Biomolecular and histological features in pediatric essential thrombocythemia: adequacy of who diagnostic criteria

Myeloproliferative neoplasms (MPN), Essential Thrombocythemia (ET), Polycythemia Vera (PV) and primary myelofibrosis (PMF), are clonal disorders of the hematopoietic stem cell. In the recent years, somatic mutations of JAK2, CALR and MPL genes have been found in these diseases. However, these mutations do not allow the discrimination between the different diseases, being present in more than one form. World Health Organization (WHO) criteria, in fact, request in addition to the molecular study, a complete bone marrow histological evaluation. Pediatric ET is a rare disorder with an incidence about 60 times lower than the adult form albeit with similar blood finds. In children with ET, the incidence of JAK2V617F and CALR mutations is significantly lower than in adults, while hereditary cases characterized by MPL mutations are relatively more common. So far the histological evaluation of the bone marrow has not been exhaustively explored in this setting of patients. The aim of the present study was to evaluate the adequacy of current WHO criteria in pediatric ET exploring both the incidence of driver mutations and bone marrow features in a large cohort of children with a clinical diagnosis of ET. Our study was built in two steps: (i) a biomolecular study of a pediatric population with a clinical diagnosis of ET performed by Italian pediatricians, expert in hematological disorders and (ii) a histological evaluation strictly adherent to WHO criteria of BM features of a sub group of these children. Firstly, biomolecular studies of 89 children with a clinical diagnosis of ET, all having a sustained increase in platelet count (>450 x10^9/L) with no demonstrable reactive or secondary cause and no familial history of MPN or thrombocytosis, were evaluated to our central laboratory. In the second phase of our work we collected naive bone marrow (BM) biopsies of 20 children with a clinical diagnosis of ET (PedET) and, as controls, BM of 6 children (PedST) with reactive/secondary thrombocytosis, 18 children (Norm) with a normal BM histology and 36 adults (AdsET) with WHO-diagnosed ET. All BM biopsies were reviewed by two MPN’s expert pathologists, blinded to the cause of each child’s thrombocytosis. In the biomolecular study we found that 23 patients (25,8%) had a clonal disease. The JAK2V617F mutation was identified in 14 children, 1 child had the MPLW515L mutation, and 6 had CALR mutations. The HUMARA monoclonal X-chromosome inactivation pattern was demonstrate in 6 patients (two with JAK2V617F and two with CALR mutations). The other 66 patients (74,2%) had persistent thrombocytosis with no clonality. There were no clinical or hematological differences between the clonal and non-clonal patients. From the histological point of view, while cellularity was increased in all pediatric cases compared to adults (p<0.001), megakaryocytes (MK) density was higher in PedET (37.5 MK/mm2) than in PedST (9.2 MK/mm2) (p<0.001). Moreover, MK clusters (100%) and BM fibrosis (30%) were observed only in PedET but not in PedST and in Norm. The BM histology was similar in PedET and AdsET. On a whole, BM histology confirmed the diagnosis of ET in 15 children, suggested a PV in 1 child, a PMF in 3 (1 grade 1 and 2 grade 0) and secondary thrombocytosis in one. Our study shows that children with ET are mostly non-clonal, however, the relative proportion of ET-specific mutations in the clonal children was much the same as in adults. Histological WHO criteria are able to identify ET, PMF and PV and distinguish ST from primary thrombocytosis, also in pediatric population. Therefore, WHO criteria seem suitable in all age groups, making both complete biomolecular evaluation and BM assessment mandatory in children with suspected ET.