Revisiting the need for bone marrow examination in chronic myeloid leukemia
Authors: Ayalew Tefferi, Curtis A. Hanson, Rhett P. Ketterling
Journal: American Journal of Hematology (2017)

Chronic myeloid leukemia (CML) is one of the most genetically visible diseases, credit to Peter Nowell (1928–2016) and David Hungerford (1927–1993), who discovered the Philadelphia chromosome (Ph1) in 1960.1 In 1972, Janet Rowley (1925–2013) uncovered the constitution of Ph1, as a reciprocal translocation between chromosomes 9 and 22; t(9;22)(q34;q11.2).2 In 1984, the Ph1 chromosome 22 breakpoint site was narrowed to a 5.8 kb “breakpoint cluster region”3 and subsequently shown to be part of the BCR gene.4 Around the same time, a new 8.5 kb ABL transcript was described5, 6 and subsequently identified as a BCR-ABL1 fusion transcript7, 8 that translated into a P210 fusion oncoprotein.9 In 1990, retroviral infection of hematopoietic stem cells with P210 BCR-ABL1 was shown to induce CML-like disease in mice.10-12 There are currently three popular laboratory techniques for detecting the CML-associated BCR-ABL1 abnormality: karyotype, fluorescence in situ hybridization (FISH) and reverse-transcriptase polymerase chain reaction (RT-PCR). Karyotype and FISH are semi quantitative methods while quantitative RT-PCR is the preferred technique for monitoring minimal residual disease (MRD). Furthermore, the overall accuracy of these methods varies by institution and commercial vendors. At our institution, karyotype is performed on fresh bone marrow aspirates and reported according to the International System for Human Cytogenetic Nomenclature (ISCN) criteria.13 We utilize a dual-fusion FISH (D-FISH) method to ensure adequate sensitivity and also determine/predict balanced 9;22 translocation (1R1G2F), complex translocation (2R2G1F), and molecular imbalances at the ABL or BCR regions (2R1G1F, 1R2G1F or 1R1G1F).14 BCR-ABL1 transcript levels are evaluated using quantitative RT-PCR with 0.01% analytical sensitivity and reported using the international scale (IS).15 Our discussion below assumes availability of adequately sensitive and accurate testing methods. In 1996, imatinib mesylate (IM) was discovered as a small molecule ATP binding site inhibitor of ABL.16 In 2003, the first results of the international randomized study of interferon and STI571 (IRIS) were published and recently updated in 2017 after 11 years of follow-up;17-19 at 18 months, the complete cytogenetic response (CCyR) rate with IM was 76.2% and progression-free survival 96.7%. At the time of the final analysis of IRIS, 6.9% of IM-treated patients experienced progression into accelerated or blast phase disease while event-free survival at 10 years was> ̴> 80% and overall survival 83%; the cumulative incidence of CCyR at the end of the trial was> ̴> 83% and CCyR loss was documented in 16%; among informative cases, major molecular response (MMR) rate at 10 years was 93% and 63% had MMR 4.5.19 One of the major lessons learned from IRIS and other similarly mature IM-based studies20 is the fact that the achievement of CCyR, and not necessarily MMR, was the most relevant milestone for overall survival. In this regard, one has to be acutely aware of the potentially flawed suggestion of an additional survival advantage from early molecular response or MMR, which does not take into account the fact that the comparator arm in such analyses includes patients who ultimately might or might not achieve CCyR; in other words, to be credible, such analysis should be restricted to patients who have otherwise managed to attain CCyR, sooner or later. Since the inception of IRIS, a number of other second generation tyrosine kinase inhibitors (2G-TKIs) have been developed and some have been compared to IM in newly diagnosed patients.21, 22 To date, however, despite a recurring theme of faster and deeper responses with 2G-TKIs (e.g. nilotinib, dasatinib, bosutinib, ponatinib, radotinib), or higher dose IM (800 mg/day), none of them have shown a significant overall survival advantage over standard dose IM (400 mg/day).20-22 Furthermore, the apparently higher drug potency displayed by 2G-TKIs might have been achievable with higher doses of IM.23 Accordingly, and also considering the better long-term drug safety profile, IM remains the first line drug of choice in chronic phase CML, regardless of risk score,24 with early switch to 2G-TKI recommended for suboptimal response.25, 26 Evidence-based guidelines for monitoring TKI treatment response in CML have been formalized by the European LeukemiaNet (ELN) but are seldom adhered to;27 the 2013 ELN recommendations consider “optimal” response in the presence of ≤10% peripheral blood (PB) BCR-ABL1 IS or partial cytogenetic remission (PCyR) at 3 months, ≤1% BCR-ABL1 IS or CCyR at 6 months, ≤0.1% BCR-ABL1 IS or MMR at 12 months. In a recently published IM-based controlled study that did not include other TKIs, these milestones were achieved by 69%, 61% and 55% of treated patients, respectively.20 Most recently, the possibility of long and safe treatment-free remissions in about half of TKI-treated patients achieving undetectable disease status (MMR 4.5) lasting for at least two years have been reported.28-30 Taken together, these observations highlight the prominent role of PB BCR-ABL1 IS monitoring in TKI-treated patients with chronic phase CML. What then is the role of bone marrow (BM) examination and PB FISH? In the 2016 edition of the WHO classification of tumors of haematopoiectic and lymphoid tissues, the authors for the CML section state “most cases of CML can be diagnosed on the basis of PB findings combined with detection of the Ph chromosome and/or BCR-ABL1 by cytogenetic and molecular genetic techniques. However, BM aspiration is essential to ensure sufficient material for a complete karyotype and for morphological evaluation to confirm the phase of disease. BM biopsy is not required to diagnose CML in most cases, but should be done if the findings in the PB are atypical or if a cellular aspirate cannot be obtained”.31 Similarly, the 2013 ELN recommendations at time of CML diagnosis include BM karyotype, FISH in case of Ph-negative cases and qualitative PCR for determination of BCR-ABL1 transcript type.32 For disease monitoring, ELN recommendations included quantitative RT-PCR for determination of BCR-ABL1 transcript level and/or BM karyotype at 3, 6, and 12 months until CCyR has been achieved and then every year; the authors allowed skipping the latter if adequate molecular monitoring or FISH can be secured. In addition, periodic BM examination was recommended in the presence of myelodysplastic changes or additional chromosome abnormalities (ACA).32 The above-stated sentiments from the 2016 WHO document and the 2013 ELN treatment recommendations for CML are internally consistent and appropriately noncommittal, in regards to the need for BM examination at diagnosis or during follow-up. Emphasis is instead placed on securing adequate blood tissue for facilitating genetic diagnosis and monitoring efficacy of treatment. The latter is also dependent on what one considers to be optimal activity and at what point during treatment; as mentioned before, current evidence is not convincing regarding long-term survival advantage, beyond that achieved with CCyR, from MMR/MMR4.5 or faster and deeper response patterns.20 BM morphology in CML is most useful in the setting of progressive disease and one might be concerned about misclassifying accelerated or blast phase disease as chronic phase disease in the absence of morphologic assessment. However, advanced phase disease is often accompanied by additional clinical and PB laboratory features that would help determine the need for BM examination. Furthermore, we do not believe that patients are necessarily harmed by the possibility of “inaccurate” disease phase designation, as long as they are closely monitored on TKI treatment, which incidentally might be a more accurate identifier of high risk patients, compared to BM morphology. In chronic phase CML, the possible prognostic value of BM fibrosis or underlying myelodysplasia requires examination in the context of large TKI-based controlled studies.33 Another line of argument used to defend the value of BM examination in CML involves clonal evolution (CE), defined by the presence of ACAs. In the current issue of the journal, colleagues from the MD Anderson Cancer Center addressed the particular issue in 603 patients with chronic phase CML treated with frontline TKIs. ACAs were seen in 5% of these patients and did not affect cumulative CCyR, MMR, MMR 4.5 or overall or event-free survival rates, regardless of whether or not the ACAs were classified as “major” or “minor” route abnormalities.34 Taken together, BM examination might no longer be essential for CML diagnosis or monitoring of TKI treatment response, unless in the presence of atypical disease features, suboptimal treatment response or signs of progressive disease. Our current practice includes performance of both PB D-FISH and RT-PCR at diagnosis and utilizing both techniques, as necessary, for monitoring treatment response; we prefer to use both techniques until CCyR is achieved by demonstrating both absence of BCR-ABL1 dual fusion signal by PB D-FISH and presence of ≤1% BCR-ABL1 IS by PB quantitative RT-PCR. In a previous publication, we have shown that PB D-FISH can effectively substitute for BM D-FISH and BM karyotype in documenting CCyR and lesser degrees of cytogenetic response; the correlation r2 in the particular study were 0.92 for BM FISH vs karyotype, 0.97 for PB FISH vs karyotype and 0.97 for PB vs BM FISH.35 Specifically, absence of BCR-ABL1 D-FISH signal was a reliable indicator of BM karyotype confirmed CCyR. In line with our observation, another large study had reported that 98.4% of patients with <1% BM FISH fusion signal were cytogenetically confirmed as CCyR.36 Quantitative RT-PCR is currently the only reliable method of ascertaining MMR or MMR 4.5 and is therefore the preferred monitoring tool after achievement of CCyR and in those patients who might be interested in discontinuing TKI treatment, where documentation of durable MMR or MMR 4.5 through serial RT-PCR before and after treatment discontinuation is mandatory. However, MRD monitoring in CML does not necessarily imply a prelude for change in treatment strategy based on the achievement or not or the loss of MMR, as long as CCyR is maintained. Instead, quantitative RT-PCR results that approach the CCyR threshold level of 1% might require concomitant PB D-FISH analysis to entertain the possibility of cytogenetic relapse. Finally, molecular tests carry the additional value of cost savings and more rapid test turnaround time.37 The authors have no conflict of interest to declare.