Scientists at the Salk Institute for Biological Studies have engineered a mouse with a mostly human liver by injecting human liver cells, or hepatocytes, into genetically engineered mice. Researchers say the mouse/human chimera could serve as a new model for discovering drugs for viral hepatitis, a disease that has been notoriously difficult to replicate and study in the lab. The team exposed the altered mice to hepatitis B and C viruses and, after treating the rodents with conventional drugs, found that the mice responded much like human patients.

In the United States, 1.2 million Americans are infected with chronic hepatitis B, and 3.2 million with chronic hepatitis C. Searching for effective treatments and drug combinations for viral hepatitis has been a frustrating challenge for years.

In the laboratory, hepatitis and the liver cells it infects can be cagey and temperamental. Human liver cells immediately change character when taken out of the body, and are difficult to grow in a petri dish. What's more, hepatitis only infects humans and chimps, having virtually no effect in other species, meaning conventional lab animals like mice and rats are useless as live models. "You could do chimp studies, but that is not very convenient, and it is of course an ethical issue," says Karl-Dimiter Bissig, first author of the group's paper, published in the Journal of Clinical Investigation. "There's really a need to develop animal models where you can make a human chimerism and study the virus."

Bissig says his group's mouse/human chimera improves on a similar model developed several years ago that was genetically engineered to give human liver cells a growth advantage when injected into a mouse liver. Researchers engineered the mouse with a gene that destroyed its own liver cells. This programmed death gave human liver cells an advantage, and when researchers injected human hepatocytes, they were able to take over and repopulate the mouse liver. However, scientists found that the genetically engineered mice tended to die off early, which required injecting human liver cells within the first few weeks after birth--a risky procedure that often resulted in fatal hemorrhaging.

Instead, Bissig and his colleagues, including Inder Verma of the Salk Institute, sought to engineer a mouse chimera in which the introduction of human liver cells could be easily controlled. The group first engineered mice with several genetic mutations, which eliminated production of immune cells so that the mice would not reject human liver cells as foreign. The researchers made another genetic mutation that interfered with the breakdown of the amino acid tyrosine. Normally, tyrosine is involved in building essential proteins. To keep a healthy balance, the liver clears out tyrosine, keeping it from accumulating to toxic levels. Bissig engineered a mutation in mice that prevents tyrosine from breaking down, instead causing tyrosine to build up in liver cells, eventually killing the mouse cells, giving the human cells an advantage.