Somatic TET2 mutations (TET2 MT) are frequent in myeloid neoplasia (MN), particularly chronic myelomonocytic leukemia (CMML). TET2 MT includes mostly loss-of-function/hypomorphic hits. Impaired TET2 activity skews differentiation of hematopoietic stem cells toward proliferating myeloid precursors. This study was prompted by the observation of frequent biallelic TET2 gene inactivations (biTET2 i ) in CMML. We speculated that biTET2 i might be associated with distinct clinicohematological features. We analyzed TET2 MT in 1045 patients with MN. Of 82 biTET2 i cases, 66 were biTET2 MT, 13 were hemizygous TET2 MT, and 3 were homozygous TET2 MT (uniparental disomy); the remaining patients (denoted biTET2 - hereafter) were either monoallelic TET2 MT (n = 96) or wild-type TET2 (n = 823). Truncation mutations were found in 83% of biTET2 i vs 65% of biTET2 - cases (P = .02). TET2 hits were founder lesions in 72% of biTET2 i vs 38% of biTET2 - cases (P < .0001). In biTET2 i , significantly concurrent hits included SRSF2 MT (33%; P < .0001) and KRAS/NRAS MT (16%; P = .03) as compared with biTET2 - When the first TET2 hit was ancestral in biTET2 i , the most common subsequent hits affected a second TET2 MT, followed by SRSF2 MT, ASXL1 MT, RAS MT, and DNMT3A MT BiTET2 i patients without any monocytosis showed an absence of SRSF2 MT BiTET2 i patients were older and had monocytosis, CMML, normal karyotypes, and lower-risk disease compared with biTET2 - patients. Hence, while a second TET2 hit occurred frequently, biTET2 i did not portend faster progression but rather determined monocytic differentiation, consistent with its prevalence in CMML. Additionally, biTET2 i showed lower odds of cytopenias and marrow blasts (≥5%) and higher odds of myeloid dysplasia and marrow hypercellularity. Thus, biTET2 i might represent an auxiliary assessment tool in MN.
© 2019 by The American Society of Hematology.