TET2 Mutations in Polycythemia Vera (PV) in Some Cases Follow Rather Than Precede JAK2 V617F Mutation, Are Not a Disease-Initiating Event, Affect Mainly Erythropoiesis, and Contribute to Increased Aggressivity of PV Clone.

Abstract 3913

Poster Board III-849

The Ten-Eleven Translocation (TET) 2 gene is a tumor suppressor gene. Its mutations of which are frequently found in PV and other myeloid malignancies. The published evidence suggests that a TET2 mutational burden is present in a higher proportion than JAK2V617F in PV stem cells; its mutations typically precede the JAK2V617F mutation, and are preferentially expressed in myeloid cells. We studied 40 PV patients. Two had known TET2 mutation, one with an intronic mutation (3954+2T>A) predicted to cause aberrant splicing, and the other with a deletion of a single nucleotide in exon 3 (3138delT) leading to the predicted truncation of the TET2 peptide. We quantitated the mutational burden of JAK2V617F and TET2 and the clonality of blood lineages using X-chromosome allelic usage ratios in the blood cells and BFU-E colonies. We also followed the mutational burden of the JAK2V617F and TET2 somatic mutations and expression of TET2 mRNA and monitored the proportion of polyclonal cells, in in vitro expanded erythroid progenitors. These data were compared to the PV patients without known TET2 mutations. Using an X-chromosome-based transcriptional clonality assay, the PV patients had predominantly clonal reticulocytes, granulocytes, platelets and, in those available, CD34-positive cells. Studies of individual BFU-E found that in two PV patients, the TET2 mutations followed rather than preceded the JAK2V617F mutation. We report that only a fraction of clonal CD34+ cells carry the TET2 mutation, and demonstrate that a small proportion of largely polyclonal T cells also carry the TET2 mutation. We report that the presence of both JAK2V617 and TET2 mutations favors accumulation of the mutated erythroid progenitors, while in similar conditions the PV JAK2V617-positive and TET2-negative cells are at a proliferative disadvantage compared to normal erythroid progenitors. We also examined the clonality of these in vitro-expanded erythroid progenitors to determine if the dormant minor populations of nonclonal hematopoietic cells are preferentially expanded along with those belonging to the PV clone. We found that some PV erythroid progenitors with JAK2V617F but no known TET2 mutations became polyclonal after in vitro erythroid expansion, while two PV patients with JAK2V617F and TET2 mutations remained clonal, suggesting that the TET2 mutated clonal progenitors retained their proliferative advantage. Lastly, compared to normal erythroid progenitors wherein TET2 mRNA increases with erythroid maturation, it decreases in PV erythroid progenitors regardless of the presence of a TET2 mutation. As predicted, the intronic TET2 mutation causing aberrant splicing had decreased TET2 expression compared to controls and other PV samples in all cells examined. However, the TET2 mRNA transcript in peripheral blood granulocytes and platelets in JAK2V617F positive PV, regardless of TET2 mutations, was significantly increased compared with normal controls. We conclude that loss-of-function TET2 mutations in the two studied PV subjects are not the PV initiating events. Our data suggest that these TET2 mutations in PV preferentially affect the erythroid lineage, contribute to increased erythroid proliferation, and cause relative inhibition of PV granulopoiesis and megakaryopoiesis. However, in aggregate, these in vitro data also suggest that the acquisition of the TET2 somatic mutations increases the aggressivity of the PV clone.