Loss of a newly discovered microRNA in Chinese hamster ovary cells leads to upregulation of N-glycolylneuraminic acid sialylation on monoclonal antibodies

Chinese hamster ovary (CHO) cells are known not to express appreciable levels of the sialic acid residue N-glycolylneuraminic acid (NGNA) on monoclonal antibodies. However, we actually have identified a recombinant CHO cell line expressing an IgG with unusually high levels of NGNA sialylation (>30%). Comprehensive multi-OMICs based experimental analyses unraveled the root cause of this atypical sialylation: (1) expression of the cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) gene was spontaneously switched on, (2) CMAH mRNA showed an anti-correlated expression to the newly discovered Cricetulus griseus (cgr) specific microRNA cgr-miR-111 and exhibits two putative miR-111 binding sites, (3) miR-111 expression depends on the transcription of its host gene SDK1, and (4) a single point mutation within the promoter region of the sidekick cell adhesion molecule 1 (SDK1) gene generated a binding site for the transcriptional repressor histone H4 transcription factor HINF-P. The resulting transcriptional repression of SDK1 led to a downregulation of its co-expressed miR-111 and hence to a spontaneous upregulation of CMAH expression finally increasing NGNA protein sialylation.