Identification of sternness 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.