1. Breast tissue samples from patients with BRCA1 mutations showed high expression of the protein Receptor Activator of Nuclear Factor κ B (RANK), which was associated with increased cell division and defective DNA repair.
2. When human breast tissue samples and a mouse model with BRCA1 mutations were treated with a RANK ligand (RANKL) inhibitor, breast cancer development was delayed and tumor growth was decreased.
Evidence Rating Level: 1 (Excellent)
Study Rundown: Although it is known that individuals with mutations in the BRCA1 gene are predisposed to developing breast and ovarian cancers, the only current way to prevent tumor development in these patients is through the removal of the target organs. Therefore, identification of a pharmacological target to prevent tumor development would be an ideal alternative to surgical intervention. To this end, this study evaluated the potential of inhibiting RANKL with antibodies, since previous studies in mouse mammary stem cells have demonstrated the importance of RANKL in mediating hormone signaling in breast tissue.
In order to determine whether RANK is important in human breast cancer development, breast tissue was isolated from women with and without BRCA mutations. In tissues with BRCA1 mutations, RANK was found to be highly expressed in luminal progenitor cells and this increased expression was associated with increased cell proliferation. BRCA1 mutant cells with high levels of RANK expression were also more sensitive to DNA damage. Upon the addition of the RANKL inhibitor denosumab, breast tissue did not respond to progesterone-induced cell division. Finally, when the RANKL inhibitor OPG-Fc was tested on a mouse model of BRCA1-mutation breast cancer, a delay in tumor onset as well as a decrease in growth was observed.
Through testing the effect of RANKL inhibitors on both human tissues and mouse models, this study demonstrated that the inhibition of RANKL in patients with BRCA1 mutations may be a potential preventative therapy for breast cancer development. In addition, due to the current use of the RANKL inhibitor denosumab in osteoporosis, this FDA-approved antibody could potentially be repurposed for patients at risk of developing breast cancer.
In-Depth [human tissue and animal study]: Human breast epithelial cells were collected from 33 women without a mutation in the BRCA gene and 24 women with a mutation in BRCA1. Using flow cytometry, these cells were separated based on cell type. These cells were stained using immunohistochemistry to evaluate the expression of RANK. This protein was located in the luminal progenitor cells of both wild-type and BRCA1-mutated tissues. The cells were then sorted and analyzed using fluorescence-associated cell sorting, revealing that tissues harboring BRCA1 mutations had a significantly higher expression of RANK (p<0.0001).
When RANK+ BRCA1-mutated cells were subjected to a 3D colony formation assay using Matrigel, they were found to have an increased rate of cell growth compared to RANK+ wild-type cells (p<0.05). The researchers then determined whether the mutated RANK+ cells were functionally different than wild-type RANK+ cells by assessing their response to two DNA damaging agents, hydroxyurea and gamma radiation. Not only were RANK+ BRCA1 mutant cells more sensitive to DNA damage, they also showed higher levels of DNA damage prior to exposure to these conditions.
Finally, the effect of RANKL inhibitors was tested on human breast tissue as well as a mouse model containing BRCA1 mutations. Using an organoid of breast tissue, denosumab treatment inhibited cell proliferation upon progesterone stimulation (p<0.05). This result was also seen in a pilot clinical study when 3 women with BRCA1 mutations were treated with this inhibitor. In the mouse model, treatment with the antibody OPG-Fc led to a delay in tumor onset (p=0.0002) as well as a significant decrease in tumor growth.
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