Mutations

I'm not sure if this answers your question, but here's a quote from wikipedia:

"The fused BCR-ABL protein interacts with the interleukin 3beta(c) receptor subunit. The BCR-ABL transcript is continuously active and does not require activation by other cellular messaging proteins. In turn, BCR-ABL activates a cascade of proteins that control the cell cycle, speeding up cell division. Moreover, the BCR-ABL protein inhibits DNA repair, causing genomic instability and making the cell more susceptible to developing further genetic abnormalities. The action of the BCR-ABL protein is the pathophysiologic cause of chronic myelogenous leukemia."

The way I think about it is that when CML is not well controlled and kept at a low level,  then there are lots of bad stem cells doing what they ought not and that makes the chances of a viable mutation much higher.

I would say usually, except for someone in the first year or so of treatment for CML.

Before someone begins TKI treatment a "normal" CML cell may have acquired a viable mutation that is resistant to a particular TKI.  That mutant cell's progeny then become a portion of the CML "tumor".  If when TKI therapy is started and the "normal" tumor is dominant then there will be a sharp reduction in the CML tumor.  However if there is a population of mutant CML cells that aren't vulnerable to the TKI then they can expand and become the dominant type as treatment progresses.  That's when mutational analysis comes into play and a change of therapy is in order.

Happily, most of us abnormal CML people have the "normal" type of CML!

OK - you all asked for it, so here's Trey's very technical and very detailed explanation of TKI response and resistane as posted by him on the LLS site:

Title: TKI Drug Response Variations and Drug Resistance Among CML Patients Posted by: Trey 

In this post I will discuss why there are such wide variations in TKI drug response among CML patients, and the major causes of drug resistance and failure.  I have promised to do this for some time, but the subject is difficult and the answers are not necessarily satisfying, to me or to you.  There is still much which needs to be discovered.     Drug response rate is probably the most important and dramatic difference among patients and the number one cause for concern.  But the answers to the question "why?" are not necessarily simple.  CML patients experience significant variations in response to CML TKI drugs, with some responding quickly, others slow but steady, some with a steadily declining test results, some with ups and downs in results, some respond for only a short time, some with long plateaus in results, some respond much better to one specific drug, and some have issues with severe drug resistance and drug failure on specific or multiple drugs.  Patients respond at vastly different rates as measured by PCR log reductions, FISH testing, and BMB cytogenetics.  Drug resistance can take the form of some level of limited response (suboptimal and/or very slow) or outright failure when the results are clearly unacceptable with little or no response.    But why such variations, some of which are extreme?  Response variations can be from several causes, including:   Drug side effects leading to drug intolerance Drug metabolism and uptake Drug discharge from the cell and plasma Differences among TKI drugs (pharmacokinetics) Cellular variations among patients Drug interference CML genetics CML Phase Other patient specific issues Failure to take the drug (adherence) Unknown causes   I will deal with each of these now in more detail.   Drug side effects/intolerance: Sometimes a patient will respond well enough to a drug but may not be able to withstand the side effects of it, or may not be able to take a large enough dosage due to side effects such as low blood counts.  This is called drug €œintolerance, not drug failure.  Failure means it won't work, while intolerance means the patient cannot remain on the drug long enough at therapeutic dosages to adequately control the disease.  Drug breaks are often required to allow the side effect to subside, and lower dosage is also commonly necessary.  For some patients a therapeutic dosage is higher than others due to individual patient factors discussed in the paragraphs below.  The most common side effect which causes drug intolerance is perpetual low blood counts.  This can be low WBC, low RBC, low PLT, or a combination of these.  Other intolerable side effects can be whatever the patient is unable or unwilling to endure.  So the first thing to understand is the difference between drug intolerance and drug failure, even though both can lead to poor response. One issue worth addressing related to drug intolerance is that some patients only require small dosages of a drug, and standard dosages create intolerance issues.  In this case the patient would do well to take less drug since they can have an adequate response while avoiding side effects which would lead to intolerance of the excess dosage.   Drug metabolism and uptake: CML TKI drugs only do their work controlling CML while inside leukemic cells which are capable of dividing.  They do not accomplish their task in the GI tract, or in the blood plasma, or inside non-blood cells, or inside normal (non-leukemic) blood cells.  So the TKI drug must go from pill form in the stomach to the drug inside leukemic blood cells if there is to be any benefit.  This occurs in two stages, and if either of the two uptake stages are deficient, drug response will be degraded.  First, the drug must break down in the alimentary canal and get into the blood plasma.  This first level of drug uptake occurs primarily through the stomach and duodenum.  This uptake occurs best in an acidic environment, which is the normal state in these organs.  If the normal acidic processes are disrupted (antacids, PPI drugs, etc) drug uptake can be reduced.  If binding agents (calcium pills, etc) are present at the same time, some of the drug may bind to those agents.  Drug uptake is generally not much of an issue unless the patient does something to limit that uptake, although it could be an issue for a small set of patients.  The level of drug uptake can be assessed through a blood plasma test, although this test may be hard to find because the US FDA virtually shut down the process at one point over a disagreement with one of the TKI drug makers.  Once the TKI drug has been absorbed from the GI tract into the plasma the second level of drug uptake is from plasma into the leukemic blood cells.  This process is not the same for each drug, and is not fullly understood.  Gleevec is taken into blood cells via the cellular OCT1 pump, while Sprycel and Tasigna do not use OCT1 but rather appear to be taken up more passively.  There is little information about the other TKI drugs.  Some people have low OCT1 activity, so Gleevec often does not work as well for those patients, but other drugs could work better.  This is just one reason why a simple drug switch can produce dramatic results.  Although not an uptake issue, if the body clears the drug too quickly from the plasma, there can be reduced effectiveness.  Less is known about what might cause excessive drug clearance to occur.  But other drugs and some herbals might be a cause of accelerated TKI drug removal from the plasma. This issue often affects dosage required to gain the same level of response.  Some patients can do as well on half or quarter the dosage another patient requires.  Leukemic cells usually have more than one BCR-ABL messenger protein (ATP binding site) which must be shut down by the TKI drug, so it is not enough to have just one molecule enter the leukemic cell.  It is mainly a matter of getting the drug into the leukemic cells, and sometimes a leukemic cell may require more than others.  http://clincancerres.aacrjournals.org/content/early/2011/01/21/1078-0432... TKI Activity Inside the Leukemic Cell: Once the TKI drug is inside the leukemic cell it must bind to multiple BCR-ABL messenger proteins (ATP binding site) to shut down the cell replication signaling process.  I say multiple since there are more than one in most leukemic cells, so there must be multiple inhibitions of the BCR-ABL ATP binding sites in each cell.  So to shut down a leukemic cell there must first be adequate drug inside that cell.  After all BCR-ABL ATP binding sites have been shut down by the TKI drug the leukemic cell cannot reproduce, so it either self-destructs or lives out its remaining life as a neuter.  This is the goal of a TKI drug, to stop leukemic cell replication and thereby prevent future generations of leukemic cells from being created.  When enough leukemic cells have been neutered by the TKI drug only the highest level leukemic cells remain, which are much harder to neuter and/or kill off.  To learn more about the genetics of CML read my paper on the issue: http://treyscml.blogspot.com/p/special-topics-on-cml.html If there is sufficient drug inside the leukemic cell but the cell continues to divide, then the drug is not doing its work for one of several reasons.  The reasons could include mutations in the BCR-ABL messenger protein (kinase mutations), BCR-ABL gene amplification, variations in the type of BCR-ABL the patient has, and unknown reasons.  There is still much to learn about the issue.   Drug Discharge Rate from Cells: Just as drug uptake into leukemic cells is important for drug response, keeping the drug inside leukemic cells in sufficient quantity is also important.  Recent studies have shown that for some patients the TKI drugs (especially Gleevec) can cause increased activity in the enzyme ABCB1 which expels drugs and other things from the cells.  This higher level of this enzyme called €œABCB1 drug efflux transporter (a type of cellular garbage man) might cause drug expulsion and decrease TKI levels inside the leukemic cells.  Since the drug only works inside leukemic cells, this more rapid drug expulsion from the cells could decrease drug response.  Recall that there are multiple BCR-ABL signaling enzymes inside each leukemic cell, so it takes numerous molecules of the TKI drug to effectively inhibit each leukemic cell and shut it down.    Kinase Mutations: Kinase mutations are changes in the BCR-ABL messenger protein (ABL portion) structure at the location where the TKI drug must bind to shut down the leukemic cell replication process.  If this "docking site" for the drug mutates and changes structure, the drug attachment may be weakened (reduced drug response) or made impossible (drug failure).  There are over 100 known ABL kinase mutations, although possibly 33 can cause some level of TKI drug resistance for Gleevec, and only a few of these 33 can cause resistance for Tasigna, Sprycel, or Bosutinib.  Ponatinib does not have any known kinase mutation resistance issues.  These kinase mutations have names such as Y253H, E355, F255K, T315i and others.  While kinase mutations can be a major cause of Gleevec resistance, slower response, or even drug failure it is not a major cause of resistance in the other drugs, although it can happen occasionally.  This leaves the cause of most TKI drug resistance to other issues, even though Oncs are often quick to blame kinase mutations.  Kinase mutations cause possibly 1/3 of drug resistance/failure for Gleevec, but only about 5% of non-Gleevec resistance/failures. TKI kinase mutations probably do not develop over time, but more likely are there from the beginning of the disease.  Often Gleevec will appear to work well for a time, but then be reduced or fail.  This may be due to Gleevec handling the non-mutated leukemic cells but as time goes by the mutated cells are not shut down and therefore continue to expand so the response is lost.  Supporting this theory is the fact that kinase mutations almost always appear within the first 2 years of diagnosis.  If a patient can get past 2 years, then kinase mutations rarely appear.  For the very few exceptions these could have been recessive or could have developed over time, but that is unknown.  We have learned that low dosage is not an inducer of kinase mutations, although that was initially a concern.   Differences among TKI drugs (Pharmacokinetics): Each TKI drug is different, making it function differently.  Each binds in a different pocket of the ABL kinase, otherwise there would be patent violations.  A good overall comparison of the various TKI drug €œpharmacokinetics (methods of drug action) can be found here: http://asheducationbook.hematologylibrary.org/content/2013/1/168.full Because each TKI drug works slightly differently, these various drugs are able to defeat certain aspects of the disease to better or lesser degrees.  Each drug has its strengths and weaknesses.  Gleevec has not been outdated by the later TKI drugs.  Some drugs are said to be stronger than Gleevec.  That is not an accurate statement, although second generation TKIs may work better for some patients.  But those difference can come at a cost, which is inhibition of more off-target kinases which do not affect the course of the disease and may cause side effects.    Alternate Signaling Pathways in Leukemic Cells: Although CML is highly dependent on the BCR-ABL signaling process, it is not solely dependent on it.  Higher level leukemic stem cells have multiple pathways to ensure survival, which makes them difficult for TKI drugs to kill off.  This is why TKI drugs are not thought to provide a cure.  It is possible that lower level leukemic cells can acquire this alternate pathway survival mechanism in some patients.  This is why there has been much research on this subject, and we see other pathways such as STAT, LCK, HEDGEHOG, LYN, CRKL and others being targeted.  In fact, Sprycel and Bosutinib are effective in some patients because they target the alternate pathway SRC in addition to BCR-ABL.    Drug Interference: There can be drug-to-drug interaction which decreases efficacy.  They appear to be rare but little is known about the issue.  I would not assume that if an interaction checker says the drug is OK to use with a TKI that this is absolute.  If there is lack of adequate response I would experiment with changing the other drugs if possible.    CML Genetics: We do not all have the same type of CML.  95% have either b2a2 (e13a2) or b3a2 (e14a2) or both.  A few have e1a2, and some have very rare other breakpoint types.  Some patients have a der 9 deletion (ASS deletion) (maybe 10%).  Some have three or four way translocations of the Philadelphia Chromosome, or other secondary chromosome mutations (trisomy 8, monosomy 7, or others) or even secondary translocations (all of these can only be found by a BMB).   So not all CML is the same.  Some leukemic cells have alternate splicing.  Most CML has a rather normal Philadelphia Chromosome rearrangement whereby the chromosomes 9 and 22 break at the normal spots and then rearrange in the expected fashion on the new chromosome 22.  Although the b2a2 (e13a2) and b3a2 (e14a2) are the most common, statistically the b2a2 responds more quickly than b3a2.  Also, within both b2a2 and b3a2 there can be alternate splicing whereby certain micro-genetic material is deleted during the translocation, changing the nature of their BCR-ABL signals slightly.  While the Philadelphia Chromosome (chromosome 22) is the cause of the BCR-ABL signals which cause CML, the chromosome 9 called der(9) may form in a way which puts out its own signals, although for most the der(9) is benign.  But this is less of an issue than it was in the days before TKI drugs. Some patients can have an amplified Philadelphia Chromosome, meaning it puts out stronger signalling.  This is because there are two copies of the BCR-ABL gene on the Philadelphia Chromosome, so it sends out twice the BCR-ABL signals.  Whether this can be seen during a BMB I am not sure, but it would be somewhat longer than the standard Philadelphia Chromosome.    CML Phase: The Phase at diagnosis is important when it comes to drug response.  I want to warn patients that not all definitions of Advanced and Blast Phase CML are properly diagnosed, so get an expert opinion.  The later the Phase, the more the CML has gained certain advantages.  And it is not just that there are 3 phases, but also late Chronic Phase can react differently for some than early Chronic Phase.  Gleevec should not be used for advanced phase CML, so Sprycel may be the best available drug option.  Blast Phase diagnosis means the patient may respond to TKI drugs for a while (maybe a year or so), but the probability percentages for long term good response are not favourable due to the morphed genetics which are irreversible once they happen.  A very few Blast Phase patients may be able to have a long term stable response using TKI drugs, but most would be well advised to consider bone marrow transplant, or at least try to find a matched donor just in case.  For Advanced Phase CML the TKI drug will normally work, but I would advise more careful monitoring than Chronic Phase patients may require.  Remember my warning to get an expert opinion about the Blast Phase diagnosis since it can be misdiagnosed.   Patient Specific Issues: Some patients have other conditions which can impact drug response.   These could include diabetes or other conditions.     Unknown Causes: Much is known about CML and the TKI drugs, but even more is unknown.  These drugs are still cutting edge, and there is much more to learn about them.  Hard as it may be to understand or accept, the major cause of TKI resistance and lack of response will often be unknown to the patient.   Adherence: Some patients do not take the drugs as prescribed.  I put this last because this is less often the case with drugs that are clearly life savers.  Of course, Oncs suspect this first, not last.  But if a person is interested enough to be reading this, then they likely do not have this problem.   What level of response is good enough? While there is no clear answer, it appears that a stable CCyR is good enough for long term survival, so stable CCyR should not be considered a poor response in the sense of survival.  Below average or suboptimal response is not drug failure as long as the response trend line is stable or downward.  And some can overcome this lackluster response with higher dosage, or by switching drugs, so if someone is stable CCyR and wants to switch drugs to get a better response, that is understandable and is what I would do in that situation.  Stats show that approximately 40% of patients move to a second line drug, but many of those are not actual failure or suboptimal response but intolerance of certain side effects.  Stable MMR has been shown to have nearly 100% survival rate (I must add here this assumes continued taking the drug, not MMR in cessation where there is no real data).  Anything better than MMR such as PCRU is just more better. But Oncs and patients have set the standard for good response very high, which leads to more questioning about why a patient is so slow in responding.  While it may not help much, today's "slow" was called "unbelievable" just 15 years ago.  And many Oncs improperly compare patient responses to each other instead of to NCCN treatment guidelines, causing patients to look at failure to achieve MMR in 1 year as unsatisfactory.  Such "expectation inflation" is unprofessional and a disservice to patients.   Good Initial Response Followed by Loss of Response: If the patient has a good initial response followed by loss of that response, normally within the first 2 years after diagnosis, it can be assumed there is not an issue of the two stages of drug uptake, unless something has changed such as taking other drugs.  Drug resistance due to kinase mutations occurs most often during the first 2 years after diagnosis, and after that it does not normally appear.  Other causes of loss of initial good response are likely due to rapidly killing off the leukemic cells the drug can handle, then the harder to handle leukemic cells come to the forefront and the response is lost.  If the patient makes it through 2 years and has a good response, that good response will probably continue over the longer term.  The first 2 years are the critical time for potential loss of response.   Multi-TKI Drug Failure: Here is an article about how to proceed after multiple TKI drug failures, and there is no reason for me to try to summarize what it says: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4344171/   At this point you may say: That does not tell me why I have a poor drug response.  Probably true.  I do not know, and neither does anyone else.  If your response is truly unacceptable (no sustained CCyR), the best you can do is change drugs until you find one that has the best response with the fewest side effects, or else increase dosage of the present drug.  If you have severe side effects no matter the drug, then the best you can do is take as much of the drug as is tolerable and deal with the side effects as best you can.  If you are sustained MMR and want to complain, changing drugs is a reasonable option just to see what happens -- these drugs are not best friends, just tools to use.  But if you are sustained MMR and just want to complain that it is resistance and sucks, tell someone else.  At this point you may say That's not fair€.  Of course it isn't.  Then at this point you may say is that the best you have to offer?.  Yes, it is.  But as more is learned I will edit and add to this post.   Reading Room for Overachievers:   Dynamics of TKI Response http://www.haematologica.org/content/99/11/1701

Thanks for posting Trey's extensive, fascinating and elucidating words, which I have been lucky enough to read before and benefit from greatly.

However.

Nimbus's question still stands:  If there are ANY detectable CML cells left, are they not capable of mutating?  (And remember, even when "undetectable," untold numbers of leukemic cells are still there, just not countable.)  After all, the accumulated experience we have of all cancer treatment is that eventually the cancer finds a workaround the treatment.  That's why Druker et al's work was so astounding - it seems (cross all our collective fingers) that this is not the case with CML and the TKI treatment.  When I finished Mukharjee's fabulous The Emperor of All Maladies, I was pretty darned depressed.  The bottom line seemed to be:  cancer ALWAYS wins, in the end.  Then I read The Philadelphia Chromosome and cheered up a smidge.  What I remember as a real turning point for me, in the early days of exploration in this survival thing, was a speech Druker gave in which he was quoted as believing that CML patients on TKI's could expect to survive even 30 years or more.  I felt that was a much better deal than I had feared!  Now, of course, I want much more time, for all of our younger CML friends.  

But we still don't know.  And we still can't get at the original stem cell providing the initial mutation.  I've said this before, and I make people exasperated and angry, but my oncologist, who is a world-known CML research specialist, is a skeptic of low-dosing.  He tried to explain his worry to me by using an umbrella analogy:  A lot of a drug will cover - like a big umbrella - hopefully all or almost all of the CML;  a smaller umbrella will cover only some, leaving CML cells outside the coverage that are uninhibited and they can go on and thrive and replicate.  His fear is that some of those, given enough time and if the environment is right, can mutate.  Then you're talking blast crisis, presumably.  That's his worry, and he HATES me being at 20 mg Sprycel to keep the pleural effusion I have at a minimal level (my only known side effect).  So far, I remain in the 0.005% IS range.  But, for how long?

Scuba patiently explained the concept of keeping a cell population low enough so as to not be a viable threat.  I have forgotten the scientific name of it - sorry, Scuba!  I hope he will come in here and restate it for all.  It has helped me not be afraid, and I hope he's right.

And we could all get hit by lightning tomorrow and die - just kidding.  I try really hard not to focus on whether or not I will lose response.  Most days I'm successful at this, but beginning about 4 - 6 weeks before my next PCR, I start thinking about it more.  At the end of the day, I think but cannot prove that we all have cancer, but in most cases the immune system finds it and deals with it before it gets out of control.  Problem is that once you get diagnosed, any thought of invulnerability leaves you forever. I used to feel invulnerable  - my mother is almost 100 and is generally quite healthy. My father lived to be 92 despite smoking 5 packs a day for 45 years and his mother lived to 105 - great genes right???

I feel anything but invulnerable now, and that's perhaps one of the more difficult mindset changes that I have had to deal with once I got over the initial shock of diagnosis and realized that I wasn't going to die in a year or 2. My wife keeps reminding me that we all have an expiration date and I just laugh at her, then I go off in a corner and worry about when mine is. I remember when I was first diagnosed reading a post from someone who 8 years after Dx was going to start trying to stop letting CML define her life.  At the time, I couldn't imagine how that could be possible.  But now, 2+ years into this and holding steady at <.003% for more than a year, I think it might actually be possible. I'm not there yet, but I'm closer than I was.  I want to be educated, thus I read Trey's post with great interest, but I also need to fight the urge to be obsessed with CML and try to live as normal a life as possible for however long I have.

Excuse my rambling, just needed to share.  We are the lucky ones within a very unlucky subset of the population.  We have TKI's and they work amazingly well for most of us. Could be worse; could be better, so for now carpe diem if you can and Happy Thanksgiving to all the yanks out there.

You may run into the K562 designation as you search the web for information about wild-type versus mutant BCR-ABL proteins.

Here's a link to "The K562 Story":  http://cmlc.ml/community.lls.org/topic/1177-the-k562-story/index.html