By David N. Leff

A breathalizer test tells the cop how far over the line to intoxication is the weaving driver's alcohol blood level. A warning, a ticket or license suspension may punish driving under the influence. But what's the penalty for chronic, lifelong ethanol overdose?

The indictment can often lead to a death sentence, carried out not by lethal injection, hanging or electrocution, but by cirrhosis of the liver.

"Hundreds of millions of people succumb to cirrhosis each year," observed cancer geneticist Ronald DePinho, an endowed professor at Harvard Medical School. It is the common endpoint," he pointed out, "of many different insults that impact on the liver - mainly alcoholism, chronic hepatitis B and C induction and parasite infections.

All these diverse etiologies basically do one thing: They trash the liver cells - the hepatocytes - which die.

"But the liver has the remarkable capacity to regenerate, and it does so, maintaining liver function for years and years in the face of this chronic destruction. Then, for some mysterious reason, after a couple of decades of chronic cellular disruption and turnover, the liver suddenly and mysteriously runs out of regenerative potential. The hepatocytes don't proliferate any more.

"Coincident with this hepatocyte arrest," DePinho continued, "the liver gets replaced by scar tissue - by collagen from stellate cells. Those are the two signal events of liver cirrhosis: hepatocyte liver arrest and stellate cell activation."

Long And Short Of It: Telomeres

He and his colleagues asked: Why does that happen? "One intriguing hint came from other groups," he recalled, "that in the chromosomes of cirrhotic liver cells, telomeres are short, compared to those in age-matched livers. Telomeres," DePinho explained, "are repeat structures at the ends of chromosomes. They function to cap, to insulate chromosomes, and maintain their integrity. If you were to break an uncapped chromosome in the middle, it would start fusing with its buddies. That is, the exposed free DNA would fuse to other DNA molecules."

He continued: "So imagine you're a liver cell that needs to continually divide to replace the liver because it's getting trashed on a regular basis because you're drinking too much alcohol every night. Eventually you're going to walk this telomere plank. It turns out that there's a special enzyme called telomerase," DePinho went on, "that can maintain telomeres. That enzyme solves this replication problem by synthesizing and extending the telomeres de novo. Normally, it's not turned on in hepatocytes, so when these liver cells are turning over, the telomerase doesn't do its job of synthesis, and the telomeres get shorter and shorter until they reach a critical length at which the hepatocytes are no longer able to proliferate."

DePinho and his colleagues proved this picture in an article - of which he is senior author - appearing in today's issue of Science, dated Feb. 18, 2000. Its title: "Inhibition of experimental liver cirrhosis in mice by telomerase gene delivery."

He and his co-authors performed three preclinical experiments at the Dana Farber Cancer Institute in Cambridge, Mass., to demonstrate their telomerase therapy.

"In the first one," he told BioWorld Today, "we used a transgenic mouse model. These animals carry the gene for a liver toxin, urokinase plasminogen activator (UPA), that kills the hepatocytes in which it's expressed. But a rare hepatocyte will spit out the UPA transgene by undergoing intrachromosomal recombination. And those few, rare liver cells can now proliferate - charged with responsibility for clonally expanding to a tremendous amount to give rise to a regenerated liver mass. And when we placed that transgene against the backdrop of a mouse that had no telomerase and critically short telomeres, we showed that the ability to generate these large red liver nodules was markedly compromised.

"Next, we surgically removed two-thirds of the liver from two groups of regular mice that did or didn't have short telomeres. What we saw is that liver regeneration following surgical hepatectomy was delayed or impaired in the short-telomere mice, but not in the ones with ample, long telomere reserve. Their liver mass, amazingly, reconstituted in one week.

"The third experiment was chemical ablation of the liver by repeated rounds, every other day, of carbon tetrachloride, a hepatotoxin [and poisonous industrial solvent]. Each time we administered carbon tet to mice, 60 percent of their livers were ablated. In regular mice with long telomeres, not much happened, but mice with short telomeres developed a picture very similar to what we see in human cirrhotic patients. This showed that we can block chemically induced liver cirrhosis."

Now for the downside:

"There's a flip side to this story," DePinho observed. "that when aspiring cancer cells are dividing, and have yet to turn on telomerase, there's a period in their development when the telomeres erode. This leads to chromosomal translocation and all kinds of genetic chaos. Most of the cells will die from this genomic instability, but from the ashes of the crisis emerge rare cells that now have telomerase and so maintain chromosomal integrity in the cancer cells.

"It's really a double-edged sword," he pointed out. "Putting telomerase back in early enough, as cirrhosis therapy, may actually be anti-neoplastic, because you'd prevent the genetic instability phase. If you were already in that phase, and applied telomerase, you could potentially cause more cancer. But in the liver-cirrhotic patient, even if that were to happen, many of those livers would be removed at the time of organ transplantation. It's really a good clinical setting in which to test this hypothesis.

"As a therapeutic modality," DePinho concluded, "this is a first proof of principle, and there's still a great deal of preclinical work to be done before we move towards the clinic."

Editor's note: Back to back with this Science article is a separate but relevant report titled, "Prevention of acute liver failure in rats with reversibly immortalized human hepatocytes." Its senior author is molecular biologist Philippe Laboulche of Harvard Medical School. BioWorld Today will cover this story in Tuesday's issue.