Cytogenetic, fluorescence in situ hybridization, and genomic array characterization of chronic myeloid leukemia with cryptic BCR-ABL1 fusions

Chronic myeloid leukemia (CML) is characterized by the breakpoint cluster region (BCR)-Abelson murine leukemia (ABL1) fusion gene. In approximately 1% of CML cases, the Philadelphia chromosome associated with the BCR-ABL1 fusion gene is not present, and the BCR-ABL1 fusion gene is generated by cryptic insertion or sequential translocations. In this study, we describe the cytogenetic and molecular features of five CML patients with cryptic BCR-ABL1 fusion genes using karyotype, fluorescence in situ hybridization (FISH), and whole genome single nucleotide polymorphism array techniques. Two cases of CML in the chronic phase (CP) had a normal karyotype, and three cases of CML in the blast phase (BP) had an abnormal karyotype with neither a typical nor variant t(9;22). By BCR-ABL1 metaphase FISH analysis, we found that fusion signals were localized on chromosomes 9 (3 cases), 22 (1 case), and both 9 and 22 (1 case). In two cases of CML-BP, duplication of the BCR-ABL1 fusion gene occurred as a result of mitotic recombination between homologous chromosomes. Copy number losses involving the IKZF1 gene were observed in two patients with CML-BP. This study demonstrates for the first time the acquisition of additional BCR-ABL1 fusion genes through mitotic recombination in CML with cryptic BCR-ABL1.