33 replies to this topic

[Trey]

NOTE: This will attempt to provide a basic understanding of stem cells related to leukemia, hopefully in a way that most leukemia patients can relate to and understand.  This is my own writing (not a "cut and paste") since I could not find existing articles that were understandable for most people, and the information is scattered across many sources.  Some issues are over-simplified for ease of understanding, otherwise this would have become an entire book. This subject is the key to an eventual cure for leukemia and other blood cancers as research continues to unlock the many mysteries of stem cells.

The term "stem cell" is not a single entity, but rather there are many types and levels of stem cells that form a hierarchy in the human body. A human starts as a single stem cell. The offspring of this original stem cell will eventually become every type of cell the body needs: bone, organ, brain, skin, blood, etc, etc. Because this first cell can make any type of cell in the body, it is called an "omnipotent" stem cell. During the first several divisions, the resulting cells are still omnipotent stem cells. But after these first few divisions, the stem cells begin to specialize. The first level of specialized stem cells are called "pluripotent" stem cells, which can make several types of cells, but no longer every type of cell in the body. The pluripotent stem cells can generally be brain but not blood cells, bone but not skin cells, etc. So the specialization of stem cells has started, but they are still all stem cells, just more specialized ones. The pluripotent stem cells go through a number of divisions until, at some point, the pluripotent stem cells further specialize and become an even more specific type of tissue, such as a blood stem cell (called a "hematopoetic" stem cell - Greek Hema means blood, and Poetic means it makes things). So from that very first cell until the first hematopoetic (blood) stem cell is made, every cell in this chain has been a stem cell. And these stem cells have become increasingly specialized along the way. For instance, the mother cell that made the hematopoetic stem cell is also capable of making blood vessel cells, and possibly other related cells. But now the hematopoetic stem cell can only make blood cells, although it can make any and every type of blood cell it wants (red blood cells, white blood cells, etc).

The hematopoetic (blood making) system is made up of a hierarchy of blood cell types, and becomes increasingly specialized top to bottom. The top cells in the blood making system hierarchy are called Long Term Hematopoetic Stem Cells (LT-HSC), which can live for many years, maybe decades, and possibly the lifetime of the person. But each of its successive offspring (children, grandchildren, great-grandchildren) each have shorter lives, down to the final "working" cells (white blood cells, red blood cells, & platelets) which only stay in the body for days (WBCs) or a few months (RBCs). So the blood making process starts with the Long Term Hematopoetic Stem Cells (many millions of them), which divide and make daughter cells. But this division does not change the hematopoetic stem cell; as it divides it remains a hematopoetic stem cell (self renewal), but also makes a more specialized daughter stem cell at the same time. So the main feature of a stem cell is that it does not change itself by dividing, since it makes a clone of itself plus a daughter cell. But the daughter cells become increasingly specialized as subsequent divisions occur. The offspring of the Long Term Hematopoetic Stem Cells are also stem cells, and they are called (wait for it...wait for it....) Short Term Hematopoetic Stem Cells. In reality, within the Long and Short Term Hematopoetic Stem Cell compartments are possibly as many as 16 sub-classes of Long and Short Term Hematopoetic Stem Cells.

Long Term Hematopoetic Stem Cells have the ability to make any type of blood cell, but they each have a built-in bias to produce either myeloid cells (marrow type - neutrophils, basophils, eosinophils, along with red blood cells and platelets, and some others) or lymphoid cells (T-Cells, B-Cells, NK-Cells, etc). So there are two co-existing sets of Long Term Hematopoetic Stem Cells, one set making mostly myeloid cells, and the other set making mostly lymphoid cells. These stem cells can change bias if injury requires more of the other type of cell, but otherwise they generally stick with their bias toward one of the two main blood cell lines. The Short Term Hematopoetic Stem Cells are even more biased toward the myeloid or lymphoid line than the Long Term Hematopoetic Stem Cells, so we see the specialization process continuing. They can still make any type of blood cell, but it is harder for them to switch back and forth from myeloid to lymphoid, and vice versa.

The Short Term Hematopoetic Stem Cell is the last of the true stem cells in the blood making hierarchy. After several divisions they eventually make a more committed cell, called a Multipotent Progenitor Cell. The Multipotent Progenitor Cell can still make all types of blood cells (red, white, etc). But since it has lost stem cell status, when it divides it makes two daughter cells, not a clone of itself and a daughter. So the key to progenitor cells is that they can divide and make two new cells, but cannot self renew. While the Multipotent Progenitor Cell is not truly a stem cell, it is believed to be capable of taking on stem cell qualities when required to do so, such as injury to the marrow. (This "plasticity" or "going backward" theory has possible application to some types of leukemia.) So potentially the Multipotent Progenitor Cell can regain stem cell status under certain conditions. Recall that a stem cell can "self renew" when it divides, rather than merely make two daughters; but a progenitor cell cannot self renew. This progenitor level of the hierarchy has a number of sub-levels of types of progenitor cells, which become successively more specialized. It is the progenitor cells that do most of the real work in making blood cells. Another big difference between stem cells and progenitor cells is that stem cells can divide with no apparent limit on the number of divisions, but progenitor cells only get a set number of divisions and then they die. This is why true progenitor cells cannot be the originating leukemic cell, because the disease would go away by itself as the progenitor cell divides itself into oblivion. But if a progenitor cell regains stem cell properties, as the Multipotent Progenitor Cell can, that restriction on number of cell divisions would disappear.

After several divisions, the Multipotent Progenitor Cells become more committed, producing the next level of progenitor cells called Oligolineage (single line) Progenitor Cells, still capable of division (making new cells) and like their parent they are not stem cells. But unlike their parent cell, they have most likely lost the ability to take on stem cell characteristics, even during a severe injury to the marrow. At this point the Oligolineage Progenitor Cells can only make one line of cells, white-myeloid (plus red cells and pre-platelets) or white-lymphoid cells. These Oligolineage Progenitor Cells divide another several times, becoming even more committed to one specific type of end-stage cell, and eventually they make the end stage blood cells - neutrophils, T-Cells, red blood cells, etc, etc. These end stage cells are no longer capable of dividing. They do their work for a few days (WBCs), weeks (platelets), or months (RBCs), and then are expelled from the body. A normal adult produces many billions of new blood cells each day. The final stage white blood cells are the only blood cells with a nucleus (so they are live cells). Red cells have no nucleus. And platelets are merely cell fragments, so are not cells at all.  It is estimated that at least 20 divisions occur between the top level stem cell and the final working blood cell.

Picture of the Hematopoetic (blood cell) hierarchy:

http://stemcells.nih.gov/StaticResources/images/figure2_lg.jpg

http://www.ncbi.nlm....figure&id=A3721

It is important to understand that the higher the cell is in the hierarchy, the longer it lives. The original stem cells that were made during the first few divisions from the single original human cell remain alive in the body for the lifetime of the person. The top level hematopoetic stem cell may live for many years, decades, and some maybe a lifetime. The progenitor blood cells may live for months, or maybe a couple years. The end stage white blood cells live for only a few days. As discussed, the red cells and platelets are not even technically live cells, since they have no nucleus, and platelets are not even whole cells. So the white blood cells are the live cells, and that is where the leukemia is located. Red cells and platelets are not truly leukemic as such, although their ancestors are, so the process may cause them to be deformed and have reduced efficiency. We have also noted that the blood making process is a hierarchy of cells, starting with levels of stems cells, followed by progenitor cells, followed by end stage cells. Each stage is increasingly specific in its function. Stem cells can clone themselves and also make a daughter at the same time. Progenitor cells can only make daughters. End stage cells cannot reproduce.

So where does a blood cancer begin? That answer is likely different for the various types of leukemias, lymphomas, and myelodysplastic syndromes. CML leukemia is a disease originating with a single Long Term Hematopoetic Stem Cell that makes a genetic mistake during division, resulting in a mutant (leukemic) blood cell. This mutant cell is called a Leukemic Stem Cell (LSC). Since it is both a stem cell and also is very high in the blood making hierarchy, it has great powers to perpetuate itself, and can also make every type of blood cell (myeloid-white, lymphoid-white, red, platelets, etc). This allows for much mischief.

Another important issue is that high level stem cells live most of their lives in seclusion, hiding and resting in a state of near dormancy, coming out very rarely to divide. After that, they assume the "hide and rest" mode (quiescence). They do this in protected areas of the marrow, deep in crevasses, where they are very difficult to reach by drugs, chemotherapy, T-Cells, and other hazards. They do not "come out and fight like a man". They skulk and quiesce and....well you get the picture -- they are darn hard to reach or kill. And there is some evidence that TKI drugs and even Interferon can cause the leukemic stem cells to be more prone to go into hiding for long periods (quiescence). This adds to the problem of trying to kill them.

I previously said that CML is a disease originating with a single Long Term Hematopoetic Stem Cell. The stem cell origin is different for various types of leukemia (and myelodysplastic syndromes, lymphomas, etc). So CML leukemia starts very high in the hierarchy, at the Long Term Hematopoetic Stem Cell level. AML leukemia probably starts at the same level. But lymphoid leukemias (ALL and CLL) start lower in the hierarchy. How do we know? Because the lymphoid leukemias only affect the lymphoid cells, unlike CML and AML which affect all blood cell lines, including the lymphoid cells. So the lymphoid leukemias possibly start with a mutation at the committed progenitor line. But wait, the astute reader says -- the committed progenitor is not even a true stem cell. Very observant - but recall that I said the Multipotent Progenitor Cells can regain stem cell qualities. Possibly the lymphoid leukemias start at that level and then revert to stem cell capabilities, although they remain committed to producing only lymphoid white blood cells. But the myeloid leukemias start at a higher level, as shown by the impact on more blood cell lines, including all types of white blood cells (WBC), red blood cells (RBC), and megakaryocytes (pre-platelet cells). This is also why myeloid leukemias have more issues with anemia, platelets going too high or too low, etc. And stem cell transplants can more easily cure the lymphoid leukemias than the myeloid leukemias, since the lower the original leukemic cell is in the hierarchy, the more susceptible to chemotherapy, other drugs, and even to attack from normal T-Cells and NK-cells. The bad news for CML and AML is that their leukemic stem cell is harder to kill, so a BMT requires a harsher chemotherapy regimen and total body irradiation to attempt to kill the quiescent leukemic stem cells. But even so, relapse is more likely to occur in CML or AML because of the level of the originating stem cell in the hierarchy.

It is also important to understand that there is no test or other mechanism to determine if a specific blood cell is a leukemic hematopoetic stem cell. These higher level cells may "give off" (express) signals showing they are either higher or lower in the hierarchy, but no one can pinpoint the original leukemic mutant cell. The original leukemic cell could come up to a researcher and say "Hi, I'm the original leukemic cell" and no one would recognize it as such. Leukemic cells are probably not very truthful anyway. But research is homing in on being able to identify these cells more accurately. They generally are identified by "cluster designation" (CD numbers). A leukemic blood stem cell will be CD34+CD38- but requires more definition than that to identify it. Some research shows they could also have Thy1+, Lin-, IL1RAP+, and various other cell surface expressions.

So as we have seen, CML leukemia begins when a single hematopoetic stem cell (very high in the blood hierarchy) divides. It forms a clone of itself, and also forms a daughter cell that is slightly more differentiated than itself. So which one mutated - the clone or the daughter? No one knows. But it is during a division process where the first leukemic mutation occurs. During the division process the DNA is replicated, producing two separate strands, one for each of the two new cells (technically the clone has "self renewed", and the daughter is new). One of the two cells ends up with damaged DNA, whereby the chromosomes 9 and 22 switch pieces of the chromosomes (for CML). Often this would be detected and the cell would go into self destruct mode, and it would not be a problem. Mistakes happen all the time, and the body takes care of it. But this one is missed, and whoever was on quality control duty that day messed up. Then the T-Cells look at the mutant cell and say "you look familiar, although not from around here...so I'll let you go". So the T-Cells do not kill the mutant, since it looks "close enough" to normal. But the leukemic cells have slight variations that could make them a target under the right conditions, which makes a potential leukemia vaccine interesting (maybe some day). But the mutant stem cell survives, and because it is so high in the hierarchy, over time (possibly a year or several years) it produces a whole lineage of leukemic cells that have survival advantages over the normal cells. As the body sends signals to stop the over-production, only the normal cells obey. So the body ends up with mostly leukemic blood cells.

TKI drugs work mainly on the mutant BCR-ABL messenger system found only in leukemic cells, and specifically those capable of dividing. While BCR-ABL is found in leukemic cells at all levels of the hierarchy, the cells most reliant on BCR-ABL for proliferation and even survival are the progenitor leukemic cells. Gleevec, Tasigna, Sprycel and clinical trial TKI drugs work quickly and effectively to shut down leukemic progenitor cells. When the TKI drugs block the BCR-ABL process, the leukemic cells become confused and self destruct ("Hooray, die you suckers" the crowd cheers with glee). But the high level leukemic stem cells are not so easily affected. This is because they have more options for survival, including multiple signaling pathways that can possibly evade or ignore the BCR-ABL shutdown by TKI drugs. ("Booooo" the crowd moans). And the highest level leukemic stem cells (the hematopoetic stem cells) can hide for long periods of time in a dormant state, where they apparently do not absorb fluids that might contain drugs, so the TKI drugs cannot reach them in this dormant state (and also chemotherapy cannot reach it, which makes CML and AML more difficult to eradicate prior to a bone marrow transplant). Sprycel seems to reach higher in the stem cell hierarchy to kill off some additional levels of leukemic stem cells because it also inhibits something called SRC, which some leukemic stem cells (lower level stem cells) need for survival. Tasigna and Gleevec do not inhibit SRC. But the big question is whether any TKI drug can kill the top level leukemic stem cells when they come out of hiding. And they may hide for years in some cases, so the opportunities are limited. Research in the lab is inconclusive on this point, but there is skepticism that current TKI drugs would kill these highest level leukemic stem cells under any scenario. But things can work differently in the body, and there are more factors, such as T-Cells and other issues that could possibly come into play, so no one knows the answer.

There are many other topics that could be covered in this discussion, but this is simply meant to be an overview of stems cells and leukemia. Leukemia was the first cancer shown to be caused by mutant stem cells. There is much more that needs to be learned about this subject. It is the key to the cure.

Additional reading:

http://stemcells.nih...rt/chapter5.asp

[Goldfinger]

Trey-

Thanks for the explanation!!!  Are you by any chance a doctor?

Steve

[gianfranko]

Can the immune system actually target and kill cancer stem cells?  I always had in mind that immune cells were in the blood, are there any immune cells able to reach into the bone marrow?

[scuba]

Outstanding!  Nice job.  Very nice indeed.

[Tedsey]

Thank you for all the time and hard work you put into this.  Thank you for all you do for us.

[MACELPatient]

Nice job Trey.  Makes it easier to understand.  As a patient who has CEL I can see why Inhibitors such as Gleevec have a higher efficacy than compared to CML patients.  It would appear that I am lower down the stem cell hierarchy.  I don't have the blood issues that many here have with CML.  My anemia is almost non-existent, my platlets are normal, my RBC are normal.  For all intents and purposes you would not look at me as someone who has a disease.  My disease went into remission after only 3 months of Gleevec.

[Trey]

Goldfinger: No medical training

Gianfranco:

You asked: "Can the immune system actually target and kill cancer stem cells?  I  always had in mind that immune cells were in the blood, are there any  immune cells able to reach into the bone marrow?"

The "immune system" is the white blood cells -- that is what the WBCs are, both myeloid and lymphoid.  They are generally made in the marrow, then migrate out to the blood and lymphatic system.  So the marrow always has plenty of immune cells.  In CML the WBCs are leukemic -- both myeloid and lymphoid.  Will a leukemic T-Cell attack a leukemic neutrophil?  Not likely.  Will a normal T-Cell attack a leukemic neutrophil?  Not normally, since leukemic cells are seen as "self" (friendly) by the body, which is the problem.  Otherwise leukemia would be like the flu, and it would just go away after a few days.  The body does not attack itself unless there is an autoimmune problem.  Can normal T-Cells be trained to attack leukemic cells?  Maybe.  That is the goal of research into developing a leukemia vaccine.  Leukemic white blood cells look just slightly different, so their differences might allow T-Cells to be trained to look for them.

ET:  Yes, CEL is probably a progenitor level translocation disease which takes on stem cell characteristics.

[gianfranko]

so if I understand correctly how the vaccine is supposed to work:

antigen specific for the leukemia cells is given.  T-cells come in contact with the antigen (somewhere, not sure where, blood maybe?) and they undergo clonal expansion ending up with T-8 killer cells and Th1 helpers cells.  Then this army of cells travels to the bone marrow and is able to now kill cancer stem cells?

Is this at least partially correct?

[reedgirl]

Trey,

   Your knowledge and explanations are outstanding.  Thank you for taking the time to research and give such informative information to us all. I am learning so much by reading many posts and printing stuff out.  It's a lot to take in, hopefully if I keep reading it things will make more sense to me.

Thanks again,

Audrey

[scuba]

http://www.physorg.c...s201184703.html

"Our results demonstrate that quiescence is a double-edged sword, protecting hematopoetic stem cells from cellular stress but rendering them intrinsically vulnerable to mutagenesis following DNA damage," says senior author Emmanuelle Passegué, PhD, associate professor of medicine (division of hematology/oncology) and a member of the Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research at UCSF.

The finding, the result of "heroic" research begun four years ago by Mary Mohrin, a graduate student in the Passegué lab, upturns current dogma, says Passegué. "The thinking has been that blood stem cells must proliferate to acquire the mutations that drive tumor development. This work says, 'not so.'"

On a lighter note, she reflects, "our discovery may be good PR for blood donations." While acknowledging her rationale is a bit speculative, she says, "Bleeding is a good way to keep the blood stem cell pool in an active, or proliferative, state."

Interesting article citing research on hematiopoetic stem cell quiescence and DNA repair.  The author cites evidence that dividing cells repair DNA much more easily and better  than quiescent cells.  And that Leukemia gets triggered when the cells are quiescent.  Driving the stem cells into division may actually help in the repair process.  This makes me wonder if women in pre-menopause are less likely to develop CML than men?  Is there any statistic on this? 

This makes me wonder if I were to give blood - or simply have regular blood letting done - if this would help Gleevec perform faster by getting at the dividing stem cell population better.  Gleevec works when the cells divide - it's during that division that Gleevec can effect the cells.  Seems to me to be a good LSC population reduction technique?  Bleed - cause LSC's to get active - zap 'em with TKI's, leave fewer and fewer LSC quiescent cells.

In other words - for those of you who are PCRU (TREY) for years and believe you have quiescent LSC's around that can still create mischief - giving blood - may force those quiescent LSC's into division (have to divide to replace blood) and then the TKI is there to nail 'em - leaving an ever smaller and smaller population of LSC's until perhaps they go extinct.  Normal stem cells are unaffected.

Just thinking out of the box ...

[Trey]

There is quite a bit of speculation in that article.  And once the cell divides, the new (mutant) cell does not try to repair itself, and the stem cell thinks it already made the repair, so the bleeding theory seems odd.

[scuba]

Trey - I believe the point is that the mutation damage in the stem cell occurs "Prior" to cell division during quiescence and that the repair mechanism is inefficient causing translocation errors, etc. so the mutation sticks.   Then when division occurs, the stem cell which is now leukemic, passes that on to the new cells and the Leukemia process is started.

The article is contrasting efficiency of repair during quiescence vs. division.  Dividing cells are better able to repair themselves whereas quiescence cells are not.

Read the entire article again, I believe you will read what I was reading.

Here is an abstract from the journal article:

http://www.sciencedi...a8&searchtype=a

And by extension - if one can drive LSC's into division - then a TKI can do its work and perhaps reduce the LSC population.  A quiescent Long lived stem cell is 'when' the mutation occurs ... not during division, but by just sitting there getting old and unable to repair itself properly when insult/injury occurs (t:9;22).  The mistake is born.  And then when the cell does divide (comes out of quiescence), the leukemia begins.  There is no 'repair' to be made.

I took away from the article that regular blood donation might be a preventive measure for healthy people - especially when they get older (50's) since a dividing cell (fewer quiescent) can better repair itself should DNA damage occur.  I was never a blood donor and now I regret it.  I might have avoided CML if I was giving blood regularly.  But perhaps by doing so now, I can at least expose the dividing cells to Gleevec and reduce the population?

[CallMeLucky]

1) Are leukemia patients able to donate blood?  I know cancer is not something you could catch, but I would still think it's not a great idea to get a transfusion with blood that has leukemic cells in it.  Any science to back that up either way?

2) Make sure your platelets are not low

3) You might make the alternate argument that if you had leukemic cells dormant in your blood and you donate blood it stirs things up and potentially "wakes them up".

So speculative.  I suppose the donating blood, if we can, is good becuase it is just good to donate blood.  If it happens to help Gleevec be more efficient, then we can call that good karma.  Either way, I wouldn't beat yourself up about not donating enough beforehand.  We'll never know, but the odds that you got CML because you didn't donate blood is very unlikely.  I'm sure someone on here will respond soon that they donated blood on a regular basis and still got CML.

[Goldfinger]

I've always been one to donate blood and that was one of the first issues I brought up with my oncs after I was diagnosed. They said no...way...

Steve

[scuba]

Steve - I didn't quite follow your response.  Did you imply that donating blood 'caused' CML and your Onc's said "no way" ?

My point above was that donating blood regularly may be 'preventive' since it puts quiescent stem cells into a proliferative mode.

But since you have CML AND donated regularly - that theory may go out the window.

There still may be therapeutic benefit, however,  if we can cause our quiescent cells to "wake up" en masse due to a 'trauma' like giving blood and thereby expose them to TKI's that can kill them.  The point of the article was that if stem cells are active rather than quiescent they are better able to resist mutations and I extended the thinking to include TKI's to kill them since they are dividing rather than sitting dormant.

[CallMeLucky]

Per the Red Cross

"Eligibility depends on the type of cancer and treatment history. If you  had leukemia or lymphoma, including Hodgkin's Disease and other cancers  of the blood, you are not eligible to donate."

http://www.redcrossblood.org/donating-blood/eligibility-requirements/eligibility-criteria-alphabetical-listing

I suppose you could still look into bloodletting if you can find a physician who agrees it would be beneficial.

Keep in mind that donating or blood letting with low counts could also be very dangerous so it would have to be a very controlled situation.

[scuba]

This would apply only to people who are normal in all categories  - probably PCRU remission.

My blood for example is very close to normal now.  And I suspect that I did not having falling counts because my PH+ levels were not 100% at diagnosis (53%) - so I had plenty of 'normal' blood around.  My FISH counts, however, are not dropping as fast as I would have hoped - hence my thinking on the blood letting.

Personally - I don't think any Oncologist or other specialist would accommodate my desire to blood let (since I can't donate).  To much risk for them.  So this may all be academic unless a clinical trial was established to do this.  I sent a note to drs. Druker and Cortes for their thoughts.

I am intrigued that driving the hematopoetic blood system into a proliferative state may expose more leukemic cells to a TKI than not.

[Goldfinger]

Michael-

I used to donate  frequently (usually twice a year) before I was diagnosed with CML. I discussed donating blood after I got CML and the oncs told me in simple terms- No...

As far as your theory goes- each person is unique and what applies to one person may not apply to someone else.

Steve

[scuba]

Steve - I see ...

I still wonder if women who are menstruating have a better TKI response than men?  or if I could blood let a pint once per month - if that would help my TKI do a better job.

[CallMeLucky]

From what I have seen so far, women, in general, seem to have a tougher go at it with CML and TKI drugs.  My wife and I were speculating that maybe it is hormonal or perhaps related to menstruation.  I might be completely wrong in my perceptions, but it just seems the people I hear having the toughest time with the side effects etc seem to be the women (not that men have a walk in the park).  I also recall reading something very early on about CML being harder on women.  I could be confusing things I read, I seem to be doing that a little more lately