TORONTO, Canada--Understanding why normal cells grow old and die
while cancer cells do not could be a boon to cancer research (see
drawing on page 1). US and Canadian scientists have discovered
that a protein called telo-merase may be the cause of the "eternal
youth" of cancer cells, and they are seeking to develop drugs
to block its effects.

Researchers have found that significant levels of telomerase are
present in as many as 95% of all malignant cancer cells in humans,
but not in normal tissues, with the exception of reproductive
cells from ovaries or testes.

In a presentation at the American Association for Cancer Research
meeting, Jerry W. Shay, PhD, a cell biologist at the University
of Texas Southwestern Medical Center at Dallas, said that telomerase
may be the substance that gives cancer cells their immortality,
and if there is no telomerase activity in tumors, those tumors
may spontaneously regress.

In his study on neuroblastoma tumors, the most common solid tumor
found in children under the age of 5, Dr. Shay and his team found
telomerase in 94 of 100 tumors.

The tumors with high levels of telomerase activity also had other
genetic changes and an unfavorable prognosis. The tumors with
low telomerase activity did not have genetic mutations and were
associated with a favorable prognosis.

Three of the neuroblastoma tumors that did not show telomerase
activity were from a type of tumor that often spontaneously regresses
in children.

The investigators made similar observations in small-cell lung
cancer, with 100% of samples studied testing positive for telomerase.
In contrast, only about 80% of 136 non-small-cell lung cancers
expressed telomerase, Dr. Shay said. "This may explain why
so many more non-small-cell lung cancer patients respond well
to treatment."