A unique BCR-ABL1 transcript with the insertion of intronic sequence from BCR and ABL1 genes in a patient with Philadelphia-positive chronic myeloid leukemia: a case study.

The BCR-ABL1 fusion gene results from a reciprocal translocation rearrangement, t(9;22)(q34;q11.2), and is a hallmark of chronic myeloid leukemia (CML). The breakpoint on chromosome 9 is mostly 5' to ABL1 exon 2, whereas on chromosome 22, the breakpoint can occur in various regions involving the major breakpoint cluster region (M-bcr) in CML and the minor breakpoint cluster region (m-bcr) in acute lymphoblastic leukemia. Described here is a rare case of Philadelphia-positive CML with intronic splice sites. This atypical BCR-ABL1 transcript was detected along with a classic e13a2 transcript, using reverse transcription polymerase chain reaction (RT-PCR). Nucleotide sequence analysis revealed a joining of BCR intron 13 with ABL1 intron 1a. Both transcripts were detected when the patient was on hydroxyurea treatment; with imatinib mesylate therapy, the atypical transcript disappeared. To our knowledge, this is the first report of BCR-ABL1 transcript with breakpoint occurring within both BCR and ABL1 introns and fusion of intronic sequences from both BCR and ABL1 genes.