Tuesday, May 14, 2013


One aspect of B cell receptor (BCR - aka antibody) physiology that I think is really remarkable plays out in several types of lymphoma (mantle cell and marginal zone lymphoma) but is most striking in CLL.  It is called BCR “stereotyping.”
You make antibodies to fight off flu, e. coli, salmonella, etc.  In fact you can make an enormous number of different antibodies to fight of just about any sort of invading micro-organism imaginable.  People who study this have determined you can actually make about 1,000,000,000,000 different antibodies (one trillion).  Recall that DNA is the master plans for RNA and RNA is the template for making proteins (such as an antibody).  Even though we have a TON of DNA in each and every cell (4 billion base pairs), if you had to have a different DNA segment for every antibody you could make, you wouldn’t have enough DNA (almost sounds like the federal budget type of numbers)!
So how do we possibly get one trillion different antibodies out of four billion base pairs and still have enough DNA left over for the rest of the things our cells need to do?  The answer is best illustrated in the childhood game of Mr. Potato Head.  Recall that brown kidney bean shaped doll where you could put different arms, different hats, and different legs on it to make an infinite number of different Potato Heads?  B cells do the same thing – except instead of a trademarked game from Hasbro, it involves a process called VDJ recombination.

On chromosome 14, you have a relatively small segment of DNA where most of the BCR gets made.  In order to make an antibody, the cell has to pick a “V=variable” a “D=diversity” and “J=joining” segment sort of like picking a set of arms, legs, and bow ties for Mr. Potato Head.  I forget the exact numbers but there is something like 70 V’s to choose from, 40 D’s to choose from, and something like 10 different J segments.  The smart math guy in the back of the room would quickly point out that can only give you 28,000 different possible combinations.  The B cells have some special tricks up their sleeves though and can add in additional base pairs between the segments that they  produce out of thin air (non-templated base pair addition) and in the process called “somatic hyper-mutation” can swap out base pairs (this same process may actually cause lymphoma in the first place).  It takes three base pairs to select an amino acid (the building blocks of proteins) but if you add one to the chain it can cause a “frame shift” so that everything moves to the right and now you have one base pair from one amino acid partnering up with two base pairs from another amino acid to give you a totally different amino acid.  Everything downstream is also messed up too so you get a chain reaction.

With those sorts of tricks – you get to one trillion different antibodies.  No two people should EVER have a case of CLL where their BCR looks anything like another patients BCR.  Heck, there are something like 6 billion people on planet earth.  Even if every single one of them had CLL you would only have a 1/166 chance of having the same BCR as another CLL patient.
BUT about 1/3 patients with CLL have a BCR that looks functionally identical to somebody else – what gives?
Turns out CLL is really strongly driven by signaling through the BCR.  In the routine processing of cellular debris, some of the "cellular junk" can stimulate the B cells into action when they should probably just look the other way.  When cells die, they may expose proteins such as vimentin, myosin, etc.  Sometimes those proteins can stimulate a B cell when it shouldn’t.  Since lots of our cells are always dying there is a persistent stimulus for those B cells.  Leave that stimulus on long enough and those B cells keep growing and growing and growing trying to fight off what it thinks is an invading micro-organism when in fact it is just some cellular debris that your body will never actually get rid of.
Since we all have the same myosin, vimentin, etc. we can stimulate BCR’s that look pretty much the same.  Therefore we can make, “stereotyped” B cell receptors.   

So what you ask?
The “why it all happens” is far less interesting than what it means in the clinic.  If you accept that 1/3 patients with CLL have similar receptors, they break up into very distinct subgroups and those subgroups can behave in very characteristic ways.  Knowing about stereotypes can have a big impact on prognosis. It may help to review my prior post about new prognostic markers.

Subgroup 1?  These patients typically need treatment very soon after they walk into clinic the first time.  If you compare the typical BCR mutated/unmutated status, these guys blow them both out of the water.  They may also have a higher frequency of NOTCH mutations and trisomy 12 or other high risk abnormalities like p53 mutations.

Subgroup 2?  They are bad actors.   It is clear that they have a high frequency of SF3B1 mutations which indicate that fludarabine won’t work very well.  They tend to progress early and may also have a higher frequency of del 11q.

Subgroup 4?  The average age of a patient with subgroup 4 is actually 43 years old (whoa nelly – I thought CLL was average 71).  Furthermore, these patients have exceptionally slow growing CLL.  I recently met a young woman with CLL in her early 30’s who was pregnant at the time.  In my mind I bet she is subgroup 4 and I would love to know that because it might put her at ease that she is likely to see her baby grow up.  Their BCR is typically IgG subtype instead of the customary IgM variety.
Subgroup 8? They have an extremely high rate trisomy 12 and Notch mutations.  We know Notch is a bad thing to have – but it is really bad in this subgroup because 75% of these patients will experience Richter’s transformation within 5 years - and those patients destined to transform all have the Tri12/Notch combo.

All told, about 1/3 of patients will have a particular stereotype but there are a ton of different stereotypes someone can have.  About 1/10 CLL patients will have one of the frequent ones  (subgroups 1-8).  As we get to know these subgroups more, I think it will add quite a bit of value to knowing if you are stereotyped or not.

Some of the relationships get really interesting.  We know that certain lymphomas arise as a response to an infection.  In fact, you can sometimes cure gastric marginal zone lymphoma by treating the bacteria that causes stomach ulcers.  Subgroup 4 often has evidence of a viral infection with either CMV or EBV.  Another subgroup looks like it is trying to fight off certain types of yeast.  It is really not that farfetched (THOUGH NEVER FORMALLY TESTED – don’t try this at home) that you might be able to treat some of these cases of CLL with either anti-viral drugs or anti-fungals. 
The problem is that we don’t currently test for stereotypes. Darn!  Sort of like Notch, SF3B1 and other new markers – testing for stereotypes is not part of current CLL management.
I think we will be testing for these soon enough though and it will be interesting to see how these markers get utilized into clinical practice.  Ask most general oncologists and they are totally unaware of this topic.  Even a lot of hematology oriented docs are aware of the topic but may not have much familiarity with the different subgroups.
Frankly, the Europeans have been kicking the Americans backside on this topic.  It is the Greeks and the Italians who have been the champions of the topic. 
Anyhow – hope you find it as interesting as I do.  My flight is coming in for a landing – have to turn this off now….