Identification and characterization of novel AML1-ETO fusion transcripts in pediatric t(8;21) acute myeloid leukemia

Proc Amer Assoc Cancer Res, Volume 47, 2006 4326 t(8;21)(q22;q22) is a balanced translocation between chromosomes 8 and 21, resulting in the fusion of the 5’ end of the AML1 gene normally located on chromosome 21q22 with the 3’ end of the ETO on chromosome 8q22. t(8;21) is one of the most common cytogenetic abnormalities in childhood acute myeloid leukemia (AML), present in ∼12% of cases and has been considered a favorable prognostic factor in several studies. Transcription of the AML1 gene is regulated by two independent promoters (P1 and P2) and alternative splicing. Thus, three transcript isoforms of AML1, designated AML1a, AML1b (both transcribed from promoter P2), and AML1c (transcribed from promoter P1) have been identified. It is conceivable that the AML1-ETO gene is also regulated by these two promoters. By amplification and sequencing of cDNAs derived from RNAs prepared from 8 primary AML specimens with t(8;21), we found that: (i) both AML1b-ETO (A1bE) and AML1c-ETO (A1cE) fusion transcript forms were detected; (ii) both AML1b and AML1c sequences were in-frame with ETO ([Table 1][1]); and (iii) out-of-frame A1bE and A1cE transcripts were also identified in these specimens at high frequencies. The out of frame fusion transcripts were likely due to alternative splicing and/or internal deletions in either the AML1 or ETO portions of the fusion transcripts, and encoded truncated or forms otherwise unrecognizable as AML1-ETO protein. While out-of-frame forms of AML1-ETO have been described, our results are the first to suggest that these may represent the major forms of fusion transcript in t(8;21) myeloblasts. In contrast, no such gene alterations were found in the native AML1 gene, suggesting that the heterogeneity of the AML1-ETO fusion transcripts is AML1-ETO fusion gene specific. Cotransfection of the AML1-ETO fusion transcript constructs with a CD36 promoter (a known AML1 targeted promoter) reporter gene construct, demonstrated AML1b repressor function of the fusion proteins. Further experiments are needed to determine the functional roles of these fusion proteins in relation to chemotherapy sensitivity in t(8;21) AMLs and the possible heterogeneity in treatment response of t(8;21) AML. Table 1. Summary of in-frame forms of A1bE and A1cE View this table: [1]: #T1