The Innovative Multi-Marker Selection System Based on Tyrosine Synthesis Pathway for Monoclonal Antibody Expression in CHO Cells.

The production of complex biologics in Chinese hamster ovary (CHO) cells is constrained by the lack of selection systems capable of coordinating multiple transgenes. Conventional single-marker systems have low saturable thresholds that limit enrichment efficiency, while multi-auxotrophic platforms often impose metabolic burdens. Here, we present a rationally designed tyrosine-auxotrophic system that overcomes these limitations by establishing a high-threshold cooperative selection mechanism. This is achieved through the reconstruction of an essential pathway comprising pterin-4α carbinolamine dehydratase 1 (PCBD1), phenylalanine hydroxylase (PAH), and quinoid dihydropteridine reductase (QDPR). We generated a triple-knockout CHO host via CRISPR/Cas9, wherein survival under tyrosine deprivation became strictly dependent on the balanced co-expression of all three rescue genes. This architecture creates a selection pressure that is not saturable by any single gene, enabling efficient co-enrichment. Applied to monoclonal antibody (mAb) production, the system enriched triple-positive populations to 97.49%, resulting in significantly enhanced homogeneity and coordinated upregulation of antibody chain expression. Optimized pools achieved titers of 0.35 g/L in fed-batch and 1.60 g/L in perfusion cultures without tyrosine feeding. Consequently, pathway reconstitution rewired central metabolism, reducing byproducts and enhancing biosynthesis. This antibiotic-free multi-marker platform establishes a new paradigm for stringent multigene co-expression, advancing CHO cell engineering for next-generation biologics.