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RNA Quality Control in Mirna Expression Analysis

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RNA Quality Control in miRNA Expression Analysis Application Note Genomics Authors Abstract Christiane Becker, Michael W. Pfaffl It is generally known that total RNA quality has a distinct influence on the validity and TU München, ZIEL reliability of quantitative PCR results. In addition, the recently published MIQE Physiology Weihenstephan guidelines focus on the pre-PCR steps and state the importance of RNA quality Weihenstephaner Berg 3 assessment. 85384 Freising, Germany Various studies showed the impairing effect of ongoing RNA degradation on mRNA Martina Reiter, Michael W. Pfaffl expression results. Therefore, the verification of RNA integrity prior to downstream BioEPS GmbH, Lise-Meitner-Str. 30 applications like RT-qPCR and mircroarrays is indispensable. A fast and reliable 85354 Freising, Germany assessment of RNA integrity can be done with the Eukaryote Total RNA Nano Assay of the Agilent 2100 Bioanalyzer System. The importance of RNA quality should also be considered in new applications such as the investigation of miRNA expression profiles. With the Agilent Small RNA Assay, Agilent is offering one of the few possibilities for selectively estimating miRNA before expression analysis. However, by now little is known about factors affecting miRNA analysis. Herein, the important impact of total RNA quality on quantification of mRNA and miRNA should be considered. Introduction After extraction, RNAs are very unsta- ble and sensitive to degradation due to 1A the ubiquitous occurrence of nucleases. It is well known that expression profil- ing using microarrays or qPCR is influ- enced by RNA quality1, 2. Therefore, this hazard has to be considered during all preprocessing steps and should be min- imized with cautious handling. The recently published MIQE Guidelines3 recommend which data are essential for scientific publication. These guide- lines focused on the pre-PCR steps and stated the importance of RNA quality. The term RNA quality is defined by the composition of RNA purity and RNA integrity. Therefore, the RNA purity should be tested by the estimation of 1B the OD260/280 and the OD260/230 using photometrical methods. A fast and reli- able analysis of total RNA integrity can be performed with the lab-on-a-chip technology of the Agilent 2100 Bioanalyzer System using automated capillary electrophoresis. This is the current gold standard for standardized RNA integrity assessment. The measurement of RNA quality should also be integrated in the routine Figure 1 analysis of new applications like the Results from WBC in 11 degradation steps. Gel-like image generated by the 2100 Expert Software for the (a) Eukaryote Total RNA Nano Assay investigation of microRNAs (miRNAs)4. and the (b) Small RNA Assay. miRNAs are non-coding, regulatory RNA molecules with a length of approx- tometrical methods do not allow the tissues. This relationship was also imately 22 bp. There is interest in discrimination between different RNA demonstrated for the absolute miRNA miRNA expression profiles especially in fractions. The Agilent Small RNA Assay concentration in liver and white blood clinical diagnostics because these is one of the few analyzers available cells (WBC)4. could be used as biomarkers in pre- that can measure miRNA concentra- With ongoing total RNA degradation, diagnosis of cancer and other diseases. tion. This chip can selectively quantify the amount of smaller RNA fragments miRNAs belong to the nucleic acids miRNAs in absolute amounts [pg/µL] increases, whereas longer RNA mole- group, so they are quantified with the and as a relative percentage of small cules decrease (Figures 1A and 1B). same analytical methods as long RNAs, RNA [%]. In addition to these numerical These unspecific fragments reach the such as mRNAs. Currently, little is results, the Small RNA Assay also pro- analytical range of the Small RNA known about factors compromising vides an electropherogram and an elec- Assay and thereby lead to an overesti- miRNA analysis4, 5. Therefore, this tronic virtual gel-like image (Figures 1A mation of the absolute miRNA amount. study determined the influence of RNA and 1B) of the measured samples. Consequently, the miRNA should not be quality on quantitative expression Recent results showed that RNA degra- considered as a single fraction, but as a analysis. dation has a significant effect on mRNA part of the entire total RNA. It is recom- RNA quality and miRNA and miRNA fragmentation patterns. A mended therefore, to combine the concentration measurement negative correlation between the Small RNA Assay and the Eukaryote miRNA percentage and total RNA Total RNA Nano Assay for a complete Challenges start with concentration integrity was shown for all investigated evaluation of every total RNA sample. measurement of miRNAs because pho- 2 RNA quality and mRNA expression analysis Previous studies indicated a distinct effect of RNA quality on the perfor- mance of later reverse transcription quantitative PCR (RT-qPCR). Different samples showing a degradation gradi- ent with several steps of intact RNA down to degraded RNA were analyzed using RT-qPCR. With ongoing RNA degradation a highly significant rise in the quantification cycle (Cq) value occurs for all analyzed mRNAs (Figure 2) independent of tissue type2. The expression data then demonstrates an apparent high expression level for intact starting material and in contrast a low expression level for degraded RNA samples. Each amplified gene was tissue specifically influenced by RNA integrity demonstrating an incompre- hensible tissue-matrix effect between RNA integrity and the type of tissue Figure 2 and the analyzed transcript1, 4. Also, the Highly significant, negative correlation between the RIN and the Cq in liver tissue with p<0.001 for all influence of RNA integrity on single-run analyzed mRNAs4. PCR efficiency was determined as minor compared to its influence on Cq1,2. Additionally, the effect of the length of amplified product was tested. In this analysis, samples with different levels of RNA integrity were amplified using primers producing PCR product of different lengths. Best results could be obtained for samples with RIN values higher than 5 and PCR amplicon lengths of lower than 200 bp2. The impairing effect of low RNA integri- ty on the results of gene expression analysis can partly be reduced by a valid normalization method. For mRNA, a reference gene (RG) index calculated as the geometric mean of multiple ref- erence genes selected by Genorm6, Normfinder7 or GenEx software (MultiD, Gothenburg) is generally accepted8. This strategy has been used for mRNA expression results. For nearly all genes and all tissue, a significant reduction of the technical variance due to different Figure 3 Highly significant, negative correlation between the RIN and the Cq in liver tissue with p<0.001 for all RNA integrity levels could be shown analyzed miRNAs4. after normalization1, 4. 3 Therefore, the application of a valid these degradation steps from the calcu- 3. normalization method is indispensable lation. Therefore, the previously deter- "The MIQE guidelines: minimum infor- in the processing of RT-qPCR expres- mined threshold level for gaining reli- mation for publication of quantitative 2 sion data. able PCR results (RIN=5) was con- real-time PCR experiments," Clin.Chem. RNA quality and miRNA firmed for mRNA and stated for 4, 611-622, 2009 miRNA4. Samples with a lower RIN are expression analysis not recommended for expression analy- 4. This test also illustrated a distinct influ- sis because the results may not be reli- "mRNA and microRNA quality control ence of RNA degradation on miRNA able. for RT-qPCR analysis," Methods, 2010 expression analysis. Eight different miRNAs were quantified using RT-qPCR. Conclusion 5. For all tissues and in all eight genes, a "RNA degradation compromises the highly significant, negative correlation In conclusion, it is shown that sustain- reliability of microRNA expression pro- (p<0.001) between the RIN and Cq ing high quality RNA is critical for valid filing," BMC.Biotechnol. 1, 102, 2009 occured. The results gained for liver and reliable quantification results in 6. tissue are shown in Figure 3. With miRNA and mRNA expression analysis. "Accurate normalization of real-time ongoing RNA degradation the Cq The specific concentration measure- quantitative RT-PCR data by geometric increased, showing an impairing influ- ment of miRNAs is challenging and reli- averaging of multiple internal control ence of the RNA integrity on the perfor- able for nondegraded samples with high genes," Genome Biol. 7, RESEARCH0034, mance of the qPCR. As represented by overall RNA quality. It is recommended 2002 the lower slope of the regression line, to consider miRNA in combination with the total RNA, as opposed to a single the expression analysis of miRNAs is 7. fraction. influenced by RNA integrity to a lesser "Normalization of real-time quantitative degree than mRNA expression (1.521 The performance of RT-qPCR is reverse transcription-PCR data: a vs. 0.709, respectively). Due to their impaired by decreasing RNA quality for model-based variance estimation small length, miRNAs4 seem to be less miRNA similar to mRNA, but to a lower approach to identify genes suited for susceptible to degradation because magnitude. The use of biological sam- normalization, applied to bladder and they show less binding sites for nucle- ples showing a RIN higher than 5, and colon cancer data sets," Cancer Res. ases. Consequently, it can be assumed primers producing short PCR product 15, 5245-5250, 2004 that the amplification of miRNA in qPCR are recommended. The application of an might be less affected by a low RNA 8. appropriate normalization method can integrity compared to the longer "A novel and universal method for partly reduce the degradation problem mRNAs. Currently, there are no precise microRNA RT-qPCR data normalization" in RT-qPCR. recommendations for the normalization Genome Biol. 6, R64, 2009 of miRNA expression data. Two strate- References gies of normalization have been applied for testing.
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