A comparison of normalization methods for differential expression analysis of RNA-seq data

Normalization is an essential step with considerable impact on high-throughput RNA sequencing (RNA-seq) data analysis. Although there are numerous methods for read count normalization, it remains a challenge to choose an optimal method due to multiple factors contributing to read count variability that affects the overall sensitivity and specificity. In order to properly determine the most appropriate normalization methods, it is critical to compare the performance and shortcomings of a representative set of normalization routines based on different dataset characteristics. Therefore, University of Louisville researchers set out to evaluate the performance of the commonly used methods (DESeq, TMM-edgeR, FPKM-CuffDiff, TC, Med UQ and FQ) and two new methods they propose: Med-pgQ2 and UQ-pgQ2 (per-gene normalization after per-sample median or upper-quartile global scaling).

The researchers per-gene normalization approach allows for comparisons between conditions based on similar count levels. Using the benchmark Microarray Quality Control Project (MAQC) and simulated datasets, they performed differential gene expression analysis to evaluate these methods. When evaluating MAQC2 with two replicates, they  observed that Med-pgQ2 and UQ-pgQ2 achieved a slightly higher area under the Receiver Operating Characteristic Curve (AUC), a specificity rate > 85%, the detection power > 92% and an actual false discovery rate (FDR) under 0.06 given the nominal FDR (≤0.05). Although the top commonly used methods (DESeq and TMM-edgeR) yield a higher power (>93%) for MAQC2 data, they trade off with a reduced specificity (<70%) and a slightly higher actual FDR than the proposed methods. In addition, the results from an analysis based on the qualitative characteristics of sample distribution for MAQC2 and human breast cancer datasets show that only their gene-wise normalization methods corrected data skewed towards lower read counts. However, when the researchers evaluated MAQC3 with less variation in five replicates, all methods performed similarly. Thus, the proposed Med-pgQ2 and UQ-pgQ2 methods perform slightly better for differential gene analysis of RNA-seq data skewed towards lowly expressed read counts with high variation by improving specificity while maintaining a good detection power with a control of the nominal FDR level.

 Comparison of nine normalization methodsrna-seq

(A) Illustrated are boxplots of log2 (counts+1) for MAQC data with two replicates in two conditions (uhr and hbr). The samples in hbr and uhr conditions are in green and red, respectively. Med-pgQ2 and UQ-pgQ2 are our proposed methods. (B) Illustrated are boxplots of the intra-condition coefficient of variation (uhr and hbr), respectively. 

Li X, Brock GN, Rouchka EC, Cooper NGF, Wu D, O’Toole TE, et al. (2017) A comparison of per sample global scaling and per gene normalization methods for differential expression analysis of RNA-seq data. PLoS ONE 12(5): e0176185. [article]

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