SLC35A1 Knockout Cell Line (HEK293)

Recombinant adeno-associated viruses (AAV) have emerged as an important tool for gene therapy for human diseases. A prerequisite for clinical approval is an in vitro potency assay that can measure the transduction efficiency of each virus lot produced. The AAV serotypes are typical for gene therapy bind to different cell surface structures. The binding of AAV9 on the surface is mediated by terminal galactose residues present in the asparagine-linked carbohydrates in glycoproteins. However, such terminal galactose residues are rare in cultured cells. They are masked by sialic acid residues, which is an obstacle for the infection of many cell lines with AAV9 and the respective potency assays. The sialic acid residues can be removed by enzymatic digestion or chemical treatment. Still, such treatments are not practical for AAV9 potency assays since they may be difficult to standardize. In this study, we generated human cell lines (HEK293T and HeLa) that become permissive for AAV9 transduction after a knockout of the CMP-sialic acid transporter SLC35A1. Using the human aspartylglucosaminidase () gene, we show that these cell lines can be used as a model system for establishing potency assays for AAV9-based gene therapy approaches for human diseases.

We conducted a genome-wide CRISPR/Cas9 screen in suspension 293 F cells transduced with rAAV5. The highly selected genes revealed after two rounds of screening included the previously reported , , and , along with a cluster of genes involved in glycan biogenesis, Golgi apparatus localization, and endoplasmic reticulum penetration. In this report, we focused on solute carrier family 35 member A1 (), a Golgi apparatus-localized cytidine 5'-monophosphate-sialic acid (CMP-SIA) transporter. We confirmed that knockout (KO) significantly decreased rAAV5 transduction to a level lower than that observed in or KO cells. Although KO drastically reduced the expression of α2,6-linked SIA on the cell surface, the expression of α2,3-linked SIA, as well as the cell binding and internalization of rAAV5, was only moderately affected. Moreover, KO significantly diminished the transduction of AAV multi-serotypes, including rAAV2 and rAAV3, which do not utilize SIAs for primary attachment. Notably, the SLC35A1 KO markedly increased transduction of rAAV9 and rAAV11, which primarily attach to cells via binding to galactose. Further analyses revealed that KO significantly decreased vector nuclear import. More importantly, although the C-terminal cytoplasmic tail deletion (∆C Tail) mutant of SLC35A1 did not drastically decrease SIA expression, it significantly decreased rAAV transduction, as well as vector nuclear import, suggesting that the C-tail is critical in these processes. Furthermore, the T128A mutant significantly decreased SIA expression but still supported rAAV transduction and nuclear import. These findings highlight the involvement of the CMP-SIA transporter in the intracellular trafficking of rAAV vectors post-internalization.IMPORTANCErAAV is an essential tool for gene delivery in the treatment of genetic disorders; however, the mechanisms of rAAV transduction remain partially understood. GPR108 is vital for the transduction of most rAAV vectors, but not for rAAV5. We aimed to identify host factors that impact AAV5 transduction akin to GPR108. Using a genome-wide CRISPR/Cas9 screen in 293 F cells, we identified SLC35A1, a Golgi apparatus-localized CMP-sialic acid transporter that transports CMP-sialic acid from the cytoplasm into the Golgi apparatus for sialylation, is essential to rAAV transduction. Further studies across various AAV serotypes showed SLC35A1 significantly affects vector nuclear import post-internalization. These results underscore the crucial role of SLC35A1 in intracellular trafficking beyond the initial cell attachment of rAAV.

Cancer antigen 125 (CA125) is a serum marker associated with ovarian cancer. Despite its widespread use, CA125 levels can also be elevated in benign conditions. Recent reports suggest that detecting serum CA125 that carries the Tn antigen, a truncated O-glycan containing only N-acetylgalactosamine on serine or threonine residues, can improve the specificity of ovarian cancer diagnosis. In this study, we engineered cells to express CA125 with a Tn antigen. To achieve this, we knocked out C1GALT1 and SLC35A1, genes encoding Core1 synthase and a transporter for cytidine-5'-monophospho-sialic acid respectively, in human embryonic kidney 293 (HEK293) cells. In ClGALT1-SLC35A1-knockout (KO) cells, the expression of the Tn antigen showed a significant increase, whereas the expression of the T antigen (galactose-β1,3-N-acetylgalactosamine on serine or threonine residues) was decreased. Due to the inefficient secretion of soluble CA125, we employed a glycosylphosphatidylinositol (GPI) anchoring system. This allowed for the expression of GPI-anchored CA125 on the cell surface of ClGALT1-SLC35A1-KO cells. Cells expressing high levels of GPI-anchored CA125 were then enriched through cell sorting. By knocking out the PGAP2 gene, the GPI-anchored form of CA125 was converted to a secretory form. Through the engineering of O-glycans and the use of a GPI-anchoring system, we successfully produced CA125 with Tn antigen modification.