Different transcriptional responses by the CRISPRa system in distinct types of heterochromatin in Drosophila melanogaster

Abstract

Transcription factors (TFs) activate gene expression by binding to elements close to promoters or enhancers. Some TFs can bind to heterochromatic regions to initiate gene activation, suggesting that if a TF is able to bind to any type of heterochromatin, it can activate transcription. To investigate this possibility, we used the CRISPRa system based on dCas9-VPR as an artificial TF in Drosophila. dCas9-VPR was targeted to the TAHRE subtelomeric element, an example of constitutive heterochromatin, and to promoters and enhancers of the HOX Ultrabithorax (Ubx) and Sex Combs Reduced (Scr) genes in the context of facultative heterochromatin. dCas9-VPR robustly activated TAHRE transcription, showing that although this element is heterochromatic, dCas9-VPR was sufficient to activate its expression. In the case of HOX gene promoters, although these genes are epigenetically silenced by Polycomb complexes, both were ectopically activated. When the artificial TF was directed to enhancers, we found that the expression pattern was different compared to the effect on the promoters. In the case of the Scr upstream enhancer, dCas9-VPR activated the gene ectopically but with less expressivity; however, ectopic activation also occurred in different cells. In the case of the bxI enhancer located in the third intron of Ubx, the presence of dCas9-VPR is capable of increasing transcription initiation while simultaneously blocking transcription elongation, generating a lack of functional phenotype. Our results show that transcription can be activated in any type of heterochromatin by CRISPRa; nevertheless, its effect on transcription is subject to the intrinsic characteristics of each gene or regulatory element. SignificanceWhether transcription only depends on activating factors binding to chromatin, even though it is found in a silent state as heterochromatin, remains an open question. In this work, we addressed this question using the CRISPRa system via dCas9-VPR as a synthetic transcriptional activator in Drosophila. This activator was directed to a constitutive heterochromatin element and to promoters and enhancers of two HOX genes, which in the tissues where they are not expressed, are present as facultative heterochromatin. In all cases, the CRISPRa system was able to activate transcription, showing that its sole presence is sufficient for this to occur. Although transcription in constitutive heterochromatin was very robust, in the case of promoters and enhancers of HOX genes, the degree of expressivity, penetrance and ectopic effect was different between promoters and enhancers. These results indicate that the presence of a synthetic activator can activate transcription by binding to transcriptional regulatory elements; however, its effect depends on the particular characteristics of each one. These results show how artificial transcription factors can be used to understand transcription regulation at the organismal level.