Abstract: Relapse following initial chemotherapy remains a barrier to survival in approximately 20% of children suffering from acute lymphoblastic leukemia (ALL). Recently, to investigate the mechanism of relapse, we analysed clonal populations in 27 pairs of matched diagnosis and relapse ALL samples using PCR-based detection of multiple antigen receptor gene rearrangements. These clonal markers revealed the emergence of apparently new populations at relapse in 13 patients. In those cases where the new 'relapse clone' could be detected in the diagnosis population, there was a close correlation between length of first remission and quantity of the relapse clone in the diagnosis sample. A shorter length of time to first relapse correlated with a higher quantity of the relapsing clone at diagnosis. This observation, together with demonstrated differential chemosensitivity between sub-clones at diagnosis, indicates that relapse in ALL patients may commonly involve selection of a minor intrinsically resistant sub-clone that is undetectable by routine PCR-based methods. From a clinical perspective, relapse prediction may be improved with strategies to detect minor potentially resistant sub-clones early during treatment, hence allowing intensification of therapy. Together with the availability of relevant in vivo experimental models and powerful technology for detailed analysis of patient specimens, this new information will help shape future experimentation towards targeted therapy for high-risk ALL.

Abstract: B-cell acute lymphoblastic leukaemia (B-ALL) is a clonal malignant disease originated in a single cell and characterized by the accumulation of blast cells that are phenotypically reminiscent of normal stages of B-cell differentiation. B-ALL origin has been a subject of continuing discussion, given the fact that human disease is diagnosed at late stages and cannot be monitored during its natural evolution from its cell of origin, although most B-ALLs probably start off with chromosomal changes in haematopoietic stem cells. However, the cells responsible for maintaining the disease appear to differ between the different types of B-ALLs and this remains an intriguing and exciting topic of research, since these cells have been posited to be responsible for resistance to conventional therapies, recurrence and dissemination. During the last years this problem has been addressed primarily by transplantation of purified subpopulations of human B-ALL cells into immunodeficient mice. The results from these different reconstitution experiments and their interpretations are compared in this review in the context of normal B-cell developmental plasticity. While the results from different research groups might appear mutually exclusive, we discuss how they could be reconciled with the biology of normal B-cells and propose research avenues for addressing these issues in the future.