Tying Up Loose Ends, by Steve Baragona-- Endeavors, Fall 1999

Tying Up Loose Ends
by Steve Baragona

Mystery lurked at chromosome's end. Each chromosome is made of a long DNA double helix—two strands wrapped around each other. The chromosome ends had been thought to simply stop as if the DNA helix had been broken at that point. In cells, broken DNA triggers a suicide response and the cells die. So why don't natural chromosome ends trigger this same response?

The answer may have arrived this spring. Research from the laboratories of Carolina's Jack Griffith, professor of microbiology and immunology and member of the Lineberger Comprehensive Cancer Center, and Professor Titia de Lange at the Rockefeller University, ran as the cover story in the May 15 issue of the journal Cell.

Griffith and de Lange showed that the chromosome doesn't terminate in a simple end, but instead circles back on itself, forming what looks like a lasso—a neat tidy loop with no real end at all.

Recently it was shown that at a chromosome end, one strand of the double helix juts out beyond the other. "The dogma in the field had been that the ends were capped by proteins that bind to the extended strand," Griffith says. What de Lange describes as "hundreds of postdoc years" have gone into searching for such mysterious proteins, but none have been found in human or mouse cells. Now scientists know why.

Griffith and de Lange met when she came to speak about her work on telomeres, the special region of DNA at the ends of chromosomes. The two soon began collaborating. De Lange's laboratory had discovered a protein called TRF2 which binds to telomere DNA. They found that when TRF2 was removed from the cell, the suicide response was triggered and the cells died.

Griffith used an electron microscope to visualize how TRF2 arranged telomere DNA. "The pictures were odd and hard to interpret," Griffith says. He flew to Rockefeller, and he and de Lange laid the photos out on her office floor.

Griffith realized that, "It was as if TRF2 protein was simultaneously binding to a single strand of telomere DNA and a distant segment of a double stranded telomere DNA." This meant that in natural chromosomes TRF2 could be looping the end around and tucking it into the double-stranded middle of the telomere. They called this lasso-like structure it formed a "t"-(for telomere) loop.

All that remained was the small matter of actually proving that the theory was true. Each attempt would take two weeks and cost about $1,000. Spending so much time and money on a theory that ran so counter to the conventional wisdom seemed like a losing proposition, but Griffith and technician Laurey Comeau decided to take one stab at it. Miraculously, on this first attempt, they saw the t-loops they were looking for. Rachel Stansel, a graduate student in Griffith's lab, then was able to confirm the experiment. She recreated t-loops in the test tube, using a model telomere DNA she constructed.