The apolipoprotein E (ApoE) epsilon 4 and epsilon 2 alleles may influence the age of onset of depressive illness. Depressive illness of late onset is also a risk factor for Alzheimer's disease (AD), and there is some evidence that the ApoE epsilon 2 allele is associated with depressive symptomatology in AD. Depressive symptomatology in AD may thus share common genetic risk factors with late-onset depressive illness.
The frequency of the epsilon 2 and epsilon 4 alleles of ApoE and their effects on age of onset of disease in three independent groups of subjects, with depressive illness, with AD, and controls, were compared in a defined population from Southeast London.
The frequency of the ApoE epsilon 2 allele was significantly lower in the depressive illness group compared with the control group and was associated with a later mean age at onset. Subjects with depressive symptomatology in AD had a higher frequency of the ApoE epsilon 2 allele and had a significantly later age of onset of depressive illness compared with the nondepressed AD group.
The presence of the ApoE epsilon 2 allele in AD is found to be highly associated with depressive symptomatology, and it is proposed that this subgroup represents the presence of delayed depressive illness and that there are common genetic risk factors between AD and depressive illness.