CD90/Thy-1 is preferentially expressed on blast cells of high risk acute myeloid leukaemias

Different transformation mechanisms have been proposed for elderly acute myeloid leukaemia (AML) and secondary AML (sAML) when compared with de novo AML or AML of younger patients. However, little is known regarding differences in the immunophenotypic profile of blast cells in these diseases. We systematically analysed, by flow cytometry, 148 patients affected by de novo (100 cases) or sAML (48 cases). By defining a cut-off level of 20% of CD34⁺ cells co-expressing CD90, the frequency of CD90⁺ cases was higher in sAML (40%) versus de novo AML (6%, P <0.001), elderly AML (>60 years) (24%) versus AML of younger patients (10%, P = 0.010) and poor- versus good-risk karyotypes (according to the Medical Research Council classification, P <0.001). The correlation between CD90 expression, sAML and unfavourable karyotypes was confirmed by analysing the subset of CD34⁺ AML cases alone (91/148). Consistently, univariate analysis showed that expression of CD90 was statistically relevant in predicting a shorter survival in CD90⁺ AML patients (P = 0.042). Our results, demonstrating CD90 expression in AML with unfavourable clinical and biological features, suggest an origin of these diseases from a CD90-expressing haemopoietic progenitor and indicate the use of CD90 as an additional marker of prognostic value in AML.