Correlating measurements across samples improves accuracy of large-scale expression profile experiments
© Alvarez et al.; licensee BioMed Central Ltd. 2009
Received: 17 July 2009
Accepted: 30 December 2009
Published: 30 December 2009
Gene expression profiling technologies suffer from poor reproducibility across replicate experiments. However, when analyzing large datasets, probe-level expression profile correlation can help identify flawed probes and lead to the construction of truer probe sets with improved reproducibility. We describe methods to eliminate uninformative and flawed probes, account for dependence between probes, and address variability due to transcript-isoform mixtures. We test and validate our approach on Affymetrix microarrays and outline their future adaptation to other technologies.
Gene expression profiling is a valuable technique for studying cell phenotype at the molecular level. Microarray gene expression profiling, in particular, is unquestionably the most widely adopted molecular profiling technique, used virtually throughout the life-sciences, and deep-sequencing based approaches are slated to further improve our ability to monitor gene transcripts in the cell. However, since the inception of the technology, the accuracy of gene expression profiles has been questioned due to relatively poor reproducibility [1–3]. Numerous studies have attempted to improve accuracy and reproducibility by applying filtering methods and improving data processing and normalization [4–7], but both technology-specific and technology-independent aspects of the gathering and analysis of this data modality remain challenging. For instance, a recently addressed gene expression profile-specific challenge is posed by probe designs that become rapidly outdated due to changes in genomic sequences and their annotations, and probe-set remapping using up-to-date genomic annotation has been repeatedly shown to improve Affymetrix expression microarray accuracy [8, 9]. To improve reader comprehension, we note that in this correspondence we address microarray expression profiling technical challenges at the probe level and we are careful to distinguish between individual probes (that is, 25-mer oligonucleotide sequences), Affymetrix probe sets (sets of 25-mer probes designed to span a target region based on a UniGene cluster), and our own probe clusters. Technology-independent challenges that are at best only partially resolved are related to tissue-specific transcript isoforms, post-transcriptional modifications, and polymorphisms that can affect measurement accuracy in a context-specific fashion. Because of their complex and poorly understood nature, accounting for all of these features individually is a prohibitive task, and technological advancements alone are not likely to resolve them.
We noticed that prior efforts to address these challenges were mostly focused on improving accuracy for standalone expression measurements, and largely disregarded the increasing availability of gene-expression profiles representing a diversity of phenotypic or molecular contexts for the same cellular system [10, 11]. This diversity is quite valuable, as it allows for monitoring transcript isoforms and increasing measurement accuracy by assembling transcript-specific clusters of correlated probes across large and diverse sample sets. Statistical methods that take advantage of this diversity of expression measurements can identify and even correct biases that typically result in measurement inaccuracies. We reasoned that if distinct probes monitor the same transcript isoform, then their measurements should be highly correlated across large gene expression profile datasets. This simple idea, which has not been previously used for the identification of informative probes, can be used to substantially improve expression measurement accuracy and to monitor alternative splice variants in the cell. Our proposed algorithm, Cleaner, implements this idea for Affymetrix GeneChip microarrays and may be extended in a straightforward fashion to other technologies.
We compared Cleaner both to analyses using Affymetrix annotation and AffyProbeMiner annotation . The latter is a recent effort focused on remapping and re-clustering microarray probes, and it compares favorably to other sequence-alignment centric efforts. We measured inter- and intra-microarray consistency by computing the correlation between repeated GeneAtlas gene profiling experiments using U133A, and by comparing gene sets identified as differentially down-regulated in centroblasts relative to naïve B cells using U95Av2 and U133plus2 platforms. We show that annotation by Cleaner significantly and systematically improved consistency across experiments and platforms, thus both improving accuracy of downstream analysis and simplifying the integration of experiments from multiple sources. To quantify these improvements, we performed quantitative reverse transcription real time PCR (qRT-PCR) validation of the expression of FOXM1 and MYB in human B cells, two genes that are mapped to multiple Affymetrix probe sets. Our experiments suggest that several of these probe-sets are conflicting and uninformative in the specific cellular context. As such, they should be disregarded when analyzing samples from naïve and centroblast B cells. Unlike other annotations, by pooling all consistent matching probes on the array, Cleaner annotation produced a single probe cluster per gene and gave definitive expression estimates that we validated here. In general, we showed that thousands of genes were associated with conflicting or uninformative Affymetrix probe sets in this B cell dataset, and Cleaner identified and resolved >95% of these instances.
Cleaner's approach is virtually technology-independent and can be easily adapted to clustering probes from other microarray platforms as well as short reads from deep-sequencing based approaches. For instance, probe clustering using data from exon-arrays will improve the identification of the specific isoforms that are differentially expressed across the samples, thus removing those that are not informative and significantly simplifying downstream analysis. Similarly, clustering short overlapping transcript fragments according to read multiplicity in deep-sequencing datasets may allow for improved transcript and exon-boundary detection, help estimate the frequency of splicing events, and help deconvolve and assign origin for reads with homology to several sites in the reference genome . Finally, the substantial accuracy improvement achieved using Cleaner suggests that its use offers a unique opportunity to reevaluate inferences made from past Affymetrix gene expression profiles, which comprise 80% of the data-sets currently deposited in the Gene Expression Omnibus (GEO) , and points to Cleaner's potential impact on future microarray and deep-sequencing gene expression profile experiments. Cleaner, implemented in R and Python, is available for download from Califano Lab .
We begin by describing the Cleaner algorithm, and then show that Cleaner probe clusters improve consistency across technical replicate experiments and across platforms by eliminating biased and flawed probes. We conclude with a targeted study showing that probes in Affymetrix probe sets with inconsistent behavior are regrouped into consistent and informative probe clusters by Cleaner.
The Cleaner algorithm
Cleaner proceeds by (a) remapping individual probes to the most recent RefSeq transcripts, (b) discarding probes mapped to multiple genes or incorrect regions, (c) computing correlation between all probe pairs on the same gene, and finally (d) organizing probes in clusters that are optimally intra-correlated within the specific context. A detailed description of each of these steps is given below.
Probe mapping to RefSeq genes
We mapped probe sequences to the transcripts in the RefSeq database  dating 11 December 2008 using ZOOM  and allowing for at most one mismatch per probe (each probe sequence matched at least 24 transcript positions). We matched against the positive orientation of RefSeq transcripts only. Probes that matched multiple non-overlapping genes were discarded, and each location in each matching transcript was annotated.
Building clean probe clusters
Based on probe mapping to RefSeq transcripts and corresponding genes, we constructed transcript-focused probe clusters in three steps: 1, quality control for individual probes; 2, clustering of correlated probes; and 3, testing of probe-cluster consistency. Probe clusters were used to create CDF files for assigning quantitative probe-cluster intensity by MAS5.
Step 1: probe consistency
We first established the consistency of each individual probe based on its correlation to other probes that were mapped to the same gene (neighbors) across microarray experiments. In this gene-focused approach, the readout obtained from any given probe is informative only if it is significantly correlated with other probes mapping to the same gene. First, probe readouts were quantile normalized to abstract away correlation among probes generated by inter-sample systematic bias. Then, the consistency score of each probe was set to the 90th percentile of computed Pearson correlation coefficients across its neighbors. Statistical significance was estimated on a gene-per-gene basis using a null distribution generated by computing the correlation between the probes mapping to the gene and 1,000 probes selected uniformly at random. Probes with consistency score corresponding to P > 0.01 were discarded (Figure 1a).
Step 2: probe clusters
Neighboring probes that map to isoforms that are differentiated by alternative splicing, RNA editing, or non-representative hybridization can produce readouts of different molecular species leading to poor quantitative intensity evaluation for the probe cluster. To account for RNA isoforms, we constructed transcript-focused probe clusters by performing a non-supervised, single-linkage hierarchical clustering of the probes using Pearson correlation coefficient as a distance measure. First, clusters were formed by iteratively breaking dendrogram edges that were significantly longer than the remainder of the edges in each level according to a one-tail t-test threshold of P < 10-10 (Figure 1b, c). Then, we iteratively merged cluster pairs with distance significance greater than 0.001, where distance between clusters was defined as the distance between the closest elements across clusters, and significance was estimated using a null distribution of 1,000,000 distances between randomly selected probe pairs. For illustration, Figure 1d depicts the two probe clusters identified for MAX across three of its known isoforms.
Step 3: probe cluster consistency
where x is the shortest distance between probe starting positions across isoforms (position shift in Figure 1e); a, b and c are estimated by fitting f(x) to pairwise Pearson correlations for 1 ≤ x ≤ 24. We estimated the probe-cluster FDR for each consistency score using permutation testing, where each probe cluster constructed after permuting sample labels (individually for each probe) was considered a false positive detection. For all experiments reported in this study, we set the minimum probe-cluster consistency score to s(x) ≥ 3. As shown in Figure 1f, this minimum score corresponds to FDR <5e-03 for the U95av2 B-cell samples.
Minimum sample size
To estimate the sample-size effect on Cleaner analysis, we randomly selected subsets from the 152 U95Av2 and 200 U133plus2 microarray experiments in B cells, and estimated the FDR when constructing probe clusters with Cleaner; we selected 20 samples per sample size. Results suggest that Cleaner is not effective for analyzing data derived from fewer than 20 microarray experiments. FDR and probe-cluster sizes showed no significant change for expression sets consisting of 40 or more microarray experiments, suggesting that full statistical power is obtained at this size (Figure S1 in Additional file 1). We note that we have obtained encouraging results using datasets with as few as 30 samples; these are described later in this section.
Probe sets generated by Cleaner for U95Av2 and U133plus2 platforms
Number of probes, probe sets, probe clusters and genes represented on two popular Affymetrix GeneChip microarrays
Of the probes discarded due to consistency issues, 55% and 58% were originally mapped to genes containing no consistent probes on U95Av2 and U133plus2 platforms, respectively (Figure S2 in Additional file 1), possibly reflecting the lack of detection of the given transcripts. In fact, we observed a relationship between expression intensity and probe-cluster consistency (Figure 1g). However, while low-intensity probe clusters are significantly more likely to be eliminated, low intensity on its own is not a sufficient requirement for rejection: many high-intensity probe clusters get discarded and low intensity probe clusters kept. In total, over 40% of the discarded probes were mapped to isoforms with consistent probe clusters, suggesting that individual probes mapped to expressed genes can be inconsistent due to technical bias (Figure S2 in Additional file 1). Finally, a small proportion of remapped probes aligned to RefSeq transcripts with a single mismatch, and Cleaner eliminated these imperfectly matching probes at a significantly higher rate than that of perfectly matching probes. Consequently, imperfectly matching probes accounted for a very small portion of the consistent probes in Cleaner probe clusters.
Agreement across technical replicate experiments
To better quantify the distinct role of probe-remapping and probe-correlation analysis (consistency-testing), we included results taken at an intermediary step of the Cleaner algorithm (Remap-only in Figure 2), where RefSeq-mapped probes were used for probe-cluster construction. Results show that, without consistency-testing, the difference between Cleaner and AffyProbeMiner analyses is not statistically significant (at P ≤ 0.05; Figure 2a). This suggests that Cleaner's improvements are mostly due to the elimination of inconsistent probes and to the construction of intrinsically consistent probe clusters rather than to an improved genome-probe mapping. In addition, to demonstrate that probe-set level pruning does not bridge the performance gap between Cleaner and the other annotations, we selected the 1,000 probe sets with highest coefficient of variation across samples for each annotation and repeated the comparison (Figure 2b); coefficient of variation-based pruning is commonly used to remove poorly informative probe sets from microarray expression experiments .
Finally, we restricted the comparison to genes with at least four annotated probes according to each annotation method; these genes had a sufficient number of consistent probes to generate Cleaner probe clusters and therefore they were expected to have accurate measurements according to all methods. Surprisingly, correlation coefficient distributions using Cleaner, AffyProbeMiner and Affymetrix annotation were significantly different. Remapping probes to RefSeq transcripts improved correlation across replicate experiments, and removing remapped inconsistent probes further increased correlation (Figure 2c). Our results suggest that Cleaner probe clusters are significantly more consistent across technical replicate experiments, and that the benefit of its probe-level selection and pruning cannot be achieved using probe-set level pruning.
Consistency across platforms
To identify centroblast down-regulated gene candidates, we used a z-value cutoff of 2.33 (P < 0.01) for calling down-regulation together with an added 1.5-fold change requirement. Permutation testing estimates for the FDR in the U133plus2 analysis were 10.7%, 5.4% and 3.2% for Affymetrix, AffyProbeMiner, and Cleaner probe-cluster annotation, respectively. Focusing on genes probed by both U95Av2 and U133plus2 platforms, we identified 859 and 1,234 down-regulated genes in centroblasts by using Affymetrix annotations; 742 and 989 down-regulated genes by using AffyProbeMiner; and 677 and 801 down-regulated genes when using Cleaner. For Affymetrix annotation, 1,478 genes were called down-regulated by at least one of the platforms and 615 (41.6%) genes were called down-regulated in both platforms; this ratio improved to 550 of 1,181 (46.6%) for AffyProbeMiner, and to 562 of 919 (61.2%) for Cleaner. Finally, only 394 genes were called down-regulated by all annotations on all platforms. Note that enrichment results are independent of the actual number of differentially expressed genes identified in each method and each platform. U133plus2 included more probes, more probe sets, and more probe clusters and it was consistently more accurate. The most accurate results were produced using Cleaner, which defined the fewest probe clusters. To conclude, we note that according to differential expression analysis, expression consistency across platforms when using Cleaner annotation (61.2%) was 50% better than when using Affymetrix annotation (41.6%).
Identification of biased probes
To demonstrate that Cleaner consistency testing is sufficient to identify biased and poorly designed probes, we focused on two types of probe features that are known to affect accuracy: G-spot probes and probes matching the transcript antisense. G spots have been shown to bias expression measurements , although not all probes containing G spots are flawed; we showed that Cleaner preferentially discards G-spot probes. Probes matching the antisense of transcripts are at best noisy and at worst hybridizing with the wrong gene; we temporarily included anti-sense probe alignments when remapping probes, and showed that Cleaner discards almost all antisense probes.
We used DME and motifclass  to identify patterns that are enriched in sequences of discarded probes relative to sequences of consistent probes. To ensure that discarded probes were truly individually inconsistent and were not discarded due to obsolete genomic annotation or poorly expressed target genes, we restricted the study to consistent probe-clusters corresponding to genes that had less than 20% probe rejection rates. The most enriched motifs identified were CGGGGG and GGG [G|A] [G|C]; both motifs were significantly (P < 0.001) enriched according to permutation testing, and CGGGGG had sites in 46% of the inconsistent probes and 20% of the consistent probes. This result suggests that patterns such as G spots are strongly correlated with probe bias, but may not be sufficient criteria for probe selection and pruning. Twenty percent of the consistent probes included a CGGGGG substring but still showed significant correlation to neighboring G-spot-free probes. Thus, correlation analysis discriminates between biased and faithful probes independent of the source of the bias and outperforms feature-specific analysis.
Both Affymetrix and AffyProbeMiner probe-set annotations include probes that match the antisense orientation of genes (see discussion about FOXM1 probe set 41324_g_at for an example). Cleaner does not permit antisense remapping; however, in order to test Cleaner's ability to identify inconsistent probes, we temporarily allowed antisense remapping. Antisense remapping of U95Av2 and U133plus2 probes identified 6,521 and 21,000 probes with unique homology to the reverse strand of RefSeq transcripts. Cleaner found that 6,013 (92%) of the U95Av2 and 18,511 (88%) of the U133plus2 antisense probes are inconsistent. For the majority of probes, antisense mapping should not produce clear and stable signals across profiles, and indeed Cleaner was able to recognize the vast majority of these poorly designed probes.
Inconsistent behavior of Affymetrix probe sets
Interestingly, the six MYB probe sets responding to MYB knock-down contributed to the Cleaner cluster, while all but two probes from the non-responding probe sets were discarded during the remapping process (Figure 5c). The two remapped probes for the conflicting MYB probe set 41854_at were discarded at a later stage by Cleaner consistency analysis. Similarly, the FOXM1 probe set responding to FOXM1 knock-down was the only one contributing to the Cleaner cluster, while all the probes for the remaining probe-sets were discarded during the remapping process (Figure 5c). An expanded probe-by-probe description of Figure 5c can be found in Additional file 3. Probe alignment locations to the two genes, as well as shRNA and qRT-PCR primer target locations, are given in Figure S4 in Additional file 1. Note that the FOXM1 probe set 41324_g_at aligns to the reverse complement (antisense) of the gene and is not included in the set of remapped probes in Figure 5c. We included the probe set in Figure S4 in Additional file 1 as an example for antisense remapping and, as expected, inclusion of these probes in the remapping stage only resulted in their elimination by Cleaner due to low consistency scores.
Finally, in Figure S5 in Additional file 1 we report on a breast-carcinoma-specific test comparing HER2 protein presence and HER2 mRNA expression estimates by three Affymetrix probe sets and one Cleaner probe cluster. HER2 protein was detected in 31 of the 129 samples , and using gene set enrichment analysis we showed that while estimates by all three Affymetrix probe sets are significantly correlated with the presence of the protein, the Cleaner probe cluster provides the closest estimates.
Large scale gene expression profiles are used for applications, including constructing high quality gene networks and interaction maps [10, 11], improving the efficiency of drug target identification , developing diagnostic methods for disease stratification , and improving the understanding of the factors contributing to physiologic and pathologic differences between cellular phenotypes . Nonetheless, repeatability of gene-expression measurements is still a major issue, with consistency of differential-expressed gene calling in repeated assays or across platforms below 50%. Worse, our results show that, on average, expression array probes that are overlapping in all but one position on the same transcript achieve Pearson Correlation below 0.85 (Figure 5e). These issues, which are related to probe-degeneracy, post-transcriptional/sample-specific modifications, and experimental sample preparation are broad and will continue to affect even deep-sequencing-based gene-expression profiling methods. Additionally, there are intrinsic limits for the consistency of even technical replicates due to experimental error. While existing methods operate well below that theoretical threshold, Cleaner achieves reproducibility that is closer to the theoretical limit and addresses most of the other issues by discarding non-informative probes without sacrificing dynamic range (that is, discarding low-intensity probes). Indeed, consistent low-intensity probes can be more informative than many inconsistent high-intensity ones. We suggest that transcript-level measurement-accuracy can be efficiently improved by clustering multiple informative probes, a design that will further benefit from the introduction of deep-sequencing-based approaches.
Technology that uses multiple probes per target to estimate expression can take advantage of large-scale gene expression profiling to improve accuracy. We showed that testing probe consistency across individual measurements helps identify biased or uninformative probes, leading to increased accuracy when estimating expression intensities on the transcript and gene levels. Using relatively simple statistics, we were able to resolve ambiguity and correct for bias with no concern for its source. Moreover, we showed that pruning measurement estimates for probe sets rather than for individual probes will inevitably lead to discarding high-quality probes because their aggregates include polluting poor-quality probes. In comparison, data analysis that is focused on individual probes can eliminate non-informative expression estimates for both probes and probe clusters, thus constructing clean clusters and simultaneously and efficiently reducing data dimensionality. This last point is crucial for improving the power of downstream data analysis such as biomarker discovery, phenotype classification, and reverse engineering of transcriptional networks [10, 20, 27, 28].
Due to poor mapping to RefSeq genes, poor consistency, low gene expression or low variability across experiments, we discarded most of the individual probe measurements in each Affymetrix gene chip. We retained probe-cluster representations for approximately half of the genes that were originally included in the array design. The loss of probe-level data was offset by dramatic improvements in measurement accuracy for the remaining probed genes. Our analysis suggests that the vast majority of data discarded is at best uninformative for downstream analysis, as in the case of unexpressed transcripts, and at worst reduces the accuracy of otherwise good probe clusters, as in the case of inconsistent individual probes. Namely, if a probe is affected by systematic bias, then transcript intensity estimates that disregard this probe and are solely based on faithful probe measurements will be more accurate.
In this study we analyzed large-scale expression profiles obtained using Affymetrix microarrays, which have been used to produce a variety of large-scale, publicly available datasets. However, our methodology extends to other technologies that use multiple probes per target, including exon arrays and deep sequencing. Ideas developed here for expression arrays, including overlapping probe analysis and isoform identification, are easily adapted to resolve probe dependence in data produced by other technologies.
Deep-sequencing reads are particularly amenable to the Cleaner approach. Understanding the relationship between reads and estimating the consistency of reads helps estimate target isoform concentration, and allows for eliminating or reevaluating biased and complex reads. Such reads include overlapping reads, reads that map to multiple transcripts, and reads that are biased due to features such as base composition or uneven copy number. In addition to technology-independent challenges addressed here, deep-sequencing data with sufficient coverage and dataset size will permit a more specific mixture resolution and the estimation of the contribution of individual sources producing observed read volumes. To extend the Cleaner method to deep-sequencing technology, partition RefSeq transcripts into equal-size bins, where the number of bins depends on the reads per transcript kilobase across samples. Each bin is scored based on the number of overlapping tags, and bins are treated as quasi-probes. Instead of using the distance between probes to score probe-cluster consistency, use the distance between bins. The rest of the algorithm directly follows Cleaner's current methodology.
Genome-wide gene expression profile data suffer from technology-driven and technology-independent systematic bias. Measurement multiplicity, which is implicit to gene expression profiling technologies using multiple probes per target, can be used to assemble informative, transcript-specific probe-clusters that dramatically improve expression estimates in large cell-context-specific datasets. By harvesting the power of large-scale expression profiling we accounted for systematic biases regardless of their source. Our methods can be used to analyze both the large body of data in current repositories, and due to their technology-independent nature, they can be extended to construct transcript-specific probe-clusters using exon-array data and transcript-specific read-clusters in deep-sequencing data.
Materials and methods
Cell lines and cell culture conditions
We maintained ST486 and 293FT cells in Iscove's modified Dulbecco's medium (IMDM; Invitrogen, Carlsbad, CA, USA) and Dulbecco's minimum essential medium (DMEM; Invitrogen), respectively. Culture media was supplemented with 10% fetal bovine serum (Invitrogen) and 1% penicillin-streptomycin (Cellgro, Herndon, VA, USA).
Control shRNA (SHC002), FOXM1 shRNA (TRCN0000015546) and MYB shRNA (TRCN0000040062) cloned into pLKO.1-puro lentiviral vector (Sigma, St Louis, MO, USA) were individually co-transfected with vesicular stomatitis virus glycoprotein envelope plasmid (280 ng) and Δ8.9 packaging vector (2.5 μg) into a subconfluent 100-mm plate of 293FT cells using Fugene 6 (Roche, Indianapolis, IN, USA). The viral particles were collected at 48 h and 72 h post-transfection and concentrated by ultracentrifugation in a Beckman SW28 rotor at 25,000 rpm for 1.5 h. ST486 cells (5 × 106) were transduced with the viral particles in the presence of 8 μg/ml polybrene (Chemicon, Billerica, MA, USA) by centrifugation at 450 g for 1.5 h.
Sample processing for qRT-PCR and microarrays
Total RNA was extracted with Trizol (Invitrogen) and purified by RNeasy kit (Qiagen, Valencia, CA, USA). For qRT-PCR, total RNA was reverse transcribed by QuantiTect Reverse Transcription kit (Qiagen) and SYBR-Green based qRT-PCR analysis was performed on an ABI7300 Real-time PCR system (Applied Biosystems, Foster City, CA, USA) using QuantiTect SYBR-Green kit (Qiagen). Relative quantification was performed with the 2-ΔΔCt method , and was normalized by GAPDH expression by using the forward and reverse primers CACTGGGCCCTGACAACATC and TCACTCAGAGCTTGGGGTG for FOXM1, TGGGAGATGTGTGTTGTTGATG and TCCATGCAACAGTTCTGAGACC for MYB, and CACCCAGAAGACTGTGGATGGC and GTTCAGCTCAGGGATGACCTTGC for GAPDH. For microarray-based gene expression profiles, 5 μg of total RNA were processed following the manufacturer's instructions (Affymetrix, 701025 Rev.6), and 15 μg of fragmented and biotin-labeled cRNA were hybridized to HG-U95Av2 microarrays (Affymetrix, Santa Clara, CA, USA).
Whole cell lysates were prepared from ST486 cells by using RIPA buffer (Teknova, Hollister, CA, USA) with Complete Mini protease inhibitor cocktail (Roche). Proteins were fractionated by SDS-PAGE and analyzed by standard immunoblotting procedures using the following antibodies: anti-FOXM1 (sc-502), anti-MYB (sc-517) and anti-GAPDH (sc-32233), all from Santa Cruz (Santa Cruz, CA, USA).
Gene expression data include 102 B-cell samples profiled on U95A , 152 B-cell samples profiled on U95Av2 , 75 lung carcinoma samples on U95Av2 and U133plus2 [31, 32], 51 and 295 ovarian cancer samples on U95Av2 and U133plus2 [33, 34], 49 and 45 glioblastoma samples on U95Av2 (GSE13041) and U133plus2 [35, 36], 88 and 154 prostate cancer samples on U95Av2 and U133plus2, 40 and 129 breast carcinoma samples on U95Av2 and U133plus2 [24, 37], 200 B-cell samples profiled on U133plus2 , 60 samples from 30 human tissues profiled on U133A chips , and 55 human glial brain tumor samples hybridized on huex10stv1 Affymetrix exon arrays . All samples were obtained from the GEO database  and GeneAtlasV2 . A Bioconductor-based  implementation of MAS5  was used to quantitatively estimate and normalize the intensity levels of probe clusters. Affymetrix annotation data were obtained from Bioconductor metadata packages hgu95av2.db and hgu133plus2.db v2.2.5 .
We used a non-parametric U-test to identify down-regulated genes in centroblast B cells relative to naïve B cells. Comparisons were made using expression profiles from five biological replicates in each cell type. To identify a representative set of down-regulated genes per platform and annotation method, we used a z-value cutoff of 2.33 (P < 0.01) for calling differential expression together with a 1.5-fold change requirement. The fold change requirement was used in order to correct for the high expected FDR of this non-parametric test across thousands of probe clusters when using only five biological replicates for each cell type. The 1.5-fold decrease from naïve to centroblast B cells was based on the average intensities of the probe clusters after MAS5 normalization and log2 transformation. Analysis accuracy was measured using permutation testing repeated 20 times per annotation and platform, where the experimental source labels were shuffled for each probe cluster.
The following additional data are available with the online version of this paper: a PDF file containing Figures S1 to S6 (Additional file 1); Table S1 showing the number of probes, probe clusters, and genes represented on two popular Affymetrix Genechip microarrays after running Cleaner on different expression sets (Additional file 2); Table S2 showing the list of probes mapping to FOXM1 and MYB according to Affymetrix and Cleaner annotations (Additional file 3).
false discovery rate
Gene Expression Omnibus
quantitative reverse transcription real time PCR
short hairpin RNA.
This work was supported by the National Cancer Institute (R01CA109755), the National Institute of Allergy and Infectious Diseases (R01AI066116), and the National Centers for Biomedical Computing NIH Roadmap Initiative (U54CA121852).
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