Limited value of the FISH test
If you read our recent “Updates” article on three important blood tests, you should be able ace any pop-quiz on FISH testing.
In a nutshell, FISH test looks deep inside your CLL cells, at the very core of them, and determines if any of the more common chromosomal aberrations have occurred in the DNA of the CLL cells. Deletion of a particular snippet on the 17th chromosome, called 17p53, is known to be the most dangerous.
So, if you get your FISH test done and you see no reference to the dreaded 17p deletion, does this mean you are home free? Not so fast. FISH is an important test, but it is not the whole enchilada.
Several research papers published in recent months have identified details proving there is more to this fishy story. But before we discuss these new articles we need to fill in a bit of the background. As most of you know by now I like to use silly analogies to explain things. Here is a real-life analogy that I think is pretty nifty if I say so myself. Read on, I promise this won’t hurt one bit.
Lost, vandalized, or generally messed-up
Your home is over-run by roaches. Like most of us you dithered for awhile, hoping the nasty critters would go away all by themselves, or at the very least the problem will not get much worse than it is already. You hope. By now the roaches are eating you out of hearth and home, and it is time to do something. You are keen to get rid of the roaches; but you are not too thrilled with the idea of using off-the-shelf insecticides that may do a good job of killing roaches but are also toxic to you, your kids and pets.
You heard about this really powerful roach killer that is perfectly safe to everything else. The further good news is that you don’t have to pay some greedy pesticide company an arm and a leg to buy the stuff, you can make it yourself, it is perfectly easy to brew up a big batch of it right in your own home, provided you have accurate how-to make instructions. Sounds perfectly wonderful and you are sold!
You head to the local library to get hold of “how-to” instructions on making this wonderful “organic” roach killer. The nice lady at the front desk knows exactly the book you are looking for, in fact it is the only book in the library with the necessary instructions you need to concoct the roach killer. She directs you Isle 17, book “p”, page 53. You get it? Isle 17, book “p”, page 53 :-)
It is not your lucky day. Looks like you have to live with the roaches for quite a bit longer unless you are willing to use dangerous pesticides in order to keep them under control. Here are three possible un-lucky scenarios you may face:
- When you get to Isle 17, to your dismay some one has torn out page 53 of book “p”. Without this crucial page containing instructions, you wont know how to make the wonder roach killer.
- Or, the book is there, but some hooligan has stuck together the crucial page 53 with big wads of chewing gum. Without access to this crucial page 53 the book is next to useless since you have no hope of following the “how-to” instructions.
- Going one step further, the book is there and you check it out. But when you get home and start making the brew that is going to get rid of your roach problem, you notice someone has blacked out one or two crucial words in the instructions on page 53. It is not exactly clear what you should do. You scratch your head and decide to take a chance and guess at some of the words and make the brew anyway. But when push comes to shove, roaches continue to thrive just fine even after you sprinkle liberally the concoction you made – you just made a batch of brew that is useless as a roach killer. Bummer!
Now your only options are to live with the roaches (it may no longer be a viable option) or go to the hardware store, decide which of the available but unpleasantly toxic pesticides you are going to buy, paying through your nose in the process.
Translating this to CLL-speak
Your white blood count (WBC) has been climbing steadily, you have developed a certain ‘pregnant’ look with enlarged abdominal lymph nodes. It is getting harder to shave with the nodes around your jaw line. Watch & Worry lasted only so long, now your bone marrow is heavily infiltrated with the cancer cells, preventing proper function of stem cells and formation of healthy red blood cells, platelets etc. You are keen to get the tumor load under control but you are not too thrilled with the idea of using any of the standard issue chemotherapy drugs that kill CLL cells effectively but are also toxic to the rest of your body, healthy cells and organs.
You heard about this really powerful killer of cancer cells, a protein called TP53, that is very effective in killing CLL cells but perfectly safe to everything else. The further good news is that this is a protein your body makes by itself and you don’t have to pay some greedy drug company an arm and a leg to get treated with their latest ‘miracle’ drug with toxicity to match. It sounds like a perfect solution and you are sold!
Under normal circumstances every healthy cell in your body has the “how-to” instructions for making this wonderful protein. TP53 protein acts as a suicide pill killing the cell if it turns cancerous. The instructions on how to make this protein are located on the short “p” arm of chromosome 17, exact zip code 53 – abbreviated to 17p53 gene. When cells have access to the step by step how-to instructions located at 17p53 gene, they are able to make the tumor killing protein called TP53, as needed. When and if a particular cell turns rogue (malignant), TP53 protein is made by the cell and the cell promptly commits suicide.
Most microscopically tiny clusters of cancer cells in our bodies are killed by their own home-grown TP53 protein, before you even know there is any problem, before the tiny cluster of malignant cells can grow fangs and become a real threat to your body. TP53 protein initiates a mandatory suicide (“apoptosis”) command that few cells can disobey. Lack of robust TP53 protein function is one of the ways in which cancer cells can turn a deaf ear to suicide commands and continue living and multiplying.
Cancer cells that have managed to turn off the ability to make proper TP53 protein are that much more likely to live long and prosper, since they have learned how to get around the suicide safety switch built into every cell specifically to administer the kiss of death when cells turn cancerous. Now it is easy to see why 17p53 deletion is such a strong prognostic indicator of an aggressive variety of CLL (or any other cancer, for that matter).
Here are some possible un-lucky scenarios by which your CLL cells may have learned to turn a deaf ear to suicide signals, by virtue of forgetting how to make good TP53 protein.
- FISH test reveals you are likely to be an aggressive case, a large percentage of your CLL cells have lost the particular snippet of information contained in the 17p53 gene – your FISH test reports the dreaded 17p53 deletion. Without the right instructions found only on gene 17p53, your CLL cells cannot make TP53 protein and can therefore avoid your body’s commands to commit suicide. Such cancerous cells continue to live and produce babies with abandon, leading to a rapidly proliferating cancer.
- Your FISH test says nothing about any 17p53 deletion and everything seems to be in order at first blush. But if your CLL cells have their 17th chromosome at the crucial p53 is gunked over with excessive methyl groups, your CLL cells cannot read the DNA directions, cannot make the necessary TP53 protein and can therefore avoid committing suicide. In this instance, methyl groups are biologic versions of ugly wads of chewing gum in our roach analogy. This is called epigenetic silencing, one of the mechanisms by which cancers can initiate and progress unchecked. I strongly urge you to read our earlier article on epigentics. Better understanding of this important process may yield better therapy options in the near future. Already several de-methylating agents (scrubbers to remove the chewing gum, as it were) are in clinical trials, especially at OSU.
- Even if 17p53 gene is still there in your CLL cells, and it is not gunked over by methyl groups and therefore not epigentically silenced, it is possible that an error has crept into the DNA sequence at that particular location. A mutation has occurred that garbles up the instructions just a tad on how to make proper TP53 protein. When the how-to instruction in your genetic code have been corrupted due to mutation, your CLL cells make an equally mangled and garbled version of TP53 protein. After all, the protein made is only as good as the instructions that went into its making. Mangled TP53 protein cannot do the job it is supposed to do, namely force cancer cells to die.
If for any of the above reasons your CLL cells cannot make the correct form of TP53 protein, you are up the proverbial creek without a paddle. Without this home-grown cancer killer to keep them under check, the CLL cells can turn a deaf ear to any suicide commands. Your body has a much harder time dealing with such rebellious cancer cells on its own. You have no choice but go to your local oncology practice and choose between the various expensive and often times toxic chemotherapy options available, in order to control CLL.
One last point and we can wrap this up. The library in its infinite wisdom chose to order two identical books, two identical 17p books with the same instructions on page 53. Even if one of the two books got stolen, page stuck together or otherwise vandalized, it is possible you will get lucky and find the second copy with the necessary instructions on how to make the roach killer.
In CLL lingo, each cell in your body (including CLL cells) has a pair of #17 chromosome, each one has the precious 17p53 gene. Having both copies in pristine condition is the best case scenario, but having even just one of the pair working right is acceptable and TP53 protein can still be made correctly.
All hell breaks loose if both copies of 17p53 are lost, silenced or otherwise asleep at the dead-man switch. Such cells are very rebellious and will not commit suicide on command. When cancer cells continue to have babies but refuse to die, we have a case of rampant population explosion, a rapidly growing cancer. Cancer cells without the ability to make TP53 are also a lot harder to kill by externally administered kick in the pants and patients with this chromosomal defect are likely to have poorer response to chemotherapy.
A well conducted FISH test can only speak to the presence or absence of the 17p53 gene. It can tell you whether one or both copies of this gene are physically missing. FISH testing cannot say anything about 17p53 genes that are physically present, but buggered up or gunked over in some fashion and therefore unable to do their job.
We need this new lab test
If you are with me this far, it is obvious that what we need is a lab test that looks for properly made TP53 protein, since the protein is the business end of the whole suicide switch. When rubber meets the road, the zillion dollar question is this: can your CLL cell make this suicide protein in sufficient quantities, and make it with no mistakes?
Right now, only a few of the expert centers are able to test for the TP53 protein. Most commercial labs do not offer this test, and I am willing to bet most local oncologists will not order it even if such a test is available, not until they have a lot more CME units under their belts and there is expert consensus on the need for such a test. And a lot more patients are aware of it and ask for it.
A couple of years ago CLL Topics negotiated with Quest Diagnostics to get the company to offer a comprehensive CLL diagnostic package at reasonable cost. FISH test was included in this package. It looks like we need to get back into that game once more, get our CLL experts to develop consensus on how exactly to test for TP53 protein and then get one of the commercial labs to offer the test at an affordable cost.
After a waiting a few days for members’ questions and discussion, I will review recent papers that highlight the importance of understanding the role of TP53 protein in cancer prognostics.
Did you like my roach analogy? I must say I got a chuckle out of it. PC would have made me change it to something less gross. Hah. He is no longer here to go tut-tut and wag an editorial finger at me, so roaches it will be.