Why CRISPR cell editing order changes the fate of living factories

A slow leak in the medicine vial

Medicines made of fragile proteins need a shield to stop them clumping on pharmacy shelves. Drug makers usually add polysorbate, a gentle soap-like molecule, to protect them. The trouble begins when CHO cells, tiny hamster cells used to brew these treatments, release an unwanted enzyme called a hydrolase that eats the protective coating away. To stop this breakdown, researchers set out to disable nine hydrolase genes. They compared two ways to carry out CRISPR cell editing: snipping the genes one after another in a sequential workflow, or tackling several together in a multiplex approach.

One big overhaul beats seven moves

Think of it like fixing nine broken pipes in an old house. Knocking out genes one by one was like hiring seven separate builders over a year. Each visit meant packing up the house and breaking walls. It dragged on for 383 days and required seven rounds of single-cell cloning, where one surviving cell grows into a fresh population. In contrast, the multiplex plan handled five targets at once near the end. It wrapped up in 250 days across just four cloning rounds. Both paths guarded polysorbate well, raising its retention from around 30% to 45% in original cells to over 75% in the final edited lines.

The hidden toll of living in the lab

Both routes disabled the exact same targets, yet the cells ended up in very different shape. The stepwise cells suffered badly. Their peak viable cell density, which measures the concentration of living cells in the broth, dropped from 25 million cells per millilitre down to between 15 and 17 million. Multiplex cells held up better, reaching 17 to 21 million. Stepwise cells also saw their survival at harvest fall from over 90% down to 50% or 60%, with one line sinking to 21%. Most multiplex clones stayed between 55% and 80%.

Peeking inside using RNA sequencing revealed an even sharper contrast. Stepwise editing altered the activity of 840 genes. The multiplex route changed only 271, and the two final cell types shared surprisingly little in common. The total number of electric shocks used to deliver the CRISPR tools did not explain this harm. Multiplex cells endured eleven shocks compared to seven in the stepwise line, yet they still showed milder physical and genetic damage. Long months spent recovering during repeated single-cell cloning seemed to drive the decline instead.

What we still do not know

These findings come from tests on suspension-adapted CHO-K1 cell lines across five sequential and seven multiplex clones during 14-day bioreactor runs. Multiplexing, however, brought its own quirk: the final clones showed wider variation from one to another. Furthermore, some lines, like clone M-KO4, developed an unexplained surge in cell width and DNA content, and the cause remains unknown. Researchers also do not know if these perks will hold true when targeting other gene groups or different host cell lines.