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Open Access Research

Identification of stable endogenous control genes for transcriptional profiling of photon, proton and carbon-ion irradiated cells

Geeta D Sharungbam12, Christian Schwager12, Sara Chiblak12, Stephan Brons2, Lynn Hlatky3, Thomas Haberer2, Jürgen Debus2 and Amir Abdollahi123*

Author Affiliations

1 Molecular RadioOncology [E210], National Center for Tumor Disease (NCT), German Cancer Research Center (DKFZ), Im Neuenheimer Feld 460, 69120, Heidelberg, Germany

2 Heidelberg Ion Therapy Center (HIT), Heidelberg Institute of Radiation Oncology (HIRO), Department of Radiation Oncology, University of Heidelberg Medical School, Im Neuenheimer Feld 450, 69120, Heidelberg, Germany

3 Center of Cancer Systems Biology, NASA Specialized Center of Research, St. Elizabeth’s Medical Center, Tufts University, 736 Cambridge Street [CBR 1], 02135, Boston, MA, USA

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Radiation Oncology 2012, 7:70  doi:10.1186/1748-717X-7-70

Published: 17 May 2012

Abstract

Background

Quantitative analysis of transcriptional regulation of genes is a prerequisite for a better understanding of the molecular mechanisms of action of different radiation qualities such as photon, proton or carbon ion irradiation. Microarrays and real-time quantitative RT-PCR (qRT-PCR) are considered the two cornerstones of gene expression analysis. In interpreting these results it is critical to normalize the expression levels of the target genes by that of appropriately selected endogenous control genes (ECGs) or housekeeping genes. We sought to systematically investigate common ECG candidates for their stability after different radiation modalities in different human cell lines by qRT-PCR. We aimed to identify the most robust set of ECGs or housekeeping genes for transcriptional analysis in irradiation studies.

Methods

We tested the expression stability of 32 ECGs in three human cancer cell lines. The epidermoid carcinoma cells (A431), the non small cell lung carcinoma cells (A549) and the pancreatic adenocarincoma cells (BxPC3) were irradiated with photon, proton and carbon ions. Expression Heat maps, clustering and statistic algorithms were employed using SUMO software package. The expression stability was evaluated by computing: mean, standard deviation, ANOVA, coefficient of variation and the stability measure (M) given by the geNorm algorithm.

Results

Expression analysis revealed significant cell type specific regulation of 18 out of 32 ECGs (p < 0.05). A549 and A431 cells shared a similar pattern of ECG expression as the function of different radiation qualities as compared to BxPC3. Of note, the ribosomal protein 18S, one of the most frequently used ECG, was differentially regulated as the function of different radiation qualities (p ≤ 0.01). A comprehensive search for the most stable ECGs using the geNorm algorithm identified 3 ECGs for A431 and BxPC3 to be sufficient for normalization. In contrast, 6 ECGs were required to properly normalize expression data in the more variable A549 cells. Considering both variables tested, i.e. cell type and radiation qualities, 5 genes-- RPLP0, UBC, PPIA, TBP and PSMC4-- were identified as the consensus set of stable ECGs.

Conclusions

Caution is warranted when selecting the internal control gene for the qRT-PCR gene expression studies. Here, we provide a template of stable ECGs for investigation of radiation induced gene expression.

Keywords:
Endogenous control genes; Internal control genes; qRT-PCR; Photon; Proton; Carbon-ion; Tumour cells; A431; A549 and BxPC3