Genomic structure, alternative splicing and expression of TG-interacting factor, in human myeloid leukemia blasts and cell lines.

TG-interacting factor (TGIF) is a homeobox transcriptional repressor that has been implicated in holoprosencephaly and various types of cancer, including leukemias. In this study, we provide the first detailed description of the TGIF locus characterizing 12 TGIF splice isoforms. These isoforms have similar open reading frames but different 5' untranslated regions. TGIF expression data are presented from multiple tissues, cell lines and primary leukemia cells. Isoform-specific real-time PCR analysis showed that even though these isoforms were broadly expressed all except isoform 4, had very low level of expression. In fact, isoform 4 was the predominant TGIF isoform expressed in all tissues analyzed. Since TGIF, levels have recently implicated to play a role in acute myelogenous leukemia we proceeded to characterize the minimal promoter region of isoform 4 as a first step in understanding mechanisms of TGIF expression. As expected for homeobox genes, the minimal promoter region for isoform 4 has multiple Sp1 binding sites and a CpG island raising the possibility that the low TGIF expression seen in some AML patients and leukemia cell lines may be secondary to methylation. Further characterization of expression from this promoter using 5-Aza-2'-deoxycytidine treatment and transient expression assays showed that decreased TGIF expression is likely secondary to active repression and not because of promoter methylation. A detailed characterization of this complex locus is important as it may help to clarify the functions of this gene in brain development and leukemia biology.