Fluorescence in situ hybridization identifies cryptic t(16;16)(p13;q22) masked by del(16)(q22) in a case of AML-M4 Eo

We report a patient presenting with acute myeloid. leukemia (AML)-M4 Eo, in whom conventional cytogenetic analysis revealed a 46, XY, del(16)(q22) karyotype. Molecular analysis of the bone marrow cells using reverse transcriptase polymerase chain reaction (RT-PCR) identified a CBFbeta-MYH11, "type A" fusion transcript. However, despite a thorough re-evaluation, a balanced chromosome 16 abnormality could not be definitively identified by cytogenetics. Since there exists a small possibility of obtaining a false-positive PCR result, fluorescence in situ hybridization (FISH) analysis using dual-color, break-apart probes for CBFbeta was performed to elucidate the mechanism of fusion gene formation and thus confirm the RT-PCR results. FISH analysis clearly revealed a cryptic t(16;16), which was probably masked by the del(16)(q22). FISH is the preferred diagnostic procedure to elucidate the CBFbeta-MYH11 fusion in this situation, and resolves the possibility of both false-positive and false-negative results with RT-PCR technique. Due to the improved prognosis of AML associated with the CBFbeta-MYH11 fusion compared to AML generally, we recommend the use of FISH for detection of inv(16)/t(16;16)/CBFbeta-MYH11 in patients with failed, complex, or apparently normal cytogenetics, and in whom the cell morphology indicates the strong possibility of this gene fusion.