The Genetics of the Human APOE Polymorphism

The genetic origin of the three common variants of the human apolipoprotein E (apoE) protein, known as E2, E3 and E4, was understood in 1981, and since the mid 1980s these are probably the most-studied protein variants in human races. They have been related to a number of age-related diseases, including Alzheimer disease, as well as to healthy aging and longevity. The gene variants underlying these protein isoforms, known as epsilon 2, epsilon 3, and epsilon 4, are allelic forms of the APOE gene, resulting from different haplotypes at the APOE locus (19q13.31). In particular, they result from three of the four haplotypes expected by the combinations of the alleles of the two single-nucleotide polymorphisms rs429358 and rs7412. The fourth missing haplotype, known as epsilon 3r, has been identified in only two Caucasian families from Italy and in one Yoruba family from Nigeria worldwide. Thus, this fourth APOE gene variant is rare, and it encodes a protein isoform, identified as E3r, showing identical physical characteristics to E3, that conversely, is the most common form of apoE in humans. In this review article, we report the identification of the haplotype epsilon 3r in a third Caucasian family from Italy, and then attempt to re-examine the current knowledge regarding the APOE polymorphism, taking into account this fourth haplotype. We also focus on the commonly accepted hypothesis for the evolution of the common APOE gene variants, in which we include the epsilon 3r haplotype, previously not considered.