Searching for mouse models of human
Dabney Johnson watches a mutant mouse.
The lowly mouse is highly regarded as a key to solving a
number of medical mysteries. The mouse has long been known
to be genetically similar to the human. The recently obtained
drafts of the human and mouse genomes suggest that the two
genomes are 85% identical: The differences involve a few
hundred of the approximately 35,000 genes in both organisms.
From an economic standpoint, mice and rats are small and
inexpensive to maintain, so it is not surprising that they are
used in 90% of the research involving animals.
Many mice born with mutations in at least one gene are good
models for human diseases. For example, some mutant mice
produced by former ORNL geneticist Ray Popp have
sickle-cell anemia. They are being studied as models of the
human disease at Meharry Medical College, a historically black
institution and a participant with ORNL in the Tennessee
Mouse Genome Consortium (TMGC).
When a mouse is a model for a human disease, different
treatments can be tested on it. Treatments found to control or
cure the disease in the mouse could lead to the development of
a therapy that works in humans with a similar disease.
Not all mouse models of human disease are perfect yet they
may still be useful, according to Dabney Johnson, head of the
Mammalian Genetics and Genomics Section in ORNL's Life
Sciences Division. "Children with cystic fibrosis die of lung
problems, whereas mice with CF die of intestinal blockages.
Mice with the CF gene can survive if put on a liquid diet.
Perhaps mice have a gene that makes a protein that enables
them to compensate for the lung disorder caused by the CF
gene. If so, knowledge of the structure and function of this
protein could be the key to developing a drug that could benefit
humans with CF."
To determine whether ORNL's mutant
mice are good models for human
diseases of the central nervous system
(CNS), Johnson is collaborating on
projects with the partners of TMGC.
The TMGC recently received a $12.7
million grant from the National Institute
of Mental Health (NIMH) to create 25
to 50 new strains of mutant mice that
will be used to study neurological
disorders. The partners include the
University of Tennessee at Knoxville,
UT-Memphis, St. Jude Children's
Research Hospital, Vanderbilt
University, and Meharry Medical
College. The TMGC is interested in mutations leading to new
genetic information about neurological conditions ranging from
Alzheimer's and Parkinson's diseases to depression and
"We produce mutant mice by treating males with the powerful
mutagen ENU (ethylnitrosourea), mating them with females
with particular genetic characteristics that help trace the newly
induced mutations, and screening their descendants for
disorders in the brain and central nervous system," Johnson
says. "The screening is done using several tests. For example,
mice are placed on a spinning rod to see how well they can
maintain their balance there before falling off. Mice having
certain mutations lack the coordination and balance of normal
mice and fall off this rotor-rod more quickly. We can detect
whether a mouse is depressed by observing its behavior in a
swimming test. If it tends to float rather than swim vigorously
to try to get out of the water, we classify it as a depressed
"An activity test is used to determine if a mouse is underactive
or overactive as a result of a CNS mutation. In this test to
gauge a mouse's activity in a box, a photobeam sensor counts
the number of times per minute that a mouse interrupts light
beams sent across the box."
Johnson and her associates also use this test to measure a
mouse's anxiety level. Mice are, by nature, anxious creatures.
"A normal mouse stays near the wall where it is more protected
rather than going to the middle of the box where it would be
out in the open and feel more exposed to predators," she says. "A less anxious mouse,
one that is calmer than the normal mouse and, therefore, likely to have a CNS
mutation, ventures forth into the open space."
Johnson's group also uses an array of tests to
measure learning abilities and memory in mice to
screen for CNS mutations. For example, the
researchers administer a mild foot shock and play a
sound at the same time. A day later, when a normal
mouse hears that sound, it will freeze in fear that
the unpleasant shock may occur again. Mice with
certain CNS disorders will ignore the sound and
continue their activity.
As a part of that same test, a normal mouse
returned to the same box 24 hours later will
recognize the box as the site of the shock and
freeze. However, a mouse with a CNS disorder will
be just as active in the box as it was 24 hours
before, prior to the administration of the shock.
The two parts of this test measure two different
kinds of memory.
Another CNS test is the startle test. "A normal mouse will have a measurable startle
response when it hears a loud noise," Johnson says. "It will flinch, and this action will
be detected by a load sensor. But an abnormal mouse may not startle at all, perhaps
because it is deaf. Or a mouse with a CNS disorder could startle too much rather than
simply jump or flinch."
The TMGC partners help ORNL screen mice for new mutations and analyze confirmed
mutations in more detail. One new mutation recently discovered by ORNL's Eugene
Rinchik causes the mouse born with it to have continuous seizures. Mice with this
mutation have been sent to consortium researchers who then conduct studies to
determine if the cause of the seizures is neurochemical or neurophysical. They will try
to determine if this "seizure" mouse is a good model for some form of human epilepsy.
At ORNL, researchers run automated analyses of blood and urine samples from mice,
measuring their white and red blood cell counts and hemoglobin concentrations. The dip
stick urine test is used to measure for sugar concentration and excess protein. This
information tells the researchers whether mice are anemic or diabetic or suffer from
infections, leukemia, or blood-clotting problems typical of hemophilia.
"Copies of our mouse mutants go to UT at Memphis, which screens mice for mutant
genes that cause addiction to alcohol and drugs such as cocaine," Johnson says. "For
example, our mice are given the two-bottle test in which one bottle contains water and
the other, alcohol. A mouse with a genetic predisposition for alcoholism might drink
from the alcohol bottle when thirsty, but a normal mouse drinks only from the water
bottle. A normal mouse injected with alcohol falls off the rotor-rod quickly, and it loses
its inhibitions and shows less anxiety-like most people who have too much to drink."
Mice suspected of having CNS disorders, based on ORNL tests, are sent to the UT
Memphis Health Sciences Center. The mice are sacrificed and their brains are sliced,
stained, and studied to determine if they have an abnormal anatomy. The eyes of these
mice are also examined to determine whether the retina, neural connections, and other
components are formed normally. Researchers also check eye samples for signs of
macular degeneration and other predictors of impaired vision.
For many people, mice can be a nuisance, but the results obtained from research using
mice could give victims of some diseases a new lease on life.