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Huntington disease (HD) is a neurodegenerative disease that afflicts about
1 in 10,000 individuals. Symptoms include uncontrollable choreiform or
dance-like movements, impairment in memory and reasoning ability, and
alterations in personality. The average age of onset of symptoms is about
40 years. The symptoms become increasingly disabling with time, and patients
die 15 to 20 years after the first signs appear. Children constitute approximately
10% of HD cases. They are more severely affected than adults and suffer
more rigidity in movement.
HD is an autosomal dominant disorder. This means that any individual with
one affected parent has a 50% chance of inheriting the gene. The gene
that causes HD was identified in 1993 and was initially termed IT15.
The mutation that was discovered within the gene was an unstable expansion
of the trinucleotide CAG (please see last monthıs column). This trinucleotide
encodes the amino acid glutamine and is found within the region of the
gene that encodes for protein. As a result, the gene defect in HD causes
more glutamines to be placed near the N-terminus of the protein, which
is called huntingtin. The normal number of glutamines in huntingtin is
between 6 and 34. It expands to 37 or more in the mutated protein. For
unknown reasons, there is greater instability and expansion of the CAG
repeat region when the gene is transmitted from the father than from the
mother.
As has been found with other disorders caused by an expansion of triplet
repeats, there is an inverse correlation between the degree of expansion
and the time of onset of the disease. Thus, large expansions with 55 or
more glutamines are associated with onset of symptoms in childhood, while
minimal expansions of only 37 to 39 glutamines may result in no symptoms
at all. A simple DNA test can now be performed to confirm the presence
of the mutation, and genetic counseling is imperative before testing to
review the implications of a positive or negative outcome.
The function of the normal huntingtin protein remains unknown. The protein
is present in diverse organ tissues, but its highest level of expression
is in the brain and the testes. Deletion of the huntingtin gene in mice
is lethal in the embryo before the brain is formed. This suggests an important
function for the normal huntingtin protein during development. Huntingtin
is expressed in the developing brain, rises markedly in the postnatal
period in parallel with the differentiation of neurons, and reaches maximum
levels in the adult nervous system. Most neurons throughout the brain
contain huntingtin, distributed throughout the cell body and into dendrites
and axons. Huntingtin exists in a soluble form in the cytoplasm and attaches
to microtubules as well as to intracellular and plasma membranes. Based
on this localization pattern, huntingtin was suggested to be involved
in the transport of vesicles from one compartment of the neuron to another.
Huntingtin is sometimes found in the nucleus. Since many proteins that
contain a long tract of the amino acid glutamine are known to function
in the nucleus as regulators of gene expression, such a role for huntingtin
has also been proposed. (Fig.
1)
It is believed that the mutated protein does not interfere with the function
of the normal huntingtin protein, but rather acquires a new property that
leads to cell death. This point underscores the fact that mutations in
genes do not function only by disrupting the activity of the protein.
Sometimes a mutation will lead to a completely new activity of the protein
in question. These mutations are called gain of function mutations,
and this explains why they are autosomal dominant mutations. The presence
of a normal copy of the gene will not counteract the deleterious effects
of the abnormally active protein.
Individuals carrying the HD gene express mutant huntingtin in neurons
throughout the brain, along with normal huntingtin expressed from the
normal copy of the gene on the other chromosome. The brain regions that
show the most severe loss of neurons in HD are the striatum and the cortex.
What causes greater cell death in striatal and cortical cells than in
other neurons is an enigma. Striatal cells are known to be particularly
sensitive to metabolic stress and to injury by excitatory amino acids.
These factors have long been suspected to contribute to HD pathology,
but how they are influenced by mutant huntingtin is unclear. Peptides
consisting solely of glutamine residues can form insoluble complexes.
This led to speculation that mutant huntingtin, and other disease proteins
with expanded polyglutamine regions, could aggregate in the brain. Indeed,
studies subsequently showed that the mutant huntingtin with an expanded
N-terminal polyglutamine region aggregates in the nucleus and in the cytoplasm
of the cell. Aggregates also form in axons, which degenerate into what
are now called dystrophic neurites.
The aggregated mutant huntingtin colocalizes in complexes with ubiquitin,
a protein that is known to assist in the removal of abnormal or misfolded
proteins. Ubiquitin is often associated with a protein that is targeted
for destruction by the proteasome, a cellular machine that degrades proteins.
The occurrence of these complexes, or inclusion bodies, in the nucleus
and cytoplasm of the patientıs brain is dependent on the length of the
polyglutamine tract. When there is a long expansion, as occurs in children
with HD, up to 50% of neuronal nuclei in the cortex may have inclusions.
In adult cases, only 5% to 7% of neurons in the cortex develop inclusions
in the nucleus, while more protein aggregates in axons.
Cultured striatal cells overexpressing mutant huntingtin form inclusions
in the nucleus and cytoplasm similar to those seen in the HD brain. These
inclusions do not appear to be directly involved in cell death. Treatment
of cells with a caspase inhibitor increases the survival of the cells
but does not alter the rate of inclusion formation. Conversely, inclusion
bodies are reduced by treatment with another caspase inhibitor; however,
that treatment does not alter cell survival.
Although nuclear inclusions are not sufficient to cause cell death, the
presence of mutant huntingtin in the nucleus may be necessary. This was
determined by excluding mutant huntingtin from the nucleus. One can do
this experimentally by adding a nuclear export signal of several amino
acids to the N-terminus of the protein. When this was done in cultured
rat striatal neurons, mutant huntingtin was effectively excluded from
the nuclei and cellular death, or apoptosis, was blocked.
Inclusions have been identified in other neurodegenerative diseases such
as spinocerebellar atrophy, Machado-Joseph disease, and dentatorubropallidoluysian
atrophy. All of these disorders are caused by expansion of a triplet repeat,
and a polyglutamine expansion is therefore found in the disease protein.
Although symptoms and brain pathology differ in each condition, the formation
of inclusion bodies suggests that they share a similar disease process.
The inclusions have become a pathological signature of polyglutamine expansion
diseases and, for biological research in cells, a useful indicator that
the mutant protein has exerted some effect on the cells.
One possibility that is being tested in a number of laboratories is that
mutant huntingtin binds to proteins differently than does normal huntingtin
and thereby alters cell function. The search for binding partners for
huntingtin has uncovered at least 14 potential interactors, but only a
few of these proteins have known functions. Many of the proteins identified
in this manner were found to associate with the N-terminal polyglutamine
enriched region of huntingtin. It is interesting that some of the proteins
that are known and bind differently to normal and mutant huntingtin have
a role in organelle and vesicle transport. Therefore, one possible effect
of mutant huntingtin may be to disrupt protein transport from neuronal
cell body to both dendrites and axons. Consistent with this idea, studies
of HD brains show that there is early pathology in the dendrites of cortical
and striatal neurons and degeneration of corticostriatal axons that accumulate
N-terminal mutant huntingtin. The diversity of protein partners for huntingtin,
including some that localize or function in the nucleus, suggest that
multiple functions could be disrupted in neurons in HD.
The expression of polyglutamines in the mouse brain is highly toxic. Mice
were genetically engineered to express a large polyglutamine tract inserted
into a small protein that is normally innocuous. These mice now develop
seizures, motor disturbances, and ubiquitin-positive inclusion bodies.
When mice express the N-terminal portion of human huntingtin with a highly
expanded glutamine stretch, they develop motor deficits and die within
3 months of birth. These animals show numerous inclusions positive for
huntingtin and ubiquitin and a reduced brain size. Despite the severity
of this phenotype, however, there is no loss of neurons in the striatum,
which is the hallmark of HD pathology.
Mice develop motor deficits more slowly when the polyglutamine expansion
is placed in a larger N-terminal fragment of mutant huntingtin or in the
full-length mutant huntingtin. Under these conditions, the mice also show
cell loss in the striatum. This suggests that the context in which polyglutamine
expansions are presented to the cell is important in conferring an HD-like
phenotype in the mouse. The reason for the selective loss of striatal
neurons in HD, though still elusive, clearly involves characteristics
of huntingtinıs function that are unique to these cells.
The successful development of HD mice and cell culture models has helped
move scientists into the most exciting and challenging phase of HD research,
namely, the search for more effective therapies and eventually a cure.
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