BCR-ABL Fusion Gene and BCR-ABL Transcript - Correlation with Response to Therapy in Chronic Myeloid Leukaemia.

INTRODUCTION: The quantification of BCR-ABL transcript (RNA) has been consistently associated with the evolution of Chronic Myeloid Leukaemia (CML), frequently determining therapeutic decisions. We did not find in the literature any attempt of measuring tumour burden directly by its genomic marker (DNA).

AIMS: To correlate the quantification of BCR-ABL fusion gene with the quantification of BCR-ABL transcript, in the evaluation of response to therapy in patients (pts) with CML.

METHODS: Between October/2002 and July/2005, we analysed 393 samples of peripheral blood of 33 pts, M/F:17/16, median age 52 (27–76) years, with chronic phase CML (2 pts developed blast crisis), median time since diagnosis 60,5 (19–170) months, under Imatinib or alpha Interferon (INF) + Cytarabin (Ara-C) therapy. Pts total RNA and genomic DNA were extracted from peripheral blood samples and total RNA was reverse transcribed into cDNA. Real-time PCR absolute quantification with TaqMan® probes was used to measure the fusion transcript’s levels and Sybr Green I® dye was used to measure the fusion gene levels. Imatinib was initiated in pts with cytogenetic responses (CyR) inferior to 90%, after more than 1 year of INF+Ara-C therapy. Criteria used in the evaluation of response: molecular response (MR) - reduction of transcript ≥ 3 logs, major MR (MMR) - reduction ≥ 4 logs, complete MR (CMR) - absence of transcript in 2 consecutive analyses.

RESULTS: From 33 studied pts, 26 were under Imatinib therapy, after a median of 21 (2–38) months, 6 pts maintain therapy with INF+Ara-C and 1 pt died (lower respiratory tract infection), in therapy with INF+Ara-C. The evaluation of therapy demonstrated MR in 18 pts (55%), 5 of which with IFN+Ara-C. From these 18 pts, 7 achieved CMR and 6 MMR. In the 15 pts without MR, 8 presented reduction of transcript of 1 to 2 logs and 7 pts maintained stable levels of transcript. Two pts developed blast crisis, after 4 and 20 months of therapy with Imatinib, the latter 12 months after MMR. We observed a higher incidence of transcripts b3a2 (23 pts) compared with b2a2 (11 pts), but we did not find any correlation between the type of transcript and response to therapy. In all pts BCR-ABL fusion gene was present, including in those pts in CMR. We did not observe any correlation between the quantification of BCR-ABL fusion gene and the quantification of BCR-ABL transcript.

CONCLUSIONS: The present data demonstrates that MR can be achieved in pts with CML by INF+Ara-C therapy. If INF+Ara-C fails to induce CyR, MR can be obtained with Imatinib. In the 33 CML presented pts, the evaluation of response to therapy showed that 55% pts obtained MR. No correlation between the quantification of BCR-ABL fusion gene and the quantification of BCR-ABL transcript was observed. These preliminary results suggest that the absence of BCR-ABL transcripts does not exclude the presence of BCR-ABL fusion gene. However more studies with larger series of pts are needed to confirm these results.