Molecular cytogenetic characterization of rearrangements involving 12p in leukemia

Translocations involving the short arm of chromosome 12 are frequent events among patients with various hematologic malignancies. In approximately half of these patients, fluorescence in situ hybridization (FISH) analysis has shown that the breakpoints are clustered within the ETS-variant gene 6 (ETV6) at 12p13, leading to its fusion with a variety of partner genes on different chromosomes. The remaining patients have breakpoints centromeric or telomeric to ETV6 or, less frequently, interstitial 12p13 deletions that invariably involve this gene. In most cases reported, 12p translocations were found to be associated with other structural and/or numerical abnormalities as part of a complex karyotype. Initially using conventional cytogenetic analysis, we characterized the chromosomal breakpoints of three leukemia patients (two with B-acute lymphoblastic leukemia and one with myelodysplastic/myeloproliferative disorder) presenting a t(5;12)(q13;p13), t(12;15)(p13;q22), and dic(9;12)(p11;p11), respectively, as the only structural abnormalities in the karyotype. These rearrangements were further investigated using FISH and molecular studies. Two cases revealed cryptic three-way translocations that had gone undetected in the conventional cytogenetic analyses. One of the cases presented an ETV6 rearrangement with an unsuspected fusion, with the CBFA2 gene at 21q22. In the other two, small and large 12p deletions that included ETV6 were found. This report illustrates the chromosomal and molecular heterogeneity of rearrangements underlying 12p chromosome translocations in leukemia.