Q. I have a question about the Philadelphia chromosome. The Philadelphia chromosome is present in chronic myeloid leukemia. It’s also present in some case of precursor-B acute lymphoblastic leukemia. I guess I am assuming that the translocation will result in a similar gene product, but just in a different cell line. You mentioned that the Ph+ B-ALL in adults is a bad prognosis. I am wondering if this malignancy would be treated with imatinib, like CML, due to the drug targeting the specific protein that is formed (bcr-abl). If so, would there still be a bad prognosis? Thanks!

A. Great question! You are correct: a t(9;22) translocation involving the bcr and abl genes occurs in both CML and rare cases of precursor-B ALL. In some of the cases of Ph+ ALL, the genetic mutation (and gene product) is exactly the same as it is in CML; in others the breakpoint is slightly different, and a slightly different fusion protein is produced (but it is still a mutant tyrosine kinase with presumably similar effects on the malignant cells). Most cases of Ph+ ALL occurring in children have this different fusion protein, whereas only half of the cases occurring in adults have this different fusion protein (the other half of the adult cases have exactly the same breakpoint and product as is present in CML). It doesn’t seem to have any effect on prognosis whether you have the same fusion protein as in CML or the slightly different one.

You’re also correct that the genetic mutation occurs in different cell lines in each disease: in CML it is in the myeloid cell line, whereas in Ph+ ALL it is in the lymphoid cell line. One weird thing, though, is that in CML you can often find the Ph’ in lymphoid cells too. Strange. It indicates that the translocation between 9 and 22 occurs way back in a very early precursor stem cell, one that has not yet committed itself to the myeloid or lymphoid lineage. The occasional appearance of lymphoid blast crisis in CML indicates the same thing. We don’t see a similar picture in Ph+ ALL, though (there is no phase of the disease that involves myeloid cells). So perhaps in Ph+ ALL the translocation occurs in a lymphoid stem cell.

As for the treatment, it appears that imatinib is useful in Ph+ ALL. It doesn’t appear to get the patients into a long-term remission like it does in CML, unfortunately. But it does help get the patient into at least a first remission so that they can go on to bone marrow transplant (the official way to say this is that it improves early event-free survival). So the prognosis has improved somewhat for Ph+ ALL – but the outcome is nothing like the miraculous outcome in CML. Prognosis for Ph+ ALL is still very poor, both for kids and adults – in fact, Ph+ ALL has the worst prognosis of all the types of ALL.