Identification of stemness in primary retinoblastoma cells by analysis of stem-cell phenotypes and tumorigenicity with culture and xenograft models.

Retinoblastoma (RB) is a primary intraocular malignancy in childhood, and may develop relapse and metastatic disease. This study was to identify the stem-cell properties of primary retinoblastoma cells critical to tumorigenesis and metastasis. Primary cells were isolated from fresh human RB tissues after enucleation, and cultured in serum-free or serum-enriched conditions, with two RB cell-lines Weri-RB1 and Y79 for comparison. Proliferation of primary RB cells were well-maintained in serum-free condition of DMEM/F12 medium, and formed stem-cell like spheroids. The immaturity of cultured primary RB cells was demonstrated by tendency of highly expressed stem-cell markers (CD133, Nestin and OCT4) and suppressed mature retinal-cell markers (GFAP, MAP2 and Recoverin). CD133, a neural stem-cell marker being exclusively studied in RB, was found positive in small patches of cells in archival human RB by immunohistochemistry. Meanwhile, at initial isolation, insignificant CD133 cells were detected by flow-cytometry, and substantial increase of positivity was observed after several days cultivated in serum-free condition. Cultured primary RB cells were engrafted in subretinal region of BALB/c nude mice for assessment of tumorigenicity. Strong tumorigenic activity and extensive progression of the xenograft retinoblastoma was induced by primary cells as compared with the two cell-lines. Again, immunohistochemistry confirmed that the stem-cell markers were emphasized in the xenograft tumor in mice. Our findings demonstrated that in comparison to the well-established RB cell-lines, cultured primary RB cells possess stem-cell like properties with highly expressed stem-cell markers, self-regenerative growth in culture, and strong in vivo oncogenic potentiality.