Mouse models for the human disease of chronic hereditary tyrosinemia
researchers found that a nearby gene that is also knocked out by irradiation causes
chronic hereditary tyrosinemia in mice, a disease that also afflicts humans. Patients with
this disease can develop a wide range of liver and kidney problems, as well as problems
affecting the eye, the skin of the feet and hands, and the central nervous system.
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
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.