Work

BCR-ABL1INS35 Is Not Uncommon in CML Patients and Is Related to Resistance and Sensitivity to Inhibitors in CML Treatment - Response

Year: 2010

Type: article

Abstract: Santamaria et al. report the detection of the BCR-ABL1INS35 in 27% of 18 patients with CML being treated with imatinib. This is consistent with our finding of 2 of 21 (10%) at 3 months, 1 of 20 (5%) at 6 months, 4 of 25 (16%) at 9 months, and 6 of 23 (26%) at 12 months of imatinib therapy (1). However, it is important to point out that the relative level of expression of the BCR-ABL1INS35 as compared with the wild-type fusion BCR-ABL1 transcript is very low in these patients (<20%). As we discussed in our article and based on additional data that we generated, the level of resistance is “dose-dependent”.11Jean-Marie B, Kantarjian H, Ma W, Yeh C-H, Peralta R, Lee T-S, O'Brien S, Giles F, Estrov Z, Cortes J, and Albitar M. Common but overlooked mechanism of ABL kinase inhibitor resistance in chronic myeloid leukemia. Submitted for publication. In that, the level of resistance to imatinib is dependent on the level of the BCR-ABL1INS35 expression as compared with the wild-type BCR-ABL1 and not with any other control, such as GUS as reported by Santamaria et al. Our data show that the longer the patients are on therapy, the higher the percentage of patients expressing low levels of BCR-ABL1INS35. For these patients, becoming overtly resistant to therapy requires either a substantial increase in the level of expression of the BCR-ABL1INS35 or the development of additional mutations in BCR-ABL1 or somewhere else in the genome.As for the detection of alternative splicing in normal ABL1 transcript, we have detected the presence of low levels of this alternatively spliced ABL1 transcript in normal tissue using a primer set that amplifies only the ABL1.1 This has also been reported by others (2). We think that this low expression of the ABL1INS35 in normal tissue may provide some protection to normal tissue against the effects of imatinib. Therefore, in the quantification of the BCR-ABL1INS35, it is important to be careful and not amplify the native ABL1 from normal cells. As we have described in our article, we used a nested assay in which the fusion BCR-ABL1 transcript was first amplified, then we took the fusion transcript in the next step of the nested PCR and quantified the BCR-ABL1INS35, so we prevented the amplification of the normally expressed ABL1INS35. In our hands, using our procedure, we could not detect ABL1INS35 in normal tissue. If this procedure is not stringent enough, the ABL1INS35 transcript from the normal cells may contaminate final product of the BCR-ABL1INS35 and this may confuse results.No potential conflicts of interest were disclosed.

Source: Molecular Cancer Therapeutics

DOI: 10.1158/1535-7163.mct-09-1116