Mitochondrial Ferritin Expression and Clonality of Hematopoiesis in Patients with Refractory Anemia with Ringed Sideroblasts.
Authors: Della Porta Matteo G. , Malcovati Luca , Galli Anna , Boggi Sabrina , Travaglino Erica , Marseglia Carmela , Maffioli Margherita , Levi Sonia , Arosio Paolo , Invernizzi Rosangela , Lazzarino Mario , Cazzola Mario 
Published: April 2020

Abstract
Sideroblastic anemias are a heterogeneous group of disorders that have in common the presence of erythroblasts with iron-loaded mitochondria defined as ringed sideroblasts. We have previously demonstrated that mitochondrial iron deposition in these disorders is in the form of mitochondrial ferritin (MtF), suggesting that this latter may be a specific marker of sideroblastic anemia (Cazzola et al, Blood2003;101:1996–2000). The most common type of acquired sideroblastic anemia is the myelodysplastic syndrome (MDS) defined as refractory anemia with ringed sideroblasts, which is generally associated with a relatively benign clinical course. In the present work, we studied the relationship between MtF expression and clonality of hematopoiesis in 55 consecutive female patients with low-risk MDS, including 20 cases with ringed sideroblasts and 35 cases without ringed sideroblasts. The expression of MtF, as well as that of cytosolic ferritin (H and L subunits) and of transferrin receptor (CD71), was evaluated by flow cytometry in bone marrow erythroid cells; in selected cases, these immunophenotypic investigations were also performed on liquid cultures of purified CD34-positive cells. X-chromosome inactivation patterns (XCIPs) were assessed in peripheral blood granulocytes and in bone marrow CD34-positive cells by analysis of both DNA methylation at the HUMARA and PGK loci and of IDS gene expression. Within informative females, 11 out of 12 patients with ringed sideroblasts displayed clonal XCIPs in granulocytes; by contrast, only 9 out of 22 patients without ringed sideroblasts displayed clonal XCIPs. Purified CD34-positive cells showed clonal XCIPs in 6 out of 7 patients with ringed sideroblasts but had polyclonal XCIPs in 4 out of 5 individuals without ringed sideroblasts. Flow cytometry evaluation of bone marrow erythroid cells showed that MtF expression was restricted to MDS patients with ringed sideroblasts. A close positive relationship was found between MtF and CD71 expression (r=.49, P=.001); this association may reflect the cytosolic iron deprivation induced by MtF overexpression. Analysis of cultured erythroid progenitor cells showed that MtF was detectable at a very early stage of differentiation from CD34-positive cells. Addition of erythropoietin to the culture system sustained the appearance of a polyclonal erythroid population in 2 out of 4 patients with ringed sideroblasts and clonal CD34-positive cells. These observations suggest that refractory anemia with ringed sideroblasts is a truly clonal stem cell disorder, while more than 50% of patients with refractory anemia without ringed sideroblasts have evidence of polyclonal hematopoiesis. The clonal pattern of CD34-positive cells and the early appearance of MtF during erythroid differentiation suggest that - despite be benign natural history of refractory anemia with ringed sideroblasts - the initial pathogenetic event in this condition occurs in multipotent stem cells. Although the mechanisms responsible for overexpression of MtF are still unclear, flow cytometry evaluation of this protein is a useful diagnostic tool that also provides helpful prognostic information.