Evolution of BCR/ABL gene mutation in CML is time dependent and dependent on the pressure exerted by tyrosine kinase inhibitor

Abstract

Background: Mutations in the ABL kinase domain and SH3-SH2 domain of the BCR/ABL gene and amplification of the Philadelphia chromosome are the two important BCR/ABL dependent mechanisms of imatinib resistance. Here, we intended to study the role played by TKI, imatinib, in selection of gene mutations and development of chromosomal abnormalities in Indian CML patients.

Methods: Direct sequencing methodology was employed to detect mutations and conventional cytogenetics was done to identify Philadelphia duplication.

Results: Among the different mechanisms of imatinib resistance, kinase domain mutations (39%) of the BCR/ABL gene were seen to be more prevalent, followed by mutations in the SH3-SH2 domain (4%) and then BCR/ABL amplification with the least frequency (1%). The median duration of occurrence of mutation was significantly shorter for patients with front line imatinib than those pre-treated with hydroxyurea. Patients with high Sokal score (p = 0.003) showed significantly higher incidence of mutations, as compared to patients with low/intermediate score. Impact of mutations on the clinical outcome in AP and BC was observed to be insignificant. Of the 94 imatinib resistant patients, only 1 patient exhibited duplication of Philadelphia chromosome, suggesting a less frequent occurrence of this abnormality in Indian CML patients.

Conclusion: Close monitoring at regular intervals and proper analysis of the disease resistance would facilitate early detection of resistance and thus aid in the selection of the most appropriate therapy.

Fig 1. Electropherogram showing the insertion mutation ΔCAGG at 303^th amino acid position between P-loop and gatekeeper region.

Fig 2. Diagram illustrating gradual disappearance of T315I mutation with hydroxyurea and spleenic irradiation treatment.

Fig 3. Bar graph shows the distribution of IM resistant CML patients according to the first line treatment and the total count at the time of diagnosis.

As the WBC count increases, the difference between the lengths of the blue and the red bars increases to a greater extent as compared to the difference in lengths between the green and violet bars, indicating less number of mutations in patients pre-treated with a non-TKI (HU) as compared to the patients with first line imatinib.

Fig 4. Kaplan-Meier survival curves according to the mutations and the disease phase.

Group 1: Chronic Phase (P-Loop); Group 2: Chronic Phase (non P-Loop); Group 3: Accelerated Phase (P-Loop); Group 4: Accelerated Phase (non P-Loop); Group 5: Blast Crisis (P-Loop); Group 6: Blast Crisis (non P-Loop).