JAK2V617F homozygosity arises commonly and recurrently in PV and ET, but PV is characterized by expansion of a dominant homozygous subclone

Abstract

Subclones homozygous for JAK2V617F are more common in polycythemia vera (PV) than essential thrombocythemia (ET), but their prevalence and significance remain unclear. The JAK2 mutation status of 6495 BFU-E, grown in low erythropoietin conditions, was determined in 77 patients with PV or ET. Homozygousmutant colonies were common in patients with JAK2V617F-positive PV and were surprisingly prevalent in JAK2V617F-positive ET and JAK2 exon 12-mutated PV. Using microsatellite PCR to map loss-of-heterozygosity breakpoints within individual colonies, we demonstrate that recurrent acquisition of JAK2V617F homozygosity occurs frequently in both PV and ET. PV was distinguished from ET by expansion of a dominant homozygous subclone, the selective advantage of which is likely to reflect additional genetic or epigenetic lesions. Our results suggest a model in which development of a dominant JAK2V617F-homzygous subclone drives erythrocytosis in many PV patients, with alternative mechanisms operating in those with small or undetectable homozygous-mutant clones. (Blood. 2012;120(13):2704-2707) IntroductionThe JAK2V617F mutation is found in ϳ 95% of patients with polycythemia vera (PV) and 60% of those with essential thrombocythemia (ET), 1-4 but the mechanisms responsible for the different disease phenotypes remain unclear. Several lines of circumstantial evidence suggest that increased signaling through mutant JAK2 is important: (1) A "homozygous" sequence pattern in granulocytes was identified in ϳ 30% of patients with PV but was rare in ET 1-4 ; (2) in PV patients, JAK2V617F allele burden correlates with higher hemoglobin levels and white cell counts but lower platelet counts 5,6 ; (3) the copy number of mutant JAK2 influences phenotype in mouse models 7,8 ; and (4) JAK2 exon 12 mutations are reported to signal more strongly than JAK2V617F and are associated with PV but not ET. 9 Homozygosity for JAK2V617F results from mitotic recombination, 1-4 and homozygous-mutant BFU-E were present in most patients with PV but not in those with ET. 10 This observation raised the possibilities that patients with PV are more prone to develop a homozygous subclone or that homozygous-mutant cells have a selective advantage in patients with PV but not in those with ET. In either case, it has been widely assumed that homozygousmutant cells in a given patient are usually members of a single clone with a selective advantage and that JAK2V617F homozygosity plays a causal role in PV pathogenesis. However, this model is complicated by several observations: some patients with PV have very small homozygous-mutant clones 10 ; 2 individual patients have been reported to harbor 2 distinct homozygous clones 11,12 ; a defect in STAT1 phosphorylation has been identified in PV patients 13 ; and reports of small numbers of ET patients who harbor homozygousmutant BFU-E 14-16 and of PV patients with none. 14, 15 We have therefore systematically assessed the prevalence and clonal relationship of homozygous-mutant BFU-E precursors in patients with JAK2-mutated...

Molecular determinants of pathogenesis and clinical phenotype in myeloproliferative neoplasms

“…109 Furthermore, even within the context of ET, greater JAK2V617F allele burdens are associated with a higher degree of erythrocytosis and leukocytosis. 110 However, some PV patients do not carry a homozygous JAK2V617F subclone, 111 and JAK2V617F-homozygous subclones have been detected in patients with ET 107,[110][111][112] (although these were generally small (<10%), in contrast to those in PV where they tended to represent the dominant MPN clone). Together, these findings demonstrate that JAK2V617F homozygosity is neither necessary nor sufficient for PV.…”

How I treat essential thrombocythemia

“…1,2 In the last few years, there have been significant advances in our understanding of the genetic basis, pathophysiology, and clinical course of ET. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Our article aims to offer up-to-date information and guidance regarding diagnosis and treatment of ET patients. To provide directions in the therapeutic management of common or complex clinical situations of the disease,…”

Clonal analyses reveal associations of JAK2V617F homozygosity with hematologic features, age and gender in polycythemia vera and essential thrombocythemia

“…Demographic and clinical features, together with methods used for colony culture and genotyping, have been previously described. 4 Patients met diagnostic criteria for JAK2V617F-positive PV or ET according to the British Committee for Standards in Haematology. 15,16 The study was approved by the Cambridge and Eastern Region Ethics Committee, patients gave written informed consent, and research was carried out in accordance with the Declaration of Helsinki.…”

“…Clonal analyses indicate that, in fact, small homozygousmutant clones arise frequently and recurrently in both PV and ET, suggesting that the mere presence of homozygosity is insufficient for a PV phenotype, and additional factors are required. 4 Studies of some, [9][10][11] but not other 12 knock-in mouse models have reported that heterozygosity for JAK2V617F is associated with marked erythrocytosis, with no further increase in hemoglobin levels in homozygous mice. 10 Associations of JAK2V617F allele burden with blood counts show inconsistencies between studies 13 and have not determined the extent to which allele burdens reflect the relative proportions of heterozygous and homozygous-mutant cells.…”

“…1 but the additional mechanisms which determine their distinct clinical phenotypes remain unclear. Circumstantial evidence suggests that JAK2V617F homozygosity may have a role in determining the PV phenotype: i) homozygous-mutant hematopoietic precursors are isolated from a higher proportion of patients and form larger clones in PV compared to ET; [2][3][4] ii) mouse models have demonstrated PV-like, rather than ET-like, phenotypes associated with higher JAK2V617F gene dosage; 5,6 iii) higher JAK2V617F allele burdens in granulocyte DNA have been associated with higher hemoglobin levels and lower platelet counts in PV. 7,8 However, several lines of evidence indicate that the role of JAK2V617F homozygosity in PV pathogenesis is not simple.…”