S102: OBLIGATE N-TERMINAL BUT NOT C-TERMINAL MONOFERRIC TRANSFERRIN AMELIORATES ANEMIA IN β-THALASSEMIC MICE
Authors: A. Guerra, N.L. Parrow, P. McVeigh, R.E. Fleming, Y.Z. Ginzburg, S. Rivella
Journal: HemaSphere (2022)

Transferrin (TF) is a bilobed 80kD glycoprotein with N- and C-lobe iron binding sites. TF circulates as four forms: unbound to iron (apo-TF), iron bound to the N-lobe (monoferric N-TF), the C-lobe (monoferric C-TF), or to both lobes (diferric-TF). Most circulating TF under physiological conditions is monoferric. The iron-bound TF forms interact with TF receptor-1 (TFR1), which is ubiquitously expressed and serves as the main mechanism for cellular iron delivery. Iron-bound TF also interacts with TF receptor-2 (TFR2) which is expressed on hepatocytes, erythroblasts, and bone cells. Whereas TFR1 serves primarily as a cargo receptor, TFR2 serves primarily to influence cellular signaling events regulating hepcidin expression, erythropoiesis, and bone formation. We proposed that different transferrin forms provide differential signaling properties in this regulation. We thus generated TF mutant mice in which all iron-containing TF was either monoferric N (TfmonoN) or monoferric C (TfmonoC). Compared with TfmonoC mice, the TfmonoN mice demonstrated increased RBC production and increased hepcidin expression relative to iron status (Parrow et al. Blood). Based on observations in β-thalassemic mice treated with exogenous TF (Li et al. Nat Med), we hypothesized that β-thalassemic mice obligate for monoN TF would demonstrate improved erythropoietic and iron parameters compared with β-thalassemic mice obligate for monoC TF. To address this hypothesis, we crossed Hbbth3/+ mice, a mouse model of β-thalassemia intermedia (BT), with TfmonoN and TfmonoC mice. Compared with Hbbth3/+Tf+/+mice, in Hbbth3/+TfmonoN mice demonstrated significantly increased RBC counts, elevated hemoglobin, improved erythrocyte morphology (Figure 1A-B), decreased splenomegaly, fewer bone marrow erythroblasts, and improvement of ineffective erythropoiesis (as measured by the ratio of progenitors to RBC in the bone marrow). Additionally, serum erythroferrone (ERFE) was significantly reduced and hepcidin levels were increased in Hbbth3/+TfmonoN relative to Hbbth3/+Tf+/+ controls. Conversely, hematological parameters from Hbbth3/+TfmonoC mice were comparable to Hbbth3/+Tf+/+ mice. Similarly, Hbbth3/+TfmonoC mice had no improvements in markers of ineffective erythropoiesis in the bone marrow compared with Hbbth3/+Tf+/+ mice. In summary, we demonstrate that the differential regulatory effects of monoN and monoC TF on erythropoiesis are relevant not only in steady-state, but also in the ineffective erythropoiesis that is characteristic of β-thalassemia. Because both monoN and monoC TF forms can deliver only one iron atom per TF-TFR1 binding event, our findings suggest that the improvements observed only in the Hbbth3/+TfmonoN mice were not due to iron restriction alone. We are now elucidating the mechanisms by which the two TF lobes exert their differential effects on ineffective erythropoiesis and exploring the translational potential of obligate monoN TF in the treatment of β-thalassemia. References 1. Li et al, Nat Med 2010; 16:177 2. Parrow et al, Blood 2019; 134:1373