The JAK2V617F mutation, present in >90% of pts with polycythemia vera (PV) and in approximately 50% of pts with primary myelofibrosis (PMF) and essential thrombocythemia (ET), has been identified in CD34+ cells from pts with PV and PMF, consistent with the notion that this mutation arises in an early hematopoietic progenitor cell (HPC). The relationship between the recurrent chromosomal abnormalities, occurring occurring in 8% of ET, 29% of PV and ~50% of PMF pts, to the JAK2V617F mutation is uncertain. In order to determine a relationship between the JAK2V617F allele burden and the frequency of HPC with chromosomal abnormalities, we identified 5 pts (3 PV, 1 PMF and 1 ET) who were JAK2V617F positive and had different cytogenetic abnormalities. At the time of the study Pt 1 (PMF) had +9, del(20)(q11q13) in 50% of cells and del(20)(q11q13),+der(9)t(1;9)(q21;q13) in 50% cells. Pt 2 (PV) had 85% of cells with +9. Pt 3 (PV) had 15% of cells with del(20)(q11q13). Pt 4 (ET) had 36% of cells with dup(1)(q21q31) and Pt 5 (PV) had +der(9)t(1;9)(q21;q13) in 55% cells. The JAK2V617F allele burden in granulocytes and CD34+ cells ranged from 55% to >95%. HPC (BFU-E, CFU-GM and CFU-Meg) were assayed from CD34+ cells and each individual colony was plucked, split in half, and tested for both the JAK2V617F mutation and the presence of the marker cytogenetic abnormality using FISH. Among the 289 colonies studied, 82% were JAKV617F homozygous, 8% were heterozygous for JAK2V617F and 10% contained only wild type (WT) JAK2. Cytogenetically abnormal colonies, containing the marker chromosome, as detected by FISH, were observed in 80% of the JAK2V617F homozygous, 64% of JAK2V617F heterozygous and 30% of the WT JAK2 colonies. Despite 100% of the colonies being homozygous for JAK2V617F in Pt 3 only 7% of colonies exhibited del(20q). In Pt 1, with two distinct cytogenetically abnormal populations, 2% of JAK2V617F homozygous colonies contained del(20q) alone, while 40% of colonies had both del(20q) and +9p and 52% had a combination of all three abnormalities. Further analysis of Pts 2 and 5 demonstrated that 87% and 47%, respectively, of JAK2 homozygous colonies contained gain of 9/9p. Duplication (1)(q21q31), a characteristic chromosomal abnormality of Pt 4, was identified in 18% of JAK2 WT colonies while its frequency in homozygous JAK2V617F HPC was 76%. These observations suggests that an increased JAK2V617F allele burden is associated with an increased frequency of cytogenetically abnormal HPC. A small proportion of homozygous JAK2 V617F HPC are involved by del(20q) (2%–7%) indicating their low proliferative capacity. Acquisition of trisomy 9 or trisomy 9p either in the presence of del(20q) or alone, involved a much larger population of JAK2V617F homozygous HPC (61%–93%) suggesting their higher proliferative capacity. In colonies containing only WT JAK2, 12% to 26% of cells within colonies had either +9 or dup(1q) suggesting that these chromosomal abnormalities might occur independently of JAK2V617F. These studies suggest a complex relationship between the JAK2V617F allele burden and occurrence of clonal chromosomal abnormalities in Ph-neg MPD HPC which might define important biological differences and clinical outcomes in MPD.

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