BCL2L11 (BIM) Deletion Polymorphism Is Associated with Molecular Relapse after ABL Tyrosine Kinase Inhibitor Discontinuation in Patients with Chronic Myeloid Leukemia with Complete Molecular Response

Background:

The inhibition of BCR-ABL1 kinase with tyrosine kinase inhibitors (TKIs) has markedly improved the prognosis of chronic myeloid leukemia (CML). Recently, it has been recognized that some CML patients with a complete molecular response (CMR) are able to maintain treatment-free remission (TFR) after discontinuation of TKIs. However, no predictive prognostic factors for successful discontinuation of the treatment have yet been identified. We set out to further clarify the role of predictive biomarkers in molecular relapse and non-relapse after ABL TKI discontinuation.

Materials and methods:

Patients in sustained CMR (MR 4.5) undergoing TKI therapy were eligible for inclusion in the study. Molecular relapse was defined as loss of major molecular response (MMR) of at least one point. Genomic DNA was obtained from whole blood using a DNA Extractor WB Kit (Wako, Osaka, Japan), and was subjected to polymerase chain reaction (PCR) amplification using primers designed to detect a deletion site (2903 bp) in intron two of the BCL2L11 gene (forward: 5′-AATACCACAGAGGCCCACAG-3′; reverse: 5′-GCCTGAAGGTGCTGAGAAAG-3′) and JumpStart RedAccuTaq LA DNA polymerase (Sigma Aldrich, St. Louis, MO, USA).

Results:

32 CML patients (17 men, 15 women, median age 58.4 years) were included in this study (Sokal category; low 24, intermediate 7, high 1). Six patients were treated with IFNα before TKI treatment, and 3 were treated with IFNα after stopping TKI. Median duration from TKI initiation to discontinuation was 79.3 months (range; 22 to 138 months); median duration of CMR before TKI discontinuation was 47.3 months (range; 5 to 97 months). Seven patients showed loss of MMR; 6 relapsed within 6 months and one showed late relapse at 25 months after discontinuation. The cumulative incidence of MMR loss was estimated as 18.8% at 12 months and at 24 months. Fluctuation of BCR-ABL transcript levels below the MMR threshold (> two consecutive positive values) was observed in 6.25% of patients at 24 months after ABL TKI discontinuation. Treatment-free remission was estimated as 81.2% at 12 months and at 24 months. The median period of restoration of second CMR was 6.0 months in re-treated patients. No patient died during the follow-up period. TKI-free remission was estimated as 78.1% at 30 months. There was only a significant difference in BCL2L11 (BIM) deletion polymorphism between the patients who maintained and those who lost MMR (p = 0.0253). No significant difference was observed in prior IFNα therapy, time to complete cytogenetic response (CCyR), time to MMR, and time to CMR between relapsing and non-relapsing patients.

Conclusion:

Our study shows a specific association between BCL2L11 (BIM) deletion polymorphism and clinical outcome after ABL TKI discontinuation in patients with long-lasting molecular undetectable residual disease. BCL2L11 (BIM) deletion polymorphism may predict relapse after ABL TKI discontinuation, which may have an impact on future ABL TKI discontinuation trials. These results further illustrate the importance of single nucleotide polymorphisms in successful long-term treatment of CML.