Biology Direct | |
Microarray experiments and factors which affect their reliability | |
Roman Jaksik2  Marta Iwanaszko1  Joanna Rzeszowska-Wolny2  Marek Kimmel1  | |
[1] Department of Statistics, Rice University, Houston, TX, USA | |
[2] Systems Biology Group, Faculty of Automatic Control, Electronics and Informatics, Silesian University of Technology, Gliwice, Poland | |
关键词: Measurement bias; Transcriptome profiling; Quality control; Microarray pre-processing; Microarrays; | |
Others : 1225793 DOI : 10.1186/s13062-015-0077-2 |
|
received in 2015-04-15, accepted in 2015-08-24, 发布年份 2015 |
【 摘 要 】
Oligonucleotide microarrays belong to the basic tools of molecular biology and allow for simultaneous assessment of the expression level of thousands of genes. Analysis of microarray data is however very complex, requiring sophisticated methods to control for various factors that are inherent to the procedures used. In this article we describe the individual steps of a microarray experiment, highlighting important elements and factors that may affect the processes involved and that influence the interpretation of the results. Additionally, we describe methods that can be used to estimate the influence of these factors, and to control the way in which they affect the expression estimates. A comprehensive understanding of the experimental protocol used in a microarray experiment aids the interpretation of the obtained results. By describing known factors which affect expression estimates this article provides guidelines for appropriate quality control and pre-processing of the data, additionally applicable to other transcriptome analysis methods that utilize similar sample handling protocols.
Reviewers This article was reviewed by Dr. Janet Siefert, Dr. Leonid Hanin, and Dr. I King Jordan.
【 授权许可】
2015 Jaksik et al.
Files | Size | Format | View |
---|---|---|---|
Fig. 5. | 62KB | Image | download |
Fig. 4. | 12KB | Image | download |
Fig. 3. | 14KB | Image | download |
Fig. 2. | 43KB | Image | download |
Fig. 1. | 48KB | Image | download |
Fig. 5. | 62KB | Image | download |
Fig. 4. | 12KB | Image | download |
Fig. 3. | 14KB | Image | download |
Fig. 2. | 43KB | Image | download |
Fig. 1. | 48KB | Image | download |
【 图 表 】
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
【 参考文献 】
- [1]Ramsay G: DNA chips: state-of-the art. Nat Biotechnol 1998, 16(1):40-4.
- [2]Stoughton RB: Applications of DNA microarrays in biology. Annu Rev Biochem 2005, 74:53-82.
- [3]Lockhart DJ, Dong H, Byrne MC, Follettie MT, Gallo MV, Chee MS, et al.: Expression monitoring by hybridization to high-density oligonucleotide arrays. Nat Biotechnol 1996, 14(13):1675-80.
- [4]Erickson S, MacLeod SL, Hobbs CA: Cheek swabs, SNP chips, and CNVs: assessing the quality of copy number variant calls generated with subject-collected mail-in buccal brush DNA samples on a high-density genotyping microarray. BMC Med Genet 2012, 13:51. BioMed Central Full Text
- [5]Gardner S, Thissen JB, McLoughlin KS, Slezak T, Jaing CJ: Optimizing SNP microarray probe design for high accuracy microbial genotyping. J Microbiol Methods 2013, 94(3):303-10.
- [6]Clarke W, Parkin IA, Gajardo HA, Gerhardt DJ, Higgins E, et al.: Genomic DNA enrichment using sequence capture microarrays: a novel approach to discover Sequence Nucleotide Polymorphisms (SNP) in Brassica napus L. PLoS One 2013, 8(12):e81992.
- [7]Masimba P, Gare J, Klimkait T, Tanner M, Felger I: Development of a simple microarray for genotyping HIV-1 drug resistance mutations in the reverse transcriptase gene in rural Tanzania. Trop Med Int Health 2014, 19(6):664-71.
- [8]Birney E, Stamatoyannopoulos JA, Dutta A, Guigo R, Gingeras TR, et al.: Identification and analysis of functional elements in 1 % of the human genome by the ENCODE pilot project. Nature 2007, 447:799-816.
- [9]Kaufmann K, Muiño JM, Østerås M, Farinelli L, Krajewski P, Angenent GC: Chromatin immunoprecipitation (ChIP) of plant transcription factors followed by sequencing (ChIP-SEQ) or hybridization to whole genome arrays (ChIP-CHIP). Nat Protoc 2010, 5(3):457-72.
- [10]Cauchy P, Benoukraf T, Ferrier P: Processing ChIP-chip data: from the scanner to the browser. Methods Mol Biol 2011, 719:251-68.
- [11]Dowell N, Sperling AS, Mason MJ, Johnson RC: Chromatin-dependent binding of the S. cerevisiae HMGB protein Nhp6A affects nucleosome dynamics and transcription. Genes Dev 2010, 24(18):2031-42.
- [12]Makeyev A, Bayarsaihan D: ChIP-chip identifies SEC23A, CFDP1, and NSD1 as TFII-I target genes in human neural crest progenitor cells. Cleft Palate Craniofac J 2013, 50(3):347-50.
- [13]Hegde M, Chin EL, Mulle JG, Okou DT, Warren ST, et al.: Microarray-based mutation detection in the dystrophin gene. Hum Mutat 2008, 29:1091-9.
- [14]Rouleau E, Lefol C, Tozlu S, Andrieu C, Guy C, et al.: High-resolution oligonucleotide array-CGH applied to the detection and characterization of large rearrangements in the hereditary breast cancer gene BRCA1. Clin Genet 2007, 72:199-207.
- [15]Aston E, Whitby H, Maxwell T, Glaus N, Cowley B, et al.: Comparison of targeted and whole genome analysis of postnatal specimens using a commercially available array based comparative genomic hybridisation (aCGH) microarray platform. J Med Genet 2008, 45(5):268-74.
- [16]Ahn J, Mann K, Walsh S, Shehab M, Hoang S, et al. Validation and implementation of array comparative genomic hybridisation as a first line test in place of postnatal karyotyping for genome imbalance. Mol Cytogenet. 2010;3(9). doi:10.1186/1755-8166-3-9.
- [17]Hartmann A, Thieme M, Nanduri LK, Stempfl T, Moehle C, et al.: Validation of microarray-based resequencing of 93 worldwide mitochondrial genomes. Hum Mutat 2009, 30(1):115-22.
- [18]Zwick M, Kiley MP, Stewart AC, Mateczun A, Read TD: Genotyping of bacillus cereus strains by microarray-based resequencing. PLoS One 2008, 3(7):e2513.
- [19]Berthet N, Deletoile A, Passet V, Kennedy GC, Manuguerra JC, et al.: Reconstructed ancestral sequences improve pathogen identification using resequencing DNA microarrays. PLoS One 2010, 5(12):e15243.
- [20]Kathiravel U, Keyser B, Hoffjan S, Kötting J, Müller M, et al.: High-density oligonucleotide-based resequencing assay for mutations causing syndromic and non-syndromic forms of thoracic aortic aneurysms and dissections. Mol Cell Probes 2013, 27(2):103-8.
- [21]Vanhomwegen J, Berthet N, Mazuet C, Guigon G, Vallaeys T, et al.: Application of high-density DNA resequencing microarray for detection and characterization of botulinum neurotoxin-producing clostridia. PLoS One 2013, 8(6):e67510.
- [22]Hadiwikarta W, Van Dorst B, Hollanders K, Stuyver L, Carlon E, Hooyberghs J: Targeted resequencing of HIV variants by microarray thermodynamics. Nucleic Acids Res 2013, 41(18):e173.
- [23]Barnes M, Freudenberg J, Thompson S, Aronow B, Pavlidis P: Experimental comparison and cross-validation of the Affymetrix and Illumina gene expression analysis platforms. Nucleic Acids Res 2005, 33:5914-23.
- [24]Beekman J, Boess F, Hildebrand H, Kalkuhl A, Suter L: Gene expression analysis of the hepatotoxicant methapyrilene in primary rat hepatocytes: an interlaboratory study. Environ Health Perspect 2006, 114:92-9.
- [25]Dobbin K, Beer DG, Meyerson M, Yeatman TJ, Gerald WL, et al.: Interlaboratory comparability study of cancer gene expression analysis using oligonucleotide microarrays. Clin Cancer Res 2005, 11:565-72.
- [26]Saitoh T, Yamamoto M, Miyagashi M, Taira K, Nakanishi M, et al.: A20 is a negative regulator of IFN regulatory factor 3 signaling. J Immunol 2005, 174:1507-12.
- [27]Shi L, Reid LH, Jones WD, Shippy R, Warrington JA, et al.: The microarray quality control (MAQC) project shows inter- and intraplatform reproducibility of gene expression measurements. Nat Biotechnol 2006, 24:1151-61.
- [28]Guo L, Lobenhofer EK, Wang C, Shippy R, Harris SC, et al.: Rat toxicogenomic study reveals analytical consistency. Nat Biotechnol 2006, 24:1162-9.
- [29]Irizarry R, Warren D, Spencer F, Kim IF, Biswal S, et al.: Multiple-laboratory comparison of microarray platforms. Nat Methods 2005, 2:345-50.
- [30]Hockley S, Mathijs K, Staal YC, Brewer D, Giddings I, et al.: Interlaboratory and interplatform comparison of microarray gene expression analysis of HepG2 cells exposed to benzo(a)pyrene. OMICS 2009, 12(2):115-25.
- [31]Bolstad BM, Irizarry RA, Astrand M, Speed TP: A comparison of normalization methods for high density oligonucleotide array data based on variance and bias. Bioinformatics 2003, 19(2):185-93.
- [32]Li C, Hung Wong W. Model-based analysis of oligonucleotide arrays: model validation, design issues and standard error application. Genome Biol. 2001;2(8). RESEARCH0032.. http://www.genomebiology.com/2001/2/8/research/0032 webcite
- [33]Johnson WE, Li C, Rabinovic A: Adjusting batch effects in microarray expression data using empirical bayes methods. Biostatistics 2007, 8(1):118-27.
- [34]Vardhanabhuti S, Blakemore SJ, Clark SM, Ghosh S, Stephens RJ, Rajagopalan D: A comparison of statistical tests for detecting differential expression using affymetrix oligonucleotide microarrays. OMICS 2006, 10(4):555-66.
- [35]Smyth GK. Linear models and empirical bayes methods for assessing differential expression in microarray experiments. Statistical applications in genetics and molecular biology. 2004;3:Article3. doi:10.2202/1544-6115.1027.
- [36]Calza S, Raffelsberger W, Ploner A, Sahel J, Leveillard T, Pawitan Y: Filtering genes to improve sensitivity in oligonucleotide microarray data analysis. Nucleic Acids Res 2007, 35(16):e102.
- [37]Suarez-Farinas M, Pellegrino M, Wittkowski KM, Magnasco MO: Harshlight: a “corrective make-up” program for microarray chips. BMC Bioinformatics 2005, 6:294. BioMed Central Full Text
- [38]Moffitt RA, Yin-Goen Q, Stokes TH, Parry RM, Torrance JH, Phan JH, et al.: caCORRECT2: Improving the accuracy and reliability of microarray data in the presence of artifacts. BMC Bioinformatics 2011, 12:383. BioMed Central Full Text
- [39]Jaksik R, Polanska J, Herok R, Rzeszowska-Wolny J: Calculation of reliable transcript levels of annotated genes on the basis of multiple probe-sets in affymetrix microarrays. Acta Biochim Pol 2009, 56(2):271-7.
- [40]Schneider S, Smith T, Hansen U. SCOREM: statistical consolidation of redundant expression measures. Nucleic Acids Res. 2011;40(6):e46. doi:10.1093/nar/gkr1270.
- [41]Dai M, Wang P, Boyd AD, Kostov G, Athey B, Jones EG, et al.: Evolving gene/transcript definitions significantly alter the interpretation of GeneChip data. Nucleic Acids Res 2005, 33(20):e175.
- [42]Ferrari F, Bortoluzzi S, Coppe A, Sirota A, Safran M, Shmoish M, et al.: Novel definition files for human GeneChips based on GeneAnnot. BMC Bioinformatics 2007, 8:446. BioMed Central Full Text
- [43]Kroll KM, Barkema GT, Carlon E: Modeling background intensity in DNA microarrays. Phys Rev E Stat Nonlinear Soft Matter Phys 2008, 77(6 Pt 1):061915.
- [44]Wu ZJ, Irizarry RA, Gentleman R, Martinez-Murillo F, Spencer F: A model-based background adjustment for oligonucleotide expression arrays. J Am Stat Assoc 2004, 99(468):909-17.
- [45]Draghici S, Khatri P, Eklund AC, Szallasi Z: Reliability and reproducibility issues in DNA microarray measurements. Trends Genet 2006, 22(2):101-9.
- [46]Blair S, Williams L, Bishop J, Chagovetz A: Microarray temperature optimization using hybridization kinetics. Methods Mol Biol 2009, 529:171-96.
- [47]Opitz L, Salinas-Riester G, Grade M, Jung K, Jo P, Emons G, et al.: Impact of RNA degradation on gene expression profiling. BMC Med Genet 2010, 3:36.
- [48]Croner RS, Lausen B, Schellerer V, Zeittraeger I, Wein A, Schildberg C, et al.: Comparability of microarray data between amplified and non amplified RNA in colorectal carcinoma. J Biomed Biotechnol 2009, 2009:837170.
- [49]Pease AC, Solas D, Sullivan EJ, Cronin MT, Holmes CP, Fodor SP: Light-generated oligonucleotide arrays for rapid DNA sequence analysis. Proc Natl Acad Sci U S A 1994, 91(11):5022-6.
- [50]Held GA, Grinstein G, Tu Y: Relationship between gene expression and observed intensities in DNA microarrays - a modeling study. Nucleic Acids Res 2006, 34(9):e70.
- [51]Affymetrix. GeneChip Expression Analysis - Technical Manual. 2004:185.
- [52]Wang Y, Miao ZH, Pommier Y, Kawasaki ES, Player A: Characterization of mismatch and high-signal intensity probes associated with affymetrix genechips. Bioinformatics 2007, 23(16):2088-95.
- [53]Schneider J, Buness A, Huber W, Volz J, Kioschis P, Hafner M, et al.: Systematic analysis of T7 RNA polymerase based in vitro linear RNA amplification for use in microarray experiments. BMC Genomics 2004, 5:29. BioMed Central Full Text
- [54]Urakawa H, El Fantroussi S, Smidt H, Smoot JC, Tribou EH, Kelly JJ, et al.: Optimization of single-base-pair mismatch discrimination in oligonucleotide microarrays. Appl Environ Microbiol 2003, 69(5):2848-56.
- [55]Deng Y, He Z, Van Nostrand JD, Zhou J: Design and analysis of mismatch probes for long oligonucleotide microarrays. BMC Genomics 2008, 9:491. BioMed Central Full Text
- [56]LaFramboise T: Single nucleotide polymorphism arrays: a decade of biological, computational and technological advances. Nucleic Acids Res 2009, 37(13):4181-93.
- [57]Schroeder A, Mueller O, Stocker S, Salowsky R, Leiber M, Gassmann M, et al.: The RIN: an RNA integrity number for assigning integrity values to RNA measurements. BMC Mol Biol 2006, 7:3. BioMed Central Full Text
- [58]Affymetrix. 3′ IVT Express Kit User Manual. 2012.. http://www.affymetrix.com webcite
- [59]Grillo G, Turi A, Licciulli F, Mignone F, Liuni S, Banfi S, et al.: UTRdb and UTRsite (RELEASE 2010): A collection of sequences and regulatory motifs of the untranslated regions of eukaryotic mRNAs. Nucleic Acids Res 2010, 38(Database issue):D75-80.
- [60]Fare TL, Coffey EM, Dai H, He YD, Kessler DA, Kilian KA, et al.: Effects of atmospheric ozone on microarray data quality. Anal Chem 2003, 75(17):4672-5.
- [61]Mignone F, Grillo G, Licciulli F, Iacono M, Liuni S, Kersey PJ, et al.: UTRdb and UTRsite: a collection of sequences and regulatory motifs of the untranslated regions of eukaryotic mRNAs. Nucleic Acids Res 2005, 33(Database issue):D141-6.
- [62]Gautier L, Cope L, Bolstad BM, Irizarry RA: affy--analysis of Affymetrix GeneChip data at the probe level. Bioinformatics 2004, 20(3):307-15.
- [63]Archer KJ, Guennel T: An application for assessing quality of RNA hybridized to Affymetrix GeneChips. Bioinformatics 2006, 22(21):2699-701.
- [64]Slomovic S, Laufer D, Geiger D, Schuster G: Polyadenylation of ribosomal RNA in human cells. Nucleic Acids Res 2006, 34(10):2966-75.
- [65]Yang L, Duff MO, Graveley BR, Carmichael GG, Chen LL: Genomewide characterization of non-polyadenylated RNAs. Genome Biol 2011, 12(2):R16. BioMed Central Full Text
- [66]Bemmo A, Benovoy D, Kwan T, Gaffney DJ, Jensen RV, Majewski J: Gene expression and isoform variation analysis using Affymetrix Exon Arrays. BMC Genomics 2008, 9:529. BioMed Central Full Text
- [67]Fasold M, Binder H: AffyRNADegradation: control and correction of RNA quality effects in GeneChip expression data. Bioinformatics 2013, 29(1):129-31.
- [68]Fasold M, Binder H: Estimating RNA-quality using GeneChip microarrays. BMC Genomics 2012, 13:186. BioMed Central Full Text
- [69]Jaksik R, Marczyk M, Polanska J, Rzeszowska-Wolny J: Sources of high variance between probe signals in affymetrix short oligonucleotide microarrays. Sensors 2014, 14(1):532-48.
- [70]Boelens MC, te Meerman GJ, Gibcus JH, Blokzijl T, Boezen HM, Timens W, et al.: Microarray amplification bias: loss of 30 % differentially expressed genes due to long probe - poly(A)-tail distances. BMC Genomics 2007, 8:277. BioMed Central Full Text
- [71]Nam DK, Lee S, Zhou G, Cao X, Wang C, Clark T, et al.: Oligo(dT) primer generates a high frequency of truncated cDNAs through internal poly(A) priming during reverse transcription. Proc Natl Acad Sci U S A 2002, 99(9):6152-6.
- [72]Wilson CL, Pepper SD, Hey Y, Miller CJ: Amplification protocols introduce systematic but reproducible errors into gene expression studies. BioTechniques 2004, 36(3):498-506.
- [73]Arezi B, Xing W, Sorge JA, Hogrefe HH: Amplification efficiency of thermostable DNA polymerases. Anal Biochem 2003, 321(2):226-35.
- [74]Degrelle SA, Hennequet-Antier C, Chiapello H, Piot-Kaminski K, Piumi F, Robin S, et al.: Amplification biases: possible differences among deviating gene expressions. BMC Genomics 2008, 9:46. BioMed Central Full Text
- [75]Kerkhoven RM, Sie D, Nieuwland M, Heimerikx M, De Ronde J, Brugman W, et al.: The T7-primer is a source of experimental bias and introduces variability between microarray platforms. PLoS One 2008, 3(4):e1980.
- [76]Duftner N, Larkins-Ford J, Legendre M, Hofmann HA: Efficacy of RNA amplification is dependent on sequence characteristics: Implications for gene expression profiling using a cDNA microarray. Genomics 2008, 91(1):108-17.
- [77]Sudo H, Mizoguchi A, Kawauchi J, Akiyama H, Takizawa S: Use of non-amplified RNA samples for microarray analysis of gene expression. PLoS One 2012, 7(2):e31397.
- [78]Sauer B, Henderson N: Site-specific DNA recombination in mammalian cells by the Cre recombinase of bacteriophage P1. Proc Natl Acad Sci U S A 1988, 85(14):5166-70.
- [79]Sykacek P, Kreil DP, Meadows LA, Auburn RP, Fischer B, Russell S, et al.: The impact of quantitative optimization of hybridization conditions on gene expression analysis. BMC Bioinformatics 2011, 12:73. BioMed Central Full Text
- [80]Koltai H, Weingarten-Baror C: Specificity of DNA microarray hybridization: characterization, effectors and approaches for data correction. Nucleic Acids Res 2008, 36(7):2395-405.
- [81]Affymetrix. Gene Expression Assay and Data Analysis - Hybridization time. 2012.. http://www.affymetrix.com/support/help/faqs/ge_assays/faq_15.jsp webcite
- [82]Tong W, Lucas AB, Shippy R, Fan X, Fang H, Hong H, et al.: Evaluation of external RNA controls for the assessment of microarray performance. Nat Biotechnol 2006, 24(9):1132-9.
- [83]Reimers M, Weinstein JN: Quality assessment of microarrays: Visualization of spatial artifacts and quantitation of regional biases. BMC Bioinformatics 2005, 6:166. BioMed Central Full Text
- [84]Li C, Wong WH: Model-based analysis of oligonucleotide arrays: expression index computation and outlier detection. Proc Natl Acad Sci U S A 2001, 98(1):31-6.
- [85]Song JS, Maghsoudi K, Li W, Fox E, Quackenbush J, Shirley LX: Microarray blob-defect removal improves array analysis. Bioinformatics 2007, 23(8):966-71.
- [86]Petri T, Berchtold E, Zimmer R, Friedel CC: Detection and correction of probe-level artefacts on microarrays. BMC bioinformatics 2012, 13:114. BioMed Central Full Text
- [87]Binder H, Krohn K, Burden CJ: Washing scaling of GeneChip microarray expression. BMC bioinformatics 2010, 11:291. BioMed Central Full Text
- [88]Skvortsov D, Abdueva D, Curtis C, Schaub B, Tavare S: Explaining differences in saturation levels for Affymetrix GeneChip arrays. Nucleic Acids Res 2007, 35(12):4154-63.
- [89]Hulsman M, Mentink A, van Someren EP, Dechering KJ, de Boer J, Reinders MJ: Delineation of amplification, hybridization and location effects in microarray data yields better-quality normalization. BMC bioinformatics 2010, 11:156. BioMed Central Full Text
- [90]Royce TE, Rozowsky JS, Gerstein MB: Assessing the need for sequence-based normalization in tiling microarray experiments. Bioinformatics 2007, 23(8):988-97.
- [91]Munier M, Jubeau S, Wijaya A, Morancais M, Dumay J, Marchal L, et al.: Physicochemical factors affecting the stability of two pigments: R-phycoerythrin of Grateloupia turuturu and B-phycoerythrin of Porphyridium cruentum. Food Chem 2014, 150:400-7.
- [92]Affymetrix. Gene Expression Assay and Data Analysis - Microarray scanning. 2012.. http://www.affymetrix.com/estore/support/help/faqs/ge_assays/faq_8.jsp webcite
- [93]Branham WS, Melvin CD, Han T, Desai VG, Moland CL, Scully AT, et al.: Elimination of laboratory ozone leads to a dramatic improvement in the reproducibility of microarray gene expression measurements. BMC Biotechnol 2007, 7:8. BioMed Central Full Text
- [94]Park T, Yi SG, Kang SH, Lee S, Lee YS, Simon R: Evaluation of normalization methods for microarray data. BMC bioinformatics 2003, 4:33. BioMed Central Full Text
- [95]Hochreiter S, Clevert DA, Obermayer K: A new summarization method for Affymetrix probe level data. Bioinformatics 2006, 22(8):943-9.
- [96]Marczyk M, Jaksik R, Polanski A, Polanska J: Affymetrix chip definition files construction based on custom probe set annotation database. Stud Comput Intell 2011, 381:135-44.
- [97]Canales RD, Luo Y, Willey JC, Austermiller B, Barbacioru CC, Boysen C, et al.: Evaluation of DNA microarray results with quantitative gene expression platforms. Nat Biotechnol 2006, 24(9):1115-22.
- [98]Webb PM, Merritt MA, Boyle GM, Green AC: Microarrays and epidemiology: not the beginning of the end but the end of the beginning. Cancer Epidemiol Biomark Prev 2007, 16(4):637-8.
- [99]Marton MJ, DeRisi JL, Bennett HA, Iyer VR, Meyer MR, Roberts CJ, et al.: Drug target validation and identification of secondary drug target effects using DNA microarrays. Nat Med 1998, 4(11):1293-301.
- [100]Shendure J: The beginning of the end for microarrays? Nat Methods 2008, 5(7):585-7.
- [101]Zheng W, Chung LM, Zhao H: Bias detection and correction in RNA-Sequencing data. BMC bioinformatics 2011, 12:290. BioMed Central Full Text
- [102]Benjamini Y, Speed TP: Summarizing and correcting the GC content bias in high-throughput sequencing. Nucleic Acids Res 2012, 40(10):e72.
- [103]Lahens NF, Kavakli IH, Zhang R, Hayer K, Black MB, Dueck H, et al.: IVT-seq reveals extreme bias in RNA sequencing. Genome Biol 2014, 15(6):R86. BioMed Central Full Text