Clonal variation in childhood acute lymphoblastic leukaemia at early and late relapse detected by analyses of phenotype and genotype.

To increase our knowledge of the clonal relationship of leukaemia relapse, the genotypes and phenotypes of ten children with acute lymphoblastic leukaemia (ALL) were examined at initial diagnosis and relapse. Seven patients were phenotyped as common ALL, two as mixed, and one as T-cell ALL (T-ALL). Comparative analyses of immunoglobulin (Ig) heavy and light chain as well as T-cell receptor beta-chain (T beta) sequences revealed clonal variations, i.e. appearance of a novel or an evoluted leukaemic cell clone in five patients coinciding with the loss of common acute lymphoblastic leukaemic antigen (CALLA) in four cases, irrespective of early or late relapse. Conversion of early B- to T-ALL or lymphoblastic to non-lymphoblastic leukaemia was not noted in any of the patients examined. Our results suggest that clonal variation is a frequent event in childhood ALL.