PRB and PML Expression in Human Retina and Retinoblastoma

Abstract

Abstract: : Purpose: Mutation of the retinoblastoma gene initiates retinoblastoma tumorigenesis in as yet undefined retinal cells. pRB, the retinoblastoma gene product, is known to regulate progression from G1 to S phase of the cell cycle. Although some of the molecular mechanisms underlying the role of pRB in cell cycle control have been defined, there may be other ways by which pRB suppresses retinoblastoma tumorigenesis. It was recently shown that pRB can increase the expression of the promyelocytic leukemia protein (PML) and the formation of subnuclear structures called PML nuclear bodies (NBs) in osteosarcoma cells. This may be relevant to pRB-mediated tumor suppression because PML is a potential tumor suppressor protein that is involved in cell growth arrest and apoptosis, and because the effects of PML may be mediated via PML NB formation. In this study, we investigated the relationship between pRB and PML expression in retinoblastoma tumor tissue as well as in human embryonic retinas. Methods: We examined five retinoblastoma tumors and two human fetal retinas for expression of pRb, PML, and the NB-associated protein SP100. Samples were fresh frozen and cryostat sectioned. The above proteins were visualized by indirect immunofluorescence and analyzed by both light and confocal microscopy. Results: In retinoblastoma tumors, PML was detected as bright nuclear foci in blood vessels and in non-transformed pRB positive cells that commonly infiltrate such tumors. However, PML was only weakly present in the pRB negative cells that comprise the majority of the tumor mass. The PML foci in the blood vessels and in the infiltrating pRB positive cells co-localized with SP100. In contrast, the weak PML foci in the pRB tumor cells did not have detectable SP100, suggesting that retinoblastoma cells lack PML NBs. In the human 22 week fetal retina, pRB was detected in all retinal cells, whereas PML nuclear foci were prominent only in cells adjacent to the apical surface of the retinal pigmented epithelium, and in a subset of cells in the ganglion cell layer. Conclusions: We have shown that PML NBs are prominent in a subset of fetal retinal cells, but are deficient in retinoblastoma tumor cells. Our data are consistent with the idea that pRB may promote PML NB formation in a subset of retinal cells as part of a tumor suppression program.

Keywords: retinoblastoma • retinal development • immunohistochemistry