By David N. Leff
Who says there's no such thing as a cure for cancer?
Clinical oncologists know that their front-line chemotherapy regimens can mop up and wipe out most tumors — if given in high enough doses. But there's a kicker: Those potent anticancer drugs, programmed to eradicate a tumor's rapidly dividing cells, will also destroy the fast-dividing cells of the patient's blood system.
An increasingly applied countermeasure to this Catch-22 is autotransplantation.
"When one is undergoing an autotransplant," explained molecular geneticist John Dick, at the University of Toronto, "someone who had, let's say, a breast tumor, the clinicians first draw and freeze a quantity of the patient's blood-forming stem cells, then give the high-dose chemotherapy to eradicate their tumor. At the same time, they administer whole-body ionizing radiation to kill off the patient's residual bone-marrow cells, along with any stray tumor cells.
"After blasting the cancer," Dick continued, "they inject the aliquot of stored cells back into the patient's circulation, where they promptly repopulate the bloodstream with the red cells and white cells it needs to function. But you want to be sure that you didn't also give back any tumor cells that were contaminating that bone marrow sample."
Dick imagines the human — or mouse — blood system as being like a pyramid. "At its base," he explained, "you have the billions upon billions of blood cells that need to be replenished in the body every day. Part way up are the progenitor cells which, when stimulated with hormones, will replace all those lost cells.
"At the very top are the hematopoietic [blood-forming] stem cells, which continuously produce blood throughout our lifetimes. And it's the stem cells that have the exclusive capacity to re-grow blood systems. (See BioWorld Today, Dec. 9, 1996, p. 1.)
Best known of those rare and elusive ancestral founder stem cells are those that fly a glycoprotein marker on their surfaces, identified as CD34. These are the classical CD34-positive stem cells that the cancer clinicians save, store and return to their post-transplant patients.
"What people have done," Dick observed, "is develop techniques of selecting out the CD34-positive cells to give back to the person, because most tumors don't express them. So they collect out all the 34+ stem cells, and throw everything else away. This trash includes tumor cells as well as other blood cells that aren't 34+.
Over the years, Dick has nurtured a suspicion that that trash contains more stem cells than meet the trash bin.
His hunch paid off in the September issue of Nature Medicine, as revealed in a paper of which he is senior author, reporting, "Identification of a newly discovered class of hematopoietic cells with SCID-repopulating activity."
The impetus for his seminal finding, Dick recalled, was work from another lab "showing that murine CD34 did not seem to mark stem cells exclusively. In other words, there was a mouse-34-minus cell, which could repopulate the blood of mice. That, of course, was very provocative, and caused us to ask: Do these cells exist in humans too?"
Pyramid's Slippery Slope Swaps Cell Markers
"We and others had shown," he told BioWorld Today, "that the vast majority of the 34-minus stem cells do not have repopulation capacity. In that pyramid, the cells at the top are positive, and as they begin to move down on the pyramid — that is, differentiate — they start to acquire lineage antigens.
"What that means is," Dick explained, "as they become differentiated towards red blood cells or other white cells, they start to acquire markers of that particular lineage on their surface. At the same time, they also lose their CD34 markers. So the vast majority of the cells in that pyramid are going to be 34-negative but lineage-positive.
"So, we wondered: Is there a lineage-minus cell? If so, that would place it at the top of the pyramid. But it also would not express CD34. Lo and behold," he said, "we found that cell, and then we posed the next question: Does this cell have any capacity to repopulate our mice? And indeed it does."
"If you want to study mouse cells, that's fine," Dick observed. "You can do it easily. But if you want to study human stem cells, you obviously can't line up 100 people and say, 'We're going to use you in our test system to watch your stem cells repopulate your blood system.'"
A couple of years ago, Dick announced a way to study this capacity of stem cells to regrow the entire human blood system, by injecting them into severe combined immunodeficient (SCID) mice, which don't recognize human cells as being foreign.
In a fast-track, four-day suspension culture, his CD34-positive human stem cells failed to repopulate in the mice, whereas the CD-minus cells — rescued from the trash, as it were — not only grew in the mice, but in fact expanded; they'd actually increased in number in those four days. So, it proved that this was really a novel population of cells.
"A CD34-positive stem cell," Dick pointed out, "has a frequency, let's say, of 1 in 100,000 or so cells, and these new CD34-minus cells would be five percent of that — about one in two million."
From Humanoid Mice To Fetal Sheep
Dick observed there are now "multiple pieces of evidence that such a cell exists. The question is: Is it also the cell that has repopulation capacity in humans? In other words, is this a cell with some relevance in human stem cell transplantation?
"If I were a cancer patient," he added, "I'd like to get as many stem cells as I could, and of the best kind. The data in the mouse is quite provocative that these 34-minus stem cells may be even more proliferative in the human population, though we don't have any evidence of that in our humanized mouse model yet.
"Stem-cell transplantation was started as a therapy for cancer," Dick pointed out, "to allow one to give higher doses of chemotherapy. But I think there are many other diseases that could be cured by bone marrow transplantation, if the transplant process was more benign, if you didn't have to give people really high doses of chemotherapy.
"That would make things like autoimmune diseases much more accessible, [as well as] a number of genetic disorders that affect the blood system, such as thalassemia and sickle cell anemia. And if these new cells have different properties," Dick concluded, "perhaps it will be easier — or harder; we don't know a priori — to put genes into them for gene therapy." *