Primary Trombocythemia in Children and Adolescents Includes Different Subtypes Compared to Adult Essential Thrombocythemia

The recent discovery of various mutations of the CALR gene that are mutually exclusive with JAK2 and MPL mutations has allowed a correct diagnosis in about 90% of adult cases of essential thrombocythemia (ET). Moreover, the mutation status of JAK2 and CARL defines subtypes of ET in adults with a substantially different clinical course and outcome. Based on our experience, we suggested that primary thrombocythemia (PT) in children is characterized by subtypes that differ from those found in adult ET. The present study was carried out in children and adolescents with PT in order to (a) characterize the various subtypes of the disease and (b) analyze their clinical and biologic features, treatment approach and outcome. PT patients aged <20 years (yrs) at diagnosis (dx) were evaluated for mutations of JAK2, thrombopoietin (TPO) and its receptor (MPL) and CALR genes, and for clonal hematopoiesis (females). The presence of MPLS505A (confirmed on DNA from buccal swabs) defined a hereditary thrombocytosis (HT). ET was diagnosed according to WHO 2008 criteria. For wild type patients, an additional inclusion criteria was a follow-up >24 months. Among 58 PT patients (males: 23; females: 35; median age at dx: 14.4 yrs), 21 (36%) had HT due to MPLS505A, 14 were JAK2V617F-mutated (24%), 9 (16%) harbored CALR mutations and 14 (24%) were wild type for JAK2, CALR and MPL (Fig 1). JAK2- and CALR-mutated were older than those with wild type ET or with HT (median age, 17.6 and 16.1 vs 10.4 and 13.7 yrs, p .028). As to the hematologic findings, HT patients showed both hematocrit values (median, 36.3%) and leukocytes counts (median, 9.53 x109/L) significantly lower than ET patients, whatever the subtypes (median, 41.2% and 11.2 x109/L, p .006 and p .029, respectively). No differences were found with regard to platelets both between HT and ET and among the different ET subtypes. JAK2-mutated patients exhibited more frequently symptoms (69%) compared to CALR-mutated (22%), wild-type ET (14%) and HT (14%) patients (p. 0057). Splenomegaly at diagnosis was recorded more frequently in JAK2-mutated than in CALR-mutated or wild type-ET or HT (50%, 33% 21% and 14% , respectively, p .122). Antiplatelet agents, mostly acetylsalicylic acid (ASA), were started less frequently in HT than in ET patients, irrespective of the subtypes (57% vs 81%, p .05). The use of ASA progressively decreased over the time; at the last follow-up, 2 patients with HT, 2 CALR-mutated and 1 JAK2-mutated patients were still receiving ASA, while no wild type ET patient was on treatment. Cytoreductive agents, hydroxyurea and/or interferon and/or anagrelide, were used in a minority of HT patients (19%) in comparison with ET patients (65%), p .001, mainly with those wild-type (78%, p <.001). At the last observation, one HT patient was still receiving cytoreductive agents compared to 30% of ET patients whatever the subtypes (p .024). After a median follow-up of 196 months (similar in the different subtypes), all patients are alive. On the whole, 5 thrombotic events were recorded in 3 patients with HT and in 2 ET patients (1 JAK2-mutated and 1 JAK2 and CALR wild-type), without any significant thrombophilic abnormalities during treatment with ASA and/or cytoreductive agents. A progressive splenomegaly was recorded in 9 (15%) patients (2 HT, 4 JAK2-mutated, 3 CALR-mutated) and it was combined with grade ≥2 medullar fibrosis in 2/4 JAK2-mutated and in 2/3 CALR-mutated patients. None of the JAK2 and CALR wild-type patients had spleen enlargement or reticulin fibrosis (p .022). Two untreated patients (1 HT and JAK2 and CALR wild-type) developed malignancies. On the whole, these data emphasize that in young patients with PT, hereditary forms can be frequently observed. Thrombotic events, recorded mainly in HT patients despite treatment with ASA, were probably due to a MRP4 protein overexpression that was found in our MPLS505A HT. Moreover, our observations highlight that, in contrast to adult ET, more than one third of young ET patients have no JAK2 or CALR mutations.