The translocation between chromosomes 8 and 21, t(8;21)(q22;q22), is the most frequent abnormality seen in approximately 46% of patients with acute myeloid leukemia with French-America-British (FAB)-M2 morphology and an aneuploid karyotype. The breakpoints in this translocation have been characterized at the molecular level, and the genes involved are AMLI on chromosome 21 and ETO (eight twenty one) on chromosome 8. AMLI has homology to the alpha subunit of the murine polyoma enhancer binding protein, pebp2, and to the segmentation gene, runt, of Drosophila melanogaster. ETO, also called MTG8 (myeloid translocation gene on 8) has no overall homology to known proteins, but it contains two DNA-binding zinc finger motifs and several regions that are proline- and serine-rich. Both AML1 and ETO are thought to be transcription factors because the motifs they contain are found in other transcription factors. Both genes are transcribed from telomere to centromere, and cytogenetic analysis of variant translocations has shown that the critical junction always conserved is on the derivative 8 chromosome. The rearrangement between the two chromosomes results in a fusion gene that contains the 5' region of AML1 including that homologous to runt fused to almost all of ETO. The fusion transcript from the der(8) chromosome is consistently detected in patients with the t(8;21). The translocation can be detected at the molecular level with selected genomic DNA probes from chromosome 21 and from chromosome 8 near the breakpoint in 80-100% of the t(8;21) patients at diagnosis and in relapse, and with reverse transcriptase-polymerase chain reaction (RT-PCR) in all of the patients at diagnosis and in long-term remission. These results indicate that leukemic clones are still circulating in patients who have been in remission for as long as 8 years.
