Leukocyte adhesion deficiency II (LAD II, congenital disorder of glycosylation IIc, CDG-IIc) is a rare, autosomal recessive disorder that results in the absence of sialyl Lewis X (sLeX), a ligand essential for mediating the interactions between E- and P-selectins, on the surface of leukocytes. This deficiency impairs the initial tethering and rolling of neutrophils along the endothelial cell wall, ultimately contributing to an immunodeficient state. This condition is caused by a mutation in the SLC35C1 gene encoding the GDP-fucose transporter (SLC35C1, C1). The predominant clinical manifestations of LAD II encompass recurrent infections, dysmorphic features, diminished stature, and profound cognitive impairment (Wild et al., 2002). As of 2024, 19 cases of LAD II have been diagnosed. The hallmark characteristics observed in all patients included developmental delay and intellectual disability. It is crucial to underscore that despite mutations in the same gene, several symptoms exhibited notable variation among diagnosed individuals. For instance, leukocytosis was observed in 15 out of 19 cases, short stature in 17 out of 18 cases, and recurrent infections in 18 out of 19 individuals. Among the 12 patients with available blood group information, only 7 exhibited a Bombay blood type. It is a rare blood group type, characterized by the absence of α-1–2 fucose on the red cell membrane due to mutations in both the FUT1 and FUT2 genes, leading to inhibited expression of α-1,2-fucosyltransferases (Anso et al., 2023). Due to the lack of distinct and characteristic symptoms, genome sequencing of the suspected individual remains the primary diagnostic approach for identifying this condition. Treating individuals with LAD II poses a significant challenge due to the diversity of symptoms. So far, oral L-fucose supplementation seems to be the only treatment that has helped in some cases. Out of the 19 diagnosed individuals, seven received L-fucose treatment. Three of them showed improvement, especially in reducing leukocytosis (Hüllen et al., 2021; Etzioni et al., 1992; Marquardt et al., 1999a; Tahata et al., 2022; Hidalgo et al., 2003; Knapp et al., 2020; Etzioni et al., 2002; Helmus et al., 2006; Gazit et al., 2010).
In mammals, SLC35C1 is known as the main GDP-fucose transporter in the Golgi apparatus (Lübke et al., 2001, Lühn et al., 2001). Protein analysis shows that SLC35C1 has 10 transmembrane domains, and four of these (3, 4, 7, and 9) are highly conserved. Notably, domains 4 and 9 are characterized by high hydrophilicity (Lühn et al., 2001). When overexpressed in the HEK293T cell line, SLC35C1 protein exhibits Golgi localization, with both the N and C termini exposed to the cytosolic side. Additionally, studies have shown that the cytosolic C-terminus of SLC35C1 is essential for its transport activity, yet it does not influence the correct subcellular localization of the protein (Zhang et al., 2012).
This study aimed to investigate symptom variations in LAD II individuals by analyzing 11 mutations in the SLC35C1 gene using a human cell line. We generated HEK293T cell line variants expressing specific SLC35C1 mutations found in LAD II patients. Initially, we analyzed the subcellular localization of different transporter variants. By using our original method, we were able to calculate the percentage of core fucosylated N-glycans. Our research also marks the first characterization of O-glycans generated by modified SLC35C1 variants. Finally, we observed that all of the SLC35C1 variants positively responded to fucose treatment by increasing fucosylation.
