Abstract 3306: Aberrant expression of ribosomal protein p1 and its pseudogene in breast tumors
Authors: Han Yoo J. , Olopade Olufunmilayo I. 
Published: May 2015

Introduction: Ribosome biogenesis and translation control are essential cellular processes in growing cancer cells. Defects in ribosomal components can alter the pattern of gene expression, cell cycle progression and cell growth. Human ribosomes consist of four RNA species and 79 ribosomal proteins with 2090 processed and 16 duplicated pseudogenes. Specifically, 60S Acidic Ribosomal Protein P1 (RPLP1) is expressed in a tissue specific manner with multiple processed pseudogenes. We accidentally identified the expressed pseudogene 4 of RPLP1 (RPLP1P4) and determined genetic and molecular mechanism of RPLP1 and RPLP1P4 expression in breast cancer cells.

Methods: Expression levels of RPLP1 and RPLP1P4 were quantified using qRT-PCR with total RNAs isolated from 7 normal tissues and 20 breast tumor tissues of 5 different subtypes. Cell cycle progression was analyzed cytofluorimetrically with the Beckman Coulter Gallios Flow Cytometer. Gene expression profiles were performed using Affymetrix GeneChip Human Genome U133 Plus 2.0 Array and analyzed using Partek Genomic Suite. Somatic copy number alterations (SCNA) of RPLP1/RPLP1P4 were detected with the custom designed TaqMan assays using ABI 7900HT real time PCR system. CopyCaller™ Software was used for the SCNA analysis with RNase P as a reference.

Results: Because breast cancer subtypes have distinct origins and molecular characteristics, we compared expression of RPLP1 and RPLP1P4 in 5 subtypes of breast tumors. Although RPLP1/RPLP1P4 is ubiquitously expressed in normal and cancer tissues, their expression levels are significantly increased in the basal-like subtype of breast cancer compared to other subtypes with a 2.23 ± 1.06 fold increase in basal-like subtype compared to non-basal subtypes. Interfering RPLP1 expression with siRNA dramatically decreased cell proliferation up to the level of complete inhibition of cell proliferation. FACS data revealed reduced S phase cells in RPLP1 silencing-cells compared to control, suggesting that RPLP1-mediated regulation of cell proliferation is partly mediated through regulation of cell cycle progression. Analysis of gene expression profiles displayed down-regulation of genes involved in cellular metabolism and DNA replication. When we assessed 44 formalin fixed paraffin embedded breast tissues for SCNA of RPLP1/RPLP1P4, we observed RPLP1 amplifications in 44% of tumor DNAs with no amplification observed in normal tissues. The clear difference between tumors and normal tissues demonstrates frequent RPLP1 amplification events during tumorigenesis, which possibly drives an increase in RPLP1 expression and cell proliferation in cancer.

Conclusion: Our data suggest a subtype-specific increased expression of a ribosomal protein and its pseudogene, potentially driven by copy number alterations in breast tumors. Future studies to understand the contributions of ribosomal protein genes and their pseudogenes to human neoplasia are warranted.

Citation Format: Yoo J. Han, Olufunmilayo I. Olopade. Aberrant expression of ribosomal protein p1 and its pseudogene in breast tumors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3306. doi:10.1158/1538-7445.AM2014-3306