SEC23B Loss-of-Function Suppresses Hepcidin Expression by Impairing Glycosylation Pathway in Human Hepatic Cells

Biallelic pathogenic variants in the SEC23B gene cause congenital dyserythropoietic anemia type II (CDA II), a rare hereditary disorder hallmarked by ineffective erythropoiesis, hemolysis, erythroblast morphological abnormalities, and hypo-glycosylation of some red blood cell membrane proteins. Abnormalities in SEC23B, which encodes the homonymous cytoplasmic COPII (coat protein complex II) component, disturb the endoplasmic reticulum to Golgi trafficking and affect different glycosylation pathways. The most harmful complication of CDA II is the severe iron overload. Within our case series (28 CDA II patients), approximately 36% of them exhibit severe iron overload despite mild degree of anemia and slightly increased levels of ERFE (the only erythroid regulator of hepcidin suppression). Thus, we hypothesized a direct role of SEC23B loss-of-function in the pathomechanism of hepatic iron overload. We established a hepatic cell line, HuH7, stably silenced for SEC23B. In silenced cells, we observed significant alterations of the iron status, due to both the alteration in BMP/SMADs pathway effectors and a reduced capability to sense BMP6 stimulus. We demonstrated that the loss-of-function of SEC23B is responsible of the impairment in glycosylation of the membrane proteins involved in the activation of the BMP/SMADs pathway with subsequent hepcidin suppression. Most of these data were confirmed in another hepatic cell line, HepG2, stably silenced for SEC23B. Our findings suggested that the pathogenic mechanism of iron overload in CDA II is associated to both ineffective erythropoiesis and to a specific involvement of SEC23B pathogenic variants at hepatic level. Finally, we demonstrated the ability of SEC23B paralog, i.e., SEC23A, to rescue the hepcidin suppression, highlighting the functional overlap between the two SEC23 paralogs in human hepatic cells.