Variability of age at onset in siblings with familial Alzheimer disease.

Background Variability of age at onset (AO) of Alzheimer disease (AD) among members of the same family is important as a biological clue and because of its clinical effects. Objective To evaluate which clinical variables influence the discrepancy in AO among affected relatives with familial AD. Setting Clinical genetic project of Spanish kindred with AD conducted by 4 academic hospitals in Madrid, Spain. Methods Age at onset of AD in 162 families and discrepancy in AO in intragenerational and intergenerational affected pairs were analyzed in relation to age, sex, maternal or paternal transmission, pattern of inheritance, and apolipoprotein E genotype. Results Maternal transmission of AD was significantly more frequent than paternal transmission ( P P  = .02). Segregation analysis showed no differences in the inheritance pattern between families with low (≤5 years) or high (>5 years) AO discrepancy. Age at onset in carriers of the apolipoprotein E e 4 allele was slightly younger. However, among siblings, an extra apolipoprotein E e 4 allele was not consistently associated with earlier onset of AD. Eighty percent of patients, independent of sex or mode of transmission, were already affected at their parents' reported AO. Conclusions There is a wide discrepancy in AO in affected siblings that is not clearly explained by a single clinical variable or apolipoprotein E genotype. The interaction of many factors probably determines AO in each affected individual. However, maternal transmission of AD seems to result in a similar AO in offspring, and the risk of developing dementia after the parent's reported AO decreases significantly.