ABSTRACT: Mononuclear cells were isolated from umbilical cord blood (UCB) using Lymphoprep sucrose-gradient centrifugation (1.077 g/ml, Nycomed Pharma, Oslo, Norway). Immunomagnetic cell separation, using magnetic beads coated with CD34 antibodies (Miltenyi Biotec, Gladbach, Germany), was performed to isolate CD34-positive hematopoietic progenitor cells (CB-CD34+ cells). To generate retrovirus, bicistronic retroviral DNA constructs were used expressing the ETV6-RUNX1 or AML1-ETO fusion gene and enhanced Green Fluorescent Protein (eGFP). As a control, a construct expressing only eGFP was used. Both vectors consisted of a pMSCV promoter region, an internal ribosomal entry site and an ampicillin resistance cassette. HEK293T cells were co-transfected with these constructs and second-generation retroviral packaging vectors using XtremeGENE 9 tranfection reagents (Roche, Basel, Switzerland). Viral particles were collected in IMDM 48 hours after transfection. CD34+ haematopoietic progenitors, pre-cultured overnight as described above, upon which cells were divided in two fractions. One fraction was transduced with ETV6-RUNX1-IRES-eGFP or AML1-ETO-IRES-eGFP, while the other fraction was transduced with control EV-IRES-eGFP. Transductions were performed with fresh retrovirus in retronectin (Takara, Otsu, Japan) coated wells. After sorting, DAPI- CD34+ GFP+ CB-CD34+ cells were lysed and RNA was extracted using Nucleospin RNA XS extraction columns according to manufacturer’s protocol (Macherey-Nagel, Düren, Germany). Quality of RNA was determined by on-chip-electrophoresis using a RNA Pico Chip according to manufacturer’s protocol (Agilent Technologies, Santa Clara, CA, USA). RNA Integrity scores (RIN) were higher than 8 for all samples. RNA was subsequently linearly amplified using the Nugen WT-Amplification™ pico system (Nugen, San Carlos, CA, USA). This system is based on RNA-dependent DNA polymerase activity and was previously reported to be most suitable for amplification and gene expression of picograms of input RNA.
