As you saw on the previous page, WHO classification is heavily based on cytogenetics and genetics. Here is how to interpret the designations, taking the example of the first one listed - AML with t(8;21)(q22;q22.1);RUNX1-RUNX1T1:
- t = translocation. The two numbers in parentheses after t are the numbers of the chromosomes involved in the translocation.
- The numbers following "q" in the next set of parenthesis refer to bands on the q or long arm of the chromosome where that translocation created a fusion of two genes that initially had not been in proximity.
The last part, RUNX1-RUNX1T1, is the name of the actual genes involved. That fusion (RUNX1-RUNX1T1) leads to the production of a chimeric protein that disrupts the normal function of a transcription factor complex that regulates normal hematopoiesis.
In other words, a part of chromosome #21 broke off and attached to chromosome #8, resulting in the fusion of genes, which resulted in a protein that interfered with the regulation of hematopoietic activity—thus, unchecked proliferation.
Also, to correlate with the FAB classification, this particular type of AML corresponds morphologically with the M2 subtype in FAB.
Other designations that you may see include the designation inv, which means that part of the chromosome was inverted, such as in AML with inv(16)(p13.1q22); p indicates the short arm of the chromosome, whereas q indicates the long arm.
The image shows a blood smear of the AML with t(8;21)(q22;q22.1);RUNX1-RUNX1T1: subtype, showing myeloblasts (note the abundant cytoplasm in these blasts and azurophilic granules).
