RB1 Haploinsufficiency Confers a Proliferation Advantage in Myeloma Cell Lines.

The retinoblastoma 1 (RB1) gene is a frequent target of mutation or inactivation in neoplastic diseases. However, the current model for RB1-mediated malignancy requires inactivation of both alleles for tumor progression. Single–copy loss of RB1 has been shown to correlate with an approximate 50% reduction in RB1 mRNA expression in patients diagnosed with multiple myeloma and survival in this cohort of patients is significantly reduced. Using siRNA to reduce Rb protein levels to 50% and Rb-expressing adenovirus to restore RB1, variations in proliferation and cell cycle were observed. The KMS–11 adherent cell line with normal Rb protein expression was labeled with carboxyfluorescein succinimidyl ester (CFSE) for tracking of cellular division and RB1 siRNA added to silence 50% of protein expression. Transfection efficiency for siRNA was measured at 95% or greater. Flow cytometric analysis was performed at 72 hours to identify changes in proliferation between controls and Rb knock–down. Using CFSE proliferation analysis software, reduction of Rb protein levels by 50% caused an average increase in the proliferation index from 2.05 to 2.50. Additionally, reduction of Rb protein levels caused an increase in the percentage of cells in S–phase, from 24 to 29%. CFSE and cell cycle experiments were performed in triplicate. Conversely, addition of Rb–expressing adenovirus to MM.1R and U266 cell lines, with mono-allelic and bi-allelic loss of RB1 respectively, resulted in a decrease in proliferation assayed by CFSE and an average decrease in the percentage of cells in S-phase from 29 to 23% compared to controls. No changes in proliferation or cell cycle compared to untreated controls were observed following null adenovirus infection. Western blot and quantitative RT–PCR (qPCR) were used to confirm reduction or addition of RB1 to cell lines. Further, qPCR was used to identify potential activation of the interferon response following addition of siRNA and adenovirus. No changes in annexin V expression were observed following reduction or replacement of Rb as compared to controls. These results suggest the addition of proliferation advantages to tumors with single copy loss of RB1. For malignancies such as multiple myeloma characterized by deletion of 13q14 coupled with an extended period of development, the proliferation advantage associated with RB1 haploinsufficiency may contribute to decreased survival in the deletion 13 cohort.