| Summary: | Abstract Background Large clonal populations of cells bearing PIG-A mutations are the sine qua non of PNH, but the PIG-A mutation itself is insufficient for clonal expansion. The association between PNH and aplastic anemia supports the immune escape model, but not all PNH patients demonstrate a history of aplasia; therefore, second genetic hits driving clonal expansion have been postulated. Based on the previous identification of JAK2 mutations in patients with a myeloproliferative/PNH overlap syndrome, we considered TET2 as a candidate gene in which mutations might be contributing to clonal expansion. Methods Here we sequenced the TET2 and JAK2 genes in 19 patients with large PNH clones. Results We found one patient with a novel somatic nonsense mutation in TET2 in multiple hematopoietic lineages, which was detectable upon repeat testing. This patient has had severe thromboses and has relatively higher peripheral blood counts compared with the other patients—but does not have other features of a myeloproliferative neoplasm. Conclusions We conclude that mutations in TET2 may contribute to clonal expansion in exceptional cases of PNH.
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