Breast cancer incidence and estrogen receptor alpha in normal mammary tissue--an epidemiologic study among Japanese women in Japan and Hawaii.

We have undertaken a study to examine whether the difference in breast cancer incidence between 2 populations of similar genetic background is reflected in a similar pattern of estrogen receptor alpha expression in normal mammary gland. Study participants were 92 Japanese women from Sapporo, Japan (mean age 48.2 years) and 49 Japanese women from Honolulu, Hawaii (mean age 45.4 years), who underwent biopsy indicating normal breast tissue or benign, nonproliferative breast disease in hospitals in Sapporo, Japan and Honolulu, Hawaii. The breast tissue samples were formalin-fixed and paraffin-embedded. The estrogen receptor immunohistochemistry assays were conducted using Dako kits. Japanese women in Hawaii, who have a higher incidence of breast cancer compared with Japanese women in Sapporo, also had, as predicted, higher mean percentage of estrogen receptor alpha-positive normal mammary cells (2-tailed test, p approximately 0.09). The results of our study are compatible with the hypothesis that estrogen receptor alpha expression in normal mammary tissue increases breast cancer risk and they also indicate that the expression of these receptors is dependent, at least in part, on nongenetic factors.