Chromosome Tips in the Spotlight

Henrietta Lacks is a pretty famous lady in cancer-research circles. She died in 1951 of cervical cancer, but in a sense she lives on in laboratories all over the world.

Eight months before her death, Henrietta donated some of her cancer cells to a research lab at Johns Hopkins University, where they earned the distinction of becoming the first human cells to successfully grow in culture. Because these HeLa (named for their donor) cells were cancerous, they are immortal, dividing well beyond the 50 or so times that a normal cell divides.

Today, HeLa cells remain a staple in many cell biology labs and can even be a nuisanceñthey grow so well that if they contaminate another type of cell culture, they quickly take over. A HeLa cell is also the star of the spectacular photograph pictured here, from Titia de Lange and her colleagues at Rockefeller University. The blue-stained rods are chromosomes, but it is the flashes of green fluorescence at their tips that are the focus of the Rockefeller researchersí work, as well as an exciting new branch of life science called telomere biology.

Chromosome tips, or telomeres, have intrigued geneticists since the late-1930s: Researchers working with fruit flies and corn noted that when chromosomes lost their tips, they either stuck to each other or vanished when the cell divided. Today, telomeres are making headlines because they are opening up a new line of attack against cancer and other illnesses.

In normal cells, telomeres shrink with each cell division, until they reach a certain point where the cells are signaled to cease dividing. In cancer cells, the telomeres do not shrink, and the cells never get the message to stop dividing. An enzyme called telomerase continually tacks more DNA onto chromosome tips, but only in certain cellsñcancer cells and some normal cells that divide many times, such as certain blood cells, sperm cells and the cells lining the small intestine.

But telomeres and telomerase arenít the whole story. The telomere DNAñconsisting of a six-building-block sequence repeated many timesñalso associates with a protein. It is this protein that de Lange and her group have tracked, using immunofluorescence microscopy to highlight precisely where it acts. The protein is called telomeric repeat binding factor, or TRF.

The Rockefeller group used the technique in two ways to reveal the suspected relationship between TRF protein and chromosome tips. First, they used fluorescence in situ hybridization (FISH) to label TRF protein and the DNA at chromosome ends with different colors. Looking at nuclei of cells caught between divisions, when the DNA is too unwound to be visible as distinct chromosomes, the researchers saw many pairs of dots, one yellow and one white in each, corresponding to the TRF proteins and the telomere DNA sequences that they cling to.

Turning to cells captured in the midst of dividing, the relationship between the protein and the chromosome tips became much clearer. The researchers used a series of fluorescently tagged antibody fragments that attract each other to reveal the telomeres. The green glow is due to the fluorescein compound FITC. "The antibody stain is fluorescent. It is FITC-bound to goat antibody, which fluoresces green. We detect a tag that we manipulated onto TRF protein by recombinant DNA techniques," de Lange said.

Identifying TRF protein contributes an important chapter to the fledgling field of telomere biology. Chromosome tips are potentially of interest not only to life scientists, but also to anyone who expects to age and/or would like to avoid degenerative diseases and cancer.

Geron Corp., based in Menlo Park, Calif., is a privately held company formed in 1992 to develop discoveries in telomere biology into therapies. The approach focuses on two areasñcancer and degenerative diseasesñbecause of the two consequences of telomeresí remaining long or shrinking.

A telomere-based treatment for cancer may one day entail blocking telomerase or disrupting the TRF protein. In the meantime, telomerase level is being developed into a diagnostic and possibly prognostic test to assess cancer. Very exciting work comes from Jerry Shay, a cell biologist at the University of Texas Southwestern Medical Center in Dallas. He has measured telomerase levels in several types of cancer and found that the greater the amount of the enzyme, the more advanced the cancer.

"Weíve looked at more than 1000 human primary cancers, and 90 percent of them have telomerase activity. There is a good correlation with the stage of disease," he says.

Telomerase level can be developed into a test that can tell the physician the extent of surgery and follow-up therapies that would be best for a particular patient. "For early-stage breast cancer, if a needle biopsy indicates telomerase activity, the surgeon can know, prior to the surgery, that the procedure will need more-extensive margins, plus adjuvant chemotherapy. If the early-stage tumor shows no telomerase activity, the surgeon can do a lumpectomy and not order chemotherapy," Shay says. He foresees similar tests for bladder cancer performed on urine samples and for colon cancer.

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