Insertion (21;8)(q22;q22q22): a masked t(8;21) in a patient with acute myelocytic leukemia

A 43-year-old man was diagnosed with acute myelocytic leukemia with cellular maturation (AML-M2, according to the French-American-British classification criteria). A cytogenetic study with a G-banding method initially reported the karyotype as 45,X,-Y; however, dual-color, dual-fusion fluorescence in situ hybridization (FISH) with probes for the AML1 and the ETO genes showed an unusual pattern of signals, presenting one fusion signal on chromosome 21. Molecular study by reverse transcriptase polymerase chain reaction revealed the presence of a typical AML1/ETO chimeric gene. FISH with whole-chromosome painting probes targeting chromosomes 8 and 21 revealed insertion of part of 8 chromosome into the long arm of chromosome 21. We concluded that complicated translocations involving chromosomes 8 and 21 in this patient resulted in the development of the chimeric gene, AML1/ETO, on the long arm of chromosome 21. This aberrant location of AML1/ETO gene and the final karyotype of 45,X,-Y,ins(21;8)(q22;q22q22) could not be determined without molecular analysis. This abnormality is considered a masked t(8;21).