Physiological regulation of transgene expression by a lentiviral vector containing the A2UCOE linked to a myeloid...

Protection against epigenetic silencing is a desirable feature of future gene therapy vectors, in particular for those applications in which transgene expression will not confer growth advantage to gene-transduced cells. The ubiquitous chromatin opening element (UCOE) consisting of the methylation-free CpG island encompassing the dual divergently transcribed promoters of the human HNRPA2B1-CBX3 housekeeping genes (A2UCOE) has been shown to shield constitutive active heterologous promoters from epigenetic modifications and chromosomal position effects. However, it is unclear if this element can be used to improve expression from tissue-specific enhancer/promoters, while maintaining tissue specificity in hematopoietic cells. Here, we evaluated the potential of the A2UCOE in combination with the myeloid-specific myeloid related protein 8 (MRP8) promoter to target transgene expression specifically to myeloid cells in vitro and in vivo from a self-inactivating lentiviral vector. The inclusion of the A2UCOE did not interfere with specific upregulation of MRP8 promoter activity during myeloid differentiation and mediated sustained and vector copy-dependent expression in myeloid cells. Notably, the A2UCOE did not protect the MRP8 promoter from methylation in the P19 embryonal carcinoma cell line, suggesting that this element maintains the inherent epigenetic state and transcriptional activity of cellular promoters in their native configuration. Thus, the A2UCOE could represent a useful protective genetic element in gene therapy vectors, ensuring physiological transcriptional regulation of tissue-specific promoters independent of the chromosomal integration site.