p53 and p21waf-1 expression correlates with apoptosis or cell survival in poorly differentiated, but not well-differentiated, retinoblastomas.

In human retinoblastomas, rare genetic mutations of the retinoblastoma gene cause massive cell proliferation, altered differentiation, and tumor formation; but paradoxically, this is accompanied by extensive apoptotic cell loss. We quantified the immunohistochemical distribution of p53, its downstream effector p21 (WAF-1), and apoptotic cells in 50 human retinoblastomas, within three concentric zones of sleeves of tumor cells surrounding blood vessels. In poorly differentiated retinoblastomas, both p53 expression and apoptosis increase toward the outer zone of tumor sleeves, whereas p21 expression occurs primarily within the inner zone. This staining pattern of p53 expression is reversed in well-differentiated tumors, whereas p21 staining and apoptotic cell distributions are unchanged. We detected no p53 mutations in four retinoblastomas and two retinoblastoma cell lines. We postulate that oxygen and cell "survival/growth factors" delivered via blood vessels protect retinoblastoma cells from apoptosis. In poorly differentiated tumors, apoptosis is spatially associated with increased p53 expression and may be p53 mediated, but in well-differentiated tumors, apoptosis does not colocalize with p53 and may be p53 independent. In retinoblastomas, p21 is involved not in cell death by apoptosis but in cell survival. Thus, p53 varies its expression (and by implication its function) with altered differentiation in retinoblastomas.