THE ROLE OF VENTX HOMEOBOX GENE DURING HUMAN HAEMATOPOIETIC DEVELOPMENT

Abstract

The Ventx genes are non-clustered homeobox transcription factors that confer a ventral phenotype on mesodermal cells in the developing embryo. Ventx genes are conserved in vertebrates but have been lost in rodents. In the human haematopoietic system, VENTX promotes myeloid differentiation and is expressed in some acute myeloid leukaemias (AML). Using a DOX (Doxycycline) inducible VENTX expression system, we found that VENTX overexpression in hPSC impaired mesoderm formation, but VENTX enforced expression after mesoderm commitment resulted in the emergence of an increased percentage of immature blood cells that co-expressed CD90 and CD34. These cells displayed high clonogenic capacity in methylcellulose, but only after DOX was removed from the media. This suggested that VENTX expression held cells in a non-proliferative state. Transcriptional profiling revealed increased expression of HOXA genes in haematopoietic cells following VENTX induction, consistent with their immature phenotype. Conversely, genes involved in myeloid differentiation were downregulated during VENTX overexpression, as were genes involved in proliferation, such as MYC and MYB. ATAC-sequencing demonstrated that VENTX closes selected chromatin loci, in particular areas targeted by HOXB13 and CDX transcription factors. We hypothesise that VENTX might act as a transcriptional suppressor during haematopoietic differentiation and we are currently investigating the genomic targets of VENTX, combining ATAC-seq and ChIP-seq with the transcriptional profiling. In summary, VENTX overexpression generated immature blood cells in a quiescent state, preventing their proliferation and differentiation into myeloid lineages.