WAXING AND WANING DNA
From the UCSF Magazine, September 1992
For Elizabeth Blackburn, telomerase opens a new chapter on one of the oldest forms of life.
Elizabeth Blackburn had never seen DNA like this before. From one moment to the next, the chromosomes of the single-celled organisms she and others were studying would mysteriously grow and shrink and then grow again - in defiance of all the laws of biology. As Blackburn was well aware, an organism can copy its own DNA, as it does when its cells divide. And she also knew that because of the peculiar way the enzymes do the copying, chromosomes do gradually shorten over a lifetime.
But no form of life that the molecular biologist had ever encountered could create new DNA for itself out of whole cloth. So what was happening with these chromosomes?
Whether you live your entire life as a single cell - as do some fungi, for example - or only start out that way - as does a human embryo - the DNA you end up with is the DNA you began with. All the DNA in the nearly 100 trillion cells that comprise us is but the product of repeated biochemical xeroxing of the original chromosome set that came in the fertilized egg from which we sprang.
When Blackburn, who has been at UCSF for the last two years, finally cracked the case of the waxing and waning DNA, the solution had some far-reaching implications. It suggested a new approach to treating a number of diseases, including some of the opportunistic infections that beleaguer AIDS patients. And it provided evidence of an evolutionarily earlier life scheme that until now had been purely theoretical.
Chromosomes shorten over an organism's lifetime because the enzymes that enable RNA to copy DNA aren't able to finish off copying the very ends of chromosomes. As a result, each daughter cell gets a slightly shaved-off version of the parent's chromosomes. For most creatures, that isn't a problem. The DNA sequences that are lost from the end of the chromosome - the region called the telomere - are just protective packaging, or chemical bookends that keep the chromosome stable. During most of our lifetime, we and other mammals have telomeres to burn.
But not so the single-celled. Protozoans and fungi, for example, can't survive prolonged telomeric lopping off. So they have worked out a way of replacing the chromosome tips lost in copying via a novel enzyme that Blackburn - the so-called "queen of telomeres" - discovered and named telomerase. (The enzyme is believed to be active in man and other mammals only in egg and sperm cells and possibly in fetuses.) The incredible shrinking and growing chromosomes in Blackburn's lab were the result of telomerase's ability somehow to create DNA.
While most enzymes belong strictly to the protein family, telomerase proved to be part protein and part RNA - and therein lay the secret of its DNA-creating powers. RNA is familiar as the midwife that carries the DNA recipe for a protein from the nucleus to other sites in the cells where the proteins actually are made. Like all the myriad chemical reactions that take place in a cell, the making of an RNA version of a stretch of DNA depends on an enzyme that can run that reaction.
Almost all life forms have an enzyme that can make RNA from DNA. Only a few, or so it was thought, have what it takes to go the other direction. Telomerase is one such enzyme; it can make DNA from an RNA template. Scientists call such an enzyme reverse transcriptase. The DNA that Blackburn saw telomerase adding onto chromosomes didn't come out of a hat but from the enzyme's own RNA.
During much of their lifetime, people apparently have no need for telomerase, but protozoans can't get along without it. When Blackburn deprived various one-celled animals of their telomerase, their DNA got shorter and shorter until they died. That gave her two ideas.
Among the few organisms that already were known to have the ability to spin DNA from RNA is the human immunodeficiency virus (HIV) that causes AIDS. Most viruses consist of DNA (in a protein coat). But HIV consists solely of RNA. HIV carries its own reverse transcriptase, using the enzyme after it gets into a host cell to transcribe its RNA into DNA, which it then stitches into a host chromosome.
AZT, the standard drug used to forestall the onset of the symptoms of AIDS, works by inhibiting HIV's ability to alter a host chromosome. With her discovery of telomerase, Blackburn began to wonder about the source of some of AZT's unfortunate side effects. Might it be that certain human cells besides sperm and ova really did produce and rely on telomerase? Could it be that AZT was affecting both telomerase and HIV's reverse transcriptase?
The answer to the first question is still not known with certainty, although it is probably no. The second question Blackburn was able to answer, at least by reference to the protozoan tetrahymena. When she dosed colonies of the organism with AZT, they were unfazed and telomerase levels were barely reduced. So that hunch wasn't borne out.
By Sheila Stavish
