By David N. Leff

Science Editor

Everyone knows that too much cholesterol can be hazardous to health. Now there's hard evidence that too much cholesterol in one place can be lethal to life itself.

Cholesterol consciousness-raising in the '80s and '90s led to entire new industries in the U.S. and other First-World countries: Lite, lo-fat and diet foods and beverages fill the supermarket shelves; exercise machines clutter TV commercials. Cholesterol-level tests, at home and in shopping malls, supplement those administered in doctors' offices.

Yet many people who wage war on cholesterol as the arch perpetrator of atherosclerosis and coronary heart disease overlook the fact that this molecule spells life more than death. For one thing, cholesterol is a major constituent of cell membranes. For another, it's a key ingredient of steroid hormones.

Cell biologists and geneticists who study how cholesterol works, in sickness and in health, look to a rare inherited malady as an extreme model of pathogenic cholesterol at its worst. This is Niemann-Pick C (NP-C) disease, which, at any one time, afflicts some 300 victims in the U.S. and perhaps 1,000 throughout the world.

Two German physicians, Albert Niemann (1880-1921) and Ludwig Pick (1868-1935) described NP-C's dire symptoms a century or so ago.

Molecular biologist Eugene Carstea, until three months ago head of molecular neurogenetics at the National Institute of Neurological Diseases and Stroke, in Bethesda, Md., describes NP-C thus:

"What one definitely sees in kids by the age of five to seven years," Carstea told BioWorld Today, "is the beginning of neurological involvement. It's highlighted by what we call vertical gaze palsy — loss of the ability for vertical eye movement. Then, gradually, loss of muscle tone, a little bit of clumsiness, difficulty in eating, swallowing, speaking. And all of these neurodegenerative symptoms tend to magnify themselves and intensify over the years."

He continued: "By the next seven to 12 years of age, typically, possible loss of the ability to walk, talk, even eat. Such NP-C kids need to be fed by gastric tube. Many become wheelchair-bound, and nearly all die by age 19 or 20."

Besides these cerebral deficits, NP-C also assaults liver and spleen from childhood on. Its assault weapon is cholesterol, piling up day and night in cells throughout the body.

"So a clinician who sees the coupling of this hepatosplenomegaly with vertical gaze palsy," Carstea observed, "would diagnose NP-C by taking a skin biopsy and looking for the accumulation of the fatty material — cholesterol, primarily — in the lysosomes."

NP-C: Another Lethal Lipid Storage Disease

Lysosomes are bags of enzymes, 3 to 5 microns in diameter, which adhere to the inner membranes of cells and digest waste products, including errant bacteria. But in NP-C, they can't get rid of cholesterol.

Unlike such better-known lysosomal storage diseases as Tay-Sachs and Gaucher's, which affect mainly Jews of Eastern European origin, NP-C, Carstea said, "is pan-ethnic. We see it in Chinese, Japanese, Caucasians, Anglos, Hispanics — in fact, we see it in dogs, cats and mice."

Developmental geneticist William Pavan saw NP-C in mice. He heads the mouse embryology section at the National Human Genome Research Institute, in Bethesda, Md. In fact, he saw the disease in two separate strains of mouse.

"It was just a serendipitous chance of nature," Pavan told BioWorld Today. They arose in a colony of wild-type mice that were genetically identical, and were being used for other experiments."

Both strains, he said, "have almost the same symptoms that we see in the human disorder, and abnormal cholesterol processing in their cells. All the key elements of NP disease. And they die young, at about eight weeks — equivalent to adolescence in humans."

Carstea is first author of a paper in today's Science, dated July 11, 1997, titled: "Niemann-Pick C1 disease gene: Homology to mediators of cholesterol homeostasis."

Back-to-back with it in the same issue of Science is an article bearing the title: "Murine model of Niemann-Pick C disease: Mutation in a cholesterol homeostasis gene." Pavan is its senior author.

Between them, these two reports, which have many co-authors in common, announce discovery of the long-sought mutated gene that causes NP-C.

Carstea's paper has 39 co-authors, from research centers in 17 countries on three continents, most of whom contributed DNA from affected families to National Institutes of Health gene-mappers.

"The actual gene-cloning project," Pavan said, "took us about a year and a half, but it climaxed a two-decade search."

Mutant Gene Aborts Cholesterol Feedback In Cells

Neither team yet knows the normal function of the protein that gene encodes. But they note a striking genomic identity between the gene sequence and the sequences of other proteins known to control normal cholesterol homeostasis in cells.

"One such," Carstea explained, "is the rate-limiting enzyme for de novo cholesterol synthesis. The presence of cholesterol coming into the cell, as detected by the cell's sterol-sensing domain, should shut that enzyme down." He and his co-authors surmise that mutations they have found in the NP-C gene cripple this feedback mechanism.

"One of the first experiments that we're doing now," Pavan said, "is to see if we can specifically mutate that sterol-sensing domain, put it back into NP-C cells in culture and ablate the mutated gene's function."

As for putative therapeutic payoffs from this gene discovery, Pavan observed: "Due to the nature of the disease, and that the many cells in the body can demonstrate a cellular defect, genetic therapies could be very difficult with the tools we have right now.

"However," he added, "we do have several NP-C model systems: the mouse model and cells in culture that have the same gene mutated, and mutatable yeasts and nematodes. Those are really powerful for doing drug testing at various levels. So I hope we're going to find some sort of compound that will alleviate, at least, some of the NP-C symptoms. We now have the tools to start that." *