Huntington\'s Disease

Background

Huntington\'s disease is inherited as an autosomal dominant disease that gives
rise to progressive, elective (localized) neural cell death associated with
choleric movements (uncontrollable movements of the arms, legs, and face) and
dementia. It is one of the more common inherited brain disorders. About 25,000
Americans have it and another 60,000 or so will carry the defective gene and
will develop the disorder as they age. Physical deterioration occurs over a
period of 10 to 20 years, usually beginning in a person\'s 30\'s or 40\'s. The gene
is dominant and thus does not skip generations. Having the gene means a 92
percent chance of getting the disease. The disease is associated with increases
in the length of a CAG triplet repeat present in a gene called \'huntington\'
located on chromosome 4. The classic signs of Huntington disease are progressive
chorea, rigidity, and dementia, frequently associated with seizures. Studies &
Research Studies were done to determine if somatic mtDNA (mitochondria DNA)
mutations might contribute to the neurodegeneration observed in Huntington\'s
disease. Part of the research was to analyze cerebral deletion levels in the
temporal and frontal lobes. Research hypothesis: HD patients have significantly
higher mtDNA deletionlevels than agematched controls in the frontal and temporal
lobes of the cortex. To test the hypothesis, the amount of mtDNA deletion in 22
HD patients brains was examined by serial dilution-polymerase chain reaction
(PCR) and compared the results with mtDNA deletion levels in 25 aged matched
controls. Brain tissues from three cortical regions were taken during an autopsy
(from the 22 HD symptomatic HD patients): frontal lobe, temporal lobe and
occipital lobe, and putamen. Molecular analyses were performed on genetic DNA
isolated from 200 mg of frozen brain regions as described above. The HD
diagnosis was confirmed in patients by PCR amplification of the trinucleotide
repeat in the IT 15 gene. One group was screened with primers that included
polymorphism and the other was screened without the polymorphism. After heating
the reaction to 94 degrees C for 4 minutes, 27 cycles of 1 minute at 94 degreesC
and 2 minutes at 67 degrees C, tests were performed. The PCR products were
settled on 8% polyacrylamide gels. The mtDNA deletion levels were quantitated
relative to the total mtDNA levels by the dilution-PCR method. When the
percentage of the mtDNA deletion relative to total mtDNA was used as a marker of
mtDNA damage, most regions of the brain accrued a very small amount of mtDNA
damage before age 75. Cortical regions accrued 1 to 2% deletion levels between
ages 80-90, and the putamen accrued up to 12% of this deletion after age 80. The
study presented evidence that HD patients have much higher mtDNA deletionlevels
than agematched controls in the frontal and temporal lobes of the cortex.
Temporal lobe mtDNA deletion levels were 11 fold higher in HD patients than in
controls, whereas the frontal lobe deletion levels were fivefold higher in HD
patients than in controls. There was no statistically significant difference in
the average mtDNA deletion levels between HD patients and controls in the
occipital lobe and the putamen. The increase in mtDNA deletion levels found in
HD frontal and temporal lobes suggests that HD patients have an increase mtDNA
somatic mutation rate. Could the increased rate be from a direct consequence of
the expanded trinucleotide repeat of the HD gene, or is it from an indirect
consequence? Whatever the origin of the deletion, these observations are
consistent with the hypothesis: That the accumulation of somatic mtDNA mutations
erodes the energy capacity of the brain, resulting in the neuronal loss and
symptoms when energy output declines below tissue expression thresholds.
(Neurology, October 95) Treatments Researchers have identified a key protein
that causes the advancement of Huntington\'s after following up on the discovery
two years ago of the gene that causes this disorder. Shortly after the
Huntington\'s gene was identified, researchers found the protein it produces, a
larger than normal molecule they called huntingtin that was unlike any protein
previously identified. The question that they did not know was what either the
healthy huntingtin protein or its aberrant form does in a cell. Recently, a team
from Johns Hopkins University found a second protein called HAP-1, that attaches
to the huntingtin molecule only in the brain. The characteristics of this second
protein has an interesting feature- it binds much more tightly to defective
huntingtin than to the healthy from, and it appears that this tightly bound
complex causes damage to brain cells. Researchers are hoping to find simple
drugs that can weaken this binding, thereby preventing the disease to progress
any further. In other Huntington-related research, scientists