Comparisons
Part 1

A comparison of HD to Alzheimer's andParkinson's diseases

Risk Factors

HD is caused by a mutation in the Huntington gene, which lies on chromosome 4. A certain sequence of DNA (C-A-G) of the Huntington gene is repeated multiple times. Generally, if a person has 35 or fewer copies of CAG on a particular segment of the Huntington gene, the person will not get HD. However, if he or she has 40 or more copies, he or she will get the disease. The greater the number ofrepeats, the more likely it is that the person will develop symptoms and the greater the chance that these symptoms will occur at a younger age. Every child of a parent with HD has a 50% chance of inheriting the disease, and the disease may occur earlier and more severely in each succeeding affected generation because the number of repeats canincrease. For more info on inheritance, click here.

Like Huntington's disease, family genetics present a significant risk factor for inheriting Alzheimer's disease. A study performed by Erasmus University Medical School in The Netherlands found that the risk of developing Alzheimer's disease for those with at least one affected immediate relative was 3.5 times greater than those with no affected relatives. The study also showed a significant association between Alzheimer's disease and family history of Down's Syndrome, as well as an increased risk in with a family history of Parkinson's disease. Perhaps the most significant risk factor of Alzheimer's disease is age. Although there is variability in the age ofHD symptom onset, the incidence of Alzheimer's increases with age, doubling every five years from 1% at 60 years to as many as 50% for those over 85 years of age. This is because normal aging is associated with altered protein metabolism, a process that often leads to the degradation of brain cells and the formation of abnormal clumps of protein in the brain over time. Other risk factors for Alzheimer's disease include Down's syndrome, untreated chronic hypertension, high cholesterol, and sustained head injuries.

Age is also the main risk factor for developing Parkinson's disease. A subtype of Parkinson's disease called young-onset Parkinson's disease affects those younger than 40; however, most of those affected do not experience symptoms until after age 55. Although controversial, researchers now think genetics may play a rolein the development of the disease. The recent discovery of an abnormal protein called a-synuclein in an Italian family with many Parkinson'ssufferers has contributed to the understanding of Parkinson's disease. As of early 2002, there were nine genetic abnormalities that had beenassociated with Parkinson's disease. New studies show that having one close relative with Parkinson's may increase the chances of developing Parkinson's three or four-fold, and having two or more relatives may increase the chances ten-fold. However, most often a definite family history is not present for most patients. In these sporadic cases, agenetic predisposition may still play a role by increasing the chance of getting the disease when patients are exposed to possible environmental risk factors such as certain pesticides and herbicides. Other risk factors that may contribute to Parkinson's disease include reduced estrogen levels and low folate levels (see section onnutrition).

Last Modified: 5-20-04

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