Tessa M. Bertozzi

Transposable elements (TEs) have made important contributions to the evolution of the placenta, and are argued to have played a role in the wide inter-species diversification of this critical developmental organ. Co-option of TEs by host genomes has led to the genesis of important placental genes, as well as trophoblast-specific gene regulatory elements. In mice, past work has demonstrated how multiple species-specific TE subfamilies are used as transcriptional enhancers in trophoblast stem cells. However, the involvement of TEs in the regulation of mouse placental gene expression in vivo remains unclear. Here, we characterised the TE regulatory and transcriptional landscape in mouse placenta and gauged their evolutionary dynamics through a comparative approach. We found that overall, TE cis-regulatory activity is greatly diminished in differentiated mouse trophoblast when compared to their stem cell counterpart. On the other hand, evolutionarily young intracisternal A particle (IAP) elements are highly expressed in the placenta and create several alternative, placenta-specific transcriptional start sites for protein-coding genes. Placenta-expressed IAP elements are genetically polymorphic between mouse strains and drive species-specific expression of associated genes. These putative co-option events are therefore recent and may represent a prime example of how TE activity can drive fast placental evolution.

Prion disease is caused by misfolding of the prion protein (PrP) into pathogenic self-propagating conformations, leading to rapid-onset dementia and death. However, elimination of endogenous PrP halts prion disease progression. In this study, we describe Coupled Histone tail for Autoinhibition Release of Methyltransferase (CHARM), a compact, enzyme-free epigenetic editor capable of silencing transcription through programmable DNA methylation. Using a histone H3 tail-Dnmt3l fusion, CHARM recruits and activates endogenous DNA methyltransferases, thereby reducing transgene size and cytotoxicity. When delivered to the mouse brain by systemic injection of adeno-associated virus (AAV), Prnp -targeted CHARM ablates PrP expression across the brain. Furthermore, we have temporally limited editor expression by implementing a kinetically tuned self-silencing approach. CHARM potentially represents a broadly applicable strategy to suppress pathogenic proteins, including those implicated in other neurodegenerative diseases.

Intracisternal A-particles (IAPs) are endogenous retroviruses (ERVs) responsible for most insertional mutations in the mouse. Full-length IAPs harbour genes flanked by long terminal repeats (LTRs). Here, we identify a solo LTR IAP variant (Iap5-1^solo) recently formed in the inbred C57BL/6J mouse strain. In contrast to the C57BL/6J full-length IAP at this locus (Iap5-1^full), Iap5-1^solo lacks DNA methylation and H3K9 trimethylation. The distinct DNA methylation levels between the two alleles are established during preimplantation development, likely due to loss of KRAB zinc finger protein binding at the Iap5-1^solo variant. Iap5-1^solo methylation increases and becomes more variable in a hybrid genetic background yet is unresponsive to maternal dietary methyl supplementation. Differential epigenetic modification of the two variants is associated with metabolic differences and tissue-specific changes in adjacent gene expression. Our characterisation of Iap5-1 as a genetically induced epiallele with functional consequences establishes a new model to study transposable element repression and host-element co-evolution.

Intracisternal A-particles (IAPs) are endogenous retroviruses (ERVs) responsible for most insertional mutations in the mouse. Full-length IAPs harbour genes flanked by long terminal repeats (LTRs). Here, we identify a solo LTR IAP variant ( C57iap1 ^solo ) recently formed in the inbred C57BL/6J mouse strain. In contrast to the C57BL/6J full-length IAP at this locus ( C57iap1 ^full ), C57iap1 ^solo lacks DNA methylation and H3K9 trimethylation. The distinct DNA methylation levels between the two alleles are established during preimplantation development, likely due to loss of KRAB zinc finger protein binding at the C57iap1 ^solo variant. C57iap1 ^solo methylation increases and becomes more variable in a hybrid genetic background yet is unresponsive to maternal dietary methyl supplementation. Differential epigenetic modification of the two variants is associated with metabolic differences and tissue-specific changes in adjacent gene expression. Our characterisation of C57iap1 as a genetically-induced epiallele with functional consequences establishes a new model to study transposable element repression and host-element co-evolution.