Transposable elements contribute to the spatiotemporal microRNA landscape in human brain development

Transposable elements (TEs) contribute to the evolution of gene regulatory networks and are dynamically expressed throughout human brain development and disease. One gene regulatory mechanism influenced by TEs is the miRNA system of post-transcriptional control. miRNA sequences frequently overlap TE loci and this miRNA expression landscape is crucial for control of gene expression in adult brain and different cellular contexts. Despite this, a thorough investigation of the spatiotemporal expression of TE-embedded miRNAs in human brain development is lacking. Here, we identify a spatiotemporally dynamic TE-embedded miRNA expression landscape between childhood and adolescent stages of human brain development. These miRNAs sometimes arise from two apposed TEs of the same subfamily, such as for L2 or MIR elements, but in the majority of cases stem from solo TEs. They give rise to in silico predicted high-confidence pre-miRNA hairpin structures, likely represent functional miRNAs and have predicted genic targets associated with neurogenesis. TE-embedded miRNA expression is distinct in the cerebellum when compared to other brain regions, as has previously been described for gene and TE expression. Furthermore, we detect expression of previously non-annotated TE-embedded miRNAs throughout human brain development, suggestive of a previously undetected miRNA control network. Together, as with non-TE-embedded miRNAs, TE-embedded sequences give rise to spatiotemporally dynamic miRNA expression networks, the implications of which for human brain development constitute extensive avenues of future experimental research. To facilitate interactive exploration of these spatiotemporal miRNA expression dynamics, we provide the "Brain miRTExplorer" web application freely accessible for the community.

KRAB zinc-finger proteins and their transposable element targets: between antagonism and cooperation

Jonas Caspar De Tribolet-Hardy

There are 377 Krüppel-associated box (KRAB) domain-containing zinc finger proteins (KZFPs) in the human genome, making them the largest family of transcription factors. KZFPs are defined by a N-terminal KRAB domain and several zinc-finger domains arranged in an array at the C-terminus of the proteins. The zinc-finger domains each form sequence specific interactions with double stranded DNA, allowing the zinc-finger array to target specific genomic sequences. The KRAB domain, through its interaction with the protein TRIM28 (also known as KAP1) allows for the stable silencing of transcription in a genomic region. Together these two domains allow KZFPs to bind specific regions of the genome and generally lead to the formation of heterochromatin and silencing of transcription allthough other functions for some KZFPs have been recently reported. KZFPs usually target transposable elements (TEs), mobile genomic elements that can move through the genome either by cut-and-paste or copy-paste mechanisms and make up almost half of the human genome. Different KZFPs bind to specific regions of TEs, limiting their expression and thus mitigating some of the threat they pose to human development, allowing both the TE and its host to survive. In recent years it became apparent that both certain TEs and KZFPs have roles beyond the previously described threat and mitigation of that threat. TEs harbor gene regulatory regions allowing, through their spread, for a dissemination of those regions through the genome and for new gene regulatory networks to form. KZFPs can affect the transcription of genes located in proximity to their binding sites leading to changes in gene regulation as well. A multitude of regulatory roles involving KZFPs and TEs have been described in recent years making them an exciting field of study. A major hurdle in understanding both KZFPs and TEs is to identify the binding sites of KZFPs, as they reveal both the targets of the KZFP and how, if at all, a TE is regulated by KZFPs. To do so, we expanded on previous efforts and aimed to perform chromatin immunoprecipitation followed by massively parallel sequencing (ChIP-seq) on every member of the human KZFP family. Here we present ChIP-seq experiments for 110 new KZFPs, which together with previously published data, allow us to study the binding of almost all human KZFPs (95%). The entirety of the data was analysed together to generate a coherent dataset and results for each KZFP are made available to the public on our web portal (https://tronoapps.epfl.ch/web/krabopedia/). The identified binding sites allowed us to witness the adaptation of the KZFP family to new TEs and showed how targeted sequences shift after segmental gene duplication events. Furthermore, we could corroborate that several KZFPs target the same TE subfamilies in a seemingly redundant fashion and show that unexpectedly these KZFPs arose independently in different genomic locations. These results represent a valuable tool for anyone studying KZFPs and should aid in the pursuit of both understanding KZFPs and TEs.

EPFL2022

A cis-acting structural variation at the ZNF558 locus controls a gene regulatory network in human brain development

Johan Jakobsson, Julien Paul André Pontis, Didier Trono

The human forebrain has expanded in size and complexity compared to chimpanzees despite limited changes in protein-coding genes, suggesting that gene expression regulation is an important driver of brain evolution. Here, we identify a KRAB-ZFP transcription factor, ZNF558, that is expressed in human but not chimpanzee forebrain neural progenitor cells. ZNF558 evolved as a suppressor of LINE-1 transposons but has been co-opted to regulate a single target, the mitophagy gene SPATA18. ZNF558 plays a role in mitochondrial homeostasis, and loss-of-function experiments in cerebral organoids suggests that ZNF558 influences developmental timing during early human brain development. Expression of ZNF558 is controlled by the size of a variable number tandem repeat that is longer in chimpanzees compared to humans, and variable in the human population. Thus, this work provides mechanistic insight into how a cis-acting structural variation establishes a regulatory network that affects human brain evolution.

2022

Integrative transcriptome analyses of Transposable Elements (TEs) and genes contribute to a better understanding of colorectal cancer

Eunji Shin

Cancer is the second leading cause of death worldwide. Cancer develops through multiple hallmark functions including apoptosis evasion, unlimited replicative potential, metastasis, and immune avoidance. Over the past few decades, researchers have reported a substantial amount of information about the role of gene expression dysregulation in cancer, whereas transposable elements (TEs) have been overlooked despite the fact they constitute nearly half of the human genome. TEs are DNA repetitive sequences that spread across the genome of living organisms throughout the evolution of species, contributing to genomic diversity and shaping the epigenomic landscape. The vast majority of TEs appear to be transcriptionally silent in adult tissues, thus avoiding deleterious mutational insertions and recombination events in somatic tissues. Due to the global epigenetic dysregulation that occurs during tumorigenesis, some TEs lose repressive marks and become transcriptionally active in cancer cells. Consequently, TE insertions into oncogenes and tumor suppressor genes may occur, thereby contributing to cancer development and disease progression. It is also widely accepted that some TEs harbor transcription factor recognition sites and have regulatory functions on gene expression. Some transcriptionally active TEs can give rise to alternative TE-derived chimeric gene products. Given these facts, integrative transcriptome analysis of TEs and genes may contribute to a better understanding of complex traits of cancer and help to better classify cancer subtypes with clinical implications for patients.To gain insights into the underlying mechanisms of cancer pathogenesis, we constructed a co-expression network based on the similarity in expression levels of TEs and genes. We focused on colorectal cancer (CRC), a heterogeneous disease with different genetic and molecular backgrounds, contributing to patient outcomes and response to therapy. We investigated the coordinated activity of TEs and genes in view of recurrent CRC chromosomal aberrations, genome organization, and functional significance. We further examined co-expression changes in network structure under the contrasting conditions of cancerous versus matched healthy colic mucosal tissues. We found that the cancer network was associated with a dramatic decrease in the number of gene interactions as opposed to an increase in TE interactions. Physical chromosomal distance affects the degree of co-expression, where the proximal distance effect of TE is contrasted with the distant effect of the gene. Our study sheds light on the complex interplay between TEs and genes and how they coordinately shape cancer and normal co-expression networks. Furthermore, we integrated gene and TE expression to develop a new consensus molecular subtype (CMS) classifier derived from multiple layers of TE and gene signatures. We demonstrated that our new approach led to better identification of CRC patients of the former CMS3 subtype, the most heterogeneous subtype with mixed biological characteristics, and a global reduction of the expected misclassification of other CMS subtypes. We further optimized our classifier for potential clinical use and obtained a set of 50 genes and 20 TE integrants with the best ability to identify CMS subtypes. Our integrative model could be a major add on for the development of future clinical trials by improving patient stratification over current gene-restricted molecular classifications.

EPFL2022

KRAB zinc-finger proteins and their transposable element targets: between antagonism and cooperation

Jonas Caspar De Tribolet-Hardy

EPFL2022

Integrative transcriptome analyses of Transposable Elements (TEs) and genes contribute to a better understanding of colorectal cancer

Eunji Shin

EPFL2022