Detection of BCR-ABL Kinase Domain Mutations in Chronic Myeloid Leukemia Patients Treated with Tirosin-Kinase Inhibitors

Background. Point mutations in the kinase domain (KD) of the BCR-ABL are the most frequent mechanism of drug resistance in CML patients treated with kinase inhibitors (TKI). More than 80 mutations with different frequency and clinical significance have been reported. One of them, the T315I confers resistance to all TKIs available. The detection of mutations in KD allows early identification of high-risk patients and therefore guides clinical therapy decisions.

Aim. To assess the mutation status of a group of CML pts resistant to TKI from Uruguay (n=35) and Brazil (n=30).

Methods. KD mutation screening was performed by RT-PCR and direct sequencing according to Branford et al. (2002). Additionally, we developed a rapid, specific, sensitive and low cost allele specific (AS)-RT-PCR assay to identify T315I, using Branford’s KD amplification primers in combination with an allele specific primer for the T315I point mutation detection. BCR-ABL transcript levels were also measured by RQ-PCR according to international recommendations.

Results and Discussion. RT-PCR and direct sequencing analyses performed in all pts showed the presence of T315l mutation in 3/65 cases. Other 11 showed the alternative mutations Y253H (n=2), E450A, G250E (n=2), E459K (n=2), E450G, F317L (n=2) and E255K; and the remaining 55 showed no mutations in the ABL KD. All 65 samples together with cDNA from 15 non-resistant CML pts and 10 cDNA from non-CML were analyzed by AS-RT-PCR assay for T315l mutation in order to validate the method. T315l was identified in the 3 samples in which the mutation was previously detected by direct sequencing and in 1 pt that had been classified as KD mutation negative. This result was then confirmed by direct sequencing of the AS-PCR product. T315 was neither detected in samples positive for other mutations nor in samples of non-resistant CML and non-CML patients, supporting the specificity of the method. Assessment of the sensitivity of the AS-RT-PCR was performed on serial dilutions experiments using RNA from T315 positive pt into RNA from CML-T315l negative pt, showing that the T315I mutation was detectable to a level of 0.01 % by AS-PCR, while through direct sequencing method the sensitivity was 10–20%. The prevalence of mutations in our study was 15/65 (23%).

Conclusions. Our results showed that the AS-RT-PCR described here is a convenient and easy tool to be used in a clinical routine laboratory for rapid screening for BCR-ABL T315. This, together with direct sequencing, constitutes a suitable approach for CML resistance monitoring and therapeutic choice.