Recently, Luke reported that I am a carrier of the E4 allele at the gene APOE; this gives me approximately double the average risk for late-onset Alzheimer’s disease. I didn’t think too much about this–it’s only double the risk, and in any case I’m 28 years old. But I recently came across the below plot, by Nick Eriksson (I’ve re-plotted it). It shows the frequency of the APOE4 allele plotted against average age in 15 cohorts of “cognitively normal elders” (data from here).
If we assume that these 15 cohorts are all from relatively similar populations, the interpretation of this is that, between the ages of 70 and 85, people with my genotype go from being cognitively normal elders to not (due to Alzheimer’s, another form of dementia, or death) at a rate about twice that of people who don’t carry the E4 allele . This, of course, is exactly what I knew before (that E4 carriers have double the risk of Alzheimer’s), but seeing this visually is quite striking.
 Could the drop in APOE4 allele frequency could be mostly due to E4/E4 homozygotes (i.e. people not of my genotype)? If we assume an initial allele frequency of 20% and Hardy-Weinburg equilibrium, then a fifth of the APOE4 alleles are present in homozygotes. So even if all of these individual developed Alzheimer’s, then this would drop the allele frequency from 20% to ~16%. The observed drop in allele frequency is much greater than that.
For many diseases we have very little ability to determine who is at high or low risk; the risk factors are unreplicated, complicated, or understudied. However, for other diseases we can do much better. Alzheimer’s disease is a form of senile dementia that is characterised by abnormal clustering of proteins in the brain (right). We know a number of important risk factors for Alzheimer’s, and knowing your own risk factors may seriously change your estimate of the chance of developing the disease. But how can you calculate this risk?
This is going to be somewhat of an information deluge, as I go through everything to think about when you predict a complex disease, including how to calculate genetic and environmental risks, and how important these risks are, both individually and all together. I will demonstrate all of the calculations on the various GNZ contributors, and in particular how I have worked out my own risk.
I’ll measure the risks in terms of odds ratios; you may want to read the introduction to Carl’s post from earlier this year to refresh your mind on what this means. I will also use the disease probability; this is simply the chance of developing Alzheimer’s, or equally, the percentage of people with this set of risk factors who will develop the disease.
Also note that an important factor to consider is the baseline lifetime risk, the total proportion of people who will develop Alzheimer’s before they die. I am going to use a lifetime risk of 9% for men and 17% for women, taken from an Alzheimer’s Association report, but getting a good estimate of this is actually very difficult, and will vary from country to country.
If you want to know more about Alzheimer’s, including prevention, diagnosis and treatment, you can read about the disease on the Mayo Clinic or NHS Choices websites.
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