MYND-less splice variants of AML1-MTG8 (RUNX1-CBFA2T1) are expressed in leukemia with t(8;21).

The AML1-MTG8 fusion gene is generated by chromosome translocation t(8;21), which is frequently observed in acute myeloid leukemia. The fusion gene produces a chimeric transcription factor that suppresses the expression of AML1-target genes via the MTG8 part of the chimeric protein, which is thought to be the primary cause of leukemia. The C-terminal region of MTG8 contains the MYND domain, represented by highly conserved zinc-finger-like protein motifs, and is known to interact with corepressor proteins. We found that, instead of the MYND domain, an alternative last exon of MTG8 encoding 27 amino acids in-frame is expressed naturally in human adult testis and in several leukemia cell lines. This type of alternative splicing also occurred in the AML1-MTG8 fusion gene at high levels in leukemia cell lines with t(8;21), as well as in blast cells of leukemia patients with t(8;21). The variant proteins of both MTG8 and AML1-MTG8 reduced transcriptional repressor activity in a mammalian two-hybrid assay. However, mixed expression of these variants with wild-type MTG8 recovered their repressor activity, suggesting that these variants also act as repressors in vivo where wild-type MTG8 and other family members exist in abundance. On the other hand, the MYND-less variants acquired a higher affinity for binding to MTG8 and formed a multimer, whereas the wild-type protein forms a dimer. Thus, expression of the MYND-less variants by the dysregulation of splicing machinery, which stimulates the oligomerization of fusion proteins in leukemia cells, may enhance malignant conversion of hematopoietic cells. Copyright 2005 Wiley-Liss, Inc.