{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Arylalkylamine N‐acetyltransferase (AANAT) is the key, rate‐limiting enzyme in melatonin biosynthesis that converts serotonin into N‐acetylserotonin, which is then converted into melatonin by hydroxyindole O‐methyltransferase. Its expression is tightly regulated by light, as illustrated by the photic suppression of AANAT transcript levels in models of nonvisual photoreception."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " Although the pineal gland is its classical source, AANAT is also expressed in several extrapituitary tissues. For example, reduced AANAT expression and enzymatic activity in placental tissue are associated with diminished melatonin levels in preeclamptic pregnancies, suggesting both diagnostic and therapeutic implications."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": " In addition, in the context of colon cancer, functional melatonin synthesis governed by AANAT (in conjunction with acetylserotonin methyltransferase) appears to oppose tumor-promoting pathways via downregulation of endothelin-1."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": " Genetic studies have identified polymorphisms in the AANAT gene that may influence its activity and thereby contribute to interindividual differences in melatonin production and drug metabolism."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": " Furthermore, expression of AANAT in the epithelial cells of salivary glands supports the notion that melatonin is locally produced in the oral cavity and could act in autocrine or paracrine roles."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": " While investigations into AANAT gene variability in adolescent idiopathic scoliosis did not reveal an association, the study reinforces the need to consider AANAT regulation in conditions potentially linked to altered melatonin synthesis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": " Likewise, common promoter variants in AANAT have been significantly associated with major depression, lending support to a role for disrupted melatonin biosynthesis in psychiatric disorders."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": " In contrast, analyses in human gallbladder tissue noted an absence of detectable AANAT, suggesting that in some peripheral tissues alternative pathways (for example, involving HIOMT) may predominate in generating serotonin derivatives."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": " Collectively, these findings underscore the pivotal role of AANAT in controlling melatonin levels—which in turn modulate circadian rhythms, reproductive functions, and cell proliferation—and highlight its clinical relevance in diverse pathological conditions.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Satchidananda Panda, Ignacio Provencio, Daniel C Tu, et al. 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