Breast cancer in monozygotic twins

Breast cancer is a multifactorial disease. Twin studies comparing the disease concordance rate in identical twin pairs serve to differentiate the influence of genetic and environmental factors in the disease development. To assess breast cancer risk for an identical twin sister of a patient with breast cancer. Five monozygotic twin families were examined during 2005- 2011 in which at least one of the monozygotic sisters developed breast cancer.In 4 breast cancer women from 4 families, molecular genetic analysis of BRCA1 and BRCA2 genes was performed. The median followup period was 12.6 years (7 to 24 years). No pair of monozygotic sisters was concordant for breast cancer. Familial breast/ ovarian cancer syndrome due to BRCA1 gene mutation was confirmed in one pair. These twins were phenotypically discordant, the first one developing breast cancer at the age of 54 years, her co sister suffering from ovarian cancer at the age of 43 years. In the other 4 nonBRCA families, breast cancer was diagnosed at the age of 38- 50 years (median 44 years) in one of the sisters; the other twins remain healthy through the followup period. We did not observe concordance for breast cancer in 5 pairs of monozygotic twins. Based on results of published studies, the life time breast cancer risk for a healthy identical twin of a breast cancer nonBRCA woman is around 20- 30 %. Other nonhereditary risk factors must exist to explain the discordant phenotype. This highlights that environmental factors play an important role in breast cancer development. In case of BRCA associated breast cancer, breast cancer risk for the healthy co twin is the same as that for other BRCA mutation carriers, i.e. 45- 85%.