Trial Information

The Sister Study: Genetic and Environmental Risk Factors for Breast Cancer

The Sister Study is prospectively examining environmental and familial risk factors for
breast cancer and other diseases in a cohort of 50,884 sisters of women who have had breast
cancer. Such sisters have about twice the risk of developing breast cancer as other women.
The frequency of any relevant genes and shared risk factors will also be higher. Studying
sisters enhances our ability to understand the interplay of genes and environment in breast
cancer risk and to identify potentially preventable risk factors. We assess exposures
before the onset of disease, thus avoiding biases common to retrospective studies. The
assembled cohort has created a framework from which to test new hypotheses as they emerge.

Cancer-free sisters aged 35-74 were recruited nationally. Enrollment was completed July
2009 with 50,884 women fully enrolled. Participants have enrolled from all 50 states and
Puerto Rico. The cohort is now being followed actively. Cohort members are occasionally
invited to participate in add-on studies, which sometimes include asking participants to
invite their family members to provide information.

Data on potential risk factors and current health status were collected with telephone
interviews and self-completed questionnaires. Blood, urine, and environmental samples were
collected and banked for future use in nested studies of women who develop breast cancer (or
other diseases) and a sample of those who don't. The cohort is being followed prospectively
for 10 or more years. Annual questionnaires update medical history and changes in
exposures. About 300 new cases of breast cancer are expected to occur in the cohort each
year. Analyses are ongoing and assess the independent and combined effects of environmental
exposures and genetic polymorphisms that affect estrogen metabolism, DNA repair, and
response to specific environmental exposures. Future analyses will focus on known and
potential risk factors (e.g. smoking, occupational exposures, alcohol, diet, obesity) and
include measurement of phthalates, phytoestrogens, insulin, growth factors, micro-nutrients,
and genes. Women who develop breast cancer during the study are followed to assess the role
of environment and genes in healthy survival following diagnosis and treatment. The cohort
will also be used to explore risk for other diseases (e.g. heart disease, osteoporosis,
other hormonal cancers, and autoimmune diseases) that are important for women.