ULK-mediated autophagy of α-globin in ß-thalassemia

PROJECT SUMMARY Our long-term goal is to better define the proteostasis pathways that coordinate erythropoiesis, a specialized process distinguished by massive hemoglobin synthesis and the elimination of most other proteins. This application investigates how protein quality control modulates ß-thalassemia, a common hemoglobinopathy caused by HBB gene mutations that impair the production of the ß-globin subunit of adult hemoglobin (HbA, α2ß2). Consequently, free α-globin forms cytotoxic precipitates that cause maturation arrest and apoptosis of erythroid precursors (ineffective erythropoiesis) and hemolysis, leading to anemia, bone deformities and iron overload. Current therapies, including red blood cell transfusion, iron chelation and hematopoietic stem cell transplantation for selected patients, are effective but not uniformly available, particularly in low/middle income countries where the disease is most prevalent. Hence, new therapies are needed. Our preliminary data demonstrate that the Unc-51–like autophagy activating kinase 1 (ULK1) mediates the autophagy of free α-globin in ß-thalassemia. In general, ULK1 is inhibited by the mammalian target of rapamycin complex 1 (mTORC1) kinase and stimulated by AMP-activated protein kinase (AMPK). Administration of the mTORC1 inhibitor rapamycin to HbbTh3/+ mice, a validated preclinical model for ß-thalassemia, stimulated the autophagy of free α- globin to reduce ineffective erythropoiesis and hemolysis in an ULK1-dependent fashion. Our data support the central hypothesis that mTORC1 inhibition or AMPK activation can alleviate the pathophysiology of ß- thalassemia by stimulating ULK1-mediated autophagic clearance of free α-globin. We will test this by: Aim 1, optimizing the pharmacological inhibition of mTORC1 for ULK1 activation in HbbTh3/+ mice and by defining the regulatory circuitry of α-globin autophagy in mouse and human ß-thalassemic erythroblasts; Aim 2, determining whether elemental iron, a known activator of mTORC1, suppresses ULK1-mediated clearance of α-globin in ß- thalassemic erythroblasts and whether this deleterious effect can be prevented by iron restriction; and Aim 3, elucidating the genetic interactions between ß-thalassemia and miR-451, an abundantly expressed erythroid microRNA that we showed to inhibit the LKB1 kinase and its substrate AMPK. In support of Aim 3, disruption of the bi-cistronic miR-144/451 locus in HbbTh3/+ mice caused a reduction in α-globin precipitates and ß-thalassemia pathologies. Overall, our studies promise to elucidate the biology of proteostasis networks that maintain balanced hemoglobin synthesis through targeted protein degradation and validate mTORC1, AMPK and ULK1 as “druggable” targets for novel ß-thalassemia therapies.