A Large Variety of Alternatively Spliced and Differentially Expressed mRNAs Are Encoded by the Human Acute Myeloid Leukemia Gene AML1

Abstract

The human chromosome 21 acute myeloid leukemia gene AML1 is frequently rearranged in the leukemia-associated translocations t(8;21) and t(3;21), generating fused proteins containing the amino-terminal part of AML1. In normal blood cells, five size classes (2-8 kb) of AML1 mRNAs have been previously observed. We isolated seven cDNAs corresponding to various AML1 mRNAs. Sequencing revealed that their size differences were mainly due to alternatively spliced 5' and 3' untranslated regions, some of which were vast, exceeding 1.5 kb (5') and 4.3 kb (3'). These untranslated regions contain sequences known to control mRNA translation and stability and seem to modulate AML1 mRNA stability. Further heterogeneity was found in the coding region due to the presence of alternatively spliced stop codon-containing exons. The latter led to production of polypeptides that were smaller than the full-length AML1 protein; they lacked the trans-activation domains but maintained DNA binding and heterodimerization ability. The size of these truncated products was similar to the AML1 segment in the fused t(8;21) and t(3;21) proteins. In thymus, only one mRNA species of 6 kb was detected. Using in situ hybridization, we showed that its expression was confined to the cortical region of the organ. The 6-kb mRNA was also prominent in cultured peripheral blood T cells, and its expression was markedly reduced upon mitogenic activation by phorbol myristate acetate (TPA) plus concanavalin A (ConA). These results and the presence of multiple coding regions flanked by long complex untranslated regions, suggest that AML1 expression is regulated at different levels by several control mechanisms generating the large variety of mRNAs and protein products.

Runx3 and Runx1 are required for CD8 T cell development during thymopoiesis

“…It has previously been reported that in adults RUNX3 is highly expressed in the hematopoietic system as well as in various hematopoietic cell lines (1,10,(28)(29)(30)(31). Using adult Runx3 KO mice (25) we now report that Runx3 function is essential for the maturation of single-positive (SP) CD8 ϩ T cells.…”

“…In adults, Runx3 is highly expressed in spleen, thymus, and blood (1,28,29,31), whereas in embryos, it is detected in liver hematopoietic precursors and in thymocytes (20). We have previously shown (25) that targeting of Runx3 exon 2 abolished expression in thymus.…”

CBFA2, frequently rearranged in leukemia, is not responsible for a familial leukemia syndrome

“…X79549), yields various protected fragments corresponding to various 3Ј alternative splicing. 39 No abnormal CBFA2 transcripts could be detected in the affected family member (AF) when compared to an unaffected sibling (JF) or the HL60 and K562 cell lines. There was no significant difference in the ratios of transcripts by phosphorimaging analysis except for a relative decrease in the 417-bp fragment obtained with probe BH in the affected family member, when compared to the normal sibling and cell lines.…”

“…Of these, four splicing variants have potential for dominant negative activity because they lack the 3Ј transactivating domain, but retain the DNA binding domain. 39,40 These splice variants, if present in excess of the full length transcript, could have an activity similar to the leukemia-associated fusions involving CBFA2.…”

Identification of a novel splice variant of AML1b in ovarian cancer patients conferring loss of wild-type tumor suppressive functions

“…AML1 encodes the a-subunit of the transcription factor PEBP2/CBFb involved in the recognition of general transcription coregulators. AML1 exhibits a complex pattern of tissue-regulated expression at the level of transcription, splicing and translation (Levanon et al, 1996). Thus far, there are four AML1-identified alternative splice isoforms, namely AML1a, AML1b, AML1 Del N and AML1c, all of which contain an RHD (Miyoshi et al, 1995;Zhang et al, 1997;Mikhail et al, 2006).…”