A research team led by Stephen Elledge, a professor in the Department of Genetics at Harvard Medical School, and his post-doctoral fellow Nicole Solimini, has now provided an answer. The most common hemizygous deletions in cancer, their research shows, involve a variety of tumor suppressing genes called STOP genes (suppressors of tumorigenesis and proliferation) that scatter randomly throughout the genome, but that sometimes cluster in the same place on a chromosome. And these clusters, said Elledge, who is also a professor of medicine at Brigham and Womens Hospital, tend to be deleted as a group. "Eliminating the cluster gives a bigger bang for the deletion buck," he said.
This finding is especially interesting in light of the two-hit model of cancer formation, which holds that both copies of a recessive gene need to be inactivated to trigger a biological effect. Thus the loss of a single tumor suppressor copy should have little or no influence on tumor cell proliferation because the remaining copy located on the other chromosome is there to pick up the slack.
Elledges research points to a different hypothesis, namely that STOP genes in a hemizygous deletion arent recessive but are instead haploinsufficient, meaning that they depend on two copies to function normally. "If a tumor suppressor is haploinsufficient, then a single gene copy lacks the potency needed to fully restrain tumorigenesis," Elledge explained, who is also a Howard Hughes Medical Institute Investigator. "So by removing clusters of haploinsufficient genes all at once, the cancer cell immediately propels its growth forward without having to wait for the other copies to also be lost."
Angelika Amon, a professor of biology at the Massachusetts of Technology, said shes surprised by the findings. "Weve known from a lot of human syndromes that haploinsufficiency is widespread in the development of complex multicellular organisms," she said. "But these data show its also critical for individual cells and cell proliferation."