P5-01-15: The Functional Role of the Estrogen-Regulated Gene GREB1: Characterization of a Novel GREB1 Knockout Mouse Model.

Background: Gene regulated in breast cancer 1 (GREB1) was initially discovered in breast cancers as an estrogen-regulated gene that mediates estrogen-stimulated cell proliferation and is a candidate clinical marker for response to endocrine therapy. However, little is known of the functional role of GREB1 protein in normal breast tissue or breast cancers. Methods: To address this unknown role, our laboratory designed and created a novel Greb1 Knockout Mouse model (C57/bl MEL Greb1 KO). This constitutive model results in the loss of Greb1 mRNA and protein expression in cells where expression of Cre recombinase promotes the cleavage of exon 1 and intron 1 of the gene encoding Greb1. ROSA26 Cre C57/b1 MEL Greb1 KO mice heterozygous for the floxed Greb1 allele were crossed to generate experimental litters. Initial experiments were designed to evaluate if the complete loss of Greb1 expression in offspring homozygous for the floxed Greb1 allele was lethal during gestation. Experimental litters were tail clipped and genotyped using gDNA and genotype-specific PCR. Results: Offspring homozygous for the floxed Greb1 allele were identified in expected Mendelian ratios with wild type and heterozygous siblings. Loss of Greb1 expression was confirmed using RT-PCR, in situ hybridization and immunoblotting. Loss of both Greb1 alleles was not observed to be lethal during gestation for either male or female pups. Preliminary gross observation of these homozygous KO mice revealed no overt anatomical differences, however, they were 25–30% smaller than their heterozygous and wild-type siblings. Breeding experiments are underway to determine the fertility of crossbred Greb1 homozygous KO mice. Imaging experiments and necropsy with histochemical analysis of tissues will reveal any alteration in architecture and function. These findings will be summarized in this presentation. Discussion: As GREB1 has been identified as an estrogen-regulated gene involved in breast cancer cell proliferation and a potential target for new therapeutic strategies, it is important to understand the contribution of GREB1 to the differentiation, development and function of normal tissues as well as in breast cancers. Characterization of this novel Greb1 KO mouse model will provide answers to these functional questions surrounding GREB1. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P5-01-15.