Junk-DNA” meets Developmental Biology: Investigating the influence of HERV-K(HML-2) on cortical neuronal differentiation

• Applicant: Dr. Michelle Vincendeau
• Subject Area: Developmental Neurobiology ; Developmental Biology ; Cell Biology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496872373

An ongoing challenge is to understand how genome elements influence biological processes and diseases. However, the functions of large parts of the human genome including human endogenous retroviral (HERV) elements remain elusive. Importantly, HERVs comprise ~500,000 “long terminal repeats” (LTRs) on all chromosomes that can direct gene transcription. It has been shown that HERVs are silenced in human neural progenitor cells (hNPCs) by epigenetic regulators such as KAP1. Around 23,000 genomic regions are co-occupied by KAP1 in hNPCs, of which over 85% are located in retrotransposable elements including HERVs. These studies suggest an important role of distinct HERV sequences in neural development. However, thorough analyses of specific HERV groups are missing to underpin their functional contribution to neuronal as well as brain development.We previously modulated the transcriptional levels of multiple LTRs of HERV-K(HML-2) by CRISPRactivation (CRISPRa) and CRISPRinterference (CRISPRi) in order to study their physiological impact on neuronal development. Excitingly, we demonstrate for the first time that elevation of HERV- K(HML-2) transcription has a detrimental impact on development, functionality as well as growth of cortical neurons, but not dopaminergic neurons. Consequently, this leads to less organized layer formations in forebrain organoids. In contrast, transcriptional repression of HERV-K(HML-2) had no effect on cortical differentiation. Moreover, HERV-K(HML-2) transcriptional activation affects the expression of a group of specific cellular genes implicated in neurodegeneration. Based on these data, we conclude that HERV-K(HML-2) plays a decisive role in cortical neuron differentiation.A major question, which remains, is to how HERV-K(HML-2) LTRs influence neighboring genes that contribute to cortical neuronal development. Therefore, in this proposal we will address the following two critical research aspects: (1) identification of activated HERV-K(HML-2) loci and their potential role as promoter or enhancer elements to activate host genes or gene networks during cortical neuronal differentiation using ChIP-seq and subsequent individual LTR knockout studies; (2) investigation of whether specific knockout of the identified HERV-K(HML-2)-regulated host genes can recover the HERV-K(HML-2) mediated phenotype in HERV-K(HML-2) activated cortical neurons. Understanding the basic principles of how HERV-K(HML-2) influences cortical neuronal development will be groundbreaking to understand how HERVs can actively contribute to human brain formation.

• DFG Programme: Research Grants