High Frequencies Of Switching To 2^nd TKIs and Failure To Maintain Standard Imatinib Dose In Japanese CML Patients With BIM Genetic Variants

Recently, it has been demonstrated that the proapoptotic protein BIM showed a deletion polymorphism at exon 3 in eastern Asian population, and some CML patients with the BIM deletion polymorphism are resistant to imatinib treatment (Ng et al. Nature Medicine, 2012). More recently, a BIM single nucleotide polymorphism (SNP) at exon 8 (c465C>T) has also been found in French CML patients and this SNP is associated with not only imatinib resistance but also the presence of BCR-ABL mutations (Mahon et al. ASH abstract, 2012).

We aimed to investigate a possible association between such genetic variations of BIM and clinical manifestation in Japanese CML patients who experienced undetectable minimal residual disease (UMRD: so-called CMR^4.5). In this study, we newly analyzed BIM SNP (c465C>T) in 47 CML-UMRD patients with known BIM deletion polymorphism status (Katagiri et al. Br J Haematol, 2013). Twenty normal subjects were used as controls.

The frequency of either BIM SNP at exon 8 or BIM deletion polymorphism did not deviate from the normal subjects in the Japanese population (P = 0.7597 and P = 0.2880, respectively). None of the subjects showed both BIM SNP at exon 8 and BIM deletion polymorphism concomitantly. We then compared the clinical features among 3 CML-UMRD groups: patients with BIM SNP, patients with BIM deletion polymorphism, and patients who showed neither BIM SNP nor BIM deletion polymorphism (no genetic variations). The frequency of CML patients who maintained 400 mg imatinib dose until stopping was significantly higher in those without genetic variations than in those with BIM SNP or BIM deletion polymorphism (P = 0.0002). Moreover, the frequency of CML patients who switched to second tyrosine kinase inhibitors (2^nd TKIs) was significantly higher in those with BIM SNP or BIM deletion polymorphism than in those without such polymorphisms (P = 0.0055).

This is apparently the first study to circumstantiate the BIM genetic variants in Japanese CML patients with UMPD. Although the number of patients is small, our results suggest that CML patients without BIM deletion polymorphism/SNP could be maintained under standard imatinib dose without switching to 2^nd TKIs, and thereby, have a possibility to stop TKIs therapy.