Genetic variegation of clonal architecture and stem cells in leukaemia

Little is known of the genetic architecture of cancer at the subclonal and single cell level or in the stem-like cells responsible for cancer clone maintenance and propagation. We have examined this issue in ALL in which ETV6-RUNX1 gene fuson is an early or initiating genetic lesion followed by a modest number of driver copy number alterations. By multiplexing FISH probes for these mutations, up to eight genetic abnormalities can be detected in single cells, a genetic signature of sub-clones identified and a composite picture of sub-clonal architecture and putative ancestral trees assembled. Sub-clones in ALL have variegated genetics and complex, non-linear or branching evolutionary histories. CNA are independently and recurrently acquired in sub-clones of individual patients, and in no preferential order. Clonal architecture is dynamic and changes in the lead up to a diagnosis and in relapse. Leukaemic stem cells, assayed by transplantation in NOD/SCID IL2Rgamma deficient mice, are also genetically variegated, mirroring sub-clonal patterns. These findings have significant implications for the cancer stem cell concept, for interpretation of cancer genome data and for therapeutic targetting in cancer.