Complex Regulation of the Homeobox Transcription Factor TGIF: Differential Expression of Multiple 5′ Isoforms.

TG-interacting factor (TGIF) is a transcriptional repressor belonging to the TALE (three amino acid loop extension) class of homeobox proteins. In addition to its involvement by mutation or deletion in the inherited craniofacial disorder holoprosencephaly, we have shown that expression of TGIF is highly predictive of relapse and survival in acute myelogenous leukemia (AML). To better understand how TGIF expression is regulated, we characterized its genomic structure using expressed sequence tag analysis, reverse transcriptase-coupled PCR, and rapid amplification of cDNA ends. These studies revealed a complex pattern of gene expression, with 15 splice isoforms and 11 alternative 5′ exons spread over 40 kb of DNA, suggesting the presence of multiple promoters. Real-time and semi-quantitative PCR analysis showed these isoforms were differentially expressed in various adult tissues, leukemic cell lines, and AML blasts. Further, isoform C was found to be the major RNA product in hematopoietic cells, contributing significantly to total TGIF expression in leukemic cell lines TF1a, U937, AML-193, KG-1a, Kasumi-1, K562, GDM-1, HL-60 and AML blasts. This analysis suggests that altered splicing and/or expression of specific isoforms could be responsible for the reduced levels of TGIF message observed in AML blasts and cell lines. The unusually complex structure of the TGIF gene may enable its precise regulation during normal hematopoiesis and may be relevant to its reduced expression in myeloid leukemias.