APOE and early-onset autosomal dominant Alzheimer disease: Subsequent studies suggest that TBI isa risk factor only in individuals with genetic predisposition related to APOE 4alleles.39 A history ofhead trauma, coupled with the presence of the E4 allele, multiplied the risk ofdeveloping Alzheimer's disease tenfold, while in the absence of the E4 allele riskwas not increased.