Overview
Chronic myeloid leukemia (CML) is a clonal myeloproliferative neoplasm, predominantly affecting granulocytes. The incidence of CML is approximately 1-2 per 100 000 adults – resulting in 50-60 new cases per year in British Columbia – with a median age of onset of 57. CML is most often characterized by the reciprocal translocation of chromosomes 9 and 22 – t(9;22)(q34.1;q11.2). The resulting derivative 22 chromosome (der22) is colloquially described as the Philadelphia chromosome (Figure 1).
Figure 1: The reciprocal translocation of chromosomes 9 and 22 resulting in the Philadelphia chromosome
This translocation generates a novel fusion gene (BCR::ABL1) composed of the 5’ portion of the BCR gene on chromosome 22 and the 3’ portion of the ABL1 gene on chromosome 9. Several variants of the BCR::ABL1 fusion transcript are known. The most common variant (major variant, p210) fuses either BCR exon 13 or BCR exon 14 to ABL1 exon 2 (e13a2 and e14a2 respectively) and is associated with a classic CML presentation. The minor variant (p190) fuses BCR exon 1 to ABL1 exon 2 (e1a2) and may be associated with monocytosis. Finally, the micro variant (p230) fuses BCR exon 19 to ABL1 exon 2 (e19a2) resulting in neutrophilic maturation (Figure 2). Fusions involving other exons of BCR or ABL1 have also been described (Note: Chronic myelomonocytic leukemia and chronic neutrophilic leukemia proper are different entities and do not harbour a BCR::ABL1 fusion.)
Figure 2: Panel A: Genomic organization of BCR and ABL1. Panel B: Genomic organization of the common BCR::ABL1 fusion variants in CML. Red bar indicates location of the ABL1 kinase domain.
Regardless of the transcript variant present at diagnosis, it is invariably an in-frame event resulting in a novel kinase – unhindered by the regulatory domains present in the parent proteins. Expression of the fusion protein then results in the inhibitor sensitive activation of the JAK-STAT, AKT, and RAS signaling pathways.
Natural history
In the absence of targeted therapy, CML typically progresses through three phases. Most patients present in chronic phase (CP). This phase is variable in duration and is characterized by leukocytosis (predominantly myelocytes and segmented neutrophils). Eosinophilia, basophilia, and monocytosis may also be variably present. Blasts are not substantially increased nor is there significant granulocytic dysplasia.
Left untreated, CML patients will progress to accelerated phase (AP) and then to terminal blast phase (BP). The former is characterized by increasing white cell count, increasing splenomegaly, thrombocytosis (or thrombocytopenia), basophilia, increase in blasts, appearance of novel cytogenetic abnormalities (in addition to the Philadelphia chromosome present at diagnosis), as well as a dysplastic bone marrow. Blast phase disease is characterized by > 20% blasts in the bone marrow or peripheral blood (or an extra medullary, infiltrative proliferation of blasts). Blasts are most often of myeloid origin however in 20-30% of cases patients present with lymphoid blast phase (most often B-cell).
Prior to the advent of BCR::ABL1 targeted therapy in ~Y2K the 10yr survival was approximately 50% (patients treated primarily with Interferon and/or stem cell/bone marrow transplant). Subsequent to the introduction of targeted therapy the overall survival of CML patients fully compliant to therapy is not appreciably different than that of the population at large.
Testing – Diagnostic
The laboratory diagnosis of CML is currently and most commonly performed by interphase fluorescent in situ hybridization (FISH) on peripheral blood derived leukocytes (see BCR::ABL1 FISH (Dx)). In the event that a diagnosis of CML be made from a peripheral blood specimen, follow-up karyotype analysis of a bone marrow specimen is indicated for disease staging purposes. Should a bone marrow specimen be obtained for other investigations both FISH and karyotype analysis may be performed on this specimen.
Testing – Pre-treatment testing (Baseline)
Should the initial diagnosis of CML be made from a peripheral blood specimen, follow-up karyotype analysis is indicated from a bone marrow specimen for disease staging purposes.
Following the initial diagnosis of CML, reflex molecular testing from peripheral blood derived RNA is indicated. This testing allows the laboratory to confirm that the molecular lesion present at diagnosis is amenable to molecular monitoring post initiation of treatment and by which method monitoring should proceed (see below). In the case of either a p190 or p210 transcript variant this testing also allows for the establishment of a baseline molecular burden for the patient.
Baseline testing follows the same process and method as subsequent MRD testing.
Testing – Monitoring Response to Therapy (MRD testing)
Once treatment with a TKI (most commonly Imatinib) has been initiated, monitoring response to this treatment is initially indicated at three month intervals.
Monitoring is performed by one of four different methods. For e13a2(p210) or e14a2 (p210) positive disease, monitoring is by quantitative RT-PCR (QRT-PCR) and reported against the international scale (internationally accepted normalized measure of molecular burden). For e1a2(p190) positive disease, monitoring is by QRT-PCR and reported simply as a molecular burden (not normalized to the international scale). For e1a3(p190like), e13a3(p210like), e14a3(p210like), and e19a2 (p230) positive disease, monitoring is by QRT-PCR but reported simply as positive or negative for the presence of the transcript variant. For all other transcript variants, monitoring is by standard RT-PCR and is reported simply as positive or negative for the presence of the transcript variant.
Monitoring continues at three month intervals for patients with uncomplicated, low risk, p210 positive disease treated with Imatinib, 400mg daily until such time as they have achieved a sustained “major molecular response” (MMR). MMR is defined as a 3 log reduction from the internationally defined average baseline value (100% at baseline to 0.1%). After achieving and maintaining MMR for a period of approximately one year, routine molecular monitoring then moves to six month intervals.
For CML patients harbouring transcript variants other than the p210 encoding e13a2 or e14a2 variants, molecular monitoring continues at 3 month intervals indefinitely. Similarly, patients being treated with a suboptimal dose of Imatinib (due to intolerance), an increased dose of Imatinib (resistant or high risk disease), or any second or third line TKI, molecular monitoring continues at 3 month intervals indefinitely.
Finally, some patients may achieve what is termed a sustained deep molecular response to therapy after several years of treatment. Depending on the clinical context, these patients may entertain discontinuing therapy in the hope of entering into and maintaining treatment free remission (TFR). Approximately 50% of patients attempting treatment discontinuation fail to enter long term TFR with the majority of these having a molecular relapse within one year of discontinuation. In order to identify these patients and to re-initiate TKI therapy prior to hematologic relapse (or symptomatic presentation), routine molecular monitoring is available at monthly intervals for one year. After one year, routine molecular monitoring moves to three month intervals.
It should be noted that the above testing intervals are for routine monitoring. Testing between intervals for non-routine instances is always available (following prior consultation with the laboratory).
MRD Testing modalities
Testing – BCR::ABL1 Kinase Domain Mutation Screening
While response rates to TKI therapy are impressive, it remains that some patients progresses on therapy. In the event that a fully compliant patient progress while on therapy, mutation screening of the BCR::ABL1 kinase domain may reveal the presence of a mutation. If a mutation is identified, the specific mutation may guide the choice of the next line of therapy. In general, kinase domain mutation screening is routinely informative only after a demonstrated 1 log increase in molecular burden and a molecular burden at the time of testing at, or in excess of, 1% (log burden of -2.00).
Selected References
- Hehlman R, Chronic Myeloid Leukemia in 2020, HemaSphere (2020), 4:e468 (PMID: 33134861)
- Hochhaus A, et al, European LeukemiaNet 2020 recommendations for treating chronic myeloid leukemia, Leukemia (2020), 34:966-984 (PMID: 32127639)
- Jabbour E and Kantarjian H, Chronic myeloid leukemia: 2020 update on diagnosis, therapy, and monitoring, Am J Hematol (2020), 95:691-709 (PMID: 32239758)
- NCCN guidelines – CML v1.2021 (https://www.nccn.org/guidelines/category_1)
- WHO Classification of Tumours of Haematopoeitic and Lymphoid Tissues – 4th edition (2017) pp