An international research collaboration led by UC San Francisco researchers has identified a genetic variant common in Latinas that protects against breast cancer

The variant, a difference in just one of the three billion "letters" in the human genome known as a single-nucleotide polymorphism (SNP), originates from indigenous Americans and confers significant protection from breast cancer, particularly the more aggressive estrogen receptor–negative forms of the disease, which generally have a worse prognosis. "The effect is quite significant," said Elad Ziv, MD, professor of medicine and senior author of the study. "If you have one copy of this variant, which is the case for approximately 20% (the range being 10% to 25%) of U.S. Latinas, you are about 40% less likely to have breast cancer. If you have two copies, which occurs in approximately 1% of the US Latina population, the reduction in risk is on the order of 80%." Epidemiological data have long demonstrated that Latinas are less susceptible to breast cancer than women of other ethnic backgrounds. According to National Cancer Institute data from 2007 to 2009, whites have about a 13% lifetime risk of breast cancer, blacks about 11%, and Hispanics less than 10%. The lifetime risk among Hispanics with indigenous American ancestry is even lower. For several years the researchers studied Latina populations in search of genetic and biological explanations for these differences. "After our earliest studies we thought there might be a genetic variant that led to increased risk in European populations," said Ziv. "But what this latest work shows is that instead there is a protective variant in Native American and Latina populations." The newly discovered SNP is on Chromosome 6, near a gene coding for an estrogen receptor known as ESR1. The scientists say that the biological basis of the association between the variant and reduced breast cancer risk is still not known, but their preliminary experiments indicate that the variant interferes with the action of transcription factors, proteins that regulate the expression of the ESR1 estrogen receptor.