Osteoporosis Drug Shows Promise for Breast Cancer Prevention in BRCA1 Carriers

Geoffrey Lindeman, BSc, MBBS, PhD, FRACP

Geoffrey Lindeman, BSc, MBBS, PhD, FRACP

TNFSF11, also known as RANKL, shows potential as a genetic pathway in the prevention of breast cancer for women carryingBRCA1mutations. Early study findings, published inNature Medicine,show that a drug currently used in the treatment of osteoporosis, denosumab (Xgeva)—an inhibitor of RANKL—could also be used for the prevention and delay of tumor growth forBRCA1-mutation carriers.

“These findings imply an integral role for the RANK pathway in tumor initiation inBRCA1-mutation carriers and lend support to clinical studies for the ‘repurposing’ of denosumab as a novel preventative therapy strategy in these and possibly other ‘high-risk’ women,” wrote study authors.

Women withBRCA1germline mutations are known to have a highly increased risk of developing breast cancer. Preventative therapy strategies for carriers of aBRCA1mutation currently include tamoxifen treatment and surgery, but researchers believe that RANKL inhibition could be a new preventative method for premenopausal women, potentially delaying the need for prophylactic surgery.

A team of researchers, led by Jane Visvader, BSc, PhD, and Geoffrey Lindeman, BSc, MBBS, PhD, FRACP, joint laboratory heads at the Walter and Eliza Hall Institute of Medical Research in Parkville, Victoria, Australia, investigated the role of RANKL receptor TNFRSF11A, known as RANK, inBRCA1-mutant breast tissue.

Researchers collected breast tissue from premenopausal women with histologically normal breast tissue (n = 33) and carriers of theBRCA1mutation (n = 24) andBRCA2mutation (n = 10). At the time, the carriers were undergoing prophylactic mastectomies.

Co-staining for RANK was applied to mature luminal cells, luminal progenitor (LP) cells, and basal cells, yet only LP cells in wild-type and BRCA1-mutant breast tissue had clinically significant levels of RANK expression. Fluorescence-activated cell sorting (FACS) analysis showed that the amount of LP cells positive for RANK expression (≤0.5%) was higher inBRCA1-mutant breast tissue (28.6 ± 2.5%) than in age-matched wild-type (14.7 ± 1.2%) andBRCA2-mutated (13.6 ± 3.5%) breast tissue. RANKL expression was only found in mature

luminal cells.

Immunostaining was performed on tumor-adjacent breast tissue in a cohort of patients withBRCA1-mutated (n = 78),BRCA2-mutated (n = 58), or wild-type tumors (n = 248).BRCA1-mutant breast tissue showed more than double the expression, frequency, and intensity of RANK than in wild-type tissue. Alternatively, RANK expression was not prominent inBRCA2-mutated tissue.

RANK+ progenitor cells fromBRCA1carriers were found to have a higher clonogenic capacity than RANK- cells, as well as higher proliferative activity than RANK+ progenitor cells in wild-type breast tissue.

Testing for potential for DNA damage, RANK+ and RANK- progenitor cells from wild-type andBRCA1-mutant breast tissue were treated with hydroxyurea or γ-irradiation.BRCA1-mutant LP cells were extremely sensitive to these treatments, especially for RANK+ cells subjected to irradiation, whereas no DNA damage was noticeable in wild-type tissue.

Damage was also found in untreated RANK+ cells, suggesting thatBRCA1-mutant tissue has abnormal DNA damage repair systems.

Molecular profiles were compared between RANK+ LP cells and luminal A and B, basal-like, HER2, and normal-like tumors through data from The Cancer Genome Atlas. RANK+ progenitor cells closely matched the gene signature of basal-like tumors (P= .0001).

In a pilot BRCA-D window study testing the responsiveness ofBRCA1-mutant breast tissue to progesterone, 3 subjects were treated with RANK inhibitor denosumab, a fully humanized monoclonal antibody that binds RANKL (ACTRN12614000694617). Posttreatment, each subject showed substantially reduced levels of Ki67 staining (mean value of 2.5 ± 1.2% from pretreatment level 0.5 ± 0.2%). TheMKI67gene that encodes Ki67 was highly upregulated in RANK+ LP cells. Denosumab reduced the breast epithelial cell proliferation inBRCA1mutations. The result was confirmed inex vivo3D-organoid system testing.

To test for a prevention strategy with RANKL inhibition, study authors used an MMTV-Cremodel with mice that develop basal-like tumors mimicking humanBRCA1-mutated breast cancer. These mammary tumors express abundant amount of RANK and reduced amounts of RANKL. In the study the mice were randomized to receive OPG-Fc, a RANKL inhibitor, or a mouse IgG1 control antibody.

A significant delay in tumor onset was discovered for the mice treated with OPG-Fc versus the control mouse group. While the control group had a median tumor onset of day 215 (P= .0002), the median latent period was not reached in the OPG-Fc arm. Eleven of 17 mice (65%) did not develop tumors before the data cutoff.

Dramatically reduced hyperplasia was discovered in the mammary glands of the treated group of mice compared with the control group.

In a second study, young adult MMTV-Cremice treated with RANKL-specific monoclonal antibody showed a significant delay in tumor onset compared to control. The median tumor onset was at day 177 for the antibody group and day 123 for the control arm (P= .0001).

Treatment of docetaxel plus RANKL inhibitor OPG-Fc significantly weakened tumor growth in a patient-derived RANK+ xenograft from aBRCA1-mutation carrier (P<.0001). RANKL could act as a therapeutic target in established RANK+ tumors, leading to the prevention of these tumors with RANKL inhibition.

The report explained preclinical study of the role of RANK in tumorigenesis and possible implications of RANKL inhibition. Studies in women withBRCA1mutations are needed to further examine denosumab and the role of RANKL inhibition in the prevention of breast cancer.

Reference:

Nolan E, Vaillant F, Branstetter D, et al. RANK ligand as a potential target for breast cancer prevention in BRCA1-mutation carriers. [Published online June 20, 2016.]Nat Med.doi:10.1038/nm.4118.