JAK2V617F, CALR and MPL mutations and bone marrow histology in patients with essential thrombocythemia.

Mutations of JAK2V617F, CALR and MPL genes are detected in 50-60%, 20-25% and 3–5% of patients with essential thrombocythemia (ET), respectively, while about 5–10% of patients do not carry any of these somatic mutations, suggesting that different types of ET might be identified by morphology. We studied 90 patients with newly diagnosed ET to investigate the correlation between JAK2V617F, CALR and MPL mutations and clinical or bone marrow (BM) morphological features. All mutational analyses were performed on DNA from BM or peripheral blood samples at diagnosis. JAK2V617F allele burden was quantified using Real Time Quantitative Polymerase Chain Reaction (RT-Q-PCR); MPLW515L/K was assessed by Allelic Discrimination RT-Q-PCR; CALR exon 9 mutations were detected by PCR fragment analysis and Sanger sequencing. Marrow cellularity, fibrosis, number of total and dysmorphic megakaryocytes, and clusters of megakaryocytes were assessed on serial sections from Bouin’s fixed, paraffin embedded BM biopsies. Overall, mutational frequencies were 58.9% for JAK2V617F, 28.9% for CALR and 4.4% for MPL; 7.8% of cases were ‘triple-negative’. Patients with JAK2V617F mutation presented with the highest haemoglobin level (Mean 14.1 g/dL, SD+1.8) and lowest platelet count (Mean 687x109/L, SD+246); those with CALR mutation had an intermediate haemoglobin level (Mean 13.4 g/dL SD+2) and platelet count (Mean 871x109/L, SD+213); MPL mutated patients had the lowest haemoglobin level (Mean 12.8 g/dL SD+1) and the highest platelet count (Mean 993x109/L, SD+90) (p= 0.001). JAK2V617F mutated ET showed a high BM cellularity, a low number of clusters of megakaryocytes and a high number of dysmorphic megakaryocytes: this morphologic features in part overlap with those observed in prefibrotic/early primary myelofibrosis (prePMF); CALR mutated ET showed a low BM cellularity, many clusters of large megakaryocytes and only few dysmorphic megakaryocytes; MPL mutated ET showed a reduced BM cellularity, a high number of clustered and large megakaryocytes, very few dysmorphic megakaryocytes and fibrosis in all cases. In conclusion, distinct morphologic patterns of ET were associated with the different gene mutations in ET, supporting the distinction of ET in at least three subtypes.