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Lactase persistence is the continued activity of the enzyme lactase in adulthood. Since lactase's only function is the digestion of lactose in milk, in most mammal species, the activity of the enzyme is dramatically reduced after weaning. In some human populations, though, lactase persistence has recently evolved as an adaptation to the consumption of nonhuman milk and dairy products beyond infancy. The majority of people around the world remain lactase nonpersistent,〔 and consequently are affected by varying degrees of lactose intolerance as adults. However, not all genetically lactase nonpersistent individuals are noticeably lactose intolerant, and not all lactose-intolerant individuals have the lactase nonpersistence allele. ==Genetics== Multiple studies indicate that the presence of the two phenotypes "lactase persistent" and "lactase nonpersistent (hypolactasia)" is genetically programmed, and that lactase persistence is not necessarily conditioned by the consumption of lactose after the suckling period. The lactase persistent phenotype involves high mRNA expression, high lactase activity, and thus the ability to digest lactose, while the lactase nonpersistent phenotype involves low mRNA expression and low lactase activity. The enzyme lactase is encoded by the gene LCT.〔 Hypolactasia is known to be recessively and autosomally inherited, which means that individuals with the nonpersistent phenotype are homozygous and received the two copies of the lactase gene from their parents, who may be homozygous or at least heterozygous.〔 Also, only one active lactase gene is required to be lactase persistent, because lactase persistence is dominant to hypolactasia.〔〔 Lactase persistence behaves as a dominant trait because half levels of lactase activity are sufficient to show significant digestion of lactose.〔 Cis-acting transcriptional silence of the lactase gene is responsible for the hypolactasia phenotype.〔〔 Furthermore, studies show that only eight cases were found where the parents of a child with lactase persistence were both hypolactasic.〔 While a variety of genetic, as well as nutritional, factors determine lactase expression, no evidence has been found for adaptive alteration of lactase expression within an individual in response to changes in lactose consumption levels.〔 The two distinct phenotypes of hypolactasia are: Phenotype I, characterized by reduced synthesis of precursor LPH, and phenotype II, associated with ample precursor synthesis, but reduced conversion of the protein to its mature molecular form. The lactase enzyme has two active sites which break down lactose. The first is at Glu1273 and the second is at Glu1749, which separately break down lactose into two separate kinds of molecules.〔 Two mutations (single-nucleotide polymorphisms - SNPs) have been associated with lactase expression. C−13910 (C at position -13910 upstream of the gene LCT) and G−22018 (G at position -22018) are related to lactase nonpersistence, while T−13910 and A−22018 are related to lactase persistence. Lactase-persistent alleles vary in their geographic distributions. Within European and descendent populations, they are almost entirely correlated with the presence of the -13,910 C/T mutation of the lactase gene (LCT). This differs from LP allelic distributions in East African and Middle Eastern, as well as Northern African, populations. Amongst East African and Middle Eastern groups, the -13915 T/G mutation is the most prominent allelic contributor to lactase persistence. In Northern Africa, the -14010 G/C allele variant is most closely correlated to the trait’s expression. In addition, the lactase gene has a higher expression when T−13910 and A−22018 are present and a lower expression when C−13910 and G−22018 are present.〔 The position -13910 has an enhancer function on the lactase promoter (the promoter facilitates the transcription of the LCT gene). T−13910 is a greater enhancer than C−13910, so this mutation is thought to be responsible for the differences in lactase expression, although not enough evidence is found to prove that lactase persistence is only caused by C−13910→T−13910.〔 In one study involving a Finnish population, a CT SNP at –14 kb was found in all lactase-persistent individuals and absent in all hypolactasia individuals. A second SNP (G-22 kbA) was concordant with phenotype in all but a few rare individuals. Both SNPs being located in the same gene has led to a genetic means of testing lactase expression in individuals. Outside of the Finnish study, a separate study also confirmed that the CT SNP at -14kb is an indicator of lactase persistence, with the exception of two individuals.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lactase persistence」の詳細全文を読む スポンサード リンク
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