QIAseq Targeted RNA Indexes

The QIAseq Targeted RNA Panels have been developed as a Sample to Insight solution for quantitative gene expression profiling using RNAseq. The panels use molecular barcodes and a two-stage PCR-based integrated library preparation to overcome the challenge of PCR duplicates and amplification bias to deliver unbiased, accurate and reproducible gene expression results.

Expression levels for 384 genes were determined by both QIAseq Targeted RNA Panel and qPCR for Human Brain Reference RNA (HBRR) and Universal Human Reference RNA (UHRR) samples. The expression levels determined by qPCR were normalized to the average of four housekeeping genes (ACTB, B2M, GAPDH and RPLP0) and fold change between the samples was calculated (HBRR/UHRR) for each gene. For QIAseq, the number of unique molecular barcodes per gene were counted and normalized to average number of molecular barcodes for the four housekeeping genes for each sample. The fold change for each gene was then calculated. The QIAseq Targeted RNA Panel and qPCR assays exhibit similar fold-changes in gene expression, highlighting the accuracy of QIAseq-results.

Sequencing libraries were prepared using 1.25, 5 or 20 ng universal reference RNA and QIAseq Targeted RNA Panels, ranging from 12-plex to 1000-plex. Sequencing was performed on the Illumina MiSeq, dedicating 1 million reads per sample. Specificity is calculated as percent of trimmed and mapped reads that map to intended targets.

Starting with 25 ng of total unfragmented RNA, cDNA is synthesized, and each cDNA molecule is tagged with a unique molecular barcode before any amplification. The uniquely tagged cDNA molecules then undergo a two-stage PCR step for enrichment and library construction. It takes only 6 hours from RNA sample to targeted library ready for sequencing.

A 917-plex gene panel was used to prepare a library from 10 ng of NA12878 reference DNA. All assays were designed to be intra-exon, and are thus single copy on genomic DNA. This allows an estimation of the uniformity of amplicon performance in the library preparation step (e.g., every unique tag equals one captured copy). In terms of raw assay performance, 97.5% of assays are within 20% of mean/median molecular tag counts. For cataloged panels, any assay below 20% is redesigned and replaced. Molecular barcodes entirely remove this variation in RNAseq counting.

Different amounts (20 ng, 5 ng or 1.25 ng) of universal reference RNA were used to determine expression levels of three genes (BNIP1, CTNND2 and DAPK1) using targeted RNAseq. QIAseq digital RNA sequencing method (A; molecular barcode counts) showed accurate quantification of all three genes corresponding to different RNA input, whereas traditional targeted RNAseq (B; read counts) revealed PCR duplication limitation and yielded inaccurate quantification.

Different amounts (20 ng, 5 ng or 1.25 ng) of universal reference RNA were used to determine expression levels of three genes (BNIP1, CTNND2 and DAPK1) using targeted RNAseq. QIAseq digital RNA sequencing method (A; molecular barcode counts) showed accurate quantification of all three genes corresponding to different RNA input, whereas traditional targeted RNAseq (B; read counts) revealed PCR duplication limitation and yielded inaccurate quantification.

QIAseq Targeted RNA Panels use a digital sequencing method, whereby a unique 12-base random molecular barcode incorporated into the gene-specific primers (GSP1) is used in the first extension step (after mRNA is converted to cDNA). Thus, every extension event yields a unique combination of molecular barcode and target sequence. At the end of sequencing, the relative amount of each mRNA target is determined by the number of unique molecular barcode-target combinations that were sequenced, thereby eliminating PCR duplicates and amplification bias, resulting in more accurate, unbiased gene expression analysis.

ERCC standards, at 86 to 705,500 copies, spiked into universal reference RNA sample and enriched using 384-plex QIASeq Targeted RNA Panel in three technical replicates. (A) Sensitivity measurement. Under standard conditions (20 ng RNA input, 0.5 million MiSeq reads), ≥~100 copies of ERCC transcripts were reliability detected, which is the equivalent of ~0.2 copies per cell. (B) Precision measurement. At >10 barcodes/gene, CV was less than 5% for all targets, indicating high technical reproducibility. This corresponds to ~100 copies target RNA in the sample.