Darwin and evolutionary tales in leukemia. The Ham-Wasserman Lecture

All cancers evolve by a process of genetic diversification and "natural selection" akin to the process first described by Charles Darwin for species evolution. The evolutionary, natural history of childhood acute lymphoblastic leukemia (ALL) is almost entirely covert, clinically silent and well advanced by the point of diagnosis. It has, however, been possible to backtrack this process by molecular scrutiny of appropriate clinical samples: (i) leukemic clones in monozygotic twins that are either concordant or discordant for ALL; (ii) archived neonatal blood spots or Guthrie cards from individuals who later developed leukemia; and (iii) stored, viable cord blood cells. These studies indicate prenatal initiation of leukemia by chromosome translocation and gene fusion (or hyperdiploidy) and the post-natal acquisition of multiple, gene copy number alterations (CNAs), mostly deletions. The prenatal or first "hit" occurs very commonly, exceeding the clinical rate of ALL by some 100x and indicating a low rate of penetrance or evolutionary progression. The acquisition of the critical, secondary CNAs requires some Darwinian selective advantage to expand numbers of cells at risk, and the cytokine TGF beta is able to exercise this function. The clonal architecture of ALL has been investigated by single cell analysis with multicolor probes to mutant genes. The data reveal not a linear sequence of mutation acquisition with clonal succession but rather considerable complexity with a tree-like or branching structure of genetically distinct subclones very reminiscent of Darwin's original 1837 evolutionary divergence diagram. This evolutionary pattern has important implications for stem cells in ALL, for the origins of relapse and for therapeutic targeting.