Sister Study

Beginnings and objectives

Since its start in 2003, the Sister Study enrolled over 55,000 women whose sisters had breast cancer, including women of all backgrounds and ethnicities. The study focused on sisters because they share genetics and early-life environments, making them more likely to participate and providing a higher chance of identifying risk factors. In the 1980s and 1990s, advocacy groups concerned about rising breast cancer rates called for more research beyond hormones, lifestyle, and reproduction.

NIEHS researchers proposed the Sister Study in response, noting that women whose sisters had breast cancer are twice as likely to develop the disease themselves. The study initially considered endocrine-disrupting chemical exposures but expanded to include individual and community-level stressors such as air pollution, psychological stress, sleep patterns, diet, and socioeconomic factors. It also aimed to address research gaps on early-life exposures from in utero environments through childhood and adolescence.

The first participant enrolled in fall 2003, and by spring 2009 the study surpassed its goal of 50,000 enrollees, reaching 50,884 women. Originally designed for a 10-year follow-up, the study continued due to high participant commitment, expanding to cover other cancers, cardiovascular diseases, respiratory outcomes, and gynecological conditions.

Recruitment and study rationale

Recruitment began in 2003, with nationwide enrollment from October 2004 through March 2009. Eligible women were aged 35 to 74, lived in the United States or Puerto Rico, had a sister diagnosed with breast cancer, and had never had breast cancer themselves. Recruiters represented targeted demographic groups to encourage diversity, specifically seeking African American, Latina, Native American, and Asian women, as well as those aged 60 and older. Study materials became available in Spanish in 2005.

The final study population included 8,311 women (16%) who self-identified as Hispanic/Latina or non-White, 8,874 women aged 65 and older (17%), and 7,805 women with a high school education or less (15%). A small percentage (n=3,066) completed only part of the study requirements but are followed through record linkage using vital statistics and cancer registries. Among those who did not fully enroll, a higher proportion were minority women (36%) and had fewer years of schooling (18%).

Later recruitment focused on these demographics. Fully enrolled participants submitted baseline data via questionnaires on medical histories, environmental exposures, and lifestyle, and provided blood or saliva samples for DNA extraction.

Findings and contributions

The study has identified factors contributing to breast cancer, including air pollution, trauma, psychosocial stress, diet, permanent hair dye and chemical hair straighteners, some personal care product ingredients, and obesity. Researchers also found a correlation between exposure-related DNA changes and advanced biological aging as a risk factor. As of August 2023, nearly 300 research papers have been published, many in Environmental Health Perspectives, supporting links between breast cancer and genetics and environmental factors.