We are working toward a new perspective in understanding and manipulating the pathophysiology of ? thalassemia, a common and debilitating inherited anemia. A hallmark of this disorder is excessive free ? hemoglobin (Hb), an unstable protein that generates reactive oxygen species (ROS) and forms cytotoxic precipitates. We identified alpha hemoglobin stabilizing protein (AHSP), an abundant erythroid protein that enhances the solubility of free ?Hb and limits its biochemical reactivity. Ahsp-/- mice exhibit hemolytic anemia with Hb precipitates and excessive ROS. Moreover, loss of AHSP exacerbates ? thalassemia in mice, raising the possibility that altered AHSP function or expression could modulate ? thalassemia phenotypes in humans. Preliminary data support both mechanisms. First, we discovered a naturally occurring missense mutation, AHSP N75I, which impairs protein function and is associated with unexpectedly severe p thalassemia in two pedigrees. Second, AHSP appears to be a quantitative trait locus (QTL) whose expression varies considerably between different individuals. Moreover, reduced AHSP expression associates with more severe clinical disease in several independent studies of small p thalassemia cohorts and pedigrees. Together, these findings lead to the hypothesis that AHSP is a genetic modifier of ? thalassemia. We will test this by analyzing thalassemic populations for AHSP gene mutations, including N75I, and determining their effects on gene expression and/or protein function. In addition, we will study how variations in erythroid AHSP expression affect nascent ?Hb pools, oxidative stress and clinical severity in p thalassemic patients. Our findings should provide new insights into the mechanisms of normal erythropoiesis and the pathophysiology of ? thalassemia. Ultimately, this information could provide a basis for developing novel therapeutic approaches to mitigate the toxicities of free ?Hb in ? thalassemia.