CRISPR-based precise methylation of specific FUT8 promoter regions allows isolation of CHO cells with a fine-tuned glycoprofile

A major advantage of producing therapeutic proteins in mammalian cells is their ability to tailor proteins with human-like posttranslational modifications such as glycosylation, which ultimately defines aspects like stability, protein folding or immunogenicity. However, producing therapeutic proteins with a consistent and reproducible glycoprofile remains a major challenge for the biopharmaceutical industry, especially with biosimilar production. While the enzymes responsible for glycosylation of proteins have been the subject of various cell engineering approaches, tuning their gene expression to precise levels is still difficult to achieve. While CRISPR/Cas9 enabled the genetic engineering of cells to drastically overexpress or remove a target gene, CRISPR/dCas9-based epigenetic editing by targeted DNA methylation promises to stably change the expression pattern of target genes after transient transfection of the CRISPR-tool. Application of targeted DNA methylation so far has mostly been used to completely silence gene expression by fully methylating the corresponding promoter regions. Here, we aim to tune expression of the associated gene by DNA methylation of confined promoter regions and to apply this technique as a new glycoengineering approach. By coupling CRISPR-based targeted DNA methylation with lectin-FACS assisted sorting we obtained CHO cell lines with a fine-tuned phenotype. First, dCas9-DNMT3A3L in combination with one single gRNA is targeted to the FUT8 promoter to induce confined DNA methylation, resulting in a phenotypically diversified population. Next, a window sorting strategy based on lectin-stained cells using five different sorting gates spanning from low to high FUT8 expression was applied to isolate single clones with a defined phenotype. Isolated clones were phenotypically assessed and re-sorted to obtain a homogenous expression profile. The resulting clonal cell lines showed either tuned or knock-down phenotypes with varying gene expression levels. Two out of seven clones that showed tuned FUT8 gene expression were phenotypically stable over 60 days. Gene expression levels, on the other hand, showed a steady decline over time that in part, however, can be explained by the general variation of FUT8 expression in different growth phases. Importantly, glycan analysis of recombinant EpoFc produced in the tuned clonal cell lines showed ranges of 35-70 % fucosylation, demonstrating that isolated clones can produce recombinant proteins with a distinct glycosylation profile. To understand why some clones showed tuned FUT8 gene expression levels while others were completely knocked-down, we analyzed the DNA methylation status of their respective FUT8 promoter. Critical areas within the FUT8 promoter were identified, with some associated with general repression and others with the tuning of FUT8 gene expression when affected by DNA methylation. Additionally, a combination of histone marks associated with active and repressed promoters was found to potentially define clones with a fine-tuned expression. Combined, the data demonstrates that using targeted DNA methylation in a manner confined to specific promoter regions opens new engineering strategies to fine-tune gene expression in mammalian cells.