Identification of the ETS Family Member ELF1 as a Transcriptional Regulator of MEIS1 Expression.

Abstract 3647

Poster Board III-583

MEIS1 is a Hox cofactor known to be a key regulator of normal hematopoietic and leukemic stem cell function. Overexpression of Meis1 is a potent collaborating event in leukemias associated with multiple Hox, NUP98-HOX and MLL fusion genes. The transcriptional regulation of MEIS1 expression however remains poorly understood. To identify potential transcriptional regulators, we searched for candidate cis-regulatory elements within and beyond the 140 kb MEIS1 genomic locus through a combination of in silico conservation analysis, DNase I hypersensitivity analysis and histone H3 acetylation profiling in several human leukemic cell lines. To date, we have identified more than eight critical cis regulatory regions in the human MEIS1 genomic locus. In the current study, we focused on the region between -500 bp to +20 bp of the annotated human MEIS1 transcriptional start site (AHTSS), the putative promoter of this gene. This region contains a CpG island which is highly conserved throughout evolution with more than 70% identity between human and zebrafish in the region between 267 bp to 403 bp upstream of AHTSS. This region also displays DNase I hypersensitivity, significant enrichment of histone H3 acetylation and DNA hypomethylation in MEIS1 expressing cell lines such as K562. To better define the regions necessary for MEIS1 promoter activity we tested a deletion series spanning the -500 bp to +20 bp relative to AHTSS region in the pGL3 luciferase reporter vector in K562 cells. Truncation of the region 305 bp to 268 bp resulted in a strong decrease (∼6-fold) in promoter activity. Within this 38 bp region we identified several predicted transcription factor binding sites, including sites for Serum response factor, RUNX1 and ETS family member. Mutations or deletions of the predicted ETS family member binding site resulted in a decrease of promoter activity to almost basal levels whereas mutations of the other predicted binding sites did not significantly alter promoter activity. We next tested the involvement of three ETS family members in MEIS1 regulation: ELF1, FLI1 and GABPa. Both ELF1 and GABPa, but not FLI1, bound to the predicted ETS family member binding site as assessed by electrophoretic mobility shift assay conducted with nuclear extracts from cells with or without detectable MEIS1 expression, K562 and HL60, respectively. However, chromatin immunoprecipitation assay revealed occupancy of ELF1, but not GABPa or FLI1, to the MEIS1 promoter only in MEIS1 expressing K562 cells. Moreover, siRNA knockdown of ELF1 in K562 cells was associated with decreased MEIS1 expression. Together, these findings implicate the ETS family member ELF1 as a key regulator of MEIS1 expression. The absence of MEIS1 expression in HL60 cells expressing ELF1 argues that additional regulators remain to be identified. Studies of additional candidate cis-regulatory regions in the MEIS1 locus which contribute to the overall chromatin status and the expression of this critical leukemogenic gene are now in progress.