Markers that reliably identify cancer stem cells (CSC) in
ovarian cancer could assist prognosis and improve strategies for
therapy. CD133 is a reported marker of ovarian CSC.
Aldehyde dehydrogenase (ALDH) activity is a reported CSC marker in several solid
tumors, but it has not been studied in ovarian CSC. Here we report that dual positivity of CD133 and ALDH defines a compelling marker set in ovarian CSC. All human ovarian
tumors and cell lines displayed ALDH activity. ALDH(+) cells isolated from
ovarian cancer cell lines were chemoresistant and preferentially grew
tumors, compared with ALDH(-) cells, validating ALDH as a marker of ovarian CSC in cell lines. Notably, as few as 1,000 ALDH(+) cells isolated directly from CD133(-) human ovarian
tumors were sufficient to generate
tumors in immunocompromised mice, whereas 50,000 ALDH(-) cells were unable to initiate
tumors. Using ALDH in combination with CD133 to analyze
ovarian cancer cell lines, we observed even greater growth in the ALDH(+)CD133(+) cells compared with ALDH(+)CD133(-) cells, suggesting a further enrichment of ovarian CSC in ALDH(+)CD133(+) cells. Strikingly, as few as 11 ALDH(+)CD133(+) cells isolated directly from human
tumors were sufficient to initiate
tumors in mice. Like other CSC, ovarian CSC exhibited increased angiogenic capacity compared with bulk
tumor cells. Finally, the presence of ALDH(+)CD133(+) cells in debulked primary
tumor specimens correlated with reduced disease-free and overall survival in
ovarian cancer patients. Taken together, our findings define ALDH and CD133 as a functionally significant set of markers to identify ovarian CSCs.