AMLI FUSION TRANSCRIPTS IN T(3-21) POSITIVE LEUKEMIA - EVIDENCE OF MOLECULAR HETEROGENEITY AND USAGE OF SPLICING SITES FREQUENTLY INVOLVED IN THE GENERATION OF NORMAL AMLI TRANSCRIPTS

The t(3;21)(q26;q22) is associated with chronic myelogenous leukemia i n blast crisis (CML-BC), leukemia evolving from (therapy-related) myel odysplasia, and with leukemia following other hematopoietic proliferat ive diseases, Molecular cytogenetic analysis and cloning of a few t(3; 21) cases indicate that the breakpoints are quite heterogeneous even w ithin a specific clinical phenotype. Interestingly some of the (3;21) breakpoints involve the AML1 gene previously found rearranged in the t (8;21) associated with acute myelogenous leukemia. AML1 is related to the Drosophila gene runt and is the human counterpart of the gene for the alpha subunit of the nuclear polyoma enhancer binding protein (PEB P2) also known as the core binding factor (CBF). In the t(3;21) AML1 w as found rearranged with EAP, a gene on chromosome 3 encoding a small ribosomal protein, as well as with EVII, another gene on chromosome 3. Here we report our study of six cases of t(3;21). By using fluorescen ce in situ hybridization (FISH) analysis and AML1 probes we could conc lude that at least in two CML-BC cases the breakpoint occurred in the AML1 intron that is disrupted by the t(8;21). An AML1/EAP fusion trans cript, different from the one described in a therapy-related myelodysp lasia, was detected in both CML-BC cases. This transcript is expected to result in a predicted protein containing the AML1 nuclear binding d omain with an attached stretch of 17 amino acids unrelated to the EAP small ribosomal protein. In the other t(3;21) patients we could not de tect an AML1/EAP transcript or an AML1/EVII transcript. This result su ggests heterogeneity of the t(3;21 I) at the molecular level. The AML1 chimeric transcripts identified so far, both in the t(3;21) and in th e t(8;21), diverge from the normal transcripts either after exon 5 or exon 6. Here we show that in normal AML1 transcripts different splicin g events are seen to occur after AML1 exon 5 as well as exon 6. (C) 19 94 Wiley-Liss, Inc.