The Relationship of BRCA1 and HMGA2 in Breast Cancer

Institution: University of California, Irvine
Investigator(s): Connie Tsai, B.S. -
Award Cycle: 2007 (Cycle 13) Grant #: 13GB-0152 Award: $36,704
Award Type: Dissertation Award
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease

Initial Award Abstract (2007)
BRCA1 gene mutations account for 50% of hereditary breast cancer. In sporadic breast cancers, although the BRCA1 gene is intact, the protein expression is often reduced. BRCA1 is known to play an essential role in maintaining genomic integrity mainly through direct involvement in repairing damaged DNA and monitoring cell proliferation. However, these functions of BRCA1 do not adequately explain how its deficiency accelerates breast cancer progression. It is postulated that BRCA1 is also important for cell growth and differentiation by its ability to regulate genes critical for these processes. We recently demonstrated that BRCA1 has a direct role in regulating the expression of an array of genes involved in tumor progression, one of which is HMGA2.

HMGA2 (High Mobility Group AT-hook2) plays an important role during embryonic development, when cells are undergoing extensive cell growth. HMGA2 expression in adult tissue is almost undetectable, but the inappropriate expression of HMGA2 in adult tissue is observed in many types of benign and malignant tumors. Intriguingly, elevated HMGA2 mRNA expression is found in the peripheral blood of breast cancer patients and may serve as a biomarker for metastasis and poor prognosis. However, it is still unknown whether up-regulation of HMGA2 expression has a direct role in development and progression of breast cancer.

Thus, the aim of this project is to show that loss of HMGA2 gene repression by BRCA1 is a key pathway in accelerating breast cancer progression. Our methods include gene regulatory promoter analysis, luciferase assays, and ChIP (chromatin immunoprecipitation) assays to understand the mechanisms of BRCA1-dependent transcriptional regulation of HMGA2. Next, we plan to express HMGA2 in normal mammary epithelial cells via retrovirus-mediated gene transfer and assess the oncogenic consequences. Lastly, we will deplete HMGA2 by a technique, called RNA inference (RNAi), in breast cancer cells to demonstrate whether this suppresses breast tumor formation in a mouse model.

These studies will provide the mechanistic basis on the regulation of HMGA2 expression by BRCA1 and demonstrate the biological consequence upon loss of regulation by BRCA1. This finding will augment our understanding of BRCA1-associated breast cancer and identify a potential target for treatment and detection of the disease.

Final Report (2008)

The goal of this project is to establish the relationship between BRCA1 tumor suppressor and HMGA2, a gene involved in promoting cell proliferation. Elevated HMGA2 expression is found in the peripheral blood of breast cancer patients and may serve as an indicator for metastasis and poor prognosis in breast cancer. What I proposed to accomplish is that loss of transcriptional repression by BRCA1 repressor complex on HMGA2 is a key pathway in BRCA1-associated mammary tumorigenesis.

The specific aims of the project were; 1) demonstrating transcriptional repression of HMGA2 mediated by BRCAl/CtIP/ZBRK1 repressor complex, and 2) determining the biological consequences of deregulated HMGA2 expression. However, when BRCA1 and CtIP was depleted in cells transfected with HMGA2 promoter constructs, no significant changes in reporter activity was observed. This supports our hypothesis that HMGA2 is subject to negative regulation by ZBRK1 via consensus ZBRK1 binding motif on HMGA2 promoter. However, it fails to establish the relationship between BRCA1 and HMGA2. Since HMGA2 promoter contains two putative ZBRK1 consensus motifs at -820/-806 and -2523/-2509 regions upstream of exon 1, chromatin immunoprecipitation (ChIP) assay was performed to determine the direct and physical localization of BRCAl/CtIP/ZBRK1 repressor complex on HMGA2 promoter. My experimental results suggested that ZBRK1 localizes to HMGA2 promoter at ZBRK1 consensus motif -820/ 806 but not at -2523/-2509. However, contrary to our hypothesis, ChIP analysis did not show the physical localization of BRCA1 and CtIP to HMGA2 promoter.

Taken together, these data suggest that HMGA2 is regulated by ZBRK1 transcriptional repressor in a BRCA1-independent manner.