Evidence for clonal evolution in pre‐B‐cell leukaemia

Summary. This case study provides evidence for clonal evolution in pre‐B‐cell leukaemia. At diagnosis, the lymphoblasts from a 3‐year‐old boy were morphologically subtyped as L1 (French–American–British classification). Their immunophenotype was CALLA⁺, CIgM⁺, SIg^‐, TdT⁺, and the karyotype was pseudodiploid with a 1;19 translocation. Striking shifts were apparent when the child relapsed 16 months later. The morphologic subtype had changed to L3, CALLA and TdT had disappeared, and a consistent karyotype was lacking. The modal chromosome number had increased through clonal evolution to 85, the 1;19 translocation was retained, and a new marker, a 14q⁺ (partial duplication) appeared and was present in a majority of cells. These cytogenetic findings are characteristic of a transforming state. However, despite the loss of TdT, the appearance of classic L3 morphology and the acquisition of a 14q⁺ marker, the cells retained a predominantly pre‐B phenotype.