Human ApoE has three isoforms differing only by 1 amino acid substitution at either position 112 or 158, resulting in E2, E3, or E4 alleles. Even 1 copy of the E4 allele raises your risk for Alzheimer’s Disease 4-fold and if you’re unfortunate enough to possess an E4 allele from each parent, your risk is increased 12-fold.
ApoE in the brain is distinct from that in the plasma. This study by Linton et al. from the Journal of Clinical Investigation 1991, looked at ApoE phenotypes in patients receiving liver transplants. The authors noted that “in all 29 patients that we studied, the postoperative apoE serum phenotype…converted virtually completely to that of the donor”. The apoE phenotype in the CSF (cerebrospinal fluid) remained unchanged, confirming the different pools of ApoE in serum and CSF/brain.
Of note, the recipients’ apoB-100 took on the donor’s phenotype while their apoB-48 remained unchanged.
Episode 34 of vlmd rounds talks about ApoE4 in the brain and its link to Alzheimer’s.
Study link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC296029/