Expression of p14ARF, MDM2, and MDM4 in human retinoblastoma

It is still not clear whether the p53 pathway is altered in retinoblastoma development. We assessed the expression of the p53 pathway genes p14ARF, mouse double minute 2 (MDM2), and mouse double minute 4 (MDM4) in human retinoblastoma compared to normal retina. Primary human retinoblastomas, retinoblastoma cell lines and normal retinas were assessed for p14ARF and MDM4 mRNA by quantitative RT-PCR. p14ARF, MDM2, and MDM4 protein were measured by immunoblot and immunohistochemistry. Compared to retina, p14ARF mRNA expression was notably increased in retinoblastoma but p14ARF protein was undetectable. MDM2 and MDM4 proteins were expressed in 22/22 retinoblastomas. MDM2 was expressed in 3/10 retinas tested, and MDM4 in 10/10 retinas. The expression level of MDM2 protein in retinoblastomas and retina was comparable, while MDM4 protein was overexpressed in one retinoblastoma cell line Y79 and two primary retinoblastomas. We observe that overexpression of MDM2 and MDM4 is not a necessary step in retinoblastoma development. However, loss of detectable p14ARF protein and resultant lack of functional inactivation of these p53 inhibitors may contribute to retinoblastoma development by constitutive inhibition of p53.