Genome-Wide Young L1 Methylation Profiling by bs-ATLAS-seq

Ultra-sensitive detection of transposon insertions across multiple families by transposable element display sequencing

BACKGROUND

Mobilization of transposable elements (TEs) can generate large effect mutations. However, due to the difficulty of detecting new TE insertions in genomes and the typically rare occurrence of transposition, the actual rate, distribution, and population dynamics of new insertions remain largely unexplored.

RESULTS

We present a TE display sequencing approach that leverages target amplification of TE extremities to detect non-reference TE insertions with high specificity and sensitivity, enabling the detection of insertions at frequencies as low as 1 in 250,000 within a DNA sample. Moreover, this method allows the simultaneous detection of insertions for distinct TE families, including both retrotransposons and DNA transposons, enhancing its versatility and cost-effectiveness for investigating complex "mobilomes." When combined with nanopore sequencing, this approach enables the identification of insertions using long-read information and achieves a turnaround time from DNA extraction to insertion identification of less than 24 h, significantly reducing the time-to-answer. By analyzing a population of Arabidopsis thaliana plants undergoing a transposition burst, we demonstrate the power of the multiplex TE display sequencing to analyze "evolve and resequence" experiments. Notably, we find that 3-4% of de novo TE insertions exhibit recurrent allele frequency changes indicative of either positive or negative selection.

CONCLUSIONS

TE display sequencing is an ultra-sensitive, specific, simple, and cost-effective approach for investigating the rate and landscape of new TE insertions across multiple families in large-scale population experiments. We provide a step-by-step experimental protocol and ready-to-use bioinformatic pipelines to facilitate its straightforward implementation.

Locus-level L1 DNA methylation profiling reveals the epigenetic and transcriptional interplay between L1s and their integration sites

Long interspersed element 1 (L1) retrotransposons are implicated in human disease and evolution. Their global activity is repressed by DNA methylation, but deciphering the regulation of individual copies has been challenging. Here, we combine short- and long-read sequencing to unveil L1 methylation heterogeneity across cell types, families, and individual loci and elucidate key principles involved. We find that the youngest primate L1 families are specifically hypomethylated in pluripotent stem cells and the placenta but not in most tumors. Locally, intronic L1 methylation is intimately associated with gene transcription. Conversely, the L1 methylation state can propagate to the proximal region up to 300 bp. This phenomenon is accompanied by the binding of specific transcription factors, which drive the expression of L1 and chimeric transcripts. Finally, L1 hypomethylation alone is typically insufficient to trigger L1 expression due to redundant silencing pathways. Our results illuminate the epigenetic and transcriptional interplay between retrotransposons and their host genome.

Evolutionary insights from profiling LINE-1 activity at allelic resolution in a single human genome

Transposable elements have created the majority of the sequence in many genomes. In mammals, LINE-1 retrotransposons have been expanding for more than 100 million years as distinct, consecutive lineages; however, the drivers of this recurrent lineage emergence and disappearance are unknown. Most human genome assemblies provide a record of this ancient evolution, but fail to resolve ongoing LINE-1 retrotranspositions. Utilizing the human CHM1 long-read-based haploid assembly, we identified and cloned all full-length, intact LINE-1s, and found 29 LINE-1s with measurable in vitro retrotransposition activity. Among individuals, these LINE-1s varied in their presence, their allelic sequences, and their activity. We found that recently retrotransposed LINE-1s tend to be active in vitro and polymorphic in the population relative to more ancient LINE-1s. However, some rare allelic forms of old LINE-1s retain activity, suggesting older lineages can persist longer than expected. Finally, in LINE-1s with in vitro activity and in vivo fitness, we identified mutations that may have increased replication in ancient genomes and may prove promising candidates for mechanistic investigations of the drivers of LINE-1 evolution and which LINE-1 sequences contribute to human disease.