Background: Reverse-transcription polymerase chain reaction [RT-PCR] and quantitative RT-PCR [RQ-PCR] are valuable tools for investigating CML patients with BCR-ABL fusion gene. Several insertion and deletion mutations can also be identified.

Objective: To determine frequencies of el4a2 and el3a2 variants and explore existence of intronic ABL mutations in Thai CML patients.

Materials and Methods: The frequency of el4a2 and el3a2 variants was analyzed in l,035 CML cases by RT-PCR and/or RQ-PCR. Cloning of pCRTM4-TOPO plasmid and sequencing were performed to confirm the insertion sequences of mutations.

Results: Collected samples of blood or bone marrow of CML patients were analyzed by RT-PCR (682 cases) and RQ-PCR (353 cases). The prevalence of el4a2, el3a2, and both variants in BCR-ABL positive cases were 6l.9% (l09/l76), 32.4% (57/l76), and 5.7% (l0/l76), respectively. Interestingly, an unexpected band larger than both el3a2 and el4a2 bands was detected in majority of the cases, which co-expressed both el3a2 and el4a2 sequences by RT-PCR. Sequencing analysis revealed that the 49-bp insertion was a part of ABL intron lb. The insertion consisted of a stop codon, leading to only the BCR part of the fusion gene translated. This particular sequence of insertion had previously been reported only in one Japanese ALL patient.

Conclusion: The frequencies of el4a2 and el3a2 BCR-ABL variants were similar to those previously reported in Western and Asian studies. The intron lb insertion was frequently observed in Thai CML
patients carrying both variants in this study. However, its clinical importance should be further characterized.

Keywords: BCR/ABL, Chronic Myeloid Leukemia [CML], Intronic insertion