Huntingtons Disease Essays - Huntingtons Disease, Senescence, DNA

Huntingtons Disease Huntington's Background Huntington's disease is inherited as an autosomal dominant disease that gives rise to progressive, elective (localized) neural cell death associated with choreic 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 genomic 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 degreesC for 4 minutes, 27 cycles of 1 minute at 94 degreesC and 2 minutes at 67 degreesC, 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,