Enhanced myeloid specificity of CD 117 compared with CD13 and CD33

The c-kit proto-oncogene encodes a 145 kd tyrosyne kinase transmembrane rec eptor, which plays a key role in haemopoiesis. The c-kit has been classifie d as CD117 and is especially useful in the differential diagnosis of acute myelogenous leukemia (AML) and acute lymphoblastic leukemia (ALL). We analy sed 104 consecutive cases (55 AML, 23 B-cell lineage ALL, three T-cell ALL, 11 blast crisis of chronic myeloproliferative disorders and 12 cases of my elodysplastic syndromes with more than 10% of blasts) referred to our Hospi tal for immunophenotypic diagnosis and compared the expression pattern of C D13, CD33 and CD117 using the same fluorochrome (phycoerythrin-PE). The rec ommendations of the EGIL group were followed in order to establish lineage involvement of the blastic population. The threshold used to assign positiv ity for CD117 was 10%. Bcr/abl, TEL/AML-1 and MLL rearrangements were asses sed by molecular methods. CD117 expression was detected in 91% of AML and M DS. All the negative cases corresponded to acute monocytic leukemias. The c alculated specificity for myeloid involvement was 0.86 for CD117, 0.36 for CD13 and 0.44 for CD33 (P < 0.005). CD117 was also positive in four cases o f ALL. None of these cases showed bcr/abl or MLL rearrangements, in the lig ht of these findings, CD117 expression should yield a higher score, at leas t one point, in the system currently applied for the diagnosis of biphenoty pic acute leukemias (BAL) as its myeloid specificity is greater than that o f CD13 and CD33. Moreover, its absence in AML could identify two subroups o f M5b cases. The coexpression of CD117 with cytoplasmic CD79a is often asso ciated with CD7 reactivity, suggesting a stem cell disorder. CD117 should b e included on a routine basis for the immunophenotypic diagnosis of acute l eukemias. (C) 1999 Elsevier Science Ltd. All rights reserved.