4. Role of MAOA allelic variants in the ontogeny of aggression
The bulk of clinical evidence on the link between
MAOA and aggression comes from genetic studies on the numerous polymorphic variants of this gene (
Table 1). The richest source of evidence on the functional role of
MAOA in aggression has come from an upstream
variable-number tandem repeat (uVNTR) polymorphism, featuring alleles with different numbers (2, 3, 3.5, 4, 5 and 6) of 30-bp repeats 1.2 kb upstream of the
transcription initiation site (
Sabol et al., 1998,
Huang et al., 2004). The two most common uVNTR alleles, harboring 3 and 4 repeats, are estimated to be present in 35–39% and 59–63% of Caucasians, respectively; conversely, 3-repeat variants are present in the majority of African (52–59%), Asian (53–61%), and Hispanic (70%) Americans (
Sabol et al., 1998,
Rosenberg et al., 2006,
Widom and Brzustowicz, 2006,
Beaver et al., 2013). The particular importance of the uVNTR polymorphism arises from its functional nature: the 3-repeat allele (and, to an even greater extent, the 2-repeat allele) is associated with low transcriptional efficiency of the
MAOA promoter, resulting in lower
enzyme activity than that of the 4-repeat variant (
Sabol et al., 1998,
Deckert et al., 1999,
Denney et al., 1999,
Jonsson et al., 2000).