The pesticide dichlorodiphenyldichloroethylene (DDE) is a metabolite of the chemical dichlorodiphenyltrichloroethane (DDT), a once widely used agricultural and antimalarial insecticide that was subsequently banned in the United States but which, because of its long half-life, is still present in the environment. When results of a small pilot study revealed elevated levels of DDE in 20 patients with Alzheimer's disease, researchers proceeded with a larger and more comprehensive study to investigate this possible association further, as well as DDE's relationship with the apolipoprotein E4 (apo E4) allele, a gene known to increase the risk of Alzheimer's disease.

The researchers used serum samples drawn from patients at two separate Alzheimer's disease research centers in the United States between 2002 and 2008. A total of 165 samples were collected; 79 served as controls, and 86 were from confirmed Alzheimer's disease patients.

The apo E4 allele was detected in 35% of the control cases and in 65% of the cases with Alzheimer's disease. DDE was present in 70% of the control cases and 80% of the Alzheimer's disease cases. The samples from patients with Alzheimer's disease had a mean level of DDE 3.8 times higher than that in the control samples. The researchers also found that the highest levels of DDE corresponded to significantly lower scores on the Mini-Mental State Examination (MMSE). Patients in this high-DDE group had 4.18 times greater odds of Alzheimer's disease than patients in the lowest tertile. Lower scores on the MMSE were especially pronounced in patients whose serum samples showed not only high levels of DDE but also the apo E4 allele.

Of note, however, was that approximately 20% of the samples from patients with Alzheimer's disease had undetectable levels of DDE and that some control patients had high levels of DDE. This suggested that exposure to DDE may not be a factor in all cases of Alzheimer's disease.-AK

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