[A clinical and laboratory study of chronic myeloid leukemia with atypical BCR-ABL fusion gene subtypes].

Objective: To explore the clinical and laboratory features of chronic myeloid leukemia (CML) with atypical e14a3 and e19a2 BCR-ABL fusion gene subtypes.

Methods: We retrospectively analyzed a cohort of CML patients with Ph chromosome positive confirmed by cytogenetic and FISH but classical e13a3(b2a2), e14a2(b3a2)and e1a2 fusion transcripts negative identified by conventional real-time quantification RT-PCR (RQ-PCR). Further RQ-PCR was done with the forward primer and reverse primer designed to detect rare atypical BCR-ABL fusion genes including e14a3 and e19a2 transcripts. Direct sequencing analysis was performed on the PCR products and mutations in the BCR-ABL kinase domain were detected. The clinical data of patients were retrospectively analyzed.

Results: Six CML patients were found to carry t(9;22) abnormality and BCR-ABL rearrangement confirmed by FISH but classical BCR-ABL fusion genes negative detected by RQ-PCR. Further RQ-PCR and sequencing analysis confirmed the fusion of BCR exon 14 and ABL exon 3 in five CML patients (case 1-5) and the fusion of BCR exon 19 and ABL exon 2 in one CML patient (case 6). E255K and I293T IM-resistant mutations were detected in case 1 and 2, respectively. Among five cases with e14a3 transcripts, four were CML-CP, one CML-AP. Four patients were male and one was female. The median age was 48 years. The patient (case 6) with e19a2 transcripts was 40-year-old female with a diagnosis of CML-CP and PLT count was more than 1 000×10⁹/L. Imatinib (IM) therapy was administer in case 1, 2, 3, 4 and hematopoietic stem cell transplantation (HSCT) was undergone in case 5 after hydroxyurea (Hu) or interferon failure. Case 1 who had E255K IM resistant mutation, responded poorly to IM but obtained a complete cytogenetic remission (CCyR) after a substitution of dasatinib for IM. Case 2 and 3 achieved CCyR 6 months later after IM treatment and had been maintained well with IM despite I293T mutation in case 2. Case 4 attained CCyR 3 months later after IM treatment but relapsed and died soon. Case 5 was still in CCyR after HSCT. Case 6 with e19a2 transcripts got complete hematologic response after Hu treatment and CCyR was achieved soon after IM therapy.

Conclusion: Incidence of CML with atypical transcripts is extremely low. They could benefit from tyrosine kinase inhibitors or HSCT. Rare and atypical BCR- ABL fusion gene subtypes could be missed by conventional RQ-PCR.