Granulocyte JAK2 (V617F) Mutation Status in Myeloid Neoplasms with Ringed Sideroblasts.

The most common type of acquired sideroblastic anemia is the myelodysplastic syndrome (MDS) defined as refractory anemia with ringed sideroblasts (RARS). We have previously demonstrated that mitochondrial iron accumulation in this condition is in the form of mitochondrial ferritin (MtF). A gain-of-function mutation of JAK2 is found in most patients with chronic myeloproliferative disorders. A high frequency of this mutation has been also reported in RARS associated with marked thrombocytosis (RARS-T), a provisional entity characterized by marked increase in platelet count, hypercellular marrow with increased megakaryocytes, and ringed sideroblasts. In this study, we investigated the granulocyte JAK2 (V617F) mutation status in 73 patients receiving a diagnosis of myeloid malignancy with ringed sideroblasts at the Division of Hematology, University of Pavia Medical School & IRCCS Policlinico San Matteo Pavia, Italy between 2001 and 2006. According to the WHO classification of the myeloid neoplasms, 23 patients had RARS, 17 had refractory cytopenia with multilineage dysplasia and ringed sideroblasts (RCMD-RS), 16 had refractory anemia with blasts excess, four had MDS with isolated del(5q), and 13 fulfilled the criteria for RARS-T. JAK2 (V617F) mutation status was analyzed on peripheral blood granulocytes through a quantitative real time PCR-based allelic discrimination assay. We compared clinical and biological features of patients with RARS-T with those of patients with refractory anemia or cytopenia with ringed sideroblasts, and found that RARS-T patients had higher neutrophil and platelet counts, lower frequency of cytogenetic abnormalities and higher incidence of the JAK2 (V617F) mutation (P values ranging from <.001 to .02). JAK2 (V617F) was detected in six out of 63 evaluable cases (9.5%), one being diagnosed as MDS with isolated del(5q) and five as RARS-T, resulting in an incidence of mutation in the latter group of 45%. The proportion of mutant alleles ranged between 2.8% and 18.4%, values commonly observed by us in essential thrombocythemia [Blood. 2006 May 1;107(9):3676–82]. Focusing the analysis on RARS-T, a significantly higher hemoglobin level at diagnosis was found in mutated (median value 11.2 g/dL, range 10.1–15.4) compared with non mutated patients (median value 9 g/dL, range 6–9.9) (P=.009). JAK2-positive patients also showed a significantly lower percentage of ringed sideroblasts in the bone marrow (P=.01), and an increased marrow reticulin fibrosis (P=.03). We then evaluated the clonality of hematopoiesis in female patients through analysis of X-chromosome inactivation patterns (XCIPs) in circulating granulocytes and bone marrow CD34-positive cells. Twenty-one out of 23 informative female patients with ringed sideroblasts (91%), as well as 5/6 RARS-T (83%) displayed a clonal pattern of X-chromosome inactivation. These observations suggest that refractory anemia with ringed sideroblasts with marked thrombocytosis is a clonal stem cell disorder significantly associated with the JAK2 (V617F) mutation. This disorder shows both dysplastic and proliferative features, the presence of the mutation being associated with a predominant myeloproliferative phenotype. The recognition of a heterogeneous genetic background in myeloid neoplasms with ringed sideroblasts suggests that different mechanisms might be involved in the induction of mitochondrial ferritin expression in these disorders.