Wednesday, August 8, 2012

Trisomy 12 in CLL / Notch Mutations

Another common FISH abnormality in CLL is trisomy 12 (literally "three body") in which there are three copies of chromosome 12 whereas the normal allocation is just two copies.  There has been a lot of recent excitement about additional abnormalities found in trisomy 12 CLL but I wager there is a lot more to be found.  Even though chromosome 12 is a "mid-size" chromosome, there are literally thousands of genes on the chromosome so I expect we will find more information in the near future.

Before we had FISH technology and the only way to study chromosomes was "metaphase cytogenetics" where you literally grew up the cells in a dish, took out the chromosomes, froze them just before the cell was going to divide (ie. metaphase) and visually inspected them, trisomy 12 was one of the most common abnormalities found.  With FISH, we've become a lot better at finding chromosome 13q changes so it has fallen from the top spot.

Trisomy 12 CLL has a distinct personality among cases of CLL.  When you look at the cells under the microscope, they have a lot more "atypical" features.  In an individual patient, there may be a higher frequency of "prolymphocytes" which look a little more aggressive.

There are several key clinical features of trisomy 12.  One feature I think is intersting though I have not seen a lot of literature about it is that it has a considerabally higher amount of CD20 on the surface of the cell.  CD20 is the target of rituximab / ofatumumab / GA-101 and a variety of other research antibodies.  One of the reasons we don't think rituxan is as effective as a single agent in CLL compared to most of the lymphomas is that it has a lot lower CD20 on the surface of the cell.  The following paper actually suggests a higher response rate to single agent rituximab in cases with trisomy 12 (actually they also gave gm-csf but the rituxan did most the heavy lifting).

Chronic lymphocytic leukaemia CD20 expression is dependent on the genetic subtype: a study of quantitative flow cytometry and fluorescent in-situ hybridization in 510 patients

This CD20 differential may become important as combination biologic regimens begin to emerge.   I look forward to anti-CD20 in combination with BCR signaling inhibitors or BCL-2 inhibitors.  It would be very interesting to me if those patients with trisomy 12 did better under those conditions.  That is purely speculative on my part for now, but something I would like to investigate.

In the landmark Dohner paper in New England Journal of Medicine that identified impact of FISH on survival in CLL, trisomy 12 was considered an "intermediate" prognostic feature.

Genomic Aberrations and Survival in Chronic Lymphocytic Leukemia

It has recently been discovered however that a considerable fraction of trisomy 12 cases (somewhere between 1/3 to 1/2 may have a mutation in a key protein called NOTCH.  A mutation in NOTCH is a really bad thing - just about as bad as having a mutation or deletion of p53 / 17p, or another bad actor Myc.  We don't currently have convenient tests for testing for NOTCH unfortunately.  I wager though that once we do, trisomy 12 will either be a good prognostic factor (in setting of normal NOTCH), or bad factor (when NOTCH is mutated)

NOTCH1 mutations in +12 chronic lymphocytic leukemia (CLL) confer an unfavorable prognosis, induce a distinctive transcriptional profiling and refine the intermediate prognosis of +12 CLL

NOTCH is a protein at the surface of the CLL cell.  It waves its receptor out to other cells and when it comes into contact with the right signal it causes the part of the protein on the inside of the cell (conveniently called the intracellular domain aka ICD) to be released from the membrane to go into the nucleus of the cell where it turns on a lot of different genes related to cellular proliferation.  It goes about turning on genes until another protein called FBXW7 comes around and attaches a tag on NOTCH that targets it for destruction.  Interestingly FBXW7 has also recently been identified as a gene periodically mutated in CLL which is a well developed theme - there are lots of ways to activate the same pathway.

Activation of the NOTCH1 pathway in chronic lymphocytic leukemia

There are a handful of drugs out there that target the NOTCH pathway.  One way to target the pathway is to inhibit the enzyme that cuts it free from the cell membrane.  These are so called "gamma secretase inhibitors."  Unfortunately, this enzyme is important in the bowel and diarrhea has been a big problem.  There are other ways to go after the NOTCH protein and the hope is that some of them may not have the same GI issues.

I bet in time, we find a number of other abnormalities associated with trisomy 12.  Whether there are other miRNA's or other features.  In the quest toward personalized medicine this is the level of detail that will be required.