Volume 18 Number 1 Winter/Spring 2001

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Related story:
Brain Gain: bone marrow
cells migrate to the brain

new tricks for old cells
Stanford lab makes a habit of showing how adult cells reinvent themselves.

THIS IS NOT THE FIRST TIME THAT HELEN BLAU'S RESEARCH INTO CELL SPECIALIZATION HAS DEFIED CONVENTION. In a key set of
experiments published in Science in 1985, Blau, then an assistant professor, showed that nearly every normal human adult cell tested -- regardless of its previous function -- was able to turn on muscle-cell-specific genes when fused with muscle fibers.

These experiments showed that the differentiated state, rather than being a dead end for cells, requires constant regulation to prevent the cell from deviating from its specialized role. When the internal balance of regulatory factors is disrupted -- in this case by fusing two cells together -- the original cell can express a different set of genes and perhaps even perform an entirely different function.

"That was mind-blowing at the time," says Blau, now chair of Stanford's molecular pharmacology department and director of the Stanford program in gene therapy technology. "The belief was 'once you're skin, you're skin,' and you couldn't change. That adult human cells could exhibit such plasticity was totally unexpected."

Subsequent research showed that even the muscle cells themselves were also up to a challenge; genetically engineered myoblasts were able to synthesize and then secrete functional versions of complicated proteins they don't normally express.

The fact that all of the nonmuscle cells kept their muscle-specific genes dusted off and ready for action, and that muscle cells could "remember" how to modify proteins they don't usually make -- such as blood-clotting factors -- implied that differentiated cells might switch their fates more readily than anyone had suspected.

Blau wondered whether the muscle cells' ability to secrete unfamiliar proteins into the circulation might make them ideal delivery vehicles for gene therapy. Over the past decade she has shown that the muscle cells can be genetically engineered to produce nonmuscle-specific proteins, such as human growth hormone. Other groups of investigators confirmed Blau's findings by showing that genetically engineered myoblasts could be used to treat hemophilia and some types of anemia. When these cells are reintroduced into mice they integrate with existing muscle tissue and continue to produce the protein and deliver it into the circulation for at least several months.

Stanford researcher Mark Kay, MD, PhD, is testing a variation of this approach to treat hemophilia B -- a version of the disease responsible for about 20 percent of hemophilia cases. Kay, associate professor of pediatrics and of genetics, directs
Stanford's program in human gene therapy. In the phase I clinical trial, conducted in collaboration with researchers at Children's Hospital in Philadelphia, a virus expressing the missing clotting factor was injected directly into the thigh muscle of human volunteers. Several weeks later, the researchers were able to detect increased amounts of the clotting factor in the blood of two of the first three patients tested. These patients also were able to scale back the amount of external clotting factor they used to treat bleeding episodes.

Blau is also studying the ability of muscle cells to produce and secrete vascular endothelial growth factor, or VEGF -- a substance that induces the growth of new blood vessels. Researchers hope that one day the technique could be employed to restore blood flow to oxygen-starved regions of the heart in heart-attack patients or to stroke-affected areas of the brain.

But Blau cautions that the dose of VEGF is critical. Too much can cause tangles of blood vessels and swelling at the injection site. She is working on developing ways to regulate the amount of VEGF delivered into the bloodstream by the engineered
muscle cells.

"Muscle is now the prime target for gene therapy directed at getting proteins into the circulation," she says, "but we need to understand how to regulate levels of expression in order to realize the full benefits of gene therapy."
--KRISTA CONGER