Erfe-Encoding FAM132B in Congenital Dyserythropoietic Anemia Type II

Recessive mutations in SEC23B gene cause congenital dyserythropoietic anemia type II (CDAII), a rare hereditary disorder hallmarked by ineffective erythropoiesis, iron overload, and reduced expression of hepatic hormone hepcidin (Iolascon, 2013). The most recently described hepcidin regulator is the erythroblast-derived hormone erythroferrone (ERFE), a member of TNF-α superfamily that specifically inhibits hepcidin production in experimental models (Kautz, 2014). However, the function of ERFE in humans remains to be investigated.

To determine whether dysregulation of ERFE expression is associated with ineffective erythropoiesis and iron-loading in CDAII, we studied the ERFE-encoding FAM132B gene expression in 48 SEC23B-related CDAII patients and 29 age and gender matched healthy controls (HCs). Twelve new cases and four novel SEC23B mutations were described. Samples were obtained after informed consent, according to the Declaration of Helsinki. Genomic DNA, mutational screening, RNA isolation, cDNA preparation, and qRT-PCR were performed as previously described (Russo, 2013).

All patients were young adults (17.0±2.5 years at diagnosis), with increased serum ferritin (395.4±67.6 ng/mL) and transferrin saturation (71.9±5.4 %). We observed a statistically significant overexpression of FAM132B gene in peripheral blood mononuclear cells from CDAII patients (9.09±0.08) compared to HCs (8.32±0.12, p<0.0001). A similar trend was obtained when evaluating FAM132B expression in reticulocytes from a subset of patients and HCs. Of note, a statistically significant correlation between peripheral blood and reticulocyte FAM132B expression from the same patients was observed (Spearman ρ= 0.78, p=0.02). Although the role of ERFE in peripheral blood is still unknown, our observations suggested that the evaluation of FAM132B mRNA in peripheral blood is a reliable and easy-to-measure marker of ERFE levels.

When we divided CDAII patients into two sub-groups accordingly to FAM132B gene expression, we observed a statistically significant reduction in hemoglobin (Hb) level in the high-FAM132B subset (8.6±0.4 g/dL) respect to low-FAM132B one (10.1±0.5 g/dL, p=0.02). Of note, the expression level of FAM132B did not correlate with the transfusion regimen. The higher amount of ERFE reflects the increased iron demand for Hb production as well as the expanding abnormal erythropoiesis, as attested by the increased RDW and sTfR (although not significant) in high-FAM132B patients. This in turn leads to reduced hepcidin in high-FAM132B group (4.2±1.8 nM) compared to low-FAM132B one (5.9±1.8 nM, p=0.05), resulting in augmented iron delivery to the erythron. Although the iron balance data do not differ significantly between the two groups, a tendency to decreased hepcidin/ferritin ratio and increased transferrin saturation was observed in high-FAM132B patients. Thus, FAM132B overexpression seems to contribute to the inappropriate suppression of hepcidin with subsequent hemosiderosis observed in CDAII.

Consistent with our previous studies, we observed a reduced SEC23B expression in our patients compared to HC. Indeed, FAM132B and SEC23B gene expression exhibited an inverse correlation (Spearman ρ=-0.36, p=0.01). We confirmed the ex vivo data about inverse correlation between FAM132B and SEC23B expression observed in our patients by establishing K562 SEC23B-silenced cells. To knockdown SEC23B gene expression in K562 cells two different pGIPZ Lentiviral shRNAmir for SEC23B (shSEC23B-70/-74) were used. We observed a higher expression of FAM132B at 5 days of erythroid differentiation in K562 SEC23B-silenced cell compared to not-silenced ones. Conversely, SEC23B expression was lower in both shSEC23B compared to sh-CTR at 2 and 5 days of differentiation. Although the mechanisms of hemin-induced differentiation are quite different from EPO-induced ones, we can hypothesize that FAM132B over-expression is related to the maturative arrest and the subsequent increased number of erythroid precursors.

This study provides the first analysis on ERFE regulation in humans. Our data suggest that ERFE over-expression in CDAII patients is the result of both physiological and pathological mechanisms leading to hepcidin suppression in condition of dyserythropoiesis. Nevertheless, it seems that ERFE cannot be the main erythroid regulator of hepcidin suppression, at least in CDAII patients.