Fig. 5.

Epigenetic regulation of sex-specific P450 genes. Female-specific Cyp genes are proposed to be repressed in male liver, and male-specific Cyp genes are proposed to be repressed in female liver by packaging in heterochromatin. Continuous GH is proposed to activate female-specific genes, such as Cyp3a genes, by a mechanism that involves the local conversion of heterochromatin to euchromatin, which enables the binding of transcription factors (TF) that activate P450 gene expression. This process could involve loss of DNA CpG methylation and/or loss of chromatin marks associated with repressed chromatin, such as histone H3 lysine 27 trimethylation, which is typically found in genes in a compact chromatin structure and is associated with a stable, inactive heterochromatic state (Cheung and Lau, 2005).