Combination of Cancer Stem Cell Markers CD44 and CD24 Is Superior to ALDH1 as a Prognostic Indicator in Breast Cancer Patients with Distant Metastases

The combination of CD44 and CD24, or aldehyde dehydrogenase 1 (ALDH1) alone, is a widely used cancer stem cell marker in breast cancer. However, no conclusion has yet been reached as to which marker is the best for characterizing cancer stemness. Immunohistochemical evaluation using cancer stem cell markers is clearly less common clinically than in basic experiments and how the expressions of these markers relate to patient outcomes remains controversial. To investigate whether combining these markers might improve the prediction of patient outcomes, we immunohistochemically examined clinical samples. Primary invasive breast cancer samples from 61 patients who eventually developed distant metastases after curative surgery were immunohistochemically examined. All patients were free of metastatic disease at the time of surgery and received standard adjuvant systemic treatments. CD44(+)/24(-) and ALDH1-positive rates in primary tumors differed according to intrinsic subtype. ER-positive patients with CD44(+)/24(-) tumors had significantly longer disease-free-survival than all other ER-positive patients (p = 0.0047). On the other hand, CD44(+)/24(-) tumors were associated with poor outcomes of ER-negative patients (p = 0.038). Finally, expression patterns of CD44 and ALDH1 in single tumors were strikingly different and there were virtually no individual double-stained cells. Thus, this combination does not allow evaluation of relationships with patient outcomes. Our results raise the possibility of CD44(+)/24(-) being a good prognostic marker, one which would allow treatment effects and outcomes to be predicted in patients with recurrent breast cancer.