Two Britons – Sydney Brenner and John E. Sulston – and an American – H. Robert Horvitz – shared the 2002 Nobel Prize for Physiology and Medicine for discoveries about how genes regulate tissue and organ development via a key mechanism called programmed cell death, or apoptosis. Their research elucidated the exquisitely tuned process in which certain cells, at the right time and place, get a signal to commit suicide.
Programmed cell death is essential for normal development in all humans. During the fetal development of humans, huge numbers of cells must be eliminated as body structures form. Programmed cell death sculpts the fingers and toes, for instance, by removing tissue that was originally present between the digits. Likewise, it removes surplus nerve cells produced during early development of the brain. In a typical adult human, about a trillion new cells develop each day; a similar number must be eliminated to maintain health and keep the body from becoming overgrown with surplus cells.
To study programmed cell death in humans, Brenner, Sulston< and Horvitz relied on a n animal model, the nematode Caenorhabditis elegans, a near- microscopic soil worm. In the early 1960s Brenner had realized the difficulties of studying organ development and related processes in higher animal, which have enormous number of cells. His research for a simple organism with many of the basic biological characteristics of humans led to C. elegans, which begins life with just 1,090 cells. Moreover, the animal is transparent, which allows scientists to follow cell divisions under a microscope; it reproduces quickly; and it is inexpensive to maintain. As researchers later learned, programmed cell death eliminates 131 cells in C. elegans, so that adults wind up with 959 body cells. Brenner’s investigations showed that a chemical compound could induce genetic mutations in the worm and that the mutations had specific effects on organ development.
Sulston in the 1970s mapped a complete lineage for C. elegans, tracing the descent of every cell, through division and differentiation, from the fertilized egg. From this he showed that, in worm after worm, exactly the same 131 cells are eliminated by programmed cell death as the animals develop into adults. Sulston also identified the first known mutations in genes invokved in the process.
Beginning in the 1970s Horvitz used C.elegans to try to determine if a specific genetic program controlled cell death. In 1986 he reported the first two “death genes,” ced-3 and ced-4, which participate in the cell-killing process. Later he showed that another gene, ced-9, protects against cell death by interacting with ced-3 and ced-4. Horvitz also established that humans have a counterpart to the ced-3 gene. Scientists later found that most of the genes involved in the controlling programmed cell death in C. elegans have counterparts in humans.
Knowledge about programmed cell death led to important advances not only in the developmental biology but also in medicine. It helped, for example, to explain how some viruses and bacteria invade human cells and cause infections. In cancer and some other diseases, programmed cell death was seen to slow down, which allows survival of cells that normally are destined to die. In cancer the result is an excessive growth of cells that invade and destroy normal tissue. Some cancer treatments are based on the strategy of shifting the cell suicide program into higher gear.
(Encyclopaedia Britanica 2006)
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