Mouse models for the human disease of chronic hereditary tyrosinemia

Using the powerful mutagen ENU to alter a single DNA base pair in this gene, ORNL biologists recently produced two mouse models that more closely mimic the gene mutation that causes chronic hereditary tyrosinemia in humans.

Normal mice and people metabolize tyrosine, an amino acid available in food, to make melanin, a type of pigment produced in large amounts by dark-skinned people. But people and mice with the disease lack a normally functioning protein (enzyme) to carry out one step of the tyrosine metabolism process, which involves a series of enzymes. So, unless people with this disease are put on a special tyrosine-free diet, a substance that is not broken down because of the absence of a normally functioning enzyme will build up to toxic levels in the liver and kidneys, a fatal condition in mice.

By sequencing the same chromosomal region from both normal and abnormal mice and comparing the sequences, ORNL biologists identified new mouse models that carry mutations in this enzyme in the tyrosine breakdown pathway. "Mice with this disease die of poisoned livers," says ORNL biologist Dabney Johnson. "A by-product of the botched metabolism process is succinyl acetone, which accumulates in the liver and is excreted in urine where it serves as a diagnostic indicator of the disease.

Because mice entirely lacking this enzyme die right away, we exposed male mice to ENU to produce mice with this enzyme in crippled form, rather than entirely missing, so we would have a live mouse model of a disease that some humans have. In this way, interested researchers could assess the effect of therapy on mice with chronic hereditary tyrosinemia."

Using X-ray crystallography, ORNL's Gerard Bunick, along with co-workers Joel Harp and David Timm, determined the structure of the enzyme that is responsible for hereditary tyrosinemia in the mouse. The mouse enzyme serves as an easily studied model for the same disease in humans.

"I found that this enzyme folds into a three-dimensional shape that has never been seen before," says Bunick. "On the basis of our structural observations, we were able to propose how the enzyme works, allowing us to identify the key amino acids that lead to dysfunction of the enzyme when mutated, resulting in tyrosinemia. We also identified the location of several sites of known mutation in the structure that could cause the protein to fold into an incorrect three-dimensional shape, which would also cause dysfunction of the enzyme."

A paper on this research has been accepted for publication in the journal Structure. A paper on the new mouse models for the disease has been published in the Proceedings of the National Academy of Sciences. A detailed understanding of the enzyme may lead to a drug to treat hereditary tyrosinemia in humans.