In silico expression-vector design for CHO cells

What an expression vector contains

A mammalian expression vector for CHO is a modular construct. The core elements are: a promoter and enhancer to drive transcription, the gene of interest (your coding sequence), a polyadenylation (polyA) signal to terminate and stabilize the transcript, a selection marker to isolate cells that took up the construct, and a bacterial origin plus antibiotic resistance for cloning the plasmid in E. coli. For secreted proteins, an N-terminal signal peptide is added.

Step 1: Choose the promoter and regulatory elements

For strong constitutive expression in CHO, the common promoters are CMV (cytomegalovirus) and EF-1α. CMV gives very high expression but can be silenced over time in stable lines; EF-1α is often more stable long term. Adding an intron in the 5′ UTR, or a UCOE (ubiquitous chromatin opening element), can raise and stabilize expression. Match the promoter to whether you need transient expression or a stable production line.

Step 2: Codon-optimize the gene for CHO

CHO cells have their own codon usage bias, so the coding sequence should be codon-optimized for Cricetulus griseus. Good codon optimization does several things at once:

• Replace rare codons with CHO-preferred synonymous codons to improve translation
• Target a balanced GC content and avoid long GC-rich or AT-rich stretches
• Remove cryptic splice sites, premature polyadenylation signals, and strong RNA secondary structure
• Remove internal restriction sites that would interfere with the cloning strategy
• Keep the amino acid sequence identical; only the DNA changes

Step 3: Add the signal peptide (for secreted proteins)

Most therapeutic proteins are secreted, which requires an N-terminal signal peptide that directs the protein to the secretory pathway and is then cleaved. The native signal peptide does not always work best in CHO, so heterologous signal peptides (for example from IL-2, albumin, or an immunoglobulin) are often screened in silico and in the lab to maximize secretion. Verify the predicted cleavage site so the mature protein has the correct N-terminus.

Step 4: Choose the selection and amplification system

Stable CHO lines use a selection system that can also amplify the gene for higher yield. The two dominant systems are:

• Glutamine synthetase (GS): the vector carries GS, and selection uses glutamine-free medium plus MSX (methionine sulfoximine), typically in a GS-null CHO host
• DHFR (dihydrofolate reductase): selection in nucleoside-free medium with MTX (methotrexate) amplification, in a DHFR-deficient CHO host

Match the vector’s selection marker to the CHO host cell line you will use.

Step 5: QC the construct in silico

Before ordering synthesis, verify the assembled sequence computationally:

• Confirm the full open reading frame is intact and in frame, with a correct start and stop
• Check for unintended internal restriction sites used in the cloning strategy
• Scan for cryptic splice sites and premature polyA signals introduced by the sequence
• Check for direct repeats or hairpins that complicate synthesis or cause instability
• Confirm the signal peptide cleavage site and the resulting mature protein sequence

Common pitfalls

• Using CMV for a stable line and losing expression to promoter silencing
• Skipping codon optimization, or optimizing for the wrong organism
• A mismatched selection marker and host (for example a GS vector in a non-GS-null host)
• A cryptic splice site inside the coding sequence that truncates the product
• A signal peptide that predicts poorly and lowers secretion

Learn more

Codon optimization and construct design are core in silico design tasks. For the broader picture of running this work autonomously, see What is an AI in silico designer for molecular biology?.