Identification of valid endogenous control genes for determining gene expression in human glioma

In human glioma, quantitative real-time reverse-transcription PCR (qPCR) is a frequently used research tool. However, no systematic analysis of suitable reference genes for reliable gene expression analysis has been performed so far. In the current study, we tested 19 commonly used reference genes for their expression stability in human astrocytoma WHO Grade II, astrocytoma WHO Grade III, and glioblastoma (WHO Grade IV) both alone and compared with normal brain. First, equivalence tests for equal expression of candidate genes were applied, and those genes showing differential expression were ruled out from further analyses. Second, expression stability of the remaining candidate genes was determined by the NormFinder software. Generally, glioblastoma exhibited the highest expression levels and largest variability of candidate genes, whereas the opposite was true for normal brain. Even though Normfinder analyses revealed a large number of genes suitable for normalization in each of the tumor subgroups and across these groups, this number was drastically reduced after inclusion of normal brain into the analyses: Only GAPDH, IPO8, RPL13A, SDHA, and TBP were expected not to be differentially expressed; NormFinder analysis indicated favorable stability values for all of these genes, with TBP and IPO8 being the most stable ones. These 5 genes represent different physiological pathways and may be regarded as universal reference genes applicable for accurate normalization of gene expression in human astrocytomas of different grades (WHO Grades II-IV) alone and compared with normal brain, thereby enabling longitudinally designed studies (eg, in astrocytoma before and after malignant transformation).