CLONALITY ANALYSIS OF HEMATOPOIESIS AND THROMBOPOIETIN LEVELS IN PATIENTS WITH ESSENTIAL THROMBOCYTHEMIA

Essential thrombocythemia (ET) is a myeloproliferative disorder, chara cterized by sustained thrombocytosis. Diagnosis requires the eliminati on of all known causes of thrombocytosis. ET is believed to be a clona l disorder, and we investigated the frequency of a clonal hematopoiesi s in this disease with the aim of using this as a positive diagnostic criterion. However, a non-random inactivation pattern can be encounter ed in normal females which mimics clonal hematopoiesis. In addition, t he percentage of normal females with skewed lyonization seems higher u sing techniques based on the difference in DNA methylation, compared t o G6PD enzyme polymorphism. Recently, new techniques based on transcri pt analysis have been developed. We report here the results of clonali ty studies of hematopoiesis in 53 ET patients using two different tech niques based on DNA and RNA polymorphisms, and T-lymphocytes as a cont rol tissue of lyonization. The majority of ET patients showed monoclon al hematopoiesis in the presence of polyclonality of T-lymphocytes. Be cause all ET patients did not show the same clonal pattern of hematopo iesis, we searched for inappropriate secretion of thrombopoietin (TPO) in patients with polyclonal disease. This assay was performed in 48 p atients, of whom 9 showed polyclonal hematopoiesis and 27 monoclonal h ematopoiesis. We found no difference in TPO levels between ET patients and normal controls, nor between patients with polyclonal hematopoies is and those with monoclonal hematopoiesis. Our results confirm the hi gh frequency of monoclonal hematopoiesis in ET, the usefulness of RNA markers, and the possibility of using T-lymphocytes as a control tissu e for X-chromosome inactivation patterns. On the other hand, TPO level s are not decreased even in ET patients with high platelet counts, sug gesting an increased production or decreased clearance of TPO in this disease.