A Tumor, the Embryo’s Evil Twin

George Johnson in The New York Times:

TumorDuring her first encounter with cancer, Susan Sontag described a tumor as a “demonic pregnancy.” “This lump is alive,” she wrote in “Illness as Metaphor,” “a fetus with its own will.” She could hardly know that the comparison would become more than a figure of speech. Since the book was published in 1978, scientists have been finding that the same genes that guide fetal cells as they multiply, migrate and create a newborn child are also among the primary drivers of cancer. Once the baby is born, the genes step back and take on other roles. But through decades of random mutations, old embryological memories can be awakened and distorted. What is born this time is a tumor. “Cancer proceeds by a science-fiction scenario,” Sontag wrote, invoking movies like “Invasion of the Body Snatchers” and “The Blob.” When Ridley Scott’s “Alien” appeared at the cinema in 1979, his imaginary space creatures, hatching their eggs inside human bodies, made her comparisons seem all the more gruesome.

There is no need, of course, for an alien impregnation. Cancer can be provoked by a carcinogen or a hormonal imbalance — or just a senseless, spontaneous mutation. Tipped from its equilibrium, a cell begins multiplying faster than it should. Two cells become four, then eight, then 16. Sontag’s demonic pregnancy, like “Rosemary’s Baby,” stirs to life. Rough similarities between the growth of a tumor and the gestation of an embryo were first suggested more than a century ago. But no one could have guessed that the parallels would turn out to be so precise. Consider the gene SHH. The name is short for sonic hedgehog. (Hedgehog genes were discovered in fruit flies and when mutated they cause the larvae to be covered with a profusion of bristles.) In a human embryo, sonic hedgehog is involved with establishing the bilateral symmetry of the brain, skeleton and other organs. Later in life it can run amok, interacting with genes like SMO (for smoothened — another fruit fly derivation) to bring on a human brain cancer called medulloblastoma and a skin cancer called basal cell carcinoma.

More here.