Cas-CLOVER: A Smarter Genome Editing Tool Transforming CHO Cell Engineering

 

Chinese Hamster Ovary (CHO) cells are the backbone of modern biopharmaceutical manufacturing, producing life-saving monoclonal antibodies such as trastuzumab, pembrolizumab, and adalimumab. Researchers are searching for genome editing technologies that can more precisely produce more stable and productive CHO cell lines as the demand for complex biologics keeps growing. Although CRISPR-Cas9 has revolutionised gene editing, the search for safer alternatives has been driven by concerns about chromosomal rearrangements and off-target mutations. Cas-CLOVER, a highly specific genome editing system intended for the development of industrial cell lines, is one of the most promising next-generation tools.

Cas-CLOVER functions differently from traditional CRISPR-Cas9, which cuts DNA using an active Cas9 nuclease and a single guide RNA (gRNA). It combines the restriction enzyme Clo051 nuclease, which can only cut DNA when two nuclease molecules combine, with catalytically inactive Cas9 (dCas9). Accordingly, two guide RNAs that bind to opposing sides of the target DNA are needed for Cas-CLOVER. The nuclease dimerises and produces a double-strand break only when both guides are positioned correctly. Cas-CLOVER is 5–25 times more specific than CRISPR-Cas9 thanks to this dual-recognition mechanism, which also minimises undesired chromosomal rearrangements. Furthermore, the system produces staggered DNA ends, which are frequently better suited for accurate genome engineering.

The development of GS knockout (GS-KO) CHO cell lines is one of the most significant uses of Cas-CLOVER. The glutamine synthetase (GS) enzyme enables cells to synthesise glutamine, an amino acid essential for survival. In the widely used GS selection system, cells are cultured in glutamine-free medium, allowing only those that successfully integrate a recombinant GS gene along with the therapeutic protein gene to survive. This facilitates the development of cell lines and enriches high-producing cell clones.

Standard CHO cells, on the other hand, have endogenous GS genes that reduce selection efficiency by enabling some cells to survive even in the absence of successful gene integration. In order to transform cells entirely dependent on the introduced GS gene, researchers knock out these endogenous GS genes. Although the major GS5 gene was the focus of earlier research, residual activity from a second, weaker GS1 gene may still lessen the stringency of selection. In this work, GS5 and GS1 were sequentially knocked out using Cas-CLOVER, resulting in a double GS knockout (GS-DKO) platform called CleanCut GS.

Cas-CLOVER successfully distinguished between the highly similar GS5 and GS1 genes—a task that is particularly difficult with conventional CRISPR systems—achieved editing efficiencies exceeding 74–90%, and revealed no detectable off-target mutations among the predicted sites examined. More significantly, compared to single GS knockout cells, the CleanCut GS platform generated significantly higher monoclonal antibody yields. Cas-CLOVER's success goes beyond GS knockout. It is a desirable platform for upcoming CHO cell engineering due to its high specificity, low off-target activity, and compatibility with sophisticated transposon-based gene integration systems. Researchers hope to use Cas-CLOVER for targeted gene integration, multiplex gene editing, glycoengineering to improve antibody function, and removal of undesirable host cell proteins. Genome editing tools must advance to provide safer, quicker, and more dependable cell line development as biologics become more complex.

REFERENCES 

Limia, C.G., Steffey, V., Cheng, H.C., Machado, D., Hart, T., McHargue, M.C., Brizzee, C. and Crawford, J., 2026. Sequential, chromosome‐specific glutamine synthetase double knockout with Cas‐CLOVER establishes enhanced CHO platforms for cell line development. Biotechnology Progress, 42(2), p.e70113.

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