Animal Molecular Breeding, 2025, Vol.15, No.2, 60-71 http://animalscipublisher.com/index.php/amb 62 Figure 1 Schematic diagram of hair follicles (Adopted from Ji et al., 2023) 3 Summary of Epigenetic Regulation Mechanisms 3.1 The role of DNA methylation in gene expression regulation DNA methylation is one of the earliest epigenetic markers discovered, referring to the addition of methyl groups on the cytosine (C) nucleotide in the DNA sequence, which usually occurs at the cytosine-guanine dinucleotide (CpG) site. On high-density CpG islands in gene promoter regions, methylation often causes transcription factors to fail to bind to DNA, thereby repressing gene transcription (Slawinska et al., 2020). Therefore, DNA methylation is often associated with gene silencing. Studies have shown that increased methylation levels can reduce the expression of muscle growth-related genes, which in turn affects the growth rate of meat poultry. DNA methylation is mediated by DNA methyltransferase (DNMT) family enzymes, in which DNMT1 is mainly responsible for maintaining methylation, and DNMT3A/3B is responsible for creating new methylation. Early in the embryo, most of the genome undergoes a "demethylation-reprogramming" process, which is then remethylated at specific sites in development (Wang and Ibeagha-Awemu, 2021). Different tissues and cell types have specific methylation maps, giving them different gene expression patterns. In poultry, DNA methylation is widely involved in the developmental regulation of muscle, fat, liver and other tissues. Studies have pointed out that changes in feeding environment (such as increased temperature) can lead to an increase in the methylation level of growth hormone receptor (GHR) and IGF1 gene promoter in duck liver, a decrease in gene expression, and ultimately inhibit growth. 3.2 Chromatin remodeling and histone modification Eukaryotic DNA wraps histones to form chromatin unit nucleosomes, and their structural dynamic changes affect the accessibility of genes. Chromatin remodeling refers to the process of changing the conformation (open or tightening) of chromatin, which is achieved by the hydrolysis of the remodeling complex using ATP. There are two key mechanisms: one is to slide or remove nucleosomes to expose specific DNA regions to facilitate transcription; the other is to covalently modify histones to change the chromatin state. A variety of covalent modifications can occur at the N-terminal tail of histones. Different histone modifications have different effects on gene expression: for example, lysine acetylation at 4 (H3K4ac) and 9 (H3K9ac) of histone H3 are usually associated with active transcription because acetylation neutralizes histone positive charge, weakens DNA-histone binding, and relaxes chromatin; while modifications such as lysine trimethylation at 27 (H3K27me3) of histone H3 are associated with transcriptional repression because they recruit silencing complexes to coagulate chromatin. During development, chromatin remodeling and histone modification closely coordinate to regulate gene spatiotemporal expression patterns. In poultry studies, although data on histone modifications for ducks are limited, similar mechanisms can be speculated based on other species. Studies have found in chicken muscles that
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