C1galt1c1, also known as COSMC, encodes a molecular chaperone in the endoplasmic reticulum and is a master regulator of O-glycosylation of mammalian glycoproteins. It is essential for the activity of T-synthase (C1GALT1), an enzyme that synthesizes the T-antigen, a crucial O-glycan core structure and precursor for extended O-glycans [1,2]. This process is involved in multiple biological pathways related to protein glycosylation, which is vital for normal cellular functions.

Mutations in C1galt1c1 have been linked to various disorders. For instance, a hemizygous variant c.59C>A (p.Ala20Asp; A20D-Cosmc) in C1galt1c1 was found in two male patients with a novel chaperonopathy, leading to impaired protein O-glycosylation, developmental delay, immunodeficiency, short stature, thrombocytopenia, and acute kidney injury resembling atypical hemolytic uremic syndrome [1]. A female patient with a de novo mosaic variant in C1galt1c1 (c.202C>T, p.Arg68*) presented with non-immune hydrops fetalis, and analysis showed reduced Cosmc and T-synthase proteins, lower T-synthase activity, and abnormal O-glycosylation [2]. In addition, in IgA nephropathy, lipopolysaccharide stimulation can inhibit C1galt1c1 expression, causing IgA1 aberrant O-glycosylation, and 5-azacytidine can reverse this by upregulating C1galt1c1 expression [3].

In conclusion, C1galt1c1 is of great significance in the regulation of O-glycosylation. Research on C1galt1c1 mutations in patients has revealed its role in multiple disease conditions, such as chaperonopathies, atypical hemolytic uremic syndrome-like conditions, and IgA nephropathy. Understanding C1galt1c1 helps in clarifying the mechanisms of protein O-glycosylation and the pathogenesis of related diseases.