A matter of life and death
(The Economists, Jan 23rd, 2006)
CONCEPTION and cancer, which mark the beginning and, often, the end of life, share some molecular details. Cancer cells copy their contents and divide rapidly; so do newly fertilized eggs. This requires genetic reprogramming. Curiously, as a result of the reprogramming, both contain appreciable amounts of an enzyme called reverse transcriptase that biologists more usually associate with certain viral infections, including HIV. Reverse transcriptase makes part of the cell’s protein publishing line work backwards, recreating DNA, a molecule in which organisms typically store genetic information, from its less stable and correspondingly less favoured cousin, RNA. Why it does so in cancer and in embryos, though, is something of a mystery.
Corrado Spadafora, of Italy’s National Institute of Health, in Rome, studies this little-known puzzle. There are thousands of reverse-transcriptase genes in the genomes of all mammals. What that job is exactly, Dr Spadafora is not sure, but data he presented at the annual meeting of the British Andrology Society, in Leeds, suggest some answers.
Dr Spadafora showed recently that reverse transcriptase is required for mouse embryos to develop. He did this by removing it in two ways. First, he exposed embryos created in Petri dishes to a common AIDS drug called nevirapine, which works by gumming the enzyme up. This halted development whenever it was added to embryos up to the stage when they were only four cells big. Adding the drug later, when the four cells had divided into eight, had no effect. Second, he checked the drug was not bad for the embryos in some other, unknown, way by specifically turning off reverse transcriptase-producing genes. The result was the same: the embryos did not die, and again, during the sensitive period, they seemed to get stuck in a juvenile stage.
There are hints that reverse transcriptase is needed for a lot of early embryonic functions associated with getting cells ready to specialize into different types of tissue. Seven of the ten genes Dr Spadafora tested were active in healthy embryos, but were shut down in the nevirapine arrested ones.
Since cancer cells also contain a lot of reverse transcriptase, Dr Spadafora wondered whether stopping the enzyme working might stop them dividing as well. He transplanted four kinds of human cancer into four groups of mice and treated some of each group with nevirapine or a similar drug. In all cases the earlier he gave the drug, the slower the tumours grew, and they always grew more slowly than tumours in mice which got neither drug. As in the embryo experiments, he then silenced the cancer-cell genes that produced reverse transcriptase, and likewise found the tumours grew more slowly.
Moreover, both the drugs and the gene-silencing technique flattened some types of cancer cells, which suggests that both methods of getting rid of reverse transcriptase’s effects caused similar molecular changes inside the cells.
That common anti-HIV drugs slow cancer growth in AIDS patients has been known for some time. Doctors, however, have attributed the fact to the healthier immune systems the drugs promote. Dr Spadafora’s results suggest a more precise mechanism.
1. According to the first paragraph, cancer cells and newly fertilized eggs are similar to each other in the following aspects except that .
A. both of them copy their contents and divide quickly
B. both of them genetic information of organisms
C. both of their division involves genetic reprogramming
D. both of them contain reverse transcriptase
2. Reverse transcriptase can .
A. recreate DNA’s cousin RNA B. make the cell’s protein publishing line work
C. flatten some cancer cells D. recreate DNA
3. What is the most important achievement of Corrado Spadafora’s experients?
A. Reverse transcriptase is necessary for embryos to develop.
B. Getting rid of reverse transcriptase’s effects change the cells.
C. Common anti-HIV drugs are effective to cure cancer.
D. A more precise mechanism is suggested to promote the healthier immune systems.
4. The word “them” (line 2, paragrapg 5) refer to .
A. cancer cells B. reverse transcriptase C. the enzyme D. healthy embryos
5. How can we slow the growth of tumors?
A. create more reverse transcriptase B. use nevirapine as late as possible
C. silence the cancer-cell genes D. promote cells dividing