microRNA  (miRNA) normalisation  (7)
microRNA  (miRNA)   &   quantitative real-time RT-PCR (1)
microRNA  (miRNA)   &   quantitative real-time RT-PCR (2)
microRNA  (miRNA)   &   quantitative real-time RT-PCR (3)
microRNA  (miRNA)   &   quantitative real-time RT-PCR (4)
microRNA  (miRNA)   &   quantitative real-time RT-PCR (5)
microRNA  reviews  (6)
mirtrons  (8)

microRNA normalisation in array experiments:

microRNA quality and effects on quantification  ... NEW !


microRNA normalisation in real-time qRT-PCR

Data normalisation in microRNA experiments using qRT-PCR is a new challenge in gene quantification analysis. The reliability of any relative RT-PCR experiment can be improved by including an invariant endogenous control (reference gene) in the assay to correct for sample to sample variations in the qRT-PCR efficiency and errors in sample quantification. A biologically meaningful reporting of target mRNA copy numbers requires accurate and relevant normalisation to some standard and is strongly recommended in microRNA qRT-PCR.

=> But the quality of normalized quantitative expression data cannot be better than the quality of the normalizer itself.
=> Which are the best endogen microRNA normalizers ?
=> Can we apply a comparable normalising strategy as done for mRNAs ?

Any variation in the normalizer will obscure real changes and produce artifactual changes. Real-time RT-PCR-specific errors in the quantification of microRNA transcripts are easily compounded with any variation in the amount of starting material between the samples, e.g. caused by sample-to-sample variation and cDNA sample loading variation. This is especially relevant when the samples have been obtained from different individuals, different tissues and different time courses, and will result in the misinterpretation of the derived expression profile of the target genes.

=> Therefore, normalisation of target gene expression levels must be performed to compensate intra- and inter-kinetic RT-PCR variations (sample-to-sample  &  run-to-run variations).

Data normalisation can be carried out against one or more endogenous unregulated reference gene transcript or against total cellular DNA or RNA content (molecules/g total DNA/RNA and concentrations/g total DNA/RNA). Normalisation according the total cellular RNA content is increasingly used, but little is known about the total RNA content of cells or even about the microRNA or mRNA concentrations. The content per cell or per gram tissue may vary in different tissues in vivo, in cell culture (in vitro), between individuals and under different experimental conditions. Nevertheless, it has been shown that normalisation to total cellular RNA is the least unreliable method. It requires an accurate quantification of the isolated total RNA or mRNA or microRNA fraction by optical density at 260 nm, Lab-on-Chip capillary electrophoresis instruments, or Ribogreen RNA Quantification Kit.

To normalize the absolute amount according to a single reference gene (or better a set of multiple stable reference genes), further sets of kinetic PCR reactions has to be performed for the invariant endogenous control(s) on all experimental samples and the relative abundance values are calculated for internal control as well as for the target gene. For each target gene sample, the relative abundance value obtained is divided by the value derived from the control sequence in the corresponding target gene. The normalized values for different biological samples can then directly be compared.

The workflow:

  • check for good total RNA integrity
  • select one or more stable internal reference microRNA or suitable smallRNAs (via Genorm or Normfinder)
  • calculate reference-gene-index of selected normalizers (geometric mean of Cq)
  • apply relative quantification strategy (comparable to mRNA relative quantification)
  • apply PCR efficiency correction (if wanted)
  • for microRNA normalistion strategies see papers below
  • or find some more ideas in the Relative Quantification Section

TALK  -  Better appreciation of true biological miRNA expression differences using an improved version of the global mean normalization strategy
by Jo Vandesompele, RNAi and miRNA world congress, Boston 2011



Identification by Real-time PCR of 13 mature microRNAs differentially expressed in colorectal cancer and non-tumoral tissues.
Molecular Cancer 2006, 5:29
E Bandrés*1, E Cubedo1, X Agirre2, R Malumbres1, R Zárate1, N Ramirez1,
A Abajo1, A Navarro3, I Moreno4, M Monzó3 and J García-Foncillas1

MicroRNAs (miRNAs) are short non-coding RNA molecules playing regulatory roles by repressing translation or cleaving RNA transcripts. Although the number of verified human miRNA is still expanding, only few have been functionally described. However, emerging evidences suggest the potential involvement of altered regulation of miRNA in pathogenesis of cancers and these genes are thought to function as both tumours suppressor and oncogenes. In our study, we examined by Real-Time PCR the expression of 156 mature miRNA in colorectal cancer. The analysis by several bioinformatics algorithms of colorectal tumours and adjacent nonneoplastic tissues from patients and colorectal cancer cell lines allowed identifying a group of 13 miRNA whose expression is significantly altered in this tumor. The most significantly deregulated miRNA being miR-31, miR-96, miR-133b, miR-135b, miR-145, and miR-183. In addition, the expression level of miR-31 was correlated with the stage of CRC tumor. Our results suggest that miRNA expression profile could have relevance to the biological and clinical behavior of colorectal neoplasia.

Expression profiling of microRNA using real-time quantitative PCR, how to use it and what is available.
Benes V, Castoldi M.
European Molecular Biology Laboratory, Heidelberg D 69117, Germany.
Methods. 2010 Apr;50(4): 244-249
We review different methodologies to estimate the expression levels of microRNAs (miRNAs) using real-time quantitative PCR (qPCR). As miRNA analysis is a fast changing research field, we have introduced novel technological approaches and compared them to existing qPCR profiling methodologies. qPCR also remains the method of choice for validating results obtained from whole-genome screening (e.g. with microarray). In contrast to presenting a stepwise description of different platforms, we discuss expression profiling of mature miRNAs by qPCR in four sequential sections: (1) cDNA synthesis; (2) primer design; (3) detection of amplified products; and (4) data normalization. We address technical challenges associated with each of these and outline possible solutions.

A novel and universal method for microRNA RT-qPCR data normalization.
Mestdagh P, Van Vlierberghe P, De Weer A, Muth D, Westermann F, Speleman F, Vandesompele J.
Center for Medical Genetics, Ghent University Hospital, De Pintelaan 185, Ghent, Belgium. pieter.mestdagh@ugent.be
Genome Biol. 2009;10(6): R64
Gene expression analysis of microRNA molecules is becoming increasingly important. In this study we assess the use of the mean expression value of all expressed microRNAs in a given sample as a normalization factor for microRNA real-time quantitative PCR data and compare its performance to the currently adopted approach. We demonstrate that the mean expression value outperforms the current normalization strategy in terms of better reduction of technical variation and more accurate appreciation of biological changes.

Systematic comparison of microarray profiling, real-time PCR, and next-generation sequencing technologies
for measuring differential microRNA expression.

Git A, Dvinge H, Salmon-Divon M, Osborne M, Kutter C, Hadfield J, Bertone P, Caldas C.
Cancer Research UK, Cambridge Research Institute, Li Ka Shing Centre, Cambridge CB2 0RE, United Kingdom
RNA. 2010 May;16(5): 991-1006
RNA abundance and DNA copy number are routinely measured in high-throughput using microarray and next-generation sequencing (NGS) technologies, and the attributes of different platforms have been extensively analyzed. Recently, the application of both microarrays and NGS has expanded to include microRNAs (miRNAs), but the relative performance of these methods has not been rigorously characterized. We analyzed three biological samples across six miRNA microarray platforms and compared their hybridization performance. We examined the utility of these platforms, as well as NGS, for the detection of differentially expressed miRNAs. We then validated the results for 89 miRNAs by real-time RT-PCR and challenged the use of this assay as a "gold standard." Finally, we implemented a novel method to evaluate false-positive and false-negative rates for all methods in the absence of a reference method.

A modified LOESS normalization applied to microRNA arrays: a comparative evaluation.
Risso D, Massa MS, Chiogna M, Romualdi C.
Department of Statistical Sciences, University of Padova, via C. Battisti 241 and Department of Biology, University of Padova, via U. Bassi 58/B, 35121 Padova, Italy.
Bioinformatics. 2009 25(20): 2685-2691
MOTIVATION: Microarray normalization is a fundamental step in removing systematic bias and noise variability caused by technical and experimental artefacts. Several approaches, suitable for large-scale genome arrays, have been proposed and shown to be effective in the reduction of systematic errors. Most of these methodologies are based on specific assumptions that are reasonable for whole-genome arrays, but possibly unsuitable for small microRNA (miRNA) platforms. In this work, we propose a novel normalization (loessM), and we investigate, through simulated and real datasets, the influence that normalizations for two-colour miRNA arrays have on the identification of differentially expressed genes. RESULTS: We show that normalizations usually applied to large-scale arrays, in several cases, modify the actual structure of miRNA data, leading to large portions of false positives and false negatives. Nevertheless, loessM is able to outperform other techniques in most experimental scenarios. Moreover, when usual assumptions on differential expression distribution are missed, channel effect has a strikingly negative influence on small arrays, bias that cannot be removed by normalizations but rather by an appropriate experimental design. We find that the combination of loessM with eCADS, an experimental design based on biological replicates dye-swap recently proposed for channel-effect reduction, gives better results in most of the experimental conditions in terms of specificity/sensitivity both on simulated and real data.
AVAILABILITY: LoessM R function is freely available at  http://gefu.cribi.unipd.it/papers/miRNA-simulation/

Improved microRNA quantification in total RNA from clinical samples.
Andreasen D, Fog JU, Biggs W, Salomon J, Dahslveen IK, Baker A, Mouritzen P.
Exiqon A/S, Skelstedet 16, DK-2950 Vedbaek, Denmark. ina@exiqon.com
Methods. 2010 Apr;50(4): S6-9.
microRNAs are small regulatory RNAs that are currently emerging as new biomarkers for cancer and other diseases. In order for biomarkers to be useful in clinical settings, they should be accurately and reliably detected in clinical samples such as formalin fixed paraffin embedded (FFPE) sections and blood serum or plasma. These types of samples represent a challenge in terms of microRNA quantification. A newly developed method for microRNA qPCR using Locked Nucleic Acid (LNA)-enhanced primers enables accurate and reproducible quantification of microRNAs in scarce clinical samples. Here we show that LNA-based microRNA qPCR enables biomarker screening using very low amounts of total RNA from FFPE samples and the results are compared to microarray analysis data. We also present evidence that the addition of a small carrier RNA prior to total RNA extraction, improves microRNA quantification in blood plasma and laser capture microdissected (LCM) sections of FFPE samples.

Measuring microRNAs: comparisons of microarray and quantitative PCR measurements,
and of different total RNA prep methods.

Ach RA, Wang H, Curry B.
Agilent Laboratories, Agilent Technologies, Santa Clara, CA 95051, USA. robert_ach@agilent.com
BMC Biotechnol. 2008 8: 69.
BACKGROUND: Determining the expression levels of microRNAs (miRNAs) is of great interest to researchers in many areas of biology, given the significant roles these molecules play in cellular regulation. Two common methods for measuring miRNAs in a total RNA sample are microarrays and quantitative RT-PCR (qPCR). To understand the results of studies that use these two different techniques to measure miRNAs, it is important to understand how well the results of these two analysis methods correlate. Since both methods use total RNA as a starting material, it is also critical to understand how measurement of miRNAs might be affected by the particular method of total RNA preparation used. RESULTS: We measured the expression of 470 human miRNAs in nine human tissues using Agilent microarrays, and compared these results to qPCR profiles of 61 miRNAs in the same tissues. Most expressed miRNAs (53/60) correlated well (R > 0.9) between the two methods. Using spiked-in synthetic miRNAs, we further examined the two miRNAs with the lowest correlations, and found the differences cannot be attributed to differential sensitivity of the two methods. We also tested three widely-used total RNA sample prep methods using miRNA microarrays. We found that while almost all miRNA levels correspond between the three methods, there were a few miRNAs whose levels consistently differed between the different prep techniques when measured by microarray analysis. These differences were corroborated by qPCR measurements. CONCLUSION: The correlations between Agilent miRNA microarray results and qPCR results are generally excellent, as are the correlations between different total RNA prep methods. However, there are a few miRNAs whose levels do not correlate between the microarray and qPCR measurements, or between different sample prep methods. Researchers should therefore take care when comparing results obtained using different analysis or sample preparation methods.

Normalization of microRNA expression levels in quantitative RT-PCR assays:
identification of suitable reference RNA targets in normal and cancerous human solid tissues.
Peltier HJ, Latham GJ.
Asuragen, Inc., Austin, Texas 78744, USA.
RNA. 2008 14(5): 844-852
Proper normalization is a critical but often an underappreciated aspect of quantitative gene expression analysis. This study describes the identification and characterization of appropriate reference RNA targets for the normalization of microRNA (miRNA) quantitative RT-PCR data. miRNA microarray data from dozens of normal and disease human tissues revealed ubiquitous and stably expressed normalization candidates for evaluation by qRT-PCR. miR-191 and miR-103, among others, were found to be highly consistent in their expression across 13 normal tissues and five pair of distinct tumor/normal adjacent tissues. These miRNAs were statistically superior to the most commonly used reference RNAs used in miRNA qRT-PCR experiments, such as 5S rRNA, U6 snRNA, or total RNA. The most stable normalizers were also highly conserved across flash-frozen and formalin-fixed paraffin-embedded lung cancer tumor/NAT sample sets, resulting in the confirmation of one well-documented oncomir (let-7a), as well as the identification of novel oncomirs. These findings constitute the first report describing the rigorous normalization of miRNA qRT-PCR data and have important implications for proper experimental design and accurate data interpretation.

Identification of suitable endogenous control genes for microRNA gene expression
analysis in human breast cancer.
Davoren PA, McNeill RE, Lowery AJ, Kerin MJ, Miller N.
Department of Surgery, National University of Ireland, Galway, Ireland.
BMC Mol Biol. 2008 9: 76.

The discovery of microRNAs (miRNAs) added an extra level of intricacy to the already complex system regulating gene expression. These single-stranded RNA molecules, 18-25 nucleotides in length, negatively regulate gene expression through translational inhibition or mRNA cleavage. The discovery that aberrant expression of specific miRNAs contributes to human disease has fueled much interest in profiling the expression of these molecules. Real-time quantitative PCR (RQ-PCR) is a sensitive and reproducible gene expression quantitation technique which is now being used to profile miRNA expression in cells and tissues. To correct for systematic variables such as amount of starting template, RNA quality and enzymatic efficiencies, RQ-PCR data is commonly normalised to an endogenous control (EC) gene, which ideally, is stably-expressed across the test sample set. A universal endogenous control suitable for every tissue type, treatment and disease stage has not been identified and is unlikely to exist, so, to avoid introducing further error in the quantification of expression data it is necessary that candidate ECs be validated in the samples of interest. While ECs have been validated for quantification of mRNA expression in various experimental settings, to date there is no report of the validation of miRNA ECs for expression profiling in breast tissue. In this study, the expression of five miRNA genes (let-7a, miR-10b, miR-16, miR-21 and miR-26b) and three small nucleolar RNA genes (RNU19, RNU48 and Z30) was examined across malignant, benign and normal breast tissues to determine the most appropriate normalisation strategy. This is the first study to identify reliable ECs for analysis of miRNA by RQ-PCR in human breast tissue.

High-throughput stem-loop RT-qPCR miRNA expression profiling using minute amounts of input RNA.
Mestdagh P, Feys T, Bernard N, Guenther S, Chen C, Speleman F, Vandesompele J.
Center for Medical Genetics, Ghent University Hospital, Ghent, Belgium.
Nucleic Acids Res. 2008 36(21): e143

MicroRNAs (miRNAs) are an emerging class of small non-coding RNAs implicated in a wide variety of cellular processes. Research in this field is accelerating, and the growing number of miRNAs emphasizes the need for high-throughput and sensitive detection methods. Here we present the successful evaluation of the Megaplex reverse transcription format of the stem-loop primer-based real-time quantitative polymerase chain reaction (RT-qPCR) approach to quantify miRNA expression. The Megaplex reaction provides simultaneous reverse transcription of 450 mature miRNAs, ensuring high-throughput detection. Further, the introduction of a complementary DNA pre-amplification step significantly reduces the amount of input RNA needed, even down to single-cell level. To evaluate possible pre-amplification bias, we compared the expression of 384 miRNAs in three different cancer cell lines with Megaplex RT, with or without an additional pre-amplification step. The normalized Cq values of all three sample pairs showed a good correlation with maintenance of differential miRNA expression between the cell lines. Moreover, pre-amplification using 10 ng of input RNA enabled the detection of miRNAs that were undetectable when using Megaplex alone with 400 ng of input RNA. The high specificity of RT-qPCR together with a superior sensitivity makes this approach the method of choice for high-throughput miRNA expression profiling.
Facile means for quantifying microRNA expression by real-time PCR.
Shi R, Chiang VL.
North Carolina State University, Raleigh, NC 27695-7247, USA
Biotechniques. 2005 39(4): 519-525


MicroRNAs (miRNAs) are 20-24 nucleotide RNAs that are predicted to play regulatory roles in animals and plants. Here we report a simple and sensitive real-time PCR method for quantifying the expression of plant miRNAs. Total RNA, including miRNAs, was polyadenylated and reverse-transcribed with a poly(T) adapter into cDNAs for real-time PCR using the miRNA-specific forward primer and the sequence complementary to the poly(T) adapter as the reverse primer. Several Arabidopsis miRNA sequences were tested using SYBR Green reagent, demonstrating that this method, using as little as 100 pg total RNA, could readily discriminate the expression of miRNAs having asfew as one nucleotide sequence difference. This method also revealed miRNA tissue-specific expression patterns that cannot be resolved by Northern blot analysis and may therefore be widely useful for characterizing miRNA expression in plants as well as in animals.


A single-molecule method for the quantitation of microRNA gene expression.
Neely LA, Patel S, Garver J, Gallo M, Hackett M, McLaughlin S, Nadel M, Harris J, Gullans S, Rooke J.
US Genomics, 12 Gill Street, Suite 4700, Woburn, Massachusetts 01801, USA
Nat Methods. 2006 (1): 41-46


MicroRNAs (miRNA) are short endogenous noncoding RNA molecules that regulate fundamental cellular processes such as cell differentiation, cell proliferation and apoptosis through modulation of gene expression. Critical to understanding the role of miRNAs in this regulation is a method to rapidly and accurately quantitate miRNA gene expression. Existing methods lack sensitivity, specificity and typically require upfront enrichment, ligation and/or amplification steps. The Direct miRNA assay hybridizes two spectrally distinguishable fluorescent locked nucleic acid (LNA)-DNA oligonucleotide probes to the miRNA of interest, and then tagged molecules are directly counted on a single-molecule detection instrument. In this study, we show the assay is sensitive to femtomolar concentrations of miRNA (500 fM), has a three-log linear dynamic range and is capable of distinguishing among miRNA family members. Using this technology, we quantified expression of 45 human miRNAs within 16 different tissues, yielding a quantitative differential expression profile that correlates and expands upon published results.

Endogenous Controls for Real-Time Quantitation of miRNA Using TaqMan® MicroRNA Assays.
Applied Biosystems - Application Note
MicroRNAs (miRNAs) are small noncoding RNAs whose function has been implicated in a wide range of fundamental cellular processes including cell proliferation, cell differentiation, and cell death. Quantitation of miRNA gene expression levels has become an essential step in understanding these mechanisms, and has shown great promise in identifying effective biomarkers correlative with human disease1,2. Applied Biosystems has developed an extensive set of TaqMan® MicroRNA Assays, novel stem-loop RT and real-time PCR assays, for the quantitation of mature miRNA expression3. TaqMan® Assays are the ideal choice for these applications because of their unsurpassed sensitivity, specificity, and wide dynamic range. Additionally, far less input material is required compared to microarrays and other alternative technologies. When performing these experiments,variation in the amount of starting material, sample collection, RNA preparation and quality, and reverse transcription (RT) efficiency can contribute to quantification errors. Normalization to endogenous control genes is currently the most accurate method to correct for potential RNA input or RT efficiency biases. Careful selection of an appropriate control or set of controls is extremely important as significant variation has been observed between samples, even for the most commonly used housekeeping genes, including ACTB (ß-Actin) and GAPDH4. An ideal endogenous control generally demonstrates gene expression that is relatively constant and highly abundant across tissues and cell types. However, one must still validate the chosen endogenous control or set of controls for the target cell, tissue, or treatment5, as no single control can serveas a universal endogenous control for all experimental conditions. When considering endogenous controls suitable for use with TaqMan MicroRNA Assays, it is important that they share similar properties, such as RNA stability and size, and are amenable to the miRNA assay design. A number of reports indicate that other classes of small non-coding RNAs (ncRNAs) are expressed both abundantly and stably, making them good endogenous control candidates. We have performed a systematic study of a set of human ncRNA species ranging in size from 45 to 200 nucleotides, including transfer RNA (tRNA), small nuclear RNA (snRNA) and small nucleolar RNA (snoRNA) 6 across a relatively wide variety of tissues and cell lines to determine their suitability as endogenous controls when quantitating miRNA expression levels using real-time PCR methods.

microRNA normalisation of microRNA arrays

Quality assessment and data analysis for microRNA expression arrays.
Sarkar D, Parkin R, Wyman S, Bendoraite A, Sather C, Delrow J, Godwin AK, Drescher C, Huber W, Gentleman R, Tewari M.
Fred Hutchinson Cancer Research Center, 1100 Fairview Ave N, Seattle, WA 98109, USA.
Nucleic Acids Res. 2009 Feb;37(2):e17

MicroRNAs are small (approximately 22 nt) RNAs that regulate gene expression and play important roles in both normal and disease physiology. The use of microarrays for global characterization of microRNA expression is becoming increasingly popular and has the potential to be a widely used and valuable research tool. However, microarray profiling of microRNA expression raises a number of data analytic challenges that must be addressed in order to obtain reliable results. We introduce here a universal reference microRNA reagent set as well as a series of nonhuman spiked-in synthetic microRNA controls, and demonstrate their use for quality control and between-array normalization of microRNA expression data. We also introduce diagnostic plots designed to assess and compare various normalization methods. We anticipate that the reagents and analytic approach presented here will be useful for improving the reliability of microRNA microarray experiments.

A comparison of normalization techniques for microRNA microarray data.
Rao Y, Lee Y, Jarjoura D, Ruppert AS, Liu CG, Hsu JC, Hagan JP. The Ohio State University, USA.
Stat Appl Genet Mol Biol. 2008;7(1): Article 22
Normalization of expression levels applied to microarray data can help in reducing measurement error. Different methods, including cyclic loess, quantile normalization and median or mean normalization, have been utilized to normalize microarray data. Although there is considerable literature regarding normalization techniques for mRNA microarray data, there are no publications comparing normalization techniques for microRNA (miRNA) microarray data, which are subject to similar sources of measurement error. In this paper, we compare the performance of cyclic loess, quantile normalization, median normalization and no normalization for a single-color microRNA microarray dataset. We show that the quantile normalization method works best in reducing differences in miRNA expression values for replicate tissue samples. By showing that the total mean squared error are lowest across almost all 36 investigated tissue samples, we are assured that the bias correction provided by quantile normalization is not outweighed by additional error variance that can arise from a more complex normalization method. Furthermore, we show that quantile normalization does not achieve these results by compression of scale.

A sensitive array for microRNA expression profiling (miChip) based on locked nucleic acids (LNA).
Castoldi M, Schmidt S, Benes V, Noerholm M, Kulozik AE, Hentze MW, Muckenthaler MU.
Department of Pediatric Oncology, Hematology and Immunology, University of
Heidelberg, Germany.
RNA. 2006 12(5): 913-920
MicroRNAs represent a class of short (approximately 22 nt), noncoding regulatory RNAs involved in development, differentiation, and metabolism. We describe a novel microarray platform for genome-wide profiling of mature miRNAs (miChip) using locked nucleic acid (LNA)-modified capture probes. The biophysical properties of LNA were exploited to design probe sets for uniform, high-affinity hybridizations yielding highly accurate signals able to discriminate between single nucleotide differences and, hence, between closely related miRNA family members. The superior detection sensitivity eliminates the need for RNA size selection and/or amplification. MiChip will greatly simplify miRNA expression profiling of biological and clinical samples.
miChip: an array-based method for microRNA expression profiling using locked nucleic acid capture probes.
Mirco Castoldi, Sabine Schmidt, Vladimir Benes, Matthias W Hentze & Martina U Muckenthaler
Nature Protocols 3, - 321 - 329 (2008)
MicroRNAs (miRNAs) represent a class of short (22 nt) noncoding RNAs that control gene expression post-transcriptionally. Microarray technology is frequently applied to monitor miRNA expression levels but is challenged by (i) the short length of miRNAs that offers little sequence for appending detection molecules; (ii) low copy number of some miRNA; and (iii) a wide range of predicted melting temperatures (Tm) versus their DNA complementary sequences. We recently developed a microarray platform for genome-wide profiling of miRNAs (miChip) by applying locked nucleic acid (LNA)-modified capture probes. Here, we provide detailed protocols for the generation of the miChip microarray platform, the preparation and fluorescent labeling of small RNA containing total RNA, its hybridization to the immobilized LNA-modified capture probes and the post-hybridization handling of the microarray. Starting from the intact tissue sample, the entire protocol takes approx3 d to yield highly accurate and sensitive data about miRNA expression levels.
A personalized microRNA microarray normalization method using a logistic regression model.
Wang B, Wang XF, Howell P, Qian X, Huang K, Riker AI, Ju J, Xi Y.
Department of Mathematics and Statistics, University of South Alabama, Mobile, AL 36688, USA.
Bioinformatics. 2010 Jan 15;26(2):228-34
MOTIVATION: MicroRNA (miRNA) is a set of newly discovered non-coding small RNA molecules. Its significant effects have contributed to a number of critical biological events including cell proliferation, apoptosis development, as well as tumorigenesis. High-dimensional genomic discovery platforms (e.g. microarray) have been employed to evaluate the important roles of miRNAs by analyzing their expression profiling. However, because of the small total number of miRNAs and the absence of well-known endogenous controls, the traditional normalization methods for messenger RNA (mRNA) profiling analysis could not offer a suitable solution for miRNA analysis. The need for the establishment of new adaptive methods has come to the forefront. RESULTS: Locked nucleic acid (LNA)-based miRNA array was employed to profile miRNAs using colorectal cancer cell lines under different treatments. The expression pattern of overall miRNA profiling was pre-evaluated by a panel of miRNAs using Taqman-based quantitative real-time polymerase chain reaction (qRT-PCR) miRNA assays. A logistic regression model was built based on qRT-PCR results and then applied to the normalization of miRNA array data. The expression levels of 20 additional miRNAs selected from the normalized list were post-validated. Compared with other popularly used normalization methods, the logistic regression model efficiently calibrates the variance across arrays and improves miRNA microarray discovery accuracy.
AVAILABILITY:  Datasets and R package are available at  http://gauss.usouthal.edu/publ/logit/


Intra-platform repeatability and inter-platform comparability of microRNA microarray technology.
Sato F, Tsuchiya S, Terasawa K, Tsujimoto G.
Department of Nanobio Drug Discovery, Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto, Kyoto, Japan. PLoS One. 2009;4(5):e5540. Epub 2009 May 14.
Over the last decade, DNA microarray technology has provided a great contribution to the life sciences. The MicroArray Quality Control (MAQC) project demonstrated the way to analyze the expression microarray. Recently, microarray technology has been utilized to analyze a comprehensive microRNA expression profiling. Currently, several platforms of microRNA microarray chips are commercially available. Thus, we compared repeatability and comparability of five different microRNA microarray platforms (Agilent, Ambion, Exiqon, Invitrogen and Toray) using 309 microRNAs probes, and the Taqman microRNA system using 142 microRNA probes. This study demonstrated that microRNA microarray has high intra-platform repeatability and comparability to quantitative RT-PCR of microRNA. Among the five platforms, Agilent and Toray array showed relatively better performances than the others. However, the current lineup of commercially available microRNA microarray systems fails to show good inter-platform concordance, probably because of lack of an adequate normalization method and severe divergence in stringency of detection call criteria between different platforms. This study provided the basic information about the performance and the problems specific to the current microRNA microarray systems.

Impact of normalization on miRNA microarray expression profiling.
Pradervand S, Weber J, Thomas J, Bueno M, Wirapati P, Lefort K, Dotto GP, Harshman K.
Lausanne DNA Array Facility, Center for Integrative Genomics, University of Lausanne, CH-1015 Lausanne, Switzerland.
RNA. 2009 Mar;15(3):493-501
Profiling miRNA levels in cells with miRNA microarrays is becoming a widely used technique. Although normalization methods for mRNA gene expression arrays are well established, miRNA array normalization has so far not been investigated in detail. In this study we investigate the impact of normalization on data generated with the Agilent miRNA array platform. We have developed a method to select nonchanging miRNAs (invariants) and use them to compute linear regression normalization coefficients or variance stabilizing normalization (VSN) parameters. We compared the invariants normalization to normalization by scaling, quantile, and VSN with default parameters as well as to no normalization using samples with strong differential expression of miRNAs (heart-brain comparison) and samples where only a few miRNAs are affected (by p53 overexpression in squamous carcinoma cells versus control). All normalization methods performed better than no normalization. Normalization procedures based on the set of invariants and quantile were the most robust over all experimental conditions tested. Our method of invariant selection and normalization is not limited to Agilent miRNA arrays and can be applied to other data sets including those from one color miRNA microarray platforms, focused gene expression arrays, and gene expression analysis using quantitative PCR.

Comparison of normalization methods with microRNA microarray.
Hua YJ, Tu K, Tang ZY, Li YX, Xiao HS.
Bioinformatics Center, The Center of Functional Genomics, Key Lab of System Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, People's Republic of China.
Genomics. 2008 Aug;92(2):122-8. Epub 2008 Jun 2.
MicroRNAs (miRNAs) are a group of RNAs that play important roles in regulating gene expression and protein translation. In a previous study, we established an oligonucleotide microarray platform to detect miRNA expression. Because it contained only hundreds of probes, data normalization was difficult. In this study, the microarray data for eight miRNAs extracted from inflamed rat dorsal root ganglion (DRG) tissue were normalized using 15 methods and compared with the results of real-time polymerase chain reaction. It was found that the miRNA microarray data normalized by the print-tip loess method were the most consistent with results from real-time polymerase chain reaction. Moreover, the same pattern was also observed in 14 different types of rat tissue. This study compares a variety of normalization methods and will be helpful in the preprocessing of miRNA microarray data.


©  editor@gene-quantification.info