Background: Huntington’s disease is an autosomal dominant disease that results in progressive neural cell death, causing uncontrollable movements of the arms, legs, and face, as well as dementia. It is one of the more common inherited brain disorders, affecting about 25,000 Americans, with an additional 60,000 carrying the defective gene and likely to develop the disorder as they age. Physical deterioration typically occurs over a period of 10 to 20 years, usually beginning in a person’s 30s or 40s. The gene is dominant and does not skip generations, meaning that having the gene results in a 92% chance of developing 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’s disease are progressive chorea, rigidity, and dementia, frequently associated with seizures. Research studies were done to determine if somatic mtDNA (mitochondrial 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. The research hypothesis was that HD patients have significantly higher mtDNA deletion levels than age-matched controls in the frontal and temporal lobes of the cortex.
To test the hypothesis, the amount of mtDNA deletion in the brains of 22 HD patients was examined using serial dilution-polymerase chain reaction (PCR) and compared to the mtDNA deletion levels in 25 age-matched controls. Brain tissues were taken from three cortical regions (frontal lobe, temporal lobe, and occipital lobe) and putamen during autopsy of symptomatic HD patients. Molecular analyses were performed on genetic DNA isolated from 200 mg of frozen brain regions. 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. Tests were performed after heating the reaction to 94 degrees C for 4 minutes, followed by 27 cycles of 1 minute at 94 degrees C and 2 minutes at 67 degrees C.
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 deletion levels than age-matched 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 increased mtDNA somatic mutation rate. Could the increased rate be 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 neuronal loss and symptoms when energy output declines below tissue expression thresholds. 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 identification of the Huntington’s gene, researchers discovered the protein it produces – a larger than normal molecule called huntingtin, which was unlike any protein previously identified. The question they did not know was what 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 only to the huntingtin molecule in the brain. The interesting feature of this second protein is that it binds much more tightly to defective huntingtin than to the healthy form. It appears that this tightly bound complex causes damage to brain cells. Researchers hope to find simple drugs that can weaken this binding and prevent the damage.
