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To determine the incidence of MLLrearrangement in acute myeloid leukemia (AMU French-American-British (FAB) type M1 and to evaluate optimal screening strategies for the characterization of such abnormalities, we analyzed specimens from 41 patients with AML by Southern blotting with two MLLgenomic probes and compared the capacities of reverse transcription-polymerase chain reaction (RT-PCR) and fluorescent in situ hybridization (FISH) to identify the types of rearrangement found in AML M1 with those observed in AML M5. MLLrearrangement was found in 6 of 29 (20%) AML M1 and 6 of 10 AML M5 cases. RT-PCR characterization of 11 cases showed four MLLself-fusions, four MLL-AF6, two MLL-AF9, including a novel AF9breakpoint, and one uncharacterized t(11;19). Only 5 of 10 MLL-rearranged cases tested demonstrated karyotypic 11q23 abnormalities. FISH analysis of nine cases with an MLL-specific yeast artificial chromosome (YAC) confirmed the cytogenetic abnormalities in two cases, clarified them in one, and did not detect six cases, including three MLLself-fusions, one case with a probable Mil-rearranged subclone not represented karyo-typically, and two MLL-AF6.A whole chromosome 11 paint detected one of these MLL-AF6, and an AF6cosmid demonstrated that the other was probably due to insertion of a submicroscopic portion of chromosome 6, including part of AF6, into an apparently normal chromosome 11. We conclude that MLLrearrangements are common in adult AML M1, that MLLself-fusion and MLL-AF6are the most frequent types of abnormalities, and that RT-PCR is preferable to 11q23 FISH analysis for their characterization.

Karyotypic detection of chromosomal 16 abnormalities classically associated with AML M4Eo can be difficult. Characterization of the two genes involved in the inv(16)(p13q22), CBFβ and MYH11, has allowed the detection of fusion transcripts by reverse-transcriptase polymerase chain reaction (RT-PCR). We have analyzed CBFβ-MYH11 fusion transcripts by RT-PCR in myelomonocytic leukemias, with or without eosinophilia, to determine whether their presence correlates with morphology. Fifty-three cases (11 AML M4Eo; 1 AML M4 with atypical abnormal eosinophils (AML M4 “Eo”); 29 AML M4; 8 AML M5; 3 CMML; and 1 AML M2 with eosinophilia) were analyzed. All 11 typical AML M4Eo were CBFβ-MYH11 positive. The single case of AML M4 with distinctive eosinophil abnormalities was negative by karyotype, RT-PCR and fluorescent in situ hybridization (FISH). Three of 29 (10%) AML M4 without abnormal eosinophils were CBFβ-MYH11 positive, 1 of which did not show any apparent chromosome 16 abnormalities by classical metaphase analysis (2 not tested). Both cases tested also showed MYH11 genomic rearrangement. None of the other leukemias were RT-PCR positive. Follow-up of three patients showed residual positivity in apparent complete remission. These data show that CBFβ-MYH11 fusion transcripts occur not only in the vast majority of typical AML M4Eo, but also in ≍10% of AML M4 without eosinophilic abnormalities, a much higher incidence than the sporadic reports of chromosome 16 abnormalities in AML M4 would suggest. Taken together with the detection of CBFβ-MYH11 transcripts in the absence of apparent chromosome 16 abnormalities by classical banding techniques, these data show that additional screening by either RT-PCR or FISH should be performed in all AML M4, regardless of morphologic features, to allow accurate evaluation of the prognostic importance of this fusion transcript.