Characterization and mutagenesis of Chinese hamster ovary cells endogenous retroviruses to inactivate viral particle release.

The Chinese hamster ovary (CHO) cells used to produce biopharmaceutical proteins are known to contain type-C endogenous retrovirus (ERV) sequences in their genome and to release retroviral-like particles. Although evidence for their infectivity is missing, this has raised safety concerns. As the genomic origin of these particles remained unclear, we characterized type-C ERV elements at the genome, transcriptome, and viral particle RNA levels. We identified 173 type-C ERV sequences clustering into three functionally conserved groups. Transcripts from one type-C ERV group were full-length, with intact open reading frames, and cognate viral genome RNA was loaded into retroviral-like particles, suggesting that this ERV group may produce functional viruses. CRISPR-Cas9 genome editing was used to disrupt the gag gene of the expressed type-C ERV group. Comparison of CRISPR-derived mutations at the DNA and RNA level led to the identification of a single ERV as the main source of the release of RNA-loaded viral particles. Clones bearing a Gag loss-of-function mutation in this ERV showed a reduction of RNA-containing viral particle release down to detection limits, without compromising cell growth or therapeutic protein production. Overall, our study provides a strategy to mitigate potential viral particle contaminations resulting from ERVs during biopharmaceutical manufacturing.

A role for alternative end-joining factors in homologous recombination and genome editing in Chinese hamster ovary cells.

CRISPR technologies greatly foster genome editing in mammalian cells through site-directed DNA double strand breaks (DSBs). However, precise editing outcomes, as mediated by homologous recombination (HR) repair, are typically infrequent and outnumbered by undesired genome alterations. By using knockdown and overexpression studies in Chinese hamster ovary (CHO) cells as well as characterizing repaired DNA junctions, we found that efficient HR-mediated genome editing depends on alternative end-joining (alt-EJ) DNA repair activities, a family of incompletely characterized DNA repair pathways traditionally considered to oppose HR. This dependency was influenced by the CRISPR nuclease type and the DSB-to-mutation distance, but not by the DNA sequence surrounding the DSBs or reporter cell line. We also identified elevated Mre11 and Pari, and low Rad51 expression levels as the most rate-limiting factors for HR in CHO cells. Counteracting these three bottlenecks improved precise genome editing by up to 75%. Altogether, our study provides novel insights into the complex interplay of alt-EJ and HR repair pathways, highlighting their relevance for developing improved genome editing strategies.

MAR-Mediated transgene integration into permissive chromatin and increased expression by recombination pathway engineering.

Untargeted plasmid integration into mammalian cell genomes remains a poorly understood and inefficient process. The formation of plasmid concatemers and their genomic integration has been ascribed either to non-homologous end-joining (NHEJ) or homologous recombination (HR) DNA repair pathways. However, a direct involvement of these pathways has remained unclear. Here, we show that the silencing of many HR factors enhanced plasmid concatemer formation and stable expression of the gene of interest in Chinese hamster ovary (CHO) cells, while the inhibition of NHEJ had no effect. However, genomic integration was decreased by the silencing of specific HR components, such as Rad51, and DNA synthesis-dependent microhomology-mediated end-joining (SD-MMEJ) activities. Genome-wide analysis of the integration loci and junction sequences validated the prevalent use of the SD-MMEJ pathway for transgene integration close to cellular genes, an effect shared with matrix attachment region (MAR) DNA elements that stimulate plasmid integration and expression. Overall, we conclude that SD-MMEJ is the main mechanism driving the illegitimate genomic integration of foreign DNA in CHO cells, and we provide a recombination engineering approach that increases transgene integration and recombinant protein expression in these cells. Biotechnol. Bioeng. 2017;114: 384-396. © 2016 The Authors. Biotechnology and Bioengineering published by Wiley Periodicals, Inc.

A role for homologous recombination proteins in cell cycle regulation.

Eukaryotic cells respond to DNA breaks, especially double-stranded breaks (DSBs), by activating the DNA damage response (DDR), which encompasses DNA repair and cell cycle checkpoint signaling. The DNA damage signal is transmitted to the checkpoint machinery by a network of specialized DNA damage-recognizing and signal-transducing molecules. However, recent evidence suggests that DNA repair proteins themselves may also directly contribute to the checkpoint control. Here, we investigated the role of homologous recombination (HR) proteins in normal cell cycle regulation in the absence of exogenous DNA damage. For this purpose, we used Chinese Hamster Ovary (CHO) cells expressing the Fluorescent ubiquitination-based cell cycle indicators (Fucci). Systematic siRNA-mediated knockdown of HR genes in these cells demonstrated that the lack of several of these factors alters cell cycle distribution, albeit differentially. The knock-down of MDC1, Rad51 and Brca1 caused the cells to arrest in the G2 phase, suggesting that they may be required for the G2/M transition. In contrast, inhibition of the other HR factors, including several Rad51 paralogs and Rad50, led to the arrest in the G1/G0 phase. Moreover, reduced expression of Rad51B, Rad51C, CtIP and Rad50 induced entry into a quiescent G0-like phase. In conclusion, the lack of many HR factors may lead to cell cycle checkpoint activation, even in the absence of exogenous DNA damage, indicating that these proteins may play an essential role both in DNA repair and checkpoint signaling.