Do genes cause Alzheimer’s?
Three causative genes have been associated with ‘autosomal dominant’ familial Alzheimer’s disease (APP, PSEN1, and PSEN2), meaning that, if one parent has these genes, and consequently the disease, then half of the offspring will 'inherit' it. The presence of these genes is associated with early-onset Alzheimer’s which can run in families, with the onset almost always occurring before the age of 60. These account for less than 1% of cases of Alzheimer’s disease .
Other genes that predispose a person to Alzheimer’s, thus potentially increasing risk, have been identified. An example is the ApoE4 gene which, if you have it, may increase your risk. One in four have this gene and whether or not apoE4 carriers will suffer from Alzheimer’s depends on the combined effect of the gene and other risk factors that they may experience .
One big area of research is what controls expression of genes such as ApoE4 which, when expressed, tends to lead to the accumulation of amyloid proteins and plaque. In a sense, drawing such a gene card could simply mean that person has less resilience against dietary or lifestyle insults than someone without the gene. Having the ApoE4 gene is thought to account for 4% of the inheritability of risk for Alzheimer’s.
A recent study identified 11 other genes, the presence of which could increase or decrease risk, but none of these genes are proven to be causative . The authors estimated that these 11 genes are thought to account for 7.8% of the inheritability of a person’s risk , however different studies quote different estimates of genetic inheritability. The BBC misrepresented this study, implying that much of Alzheimer’s disease is genetically determined, which it is not.
Studies of identical twins suggest that the inheritable part of Alzheimer’s in those who have the same genes may be as high as 60%. However, this apparent inheritable risk does not mean that the shared genes cause the disease. The expression of these genes leads to changes in the body that may interact with the environment and vice-versa. The results of such twin studies also highlight the importance of shared environmental (inc. diet) factors on ‘inheritability’, suggesting that non-genetic lifestyle factors affect both the risk and timing of Alzheimer’s disease .
The weight of the evidence to date suggests that environmental factors (diet, lifestyle etc.) play a more significant part in determining whether or not a person is likely to develop Alzheimer’s than the presence of Alzheimer’s in a family member or the presence of predisposing genes, especially given that one can change diet and lifestyle, but not one’s genes. It is important to be mindful that diet and lifestyle habits are often similar in family members and therefore can confuse the notion that the disease is ‘inherited’.
10th December 2013
Piloting and validation of a new self-administered online cognitive screening tool in normal older persons: the Cognitive Function Test.
Subtle cognitive changes have been described that may predate the onset of clinically recognizable Alzheimer's disease (AD) and may reflect pathological changes in the brain that are detectable up to 10 years before the onset of AD. Early screening for cognitive status can have benefits in terms of early management and prevention strategies for cognitive decline.
17 September 2013
Association between hypoglycemia and dementia in a biracial cohort of older adults with diabetes mellitus
Hypoglycemia commonly occurs in patients with diabetes mellitus (DM) and may negatively influence cognitive performance. Cognitive impairment in turn can compromise DM management and lead to hypoglycemia.
09 September 2013
Rising obesity. A 'dementia time bomb'
Ever-growing waistlines could result in a big increase in the number of people who develop dementia in the future, researchers have warned. Previous studies have shown that being overweight in middle age increases the odds of developing the mental disorder. Data presented at the European Congress on Obesity suggests stemming the rise in obesity will cut dementia.
Piling on too many pounds is known to be bad for the body, but there is growing evidence that it is also bad for the mind. Diabetes is strongly linked to increasing Alzheimer’s risk. The connection between weight gain and diabetes is insulin. In both cases people become increasingly ‘insulin resistant’, which means their body makes more insulin, and higher insulin levels has consistently be linked to increased risk of Alzheimer’s.
One study of 8,500 Swedish twins showed that those with a body mass index (BMI) greater than 30, who are classified as obese, were almost four times as likely to develop dementia as those with a normal BMI.
Even those who were clinically overweight, a BMI between 25 and 30, were 71% more likely to develop dementia. In England 24% of men and 26% of women are obese. Researchers from the UK Health Forum used computer models to compare what would happen if obesity rates stayed the same or increased to 46% of men and 31% of women by 2050, which has been predicted by some groups.
They said rates of dementia would go from 4,894 cases in every 100,000 people over 65 to 6,662 cases in every 100,000 people over 65. Keeping obesity levels constant would save around £940m in dementia care, the study predicted.
For more details see http://www.bbc.co.uk/news/health-22479049
30 May 2013
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