Identification and characterisation of murine metastable epialleles conferred by endogenous retroviruses
Repetitive sequences, including transposable elements, represent approximately half of the mammalian genome. Epigenetic mechanisms evolved to repress these potentially deleterious mobile elements. However, such elements can be variably silenced between individuals – so called ‘metastable epialleles’. The best known example is the Avy locus where an endogenous retrovirus (ERV) of the intracisternal A-particle (IAP) class was spontaneously inserted upstream of the agouti coat colour gene, resulting in variable IAP promoter DNA methylation, variable expressivity of coat phenotype, and environmentally modulated transgenerational epigenetic inheritance within genetically identical individuals. It is not known whether the behaviour exhibited by the ERV at Avy represents a common occurrence throughout the genome or is unusual. Taking a genetic approach in purified cell populations, I have conducted a systematic genome-wide screen of murine metastable epialleles. I have identified over 100 murine IAPs with properties of metastable epialleles. Like Avy, each exhibits a stable epigenetic state within an individual but epigenetic variability between individuals. Methylation levels are locus-specific within an individual, suggesting cis-acting control. The same screening strategy was applied for identification of metastable epialleles associated with other types of LTR-retroelements. However, many of identified candidates showed no inter-individual methylation variation upon experimental validation. These results suggest that IAPs are the dominant class of ERVs capable of acquiring epigenetic states that are variable between genetically identical individuals. I have conducted an analysis of IAP induced initiation and termination of transcription events using de novo assembled transcriptomes generated for B and T cells. 143 IAPs have been identified to overlap de novo assembled transcripts. 33 IAPs are metastable epialleles. Several of them show an inverse correlation between LTR promoter methylation and adjacent gene expression. In addition, I have shown that metastable epialleles have a characteristic pattern of histone modification and are flanked by the methylation sensitive binding factor CTCF, providing testable hypotheses concerning the establishment and/or maintenance of the variable methylation state. My findings indicate that metastability is, in general, specific to the IAP class of ERVs, that only around 1% of these elements have this unusual epigenetic property and that the ability to impact transcription, such as at agouti in Avy, is not a ubiquitous feature of these loci.
Methylation, IAP, Metastable epiallele, Transposable element
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