SNP2TFBS - a database of regulatory SNPs affecting predicted transcription factor binding site affinity

SNP2TFBS is a computational resource intended to support researchers investigating the molecular mechanisms underlying regulatory variation in the human genome. The database essentially consists of a collection of text files providing specific annotations for human single nucleotide polymorphisms (SNPs), namely whether they are predicted to abolish, create or change the affinity of one or several transcription factor (TF) binding sites. A SNP's effect on TF binding is estimated based on a position weight matrix (PWM) model for the binding specificity of the corresponding factor. These data files are regenerated at regular intervals by an automatic procedure that takes as input a reference genome, a comprehensive SNP catalogue and a collection of PWMs. SNP2TFBS is also accessible over a web interface, enabling users to view the information provided for an individual SNP, to extract SNPs based on various search criteria, to annotate uploaded sets of SNPs or to display statistics about the frequencies of binding sites affected by selected SNPs. Homepage: http://ccg.vital-it.ch/snp2tfbs/.

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

The evolution of gene expression and binding specificity of the largest transcription factor family in primates

Jeffrey David Jensen, Adamantia Kapopoulou, Lisha Mathew, Didier Trono

The KRAB-containing zinc finger (KRAB-ZF) proteins represent the largest family of transcription factors (TFs) in humans, yet for the great majority, their function and specific genomic target remain unknown. However, it has been shown that a large fraction of these genes arose from segmental duplications, and that they have expanded in gene and zinc finger number throughout vertebrate evolution. To determine whether this expansion is linked to selective pressures acting on different domains, we have manually curated all KRAB-ZF genes present in the human genome together with their orthologous genes in three closely related species and assessed the evolutionary forces acting at the sequence level as well as on their expression profiles. We provide evidence that KRAB-ZFs can be separated into two categories according to the polymorphism present in their DNA-contacting residues. Those carrying a nonsynonymous single nucleotide polymorphism (SNP) in their DNA-contacting amino acids exhibit significantly reduced expression in all tissues, have emerged in a recent lineage, and seem to be less strongly constrained evolutionarily than those without such a polymorphism. This work provides evidence for a link between age of the TF, as well as polymorphism in their DNA-contacting residues and expression levels-both of which may be jointly affected by selection.

Wiley-Blackwell2016

Computational and Statistical analyses of Molecular Evolution and Demography using Large-scale Sequencing Data

Adamantia Kapopoulou

Evolution can be described as the change of allele frequencies over time. Four forces - mutation, migration, genetic drift, and selection, drive this change. The aim of my thesis was to accurately estimate and differentiate the parameters governing each of these four mechanisms by utilizing various types of Next-Generation Sequencing datasets. More specifically, in chapters 1 and 2, I focused on investigating how the past demographic history of African and European D. melanogaster affected its genomic polymorphism. Modern genomes of flies carry signatures of past events such as migration to new regions, adaptation to new environments, and population size changes. By studying whole genome sequences of 29 wild strains from West Africa, 14 from Sweden and comparing them with genomes from Zambia (the putative ancestral range of the species), we were able to report for the first time, colonization time of the Western part of the African continent at approximately 72k years ago. Additionally, we demonstrated the importance of gene flow between the two populations, as well as, current and past effective population sizes. Our estimations confirmed already published predictions (Current Zambian and Swedish population size, ancestral African population size). Finally, we demonstrated the importance of inversions when accounting for demographic events of D. melanogaster. In chapter 3 of my thesis, I evaluated the importance of selection acting on the DNA-binding residues of the biggest family of transcription factors in the primates, namely KRAB-ZF genes. We were able to demonstrate the existence of two distinct sub-groups, based on the type of polymorphism (synonymous or not) carried by the DNA-contacting nucleotides. The two groups of genes differ by their expression breadth and intensity, as well as at the number of paralogs and orthologs and their evolutionary age. Additionally, we manually annotated the complete catalog of human KRAB-ZF genes, thereby providing a valuable resource for further investigation of this family of genes. In conclusion, the work carried during my thesis enabled to refine the evolution and demography of D. melanogaster African and Northern European populations, underlying the importance of modelling migration flows between populations for accurate estimation of split time. The second component of my thesis demonstrated the applicability of transcriptomics and epigenomics datasets to study evolution of the KRAB-ZF family. The proposed methodologies are applicable to other transcription factor gene families and our manually curated dataset is relevant to other scientists deciphering the function of these genes.