DISCORDANT EFFECTS OF MONOFERRIC FORMS OF TRANSFERRIN ON
PARAMETERS OF ERYTHROPOIESIS IN A MURINE MODEL OF B-THALASSEMIA
INTERMEDIA
Amaliris Guerra1, Sayantani Sinha1, Nermi L. Parrow2, Paige McVeigh1, Robert E. Fleming2, Yelena
Ginzburg3 and Stefano Rivella1
(
1)Children's Hospital of Philadelphia, Philadelphia
(2)Saint Louis University School of Medicine, St. Louis
(3)Icahn School of Medicine at Mount Sinai, New York
β-thalassemia (BT) is caused by mutations in the β-globin gene or its promoter and results in
absent or compromised β-globin synthesis. The decreased production of β-globin leads to
toxic aggregations of α-globin in erythrocyte precursors and results in shortened survival of
circulating red blood cells (RBCs). One of the hallmarks of BT is ineffective erythropoiesis (IE)
(Ginzburg et al. Blood). IE is defined by the cycle of increasing erythroid precursors in the bone
marrow without a commensurate increase in mature RBCs in circulation. As a consequence,
erythropoietin (EPO) production is increased and triggers further expansion of the
erythropoietic niches (Rivella Blood). Expansion of erythroblast progenitors results in
increased erythroferrone (ERFE) expression and consequently down-regulates the iron
absorption regulatory hormone hepcidin. The suppression of hepcidin in IE results in iron
overload. BT patients primarily succumb to iron toxicity. Therapeutic approaches aiming to
modulate iron homeostasis has proven to be successful at improving IE in preclinical studies
in BT mouse models (Oikonomidou et al. Ann N Y Acad Sci.). In particular, studies show that
administration of transferrin in BT mice reverses the anemic phenotype (Li et al. Nat.Med.).
Transferrin (Tf), a bilobed 80kD glycoprotein with homologous N- and C-terminal binding
domains, binds two iron atoms and interacts with transferrin receptor-1 and 2 (TFR1 and
TFR2) to regulate the erythropoiesis-hepcidin-iron axis. We are interested in understanding
whether and how the two Tf lobes function distinctly from one another in iron metabolism
and their potentially distinct roles in coordinating iron supply with erythropoietic demand. To
these aims, we generated two monoferric Tf knock-in mutants unable to bind iron on the N-terminal
(Tfn-bl/-nbl) or C-terminal (Tfc-bl/c-bl) lobes. Our published results demonstrate
that Tfn-bl/-nbl and Tfc-bl/c-bl mice exhibit differences in EPO sensitivity, pAKT activation,
liver iron loading, and hepcidin levels despite equivalently normal serum iron and mild anemia
(Parrow et al. Blood). Here, we investigate whether iron restriction and modulation of EPO
sensitivity by the mutant Tf lobes modifies IE in a mouse model of BT. To test this hypothesis,
we crossed a BT mouse model (Hbbth3/+) with Tfn-bl/-nbl or Tfc-bl/c-bl animals. Five-month-old
Hbbth3/+ Tfc-bl/c-bl mice were assessed for complete blood counts. Serum was collected
and hepcidin, EPO and ERFE levels were measured. Animals were sacrificed and IE evaluated
by flow cytometry. Our results demonstrate increased RBC and hemoglobin levels, decreased
splenomegaly, reduced bone marrow erythroblasts, and improvement of IE in Hbbth3/+ Tfc-bl/
c-bl mice. In contrast, preliminary hematological data and morphological analysis of RBC
from Hbbth3/+ Tfn-bl/-nbl mice suggest these mice are phenotypically similar
to Hbbth3/+Tf+/+ controls. Our data provides additional support for the specialized roles of
the two monoferric Tf forms. These findings also raise an interesting possibility that Tf-iron
occupancy plays a regulatory role in erythropoiesis beyond its iron delivery function, possibly
by modulating EPO responsiveness. Ongoing experiments to test this model will be discussed.
POSTER 4