Abstract
Almost half of the human genome is made up of transposable elements (TEs), and about 8% consists of endogenous retroviruses (ERVs). ERVs are remnants of ancient exogenous retrovirus infections of the germ line. Most TEs are inactive and not detrimental to the host. They are tightly regulated to ensure genomic stability of the host and avoid deregulation of nearby gene loci. Histone-based posttranslational modifications such as H3K9 trimethylation are one of the main silencing mechanisms. Trim28 is one of the identified master regulators of silencing, which recruits most prominently the H3K9 methyltransferase Setdb1, among other factors. Sumoylation and ATP-dependent chromatin remodeling factors seem to contribute to proper localization of Trim28 to ERV sequences and promote Trim28 interaction with Setdb1. Additionally, DNA methylation as well as RNA-mediated targeting of TEs such as piRNA-based silencing play important roles in ERV regulation. Despite the involvement of ERV overexpression in several cancer types, autoimmune diseases, and viral pathologies, ERVs are now also appreciated for their potential positive role in evolution. ERVs can provide new regulatory gene elements or novel binding sites for transcription factors, and ERV gene products can even be repurposed for the benefit of the host.
Highlights
Transposable elements (TEs) are a large component of all eukaryote genomes, comprising a major fraction of all their repetitive sequences
The sequences serving as binding sites for transcription factors and the various determinants of the chromatin state present on the transposable elements (TEs) all contribute to the final distribution throughout the genome over evolutionary times [5]
CpG-rich young long terminal repeats (LTRs) were found to be suppressed by DNA methylation, whereas intermediate age LTRs were predominantly silenced through posttranslational histone modifications such as H3K9me3 [107]
Summary
Transposable elements (TEs) are a large component of all eukaryote genomes, comprising a major fraction of all their repetitive sequences. These endogenized forms of viral sequences become established after germ cell infections flanked by LTRs share high similarities with exogenous retroviral proviruses by exogenous retroviruses. Once successfully integrated into the germ line genome, is probably more than 60 to 70 million years old, whereas the youngest element of the HERV-K family are transmitted vertically by standard Mendelian inheritance. This review focuses on the silencing and transcriptional regulation of ERVs. some regulatory mechanisms have been described solely or for non-LTR retrotransposons and are included briefly in this article, since they might impact the transcriptional regulation of ERVs. Due to the considerable differences between mouse and human ERVs, we discuss the elements in the two species in this review separately. ERV regulation might be applicable to human ERVs, but there may be considerable differences
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