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Introduction
The 2008 World Health Organization (WHO) Classification of Tumors defines refractory anemia with ring sideroblasts associated with marked thrombocytosis (RARS-T) as a provisional entity, with unclassifiable myelodysplastic/myeloproliferative neoplasm (MDS/MPN) status, as opposed to a confirmed entity (1). Although thrombocytosis is a poor prognostic factor in MDS patients (2), overall, RARS-T patients exhibit a more favorable prognosis. The JAK2 V617F mutation has been identified in ~50% of RARS-T patients and in only 2/89 cases of typical MDS, indicating that RARS-T should be considered as a JAK2 mutation-associated chronic MPN (3).
RARS-T is characterized by MDS characteristics and <5% blasts in the bone marrow, and is differentiated from other diseases by the presence of ≥15% ringed sideroblasts, thrombocytosis and a platelet count of >450×109/l (1). Clinical manifestations of RARS-T include symptoms associated with anemia, leucopenia and abnormalities of platelet function and quantity, for example, fatigue, infection, bleeding and/or thrombosis. The International Prognostic Scoring System (IPSS) (4), which is commonly used for MDS, is also applicable as a prognostic tool for RARS-T. The management of RARS-T is largely supportive, including transfusion support in patients exhibiting symptomatic anemia and prophylaxis, and treatment of thromboembolism. Similar to MDS, RARS-T patients exhibiting anemia and low serum erythropoietin levels may benefit from the administration of erythropoiesis-stimulating agents. An ongoing clinical trial is currently studying the efficacy of ruxolitinib, an oral JAK2 inhibitor, in JAK2 mutation-positive RARS-T patients (ClinicalTrials.gov Identifier: NCT01895842; http://clinicaltrials.gov/show/NCT01895842). Furthermore, a case study has recently documented the successful treatment of young RARS-T patients with lenalidomide (5).
MPL (6), a cellular homologue of...