CIRM funded research could lead to treatment to prevent recurrence of deadly blood cancer

Chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a cancer of the white blood cells. It causes them to increase in number, crowd out other blood cells, leading to anemia, infection or heavy bleeding. Up until the early 2000’s the main weapon against CML was chemotherapy, but the introduction of drugs called tyrosine kinase inhibitors changed that, dramatically improving long term survival rates.

However, these medications are not a cure and do not completely eradicate the leukemia stem cells that can fuel the growth of the cancer, so if people stop taking the medication the cancer can return.

Dr. John Chute: Photo courtesy UCLA

But now Dr. John Chute and a team of researchers at UCLA, in a CIRM-supported study, have found a way to target those leukemia stem cells and possibly eliminate them altogether.

The team knew that mice that had the genetic mutation responsible for around 95 percent of CML cases normally developed the disease and died with a few months. However, mice that had the CML gene but lacked another gene, one that produced a protein called pleiotrophin, had normal white blood cells and lived almost twice as long. Clearly there was something about pleiotrophin that played a key role in the growth of CML.

They tested this by transplanting blood stem cells from mice with the CML gene into healthy mice. The previously healthy mice developed leukemia and died. But when they did the same thing from mice that had the CML gene but lacked the pleiotrophin gene, the mice remained healthy.

So, Chute and his team wanted to know if the same thing happens in human cells. Studying human CML stem cells they found these had not just 100 times more pleiotrophin than ordinary cells, they were also producing their own pleiotrophin.

In a news release Chute, said this was unexpected:

“This provides an example of cancer stem cells that are perpetuating their own disease growth by hijacking a protein that normally supports the growth of the healthy blood system.”

Next Chute and the team developed an antibody that blocked the action of pleiotrophin and when they tested it in human cells the CML stem cells died.

Then they combined this antibody with a drug called imatinib (better known by its brand name, Gleevec) which targets the genetic abnormality that causes most forms of CML. They tested this in mice who had been transplanted with human CML stem cells and the cells died.

“Our results suggest that it may be possible to eradicate CML stem cells by combining this new targeted therapy with a tyrosine kinase inhibitor,” said Chute. “This could lead to a day down the road when people with CML may not need to take a tyrosine kinase inhibitor for the rest of their lives.”

The next step is for the researchers to modify the antibody so that it is better suited for humans and not mice and to see if it is effective not just in cells in the laboratory, but in people.

The study is published in the Journal of Clinical Investigation

Support cells have different roles in blood stem cell maintenance before and after stress

How-Stem-Cells-Act-When-Stressed-Versus-When-At-Rest

Expression of pleiotrophin (green) in bone marrow blood vessels (red) and stromal cells (white) in normal mice (left), and in mice 24 hours after irradiation (right). UCLA Broad Stem Cell Research Center/Cell Stem Cell

A new study published in the journal Cell Stem Cell, reveals how different types of cells in the bone marrow are responsible for supporting blood stem cell maintenance before and after injury.

It was already well known in the field that two different cell types, namely endothelial cells (which line blood vessels) and stromal cells (which make up connective tissue, or tissue that provides structural support for any organ), are responsible for maintaining the population of blood stem cells in the bone marrow. However, how these cells and the molecules they secrete impact blood stem cell development and maintenance is not well understood.

Hematopoietic stem cells are responsible for generating the multiple different types of cells found in blood, from our oxygen carrying red blood cells to the many different types of white blood cells that make up our immune system.

Dr. John Chute’s group at UCLA had previously discovered that a molecule called pleiotrophin, or PTN, is important for promoting self-renewal of the blood stem cell population. They did not, however, understand which cells secrete this molecule and when.

To answer this question, the scientists developed mouse models that did not produce PTN in different types of bone marrow cells, such as endothelial cells and stromal cells. Surprisingly, they saw that the inability of stromal cells to produce PTN decreased the blood stem cell population, but deletion of PTN in endothelial cells did not affect the blood stem cell niche.

Even more interestingly, the researchers found that in animals that were subjected to an environmental stressor, in this case, radiation, the result was reversed: endothelial cell PTN was necessary for blood stem cell renewal, whereas stromal cell PTN was not. While an important part of the knowledge base for blood stem cell biology, the reason for this switch in PTN secretion at times of homeostasis and disease is still unknown.

As Dr. Chute states in a press release, this result could have important implications for cancer treatments such as radiation:

“It may be possible to administer modified, recombinant versions of pleiotrophin to patients to accelerate blood cell regeneration. This strategy also may apply to patients undergoing bone marrow transplants.”

Another important consideration to take away from this work is that animal models developed in the laboratory should take into account the possibility that blood stem cell maintenance and regeneration is distinctly controlled under healthy and disease state. In other words, cellular function in one state is not always indicative of its role in another state.

This work was partially funded by a CIRM Leadership Award.