Molecular and Clinical Features of the Myeloproliferative Neoplasm Associated with JAK2 Exon 12 Mutations: a European Multicenter Study.

Abstract 3904

Poster Board III-840

While about 95% of patients with polycythemia vera carry the unique V617F mutation in JAK2 exon 14, several mutations in exon 12 have been described in the minority of JAK2 (V617F)-negative subjects. The initial study [N Engl J Med 2007 Feb 1;356(5):459-68] led to the conclusion that JAK2 exon 12 mutations define a distinctive myeloproliferative syndrome that affects patients who currently receive a diagnosis of polycythemia vera or idiopathic erythrocytosis. Very recent studies suggest that the 'GGCC' haplotype of JAK2 confers susceptibility to the somatic acquisition of both JAK2 (V617F) and exon 12 mutations [Nat Genet 2009 Apr;41(4):450-4, Leukemia 2009 May 14, Epub ahead of print]. Indeed, we reported pedigrees with familial polycythemia vera in which there were both JAK2 (V617F)-positive and JAK2 exon 12 mutation-positive siblings [Blood 2008 Feb 1;111(3):1686-9]. The myeloproliferative neoplasm associated with JAK2 exon 12 mutations is a rare disorder, and only small groups of patients have been reported so far by various investigators. We therefore started a collaborative study in Europe with the aim of collecting about 100 patients with this condition in order to define the molecular and clinical features of this myeloproliferative neoplasm. An ad hoc database was developed for data collection and management. As of August 1, 2009, 77 patients with the required clinical and hematologic data at diagnosis have been recruited (median follow-up 3.2 years, range 0-27 years), while complete follow-up information was available for 57 of these patients. Various approaches were employed for the detection of JAK2 exon 12 mutations, including genomic DNA sequencing, allele-specific PCR assays, and high resolution melting. Overall, 16 different exon 12 mutations were identified. The most frequent mutation were N542-E543del (26 patients), K539L (12 patients), R541-E543delinsK (6 patients), and F537-K539delinsL (6 patients); the remaining mutations occurred less frequently. With respect to the clinical phenotype at presentation, the Kruskal-Wallis test did not reveal any significant difference between the above most frequent mutations. Median age at diagnosis was 53 years (range 15-92), and the male/female ratio was 43/34. Mean hemoglobin level was 19.3 ± 2.2 g/dL, mean WBC count 8.5 ± 3.2 × 10⁹/L, and mean PLT count 334 ± 197 × 10⁹/L. Overall, 48 out of 77 (62%) patients presented with isolated erythrocytosis, 12 (16%) with erythrocytosis and leukocytosis (WBC count > 10 × 10⁹/L), 8 (10%) with erythrocytosis and thrombocytosis (PLT count > 400 × 10⁹/L), and 8 (10%) displayed a full myeloproliferative pattern (erythrocytosis, leukocytosis and thrombocytosis). Serum erythropoietin level was below the lower normal limit in 46 out of 58 (79%) patients. Twenty-one of 25 (84%) patients had endogenous erythroid colonies. During follow-up, two patients had deep venous thrombosis, two progressed to post-polycythemia vera myelofibrosis (diagnosed according to the IWG-MRT criteria) and two developed a myelodysplastic syndrome. In conclusion, the available data indicate that the myeloproliferative neoplasm associated with JAK2 exon 12 mutations is mainly associated with isolated erythrocytosis at clinical onset, but also suggest that the subsequent clinical course may be similar to that of JAK2 (V617F)-positive polycythemia vera, at least in a portion of patients.