Retinoblastoma: A Pediatric Ocular Tumor of Possible Stem Cell Origin

Abstract

Purpose: : Retinoblastoma tumors exhibit heterogeneous morphology and express proteins from different retinal phenotypes. We examined the growth properties of tumors in a transgenic murine model of retinoblastoma and compared our results to human retinoblastoma.

Methods: : Primary human and transgenic (pax6-SV40/T antigen) mouse retinoblastoma tumors were studied in vitro and in vivo for their growth characteristics, morphology and immunophenotype using neural stem cell (NSTC) (CD133, Nestin, Sox2), neuronal differentiation (synaptophysin, NSE), and glial differentiation (GFAP) markers and for T-antigen (murine) or Rb (human) expression.

Results: : Murine retinal tumors develop by embryonic day 12.5 and the majority of these cells express T-antigen. Murine and human tumor cultures produce neurospheres when grown in serum-free "stem cell" media and these cells express both NSTC and differentiation markers in a pattern similar to the primary tumors. Cells grown in serum-containing medium predominately express NSTC markers as do metastases observed in the murine model. Tumor cells grown in vitro can be transplanted into mice where they form tumors that are similar to the original primary tumors. Approximately 10% of cloned cells from neurospheres are capable of proliferating in vitro. The percentage of tumor cells that express T-antigen decreases with the age of the mouse. NSTC markers labeled a small cell population in mouse and human primary tumors. Cells that express T-antigen also express NSTC markers. Many of the cells that did not express T-antigen express Nestin and Sox-2, which are also expressed in the mature human and murine retina. The majority of tumor cells express differentiation markers. Human tumors caused by mutations that do not allow expression of Rb protein do still contain a small population of cells that coexpress Rb and Sox2.

Conclusions: : Retinoblastoma tumors have characteristics consistent with the hypothesis that they develop from a progenitor cell that gives rise to cells with predominant neuronal differentiation. There also appears to be a population of normal cells that form part of the primary ocular but not metastatic tumors.

Keywords: retinoblastoma • differentiation • oncology