Exon 7 Deletion in the bcr-abl Gene Is Frequent in Chronic Myeloid Leukemia Patients and Is Not Correlated with Resistance against Imatinib

Abstract

Chronic myeloid leukemia (CML) patients treated with imatinib develop frequent resistance generally due to a point mutation. Recently, large rearrangements of abl sequence have also been described. In this study, we focused on the complete deletion of exon 7. We screened for bcr-abl delexon7 in 63 resistant patients by high-resolution melting (HRM) analysis and direct sequencing. Moreover, we analyzed expression of abl delexon7 and bcr-abl delexon7 in 17 CML patients at diagnosis, 32 patients at resistance, and 20 negative controls by quantitative PCR or fragment length analysis. bcr-abl delexon7 was detected on 34 (54%) among 63 resistant patients by HRM, showing an increase in the sensitivity of screening, because only 3.2% could be detected by direct sequencing. This deletion was not associated with a point mutation (P = 0.3362). In addition, abl delexon7 was found in all tested samples with the same pattern of expression, suggesting an alternative splicing mechanism. In the bcr-abl component, there was no statistical difference between CML patients at diagnosis and resistant patients (P = 0.2815) as regarding bcr-abl delexon7 proportion, thus arguing against involvement of deletion in resistance. Moreover, among two patients harboring bcr-abl delexon7 at diagnosis, one experienced a complete disappearance of this transcript, and the other decreased >75% at resistance. In conclusion, bcr-abl delexon7 is frequently observed in CML patients when using sensitive techniques. It seems to be the result of an alternative splicing mechanism and to be independent from the occurrence of resistance. Mol Cancer Ther; 9(11); 3083-9. ©2010 AACR.

The BCR-ABL35INS insertion/truncation mutant is kinase-inactive and does not contribute to tyrosine kinase inhibitor resistance in chronic myeloid leukemia

“…Others have detected variant ABL transcripts at similar frequencies in CML patients and healthy individuals, which implies the existence of alternative splicing mechanisms of ABL unrelated to TKI resistance. 11,[24][25][26] In total, the results of the present study demonstrate that BCR-ABL 35INS lacks the qualities of a functional tyrosine kinase at the cellular and biochemical level. An alternative possibility, that BCR-ABL 35INS is catalytically inactive but sequesters TKIs, is untenable on stoichiometric grounds and excluded by our experiments that demonstrated that coexpression of BCR-ABL and BCR-ABL 35INS in Ba/F3 cells does not mitigate imatinib sensitivity compared with Ba/F3 cells that express only BCR-ABL.…”

Compound mutations in BCR-ABL1 are not major drivers of primary or secondary resistance to ponatinib in CP-CML patients

“…Missense mutations outside the KD were excluded (N 5 55) (supplemental Table 1 available on the Blood Web site), as were all polymorphisms (N 5 3), deletions (N 5 5), frameshift (N 5 12), nonsense (N 5 18), and synonymous (N 5 60) mutations because these are not considered relevant for TKI resistance. [17][18][19] In samples in which multiple BCR-ABL1 missense mutations were detected, the phase (whether present on the same or different alleles) and frequency of all possible mutation combinations were calculated as described in the supplemental Methods.…”

BCR‐ABL exon 7 deletion and novel point mutation in patient with chronic myelogenous leukemia and TKI resistance

“…This deletion was first described by Curvo et al3 in 2008, who suggested that this mutation appears as a result of alternative splicing and may be one of the causes of TKI resistance. However, Gaillard et al4 in 2010 showed that the frequency of deletion occurrence is not statistically different in the groups of drug‐resistant and sensitive patients. Moreover, they detected ABL1 del.…”