Unravelling the regulatory function of a short 3'UTR sequence in zebrafish and in human cells

The mechanisms of gene expression regulation are essential for cell identity and function, and their disruption usually leads to human disease. The 3 untranslated region (UTR) of mRNA contains important regulatory elements of gene expression, including upstream sequence elements (USEs) that are cis-regulatory sequences localized upstream of polyA signals (PAS). One of the best functionally characterised USEs is located in the 3UTR of the Drosophilas polo gene, which disruption leads to critical phenotypes in adult flies. In this work we found that the USE of the Drosophilas polo gene (DplUSE) is also found in in the 3UTR of vertebrate genes, including zebrafish, mouse and human genes, showing higher levels of conservation than the whole 3UTR sequence. Using reporter assays, we show that DplUSE is able to increase gene expression in vitro in human cell lines and in vivo in zebrafish embryos. Importantly, in humans, the DplUSE containing genes are enriched for genes associated to serious diseases such as Congenital abnormalities and Malignant neoplasms, illustrating the potential of this sequence to modulate genes with relevant biological functions and related with human health. Concomitantly, when sequestering the molecular machinery that operates at the DplUSE using a dominant negative strategy, we show that this is enough to dysregulate DplUSE containing genes in human cells and disrupt proper embryo development in zebrafish. Aiming to understand the molecular mechanism operating at the DplUSE, we identified three RNA binding proteins (RBP) that specifically bind to the DplUSE in vertebrates. Importantly, one of such RBPs is PTBP1, the vertebrate orthologue of the fruit flys RBP Heph, that was demonstrated to be required for the DplUSE function in Drosophila. To test if PTBP1is essential for the DplUSE function, as observed in Drosophila, we depleted PTBP1 from human cells and observed a downregulation of the expression of DplUSE containing genes, demonstrating that the molecular mechanisms that operate at DplUSE are ultra-conserved. Finally, we explored if variants in DplUSE consensus could be associated to human disease. We found a reported single nucleotide polymorphism (SNP; rs3087967) that is associated with malignant tumor of colon and generates an ectopic consensus of DplUSE in the 3 UTR of the tumorigenic POU2AF2/C11orf53 gene. We further show that this ectopic DplUSE motif causes a gain-of-function in vivo in zebrafish gut cells, suggesting its involvement in colon cancer development. These results show that a short motif present in the 3UTR of genes from phylogenetically distant bilaterians, from fruit flies to humans, control genes expression through an ultra-conserved mechanism involving RBPs binding and its dysregulation might impact in human disease.