Public Release: 

Resistance of ER-positive breast cancer to tamoxifen therapy may be driven by APOBEC3B

American Association for Cancer Research

SAN ANTONIO -- Responses to tamoxifen were significantly prolonged by reducing levels of the enzyme APOBEC3B in preclinical models of estrogen receptor-positive breast cancer and significantly shortened by increasing levels of APOBEC3B, suggesting that APOBEC3B drives resistance to tamoxifen, according to data presented at the 2015 San Antonio Breast Cancer Symposium, held Dec. 8-12.

In complementary clinical analyses, among patients with recurrent estrogen receptor-positive breast cancer treated with tamoxifen, those who had high levels of APOBEC3B in their primary tumors had shorter durations of progression-free survival than those whose primary tumors had low APOBEC3B levels (medians of 7.5 versus 13.3 months).

"Several recent studies have linked elevated levels of the enzyme APOBEC3B in estrogen receptor-positive breast cancer with poor patient outcomes, but whether APOBEC3B actually drives the poor outcomes has not been determined," said Reuben Harris, PhD, investigator of the Howard Hughes Medical Institute, professor in the Department of Biochemistry, Molecular Biology, and Biophysics at the University of Minnesota, Twin Cities, and member of the Masonic Cancer Center. "Our preclinical studies using an established xenograft model of estrogen receptor-positive breast cancer show that APOBEC3B can drive tamoxifen resistance.

"We know that APOBEC3B is not required for human life because a large proportion of people in Southeast Asia do not have APOBEC3B as a result of a natural genetic variation," Harris continued. "This, combined with the fact that it is an enzyme, makes APOBEC3B an attractive target for a therapeutic that could stop the development of resistance mutations and substantially extend the length of response to tamoxifen."

In preclinical studies, Harris and colleagues transplanted mice with a human estrogen receptor-positive breast cancer cell line called MCF-7L and then treated the mice with tamoxifen. Compared with mice receiving unmanipulated MCF-7L cells, mice receiving cells in which levels of APOBEC3B had been reduced by a technique called shRNA had significantly prolonged responses to tamoxifen and those receiving cells in which levels of APOBEC3B were increased by genetic manipulation developed tamoxifen resistance significantly more rapidly.

Harris noted that while the typical tamoxifen response of mice transplanted with unmanipulated MCF-7 cells was about 50 to 100 days, most of those transplanted with APOBEC3B-supressed MCF-7 cells responded until the experiment was ended after a year; those transplanted with APOBEC3B-overexpressing cells developed rapid tamoxifen resistance with virtually no therapeutic response.

In collaboration with John Martens, PhD, an associate professor at Erasmus MC in Rotterdam, Netherlands, and Paul Span, PhD, an associate professor at Radboud University Medical Center in Nijmegen, Netherlands, Harris and colleagues also analyzed APOBEC3B levels in tumor samples from 285 patients with recurrent estrogen receptor-positive breast cancer. The tumor samples were obtained during surgery for primary breast cancer and all patients were treated with first-line tamoxifen therapy only after disease recurrence. Those patients whose tumors had high levels of APOBEC3B had disease progression significantly sooner after starting tamoxifen therapy than those whose tumors had low levels of APOBEC3B: 7.5 months compared with 13.3 months.

Harris explained that APOBEC3B is an enzyme that causes genetic mutations and that this study suggests that by promoting genetic mutation it drives tamoxifen resistance. He also noted that this study indicates that APOBEC3B, as a dominant intracellular source of genetic mutation, could potentially be controlled to prevent cancer progression. Harris mentioned that we already know of external causes of cancer-driving genetic mutation that can be controlled--for example, ultraviolet radiation from the sun, which can be avoided by seeking shade or applying sunscreen, and cigarette smoke, which can be controlled by quitting--and APOBEC3B should be regarded as an analogous intracellular factor.


This study was supported by the Department of Defense Breast Cancer Research Program, V Foundation for Cancer Research, and Prospect Creek Foundation. Harris is a co-founder of ApoGen Biotechnologies Inc.

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The mission of the 2015 San Antonio Breast Cancer Symposium is to produce a unique and comprehensive scientific meeting that encompasses the full spectrum of breast cancer research, facilitating the rapid translation of new knowledge into better care for patients with breast cancer. The Cancer Therapy & Research Center (CTRC) at The University of Texas Health Science Center at San Antonio, the American Association for Cancer Research (AACR), and Baylor College of Medicine are joint sponsors of the San Antonio Breast Cancer Symposium. This collaboration utilizes the clinical strengths of the CTRC and Baylor and the AACR's scientific prestige in basic, translational, and clinical cancer research to expedite the delivery of the latest scientific advances to the clinic. For more information about the symposium, please visit

Abstract: S4-07

Title: Tamoxifen resistance driven by the DNA cytosine deaminase APOBEC3B in recurrent estrogen receptor positive breast cancer

Authors: Reuben Harris1, Emily Law1, Anieta Sieuwerts2, Kelly LaPara1, Brandon Leonard1, Gabriel Starrett1, Nuri A Temiz1, Fred Sweep3, Paul Span3, John Foekens2, John Martens2 and Douglas Yee1. 1University of Minnesota, Minneapolis, MN; 2Erasmus MC Cancer Institute, Rotterdam, Netherlands and 3Radboud University Medical Center, Nijmegen, Netherlands.

Body: Recent studies have implicated the DNA cytosine deaminase APOBEC3B as a major source of mutation in breast cancer. APOBEC3B explains a large proportion of both dispersed and clustered cytosine mutations, the latter of which are also called kataegis. APOBEC3B expression levels correlate with poor outcomes for patients with estrogen receptor positive breast cancer. While targeted therapies, such as tamoxifen, are available to treat these tumors, secondary drug resistance often develops. Here we suppressed endogenous APOBEC3B in the estrogen receptor positive breast cell line MCF-7L with shRNA. Lowered levels of APOBEC3B did not affect in vitro growth or sensitivity to estradiol. In a xenograft model of tamoxifen therapy, suppression of APOBEC3B associated with prolonged responses to tamoxifen (p<0.05). Furthermore, APOBEC3B over-expression did not affect in vitro cell growth but accelerated the development of tamoxifen-resistant tumors in vivo. In addition, we studied two separate cohorts of 285 breast cancer patients who received first line treatment with tamoxifen for recurrent disease. High APOBEC3B expression levels measured in the primary tumor associated significantly with unfavorable progression free survival in multivariate analysis that included the traditional predictive factors (age, dominant relapse site, disease-free interval, estrogen receptor and progesterone receptor, and adjuvant chemotherapy; HR=1.67, p=0.0001). The median period of progression free survival was 7.5 months for patients with APOBEC3B high primary tumors and 13.3 months for those with APOBEC3B low tumors (p<0.0.0001). These studies demonstrate that APOBEC3B drives resistance to endocrine treatment with tamoxifen in recurrent disease.

This research will be presented at a press conference at the 2015 San Antonio Breast Cancer Symposium, moderated by SABCS Co-director and AACR Past-president Carlos L. Arteaga, MD, director of the Breast Cancer Program at Vanderbilt-Ingram Cancer Center, Thursday, Dec. 10, 7:30 a.m. CT in Room 217D of the Henry B. Gonzalez Convention Center. Reporters who cannot attend the press conference in person can call in using the following information:

  • United States/Canada (toll-free): 866-297-6395
  • International (toll): 1-847-944-7317
  • Conference code number: 41320576

To interview Reuben Harris, contact Julia Gunther at or 267-250-5441.

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