Age-related phenotypes in breast cancer: a population- based study

Breast cancer in young (<40 years) is associated with a higher frequency of aggressive tumor types and poor prognosis. It remains unclear if there is an underlying age-related biology that contributes to the unfavorable outcome. We aim to investigate the relationship between age and breast cancer biology, with emphasis on proliferation. Clinico-pathologic information, immunohistochemical markers, and follow-up data were obtained for all patients aged <50 (Bergen cohort-1; n=355) and compared to previously obtained information on patients aged 50-69 (Bergen cohort-2; n=540), who participated in the Norwegian Breast Cancer Screening Program. Data from the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) was applied for validation and for analyses of gene expression signatures. Young breast cancer patients presented more aggressive tumor features such as hormone receptor negativity, HER2 positivity, lymph-node metastasis, the HER2-enriched and triple negative subtypes, and shorter survival. Age <40 was significantly associated with features of stemness and higher proliferation (by Ki67), also in molecular subsets. Ki67 showed weaker prognostic value in young patients. We point to aggressive phenotypes, increased tumor cell proliferation and stem-like features in breast cancer of the young. Hence, tumors of young breast cancer patients may have a unique biology, also when accounting for screen/interval differences, that may open for new clinical opportunities, stratifying treatment by age.