In olden times, before a king went into battle, he had his high priestslaughter a sheep and look at its liver, to foretell his chances. Theancients regarded the liver rather than the heart as the body's divinecenter.
Even today, there's something almost magical about the liver. Unlikethe heart, the brain or most other bodily organs and members, theliver can regenerate lost hepatic cells and tissues.
And right now the livers of certain transgenic mice in Oregon seem tobe foretelling a regeneration of optimism for gene therapy.
These model rodents imitate the inborn error of metabolism thatdooms children who come into the world with autosomal, recessive,tyrosinemia, type 1 (HT1). A deficiency in the liver enzyme fumaryl-acetoacetate hydrolase (FAH) blocks the last step in breaking downthe amino acid tyrosine in hepatocytes.
The resulting build-up of this molecule in their liver cells can belethal to human babies in the first few hours after being born; manysurvivors require liver transplant by age five. A frightening 50percent or more of HT1 children die of hepatic carcinoma by eightyears of age. About 40 HT1 infants are delivered alive each year inthe U.S.
An experimental compound, NTBC, blocks tyrosine catabolism, andthus prevents the pile-up of its toxic metabolites. At Sweden'sUniversity of Gothenborg, pediatrician Sven Lundstet is testing thisdrug on a worldwide cohort of 80 HT1 patients.
Having created a colony of mice with mutated FAH genes, a team ofmolecular and medical geneticists at Oregon Health SciencesUniversity in Portland, led a multicenter effort to alleviate HT1 inthis mouse model by a novel gene-therapy strategy. Their report inthe March issue of Nature Genetics is titled: "Hepatocytes correctedby gene therapy are selected in vivo in a murine model of hereditarytyrosinemia type 1."
Pediatrician and molecular geneticist Markus Grompe, the article'ssenior author, prefaced it by stating, "Current strategies for hepaticgene therapy are either quantitatively inefficient or suffer from lackof permanent gene expression."
He referred these two shortcomings, respectively, to "retroviral genetransfer [which] provides stable, long-lasting expression oftherapeutic genes, but achieves stable correction of only a smallpercentage of hepatocytes in vivo," and to adenoviral vectors, which"can infect 100 percent of liver cells in vivo, but the expression doesnot persist . . . ."
Shooting Healthy Hepatocytes Into Deficient Mice
Grompe's team began by transplanting 1 million liver cells fromhealthy, FAH-plus wild-type male mice into 19 FAH-minus females.These animals had been kept alive on NTBC. In 14 of the recipients,they cut off this drug.
Both transplanted cohorts _ those with and without NTBC _gained weight. In contrast, all non-transplanted control rodents diedwithin two months.
A normal-size mouse liver contains 20 million to 30 millionhepatocytes. The investigators determined that as few as 1,000transplanted FAH-expressing cells could repopulate the deficientlivers in their transgenic mice.
Thereupon, proceeding to gene therapy, they constructed a retroviralvector expressing human FAH, and after surgical removal of aportion of the organ to trigger regeneration _ injected it directly intothe portal vein, (which feeds the liver) of 56 mutant mice. Of these,26 (52 percent) survived.
To optimize this partial outcome, the group next outfitted a newcohort of 10FAH-minus animals with indwelling venous catheters,and infused the vectors for five days. The recipients have thrivedlong-term. "Today, eight months out, they remain completelycorrected," the paper's first author, Ken Overturf, told BioWorldToday. Their liver enzyme activities ranged from 20 to 80 percent ofnormal wild-type, with a 59-percent mean.
"Before we can consider them completely cured," he observed, "wehave to watch them for a full year to see if they contract hepaticcarcinoma."
The Oregonians' report, in Nature Genetics, commented genetherapist James Wilson, "breathes new life into the prospects of liver-directed gene therapy." Wilson, who heads the University ofPennsylvania's Gene Therapy Institute, last year completed a seriesof five human liver-therapy trials, aimed at correcting familialhypercholesterolemia. (See BioWorld Today, Nov. 20, 1995, p. 1.)
The results were disappointing.
In his editorial accompanying Grompe's paper, Wilson concluded,"The story of gene therapy for HT1 provides a dramatic illustrationof the impact of fundamental advances in cell and molecular biologyin overcoming insurmountable barriers to successful gene therapies.
"Right now," Overturf said, "we're working out with James Wilsonplans for human trials of the FAH-based strategy."
"Meanwhile," he added, "we're using this technique to do xenografttransplants into our transgenic mice. We've already started with rathepatocytes, and moving on, hopefully, to human cells, as a way oflooking at human diseases in this murine model."
Overturf explained: "We're planning on using the FAH selectionadvantage to see if we can create in normal livers, treatments forsome other diseases, such as hepatitis, diabetes or hemophilia, byrepopulating with another gene of interest." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.