of themselves. Dr. Zhou said “The more HOXB4 protein there is in stem cells, the more they will self-renew and expand their population.“
However, all previous studies have failed because of their applicability. HSCs are stubbornly resistant to gene transfer. So far, the most efficient means to deliver therapeutic genes into HSCs in the laboratory setting has been with virus-based vehicles.
Scientists used a virus as a vehicle to deliver a therapeutic gene into patients with SCID (severe combined immunodeficiency disease) to reestablish their immune system. However, four children developed treatment-related leukemia because it was impossible to control where in the genome the virus inserts itself. Often the virus enters the genome on the hot-spots that either inactivate tumor suppressor genes or activate oncogenes.
Other scientists have demonstrated that directly inserting HOXB4 protein into extracted bone marrow stem cells is possible. Dr. Zhou said “But the half-life of the natural protein is very short — about one hour. So that means that in order to expand blood stem cells, these HOXB4 proteins have to be added all the time. Because the proteins are very costly, this process is both expensive and impractical.”
In this latest study, Dr. Zhou, Dr. Malcolm and team took a different approach. They set out to determine why HOXB4 protein is not long-lasting in HSCs, once these cells are removed from the protective stem cell niche that they nest quietly in. They found that for the stem cells to start differentiating, it is necessary to target HOXB4 for degradation. “Differentiating” means developing (maturing) into different types of adult blood cells.
Dr. Zhou said “HOXB4 prevents blood stem cells from differentiating, while, at the same time, allows them to renew themselves.”
CUL4, another protein, is tagged onto HOXB4 when the latter protein is destined for destruction by the cell’s protein destruction apparatus, the scientists found. The team discovered that CUL4 “sees” a set of four amino acids on the protein (it recognizes the protein that way). Dr. Zhou explained that HOXB4 has within it a destruction signal which CUL4 identifies and acts on.
Zhou and team synthesized a HOXB4 protein with a scrambled destruction signal. With the use of certain bacteria, they produced large quantities of HOXB4, and then delivered it into human blood stem cells in the lab.
Dr. Zhou said:
“When you mask the CUL4 degradation signal, HOXB4’s half-life expands for up to 10 hours. The engineered HOXB4 did its job to expand the stem cell, while keeping all its stem cell properties intact. As a result, cells receiving the engineered HOXB4 demonstrated superior expansion capacity than those given natural HOXB4 protein. Animal studies demonstrated that the transplanted engineered human stem cells can retain their stem cell-like qualities in mouse bone marrow.”
To make enough blood stem cells required for a patient transplant and for banking involves administering protein HOXB4 every 10 hours or so, the scientists explained.
Dr. Zhou added “This is the ultimate goal for what we are trying to achieve. There are likely many roadblocks ahead to reach our goals, but we appear to have found ways to deal with one major hurdle of adult hematopoietic stem cell expansion.”
Written by Christian Nordqvist