Triple-selected poly(A)+ human liver tissue RNA pooled from several individuals was obtained from Ambion (Austin, TX). First-strand cDNA was generated from 4 µg RNA/rxn with (RNase H-) M-MLV reverse transcriptase, primed with equimolar concentrations of oligo(dT18) and random decamers. The reactions were carried out at 42°C for 2 hours in the presence of 0.5 mM dATP, 0.5 mM dCTP, 0.5 mM dGTP, 0.15 mM dTTP and 0.15 mM amino-allyl dUTP to facilitate the secondary labeling of an amine-reactive fluorescent conjugate. Following reverse transcription the products were heated at 95°C for 5 minutes to denature the RNA:DNA hybrids and heat-inactivate the reverse transcriptase, after which the RNA template was hydrolyzed by incubating in 0.2 M NaOH at 65°C for 15 minutes. The reaction was then neutralized in 0.3 M HEPES (pH 7.0). Reverse transcription products from 20 separate reactions were produced in this manner and pooled to reduce technical variability between samples. The cDNAs were precipitated in ethanol:isopropanol (1:1 v/v) and resuspended in 0.1 M NaHCO3 to facilitate coupling of Alexa Fluor 555 NHS esters (Molecular Probes, Eugene, OR) to the reactive groups of the amino-allyl dUTPs. Following incubation at room temperature for 1 hour, the labeled cDNAs were purified with CyScribe GFX glass fiber spin columns (Amersham Bioscience, Piscataway, NJ) and isopropanol-precipitated. Microarrays were hybridized with 2-3 µg labeled cDNA in 320 µL hybridization buffer (50 mM MES, 0.5 M NaCl, 10 mM EDTA, and 0.005% (v/v) Tween-20) for 20 hours at 50°C. Hybridizations were performed in disposable adhesive chambers (Grace BioLabs, Bend, OR) in a hybridization oven with constant agitation. After hybridization, the arrays were washed on an orbital platform in non-stringent buffer (6× SSPE, 0.01% [v/v] Tween-20) for 10 minutes at room temperature, then in stringent buffer (100 mM MES, 0.1 M NaCl, 0.01% Tween-20) for 30 minutes at 45°C. This was followed by a 5-minute wash in non-stringent buffer and a 4-minute wash in 0.2X SSC. The arrays were dried with compressed nitrogen. Fluorescence micrographs were acquired with an Axon 4000B laser scanner at 5 µm resolution and intensity data were extracted with NimbleScan software (NimbleGen Systems). Keywords = liver tissue, poly(A)+ RNA
