MDM2 Targeting sd-rxRNA® for Retinoblastoma Therapy

Abstract

Purpose: Retinoblastoma is a cancer that originates in the retina of young children. It is driven by inactivating RB1 mutations, as well as by the expression of genes involved in the “normal” signaling circuitry of retinal cells, particularly that of cone precursors. Some of these genes have been found to be critical to retinoblastoma cell growth and survival, suggesting that they may be effective therapeutic targets. RXi has developed a new class of stable, self-delivering RNAi compounds (sd‑rxRNA^®) that incorporate features of RNAi and antisense and results in spontaneous cellular uptake. Our goal is to use the sd‑rxRNA platform to develop compounds against retinoblastoma therapeutic targets. We selected MDM2 as a target because it is a crucial component of the cone circuitry underlying retinoblastoma development and is required to sustain expression of the MYCN oncogene in retinoblastoma cells. Initial studies demonstrated in vivo uptake of control sd-rxRNA by human retinoblastoma cells in a mouse model and identified candidate sd‑rxRNAs that down-regulated MDM2 in cultured retinoblastoma cells. Here, we evaluated two MDM2-targeting sd‑rxRNAs for effects on MDM2 mRNA and protein expression, as well as impact on MYCN protein expression and cell proliferation.

Methods: Two MDM2-targeting sd‑rxRNAs identified from screening studies were evaluated in cultured RB176 cells. MDM2 mRNA was evaluated using a branched DNA assay. MDM2 and MYCN protein levels were determined by Western blot. Cell proliferation was evaluated by assaying for ATP production.

Results: MDM2-targeting sd‑rxRNAs reduced MDM2 mRNA levels as well as MDM2 protein levels over time. Importantly, treatment with MDM2 targeting sd‑rxRNAs also decreased MYCN protein expression and cell proliferation, suggesting a potential therapeutic impact on retinoblastoma cell viability. MDM2 mRNA levels were significantly reduced in a dose dependent manner through Day 6. Moreover, MDM2 and MYCN protein levels were reduced through day 7.

Conclusions: MDM2-targeting sd‑rxRNAs were evaluated in vitro. Target-specific mRNA and protein reductions were observed. Treatment of cultured cells with MDM2 targeting sd‑rxRNAs also led to reduction of MYCN protein levels which importantly can lead to retinoblastoma cell death. These findings, along with our previous report of specific and extended silencing of retinal genes by sd‑rxRNA, support the potential use of sd‑rxRNA for retinoblastoma therapy.