By Mark Groudine, M.D., Ph.D., Director, Basic Sciences Division
Among his many contributions, Hartwell's crowning achievement was to show it is possible to use genetics to understand the cell-division cycle. Hartwell chose the yeast Saccharomyces cerevisiae as a model because it undergoes easily visualized morphological changes as it progresses through the cell cycle. For example the bud, the newly emerging daughter cell, is formed as the mother cell enters S phase. This enabled Hartwell to isolate a collection of cell-division cycle (CDC) mutations, mutations that prevented progression through specific parts of the cell cycle. Hartwell's collection of CDC mutants has proved to be an invaluable treasure for the cell-cycle field. This class of mutants contains essentially all of the critical regulators of cell-cycle progression. Perhaps most remarkable is that these genes, and the pathways they control, were highly conserved during evolution. The CDC genes have homologues in all eukaryotes, and are the key regulators of cell proliferation in species ranging from yeast to humans.
The fundamental insight to emerge from Hartwell's initial experiments was the idea that key cell-cycle events could be grouped into dependent, genetically definable pathways. Thus, the occurrence of specific cell-cycle events requires the prior completion of other, earlier events; for instance, assembly of the mitotic spindle is dependent upon completion of DNA synthesis. Most influential was the concept of cell cycle START, where the coordinate onset of DNA replication, bud emergence and spindle pole body duplication are all rendered dependent upon intracellular growth and extracellular mating pheromones. Hartwell recognized START to be the crucial point at which cell proliferation is integrated with extracellular and intracellular signals, a concept that extends to all eukaryotes and is critical to understanding the unconstrained proliferation characteristic of tumors. Most remarkable is that Hartwell identified CDC28 as the gene controlling START. This was the first identification of a CDK gene (cyclin-dependent kinase), a major discovery that formed the basis for enormous advances in understanding the cell cycle that have been made during the last 15 years.
It was initially thought that the dependent relationships among cell-cycle events could be explained if a late event required a product produced by an earlier event. Hartwell's most recent studies have shown that dependencies can be established in an entirely different way, through the operation of cell-cycle checkpoints. Hartwell showed that upstream, or early, events send a signal that prevents the initiation of downstream, or later, events. Mutations in checkpoint pathways allow late events to begin even if early ones have not yet been finished, thus nuclear division might occur without completion of DNA synthesis. Hartwell pointed out that checkpoints guarantee the integrity of the cell-division cycle by allowing the cell cycle to halt and repair processes to operate before errors in cell duplication are passed on to daughter cells.
The defining characteristic of a tumor cell is its genetic instability, and Hartwell proposed the importance of checkpoint abnormalities in understanding tumor-cell biology and in formulating new strategies for drug discovery and therapy. Hartwell discovered that yeast respond to DNA damage by temporarily arresting progression through the cell cycle. This checkpoint response permits the repair of DNA damage that would otherwise lead to cell death or inheritance of altered genetic information. Checkpoints are now known to operate at multiple key transitions in the cell cycle, safeguarding against mutagenic events that would arise if cell- cycle progression continued without correction of unfaithfully duplicated or missegregated chromosomes. One of the more important outcomes of this work has been new insight into the function of tumor-suppressor genes, like p53 and the Ataxia-telangectasia genes. These proteins normally perform checkpoint functions during the cell cycle; loss of their function in tumor cells causes genetic instability and rapid evolution of increasingly malignant tumor types.
Uniqueness of Hartwell's contributions
Cell duplication requires the interactions of thousands of macromolecules and the operation of countless biochemical pathways. How are these processes coordinated with one another to allow precise duplication of the cell? Is it even reasonable to imagine that one could discern and explain the underlying logic of such a complicated array of biosynthetic events? In the 1960s when Hartwell began his study of the cell cycle, he reasoned that the events of the cell cycle were under genetic control and that it would be essential to study these events in an organism such as yeast. This insight led to the profound discoveries described above and has spawned the development of research programs throughout the world.
In multiple instances, Lee Hartwell's contributions have opened new fields, provided the foundations for the work of numerous other laboratories, and have offered an insightful framework within which to interpret his own work and the contributions of others. For example, isolation of the CDC mutants provided a rigorous definition of multiple important steps in the cell cycle and opened the way for Hartwell and many others to identify, isolate and determine the function of the genes involved in the control of the cell cycle. The insights that led to the definition of START, and later to the existence of checkpoints, were major conceptual advances that now guide the cell-cycle field. Hartwell initiated another important field of research with his development of a simple color assay for chromosome loss. This development led him to define and analyze several genes involved in chromosome stability. It also initiated work by others in defining the roles of chromosome elements (centromeres, origins of replication and telomeres) in maintaining chromosome stability.
Hartwell's latest and perhaps most exciting contribution is the realization that cancer cells bypass cell-cycle checkpoints. This insight has provided a framework to think about the relations among cancer, chromosome instability, genetic loss and DNA damage. Most exciting is that this insight has led Hartwell to propose practical ways in which yeast might be used to screen for novel anti-cancer drugs. Any one of these observations/insights is deserving of an award in itself.
In essence, Hartwell deserves credit for initiating the modern era of studies into the cell cycle. For more than 30 years, he has consistently contributed experimental and theoretical insights in this field. The completeness of his work is exemplified in his profound theoretical insights, the creative yet simple design of experiments, and the remarkable observations that led to the definition of important interactions among components of the cell-cycle machinery. The widespread application of his body of work in virtually every yeast laboratory, as well as in laboratories attempting to extend Hartwell's observations to higher eukaryotes, can not be overstated. No single investigator, other than Hartwell, has made the seminal contributions that form the foundation of our current understanding of the eukaryotic cell cycle. His contributions have also been seminal to much of our current understanding about the molecular basis of cancer, and he is now attempting to apply these discoveries to finding novel agents for the treatment of cancer.