Qdot nanobarcodes for multiplexed gene expression analysis

We report a quantum dot (Qdot) nanobarcode-based microbead random array platform for accurate and reproducible gene expression profiling in a high-throughput and multiplexed format. Four different sizes of Qdots, with emissions at 525, 545, 565, and 585 nm are mixed with a polymer and coated onto the 8-mu m-diameter magnetic microbeads to generate a nanobarcoded bead termed as QBeads. Twelve intensity levels for each of the four colors were used. Gene-specific oligonucleotide probes are conjugated to the surface of each spectrally nanobarcoded bead to create a multiplexed panel, and biotinylated cRNAs are generated from sample total RNA and hybridized to the gene probes on the microbeads. A fifth streptavidin Qdot ( 655 nm or infrared Qdot) binds to biotin on the cRNA, acting as a quantification reporter. Target identity was decoded based on spectral profile and intensity ratios of the four coding Qdots ( 525, 545, 565, and 585 nm). The intensity of the 655 nm Qdot reflects the level of biotinylated cRNA captured on the beads and provides the quantification for the corresponding target gene. The system shows a sensitivity of <= 10(4) target molecules detectable with T7 amplification, a level that is better than the 105 number achievable with a high-density microarray system, and approaching the 10(3)-10(4) level usually observed for quantitative PCR (qPCR). The QBead nanobarcode system has a dynamic range of 3.5 logs, better than the 2-3 logs observed on various microarray platforms. The hybridization reaction is performed in liquid phase and completed in 1-2 hours, at least 1 order of magnitude faster than microarray-based hybridizations. Detectable fold change is lower than 1.4- fold, showing high precision even at close to single copy per cell level. Reproducibility for this proof-of-concept study approaches that of Affymetrix GeneChip microarray, with an R-2 value between two repeats at 0.984, and interwell CV around 5%. In addition, it provides increased flexibility, convenience, and cost-effectiveness in comparison to conventional gene expression profiling methods.