ETP-ALL
From haematologyetc.co.uk
ETPs are recent immigrants from the bone marrow (BM) to the thymus, derived from hematopoietic stem cells, which retain a certain level of multilineage pluripotency.7,8 By gene expression profiling, ETP cells share similarities with hematopoietic stem cells and myeloid progenitor cells.5 The definition of ETP-ALL/LBL is based on the immunophenotype of the leukemic cells, which are typically CD1a−, CD8−, CD5− (dim), and positive for 1 or more stem cell or myeloid antigens.5 In the World Health Organization (WHO) classification, ETP-ALL/LBL falls within the early T-ALL/LBL category. ETP ALL has been reported in 11% to 12% of childhood T-ALL/LBL5,9 and in 7.4% of adult T-ALL/LBL.9 ETP-ALL/LBL is also characterized by a distinct molecular profile with a lower incidence of NOTCH1 mutations and frequent presence of FLT3 and DNMT3A mutations.6,10-12
ETP-ALL had a characteristic immunophenotype: CD1a and CD8-negative, CD5-negative, or dim (<75% of blasts positive) with one or more stem cell/myeloid markers positive (≥25% of blasts positive) (6). This served as the rationale for defining ETP-ALL under the latest WHO 2017 classification: positive intracytoplasmic CD3 expression and CD7 expression, CD1a-negative, CD8-negative, negative or dim expression of CD5 (<75% positive), and positive (≥25% of blasts population) for at least one stem cell or myeloid antigen, including CD34, HLA-DR, CD13, CD33, CD117, CD11b, and CD65 (4, 7). CD2 and CD4 may also be positive in ETP-ALL under the latest WHO 2017 classification (4).