Ataxia
UK Research Report 1988
As last year,
we supported research into both Friedreichs and the
dominant cerebellar ataxias at the Royal Free Hospital in
North London and St Mary's Hospital, Paddington.
In the case
of Friedreich's Ataxia, our interest is in frataxin, which
is a protein. We know that a genetic mutation reduces the
amount of frataxin in the cells of people with
FA.
What we need
to know now is: what is the normal function of frataxin? how
does the mutation affect the level or function of frataxin?
how does frataxin deficiency damage cells?
We are fairly
sure that frataxin is involved in regulating the amount of
iron in the mitochondrion of the cell (the mitochondrion is
a small body within the cell which produces the energy that
it needs to function). The evidence for this comes mainly
from studying yeast cells. But yeast cells are not human
cells, and we need to know whether the changes in yeast
cells are like the changes in the cells of people with
FA.
Unfortunately,
it is rarely possible to study the role of frataxin directly
in living people. Yeast can cast light on what may be
happening in human cells, but is too simple an organism to
enable us to draw firm conclusions about what is happening
in human cells or how the genetic defect may be rectified.
We therefore need to develop other approaches, including the
investigation of iron levels in tissue samples and cell
cultures, and breeding genetically modified mice which mimic
the human disease.
The research
teams at the Royal Free and St Marys have been
carrying out tests on tissue samples taken from the dorsal
root ganglia, heart, skeletal muscle and cerebellum of a
number of FA patients and blood and skin cells taken from FA
volunteers and grown in the laboratory. Their tests on
tissues suggest that what happens in yeast cells closely
resembles what happens in human cells. The teams suspect
that the loss of frataxin is most likely to result in
increased iron accumulation caused by either defective iron
regulation or free radical damage. The decreases in energy
production and mitochondrial DNA are, on this theory,
secondary to those events. This is clearly of vital
relevance to developing an effective therapy. The studies of
cells grown in the laboratory are not yet complete, but they
also suggest that the reduction in energy production is a
secondary effect of the other changes that take place in
cells with reduced frataxin.
Lastly, the
Royal Free team has been working with Dr Raffaele Lodi at
the John Radcliffe Hospital using Phosphorus Magnetic
Resonance Spectroscopy to study the ability of mitochondria
in skeletal muscle or heart cells to generate the energy the
cells need to function. They noticed a significant reduction
in mitochondrial energy production following arm or leg
exercise and in the heart muscle of FA patients. This result
might be used to assess the value of therapies in the
future.
The team at
St Mary's has been attempting to breed mice (known as mouse
models) to throw light on the normal role of frataxin and
the result of the mutation. Two models are being bred. The
first model will be a mouse which produces no frataxin at
all to allow the team to study the role of the gene during
the development and maturation of the nervous system and its
involvement in maintaining healthy nerve cells. This could
be difficult, as everybody, including FA patients, has some
frataxin, and a mouse with none at all may be unable to
survive. The second model will be a mouse with similar
frataxin levels to those in people with FA, created by
inserting the human mutation into mouse DNA.
When these
models have been created, the team hopes to be able to
confirm that they show similar clinical features, including
iron accumulation, to people with FA. If so, then the way is
opened to using the mouse models to evaluate the efficacy or
potential toxicity of therapy.
The team at
St Mary's has also been developing mouse models for
autosomal dominant CA. Research into CA is complicated by
the fact that 8 different forms of the condition have been
identified, labeled SCA1, SCA2, SCA3 etc. Each SCA is due to
a mutation on a different gene. Some of the genes have now
been isolated. Each gene is likely to have a slightly
different function, but they all share a common mutation
mechanism, ie the expansion of the CAG repeat
motif.
Considerable
progress has been made in understanding how the expanded CAG
repeat causes cell death. Recent studies indicate that it
may cause proteins within the cells to aggregate and form
unusual deposits within the nucleus. Usually, toxic products
are broken down and removed from the cell, but these
aggregates are insoluble and resist being broken down.
Consequently, their presence increasingly interferes with
the normal functioning of the cell, ultimately leading to
its premature death.
Several mouse
models have been generated by other groups, by inserting the
repeat motif in isolation but these are not entirely
representative. St Mary's have therefore been trying to
insert the mutation within the full-length gene for SCA2 and
SCA3 (also known as Machado-Joseph disease, which they have
been studying for some years). The model will enable the
team to show exactly when and how quickly deposition of the
aggregates occurs and which nerve cells are primarily
affected. This information will be absolutely critical for
the future evaluation of corrective therapies, and will
provide the benchmark against which they can be
judged.
Research
strategy 1998-99
We shall be
reviewing our research strategy during 1998-99 to make sure
that we are ready to take part in the clinical trials that
we expect to start in the next few years. We particularly
want to ensure that the UK plays a leading part and that
advances are accessible to people here at the earliest
opportunity.
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