In this study, we demonstrated that the presence of ALDH1 expression is significantly higher in young breast cancer patients than in elderly patients. Moreover, we demonstrated that ALDH1 expression is an independent risk factor for decreased survival in young breast cancer patients, but not in elderly patients.
To the best of our knowledge, we are the first to show that expression of ALDH1 in breast cancer is age-dependent. A corresponding difference in the number of cancer stem cells might provide an explanation for known differences in clinical outcome between young and elderly breast cancer patients. A potential strength of our study is that it includes consecutive patients from one center, not biased by being part of a clinical trial. The age restriction of the majority of clinical trials prohibits inclusion of patients older than 70 year and, indeed, less than 10% of clinical trial participants is older than 65 years [22
]. In our study, 34% of patients were 65 years or older at diagnosis of breast cancer. Therefore, our study was not hampered by lack of statistical power to analyse the effect of ALDH1 in the elderly.
We showed that ALDH1 expression has a qualitative age interaction effect. In our study, ALDH1 is a predictor of poor prognosis in young patients, but ALDH1 did not influence clinical outcome in elderly patients. Recently, Zhou and colleagues pooled the available data on the prognostic role of ALDH1 activity in breast cancer [18
]. Their meta-analysis demonstrated that ALDH1 activity as assessed by immunohistochemistry was significantly associated with worse overall survival (unadjusted pooled relative risk, 2.83; 95% CI, 2.16 to 3.67; four patient cohorts including 1,158 patients) [18
]. However, the authors did not stratify for age. In other studies, no interaction was found between ALDH1 expression and age [9
]. However, in these studies, an age of 40 or 50 year was used as a cut-off for age stratification. We used 65 years as a cut-off point as this may better match with the bimodal age distribution of breast cancer, which suggests that breast cancer may be characterized by early- and late-onset tumor types with modes near ages 50 and 70 years [5
]. As argued by Anderson et al., these modal ages do not suggest a sharp division of distinctive tumor categories, but rather reflect central tendencies for the age distributions of biologically distinct cancer populations [5
]. In line with this bimodal age distribution, a biological explanation of the qualitative age-interaction of the prognostic effect of ALDH1 expression might be that of a changing micro-environment in elderly patients, which may result in hampered signal transduction between tumor stem cells and the micro-environment. Moreover, changes in metabolic processes might limit the role of tumor stem cells in elderly patients. Increasing evidence from the field of epigenetics demonstrates that hypermethylation-induced repression of genes required for stem cell differentiation is linearly associated with age [24
]. This suggests that, with increasing age, the role of tumor stem cells becomes more limited. Notwithstanding the need to clarify the underlying mechanism, this new finding on the age-dependent role of ALDH1 activity warrants further validation and underlines the need of age stratification when assessing biomarkers and new therapies for breast cancer patients.
A potential limitation of our study is the choice of antibody that was used for immunohistochemical detection of ALDH1 expression, which is specific for the ALDH1A1 isoform. This antibody has been generally used in studies investigating the role of ALDH1 in breast cancer patients [9
]. In a recent study, Marcato et al. investigated the expression of the different ALDH1 isoforms in breast cancer stem cells, breast cancer cell lines and fixed human breast cancer tissue using various techniques [25
]. They found that ALDH1A3 expression correlated better with ALDH1 activity and with tumor grade, metastasis and cancer stage [25
]. Therefore, future research studying breast cancer stem cells should incorporate ALDH1A3 expression in order to determine its role as a potential marker of cancer stem cell activity.