Abstract
The Period 3 and Clock genes are important components of the mammalian molecular circadian system. Studies have shown association between polymorphisms in these clock genes and circadian phenotypes in different populations. Nevertheless, differences in the pattern of allele frequency and genotyping distribution are systematically observed in studies with different ethnic groups. To investigate and compare the pattern of distribution in a sample of Asian and Caucasian populations living in Brazil, we evaluated two well-studied polymorphisms in the clock genes: a variable number of tandem repeats (VNTR) in PER3 and a single nucleotide polymorphism (SNP) in CLOCK. The aim of this investigation was to search for clues about human evolutionary processes related to circadian rhythms. We selected 109 Asian and 135 Caucasian descendants. The frequencies of the shorter allele (4 repeats) in the PER3 gene and the T allele in the CLOCK gene among Asians (0.86 and 0.84, respectively) were significantly higher than among Caucasians (0.69 and 0.71, respectively). Our results directly confirmed the different distribution of these polymorphisms between the Asian and Caucasian ethnic groups. Given the genetic differences found between groups, two points became evident: first, ethnic variations may have implications for the interpretation of results in circadian rhythm association studies, and second, the question may be raised about which evolutionary conditions shaped these genetic clock variations.
Highlights
Clock genes are a group of genes involved in the regulation of circadian rhythms such as the sleep/wake cycle, cortisol hormone secretion and core body temperature
PER3 is of special interest since it contains a rare polymorphism in the coding region of the human genome, a variable number of tandem repeats (VNTR) with four or five copies of a 54-bp sequence at exon 18, which has recently been reported to be associated with delayed sleep phase syndrome [1,2]
The frequency of the 4/4 genotype in the PER3 gene was higher in the Asian than in the Caucasian group
Summary
Clock genes are a group of genes involved in the regulation of circadian rhythms such as the sleep/wake cycle, cortisol hormone secretion and core body temperature. These rhythms play important roles as physiological processes that determine adaptation to the environmental light/dark cycle. The molecular clock machinery works through a core feedback loop of the transcriptiontranslation of the PER1, PER2, PER3, CRY 1, CRY 2, CLOCK, TIM, and BMAL1 genes These clock genes are expressed robustly in the SNC region, but they are widely expressed in different tissues of the human body as tissue-specific cellular clocks [1,2]. PER3 is of special interest since it contains a rare polymorphism in the coding region of the human genome, a variable number of tandem repeats (VNTR) with four or five copies of a 54-bp sequence at exon 18, which has recently been reported to be associated with delayed sleep phase syndrome [1,2]
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