Transcriptional interference and transcription through regulatory elements (transcriptional read-through) are implicated in gene silencing and the establishment of DNA methylation. Transcriptional read-through is needed to seed DNA methylation at imprinted genes in the germ line and can lead to aberrant gene silencing by DNA methylation in human disease. To enable the study of parameters and factors influencing transcriptional interference and transcriptional read-through at human promoters, we established a somatic cell culture system. At two promoters of imprinted genes (UBE3A and SNRPN) and two promoters shown to be silenced by aberrant transcriptional read-through in human disease (MSH2 and HBA2) we tested, if transcriptional read-through is sufficient for gene repression and the acquisition of DNA methylation. Induction of transcriptional read-through from the doxycycline-inducible CMV promoter resulted in consistent repression of all downstream promoters, independent of promoter type and orientation. Repression was dependent on ongoing transcription, since withdrawal of induction resulted in reactivation. DNA methylation was not acquired at any of the promoters. Overexpression of DNMT3A and DNMT3L, factors needed for DNA methylation establishment in oocytes, was still not sufficient for the induction of DNA methylation. This indicates that induction of DNA methylation has more complex requirements than transcriptional read-through and the presence of de novo DNA methyltransferases.

Targeted genome editing in blood and immune cells enable new therapeutic applications, especially for infectious diseases. We present a GMP-compliant protocol to manufacture CCR5-edited CD34+ hematopoietic stem and precursor cells (HSPCs) with the goal to cure patients suffering from chronic infection with human immunodeficiency virus type 1 (HIV1). We hypothesize that genetic disruption of the CCR5 gene, which encodes the major HIV1 co-receptor, in HSPCs will give rise to an HIV-resistant immune system after transplantation. We have developed engineered nucleases based on transcription activator-like effector nucleases (TALENs) targeting CCR5. Electroporation of CD4+ T-cells and CD34+ HSPCs with mRNAs encoding TALENs revealed disruption of up to 80% of CCR5 alleles in CD4+ T-cells and over 90% of alleles in HSPCs. The high gene editing frequencies in T-cells and HSPCs were confirmed by deep sequencing, and no cleavage activity above background levels were detected at the top 20 predicted off-target sites. CCR5-edited CD4+ cells preserved their proliferation capacity and their biological function. Importantly, these cells showed significantly reduced CCR5 expression and became resistant to infection with the R5-tropic HIV-1JR-FL virus. The CCR5-edited HSPCs maintained their proliferation potential and their capacity to differentiate into the various blood lineages in vitro and in vivo, and clonal analysis revealed bi-allelic CCR5 disruption in more than 75% of cells. In summary, our developed protocol enables highly efficient and GMP-compliant knockout of the CCR5 locus in clinically relevant cells, so forming the foundation for a planned phase I/II clinical study. Disclosures Gautron: Cellectis SA: Employment. Busser:Cellectis: Employment, Patents & Royalties: Cellectis. Smith:Cellectis. Inc: Employment, Patents & Royalties. Duchateau:Cellectis: Employment, Patents & Royalties: Cellectis. Cathomen:TRACR Hematology: Consultancy; Cellectis: Research Funding; Miltenyi Biotec: Research Funding. Cornu:Cellectis: Research Funding; Miltenyi Biotec: Research Funding.