Contribution of Human Retroviruses to Disease Development-A Focus on the HIV- and HERV-Cancer Relationships and Treatment Strategies

Functional Bidirectionality of ERV-Derived Long Non-Coding RNAs in Humans

Human endogenous retroviruses (HERVs) are widely recognized as the result of exogenous retroviruses infecting the ancestral germline, stabilizing integration and vertical transmission during human genetic evolution. To date, endogenous retroviruses (ERVs) appear to have been selected for human physiological functions with the loss of retrotransposable capabilities. ERV elements were previously regarded as junk DNA for a long time. Since then, the aberrant activation and expression of ERVs have been observed in the development of many kinds of human diseases, and their role has been explored in a variety of human disorders such as cancer. The results show that specific ERV elements play respective crucial roles. Among them, long non-coding RNAs (lncRNAs) transcribed from specific long-terminal repeat regions of ERVs are often key factors. lncRNAs are over 200 nucleotides in size and typically bind to DNA, RNA, and proteins to perform biological functions. Dysregulated lncRNAs have been implicated in a variety of diseases. In particular, studies have shown that the aberrant expression of some ERV-derived lncRNAs has a tumor-suppressive or oncogenic effect, displaying significant functional bidirectionality. Therefore, theses lncRNAs have a promising future as novel biomarkers and therapeutic targets to explore the concise relationship between ERVs and cancers. In this review, we first summarize the role of ERV-derived lncRNAs in physiological regulation, mainly including immunomodulation, the maintenance of pluripotency, and erythropoiesis. In addition, pathological regulation examples of their aberrant activation and expression leading to carcinogenesis are highlighted, and specific mechanisms of occurrence are discussed.

The evaluation of human endogenous retroviral env expression in normal and cancerous tissues of the breast

Background

Both internal and external risk factors can accelerate the progression of breast cancer which is the reason why clinicians have tried to find new biomarkers for this health problem. Human endogenous retrovirus-W (HERV-W) can be one of these biomarkers, as it has been mentioned that some genes of this virus are able to have either higher or lower expression in numerous cancerous cells. In this study, we aimed to compare HERV-W envelope expression in breast cancer tissues and normal ones since its effects on this malignancy have not been clear.

Materials and Methods

We collected 46 breast cancer tissues and their normal adjacent ones. After extracting the RNA of breast samples, we evaluated the expression of HERV-W envelope syncytin-1 and 2 using quantitative real-time polymerase chain reaction (PCR) in different kinds of breast cancer stages.

Results

Data showed that more than 13% of patients had a family history of breast cancer; moreover, approximately half of the tissues were estrogen receptor or progesterone receptor positive. Lymph node metastasis was seen in 52% of the patients, and about 40% of tumors were larger than 2 cm. Real-time PCR showed that syncytin-1 and 2 had upward regulation with (*P < 0.05) and (**P < 0.01), respectively.

Conclusion

As the expression of HERV-W Env (syncytin-1, syncytin-2) was higher in breast cancerous tissues in comparison with normal ones, we believe that these genes may have a role to play in monitoring patients suffering from this type of cancer. However, further studies are needed to confirm this hypothesis.

Retrotransposon-derived transcripts and their functions in immunity and disease

Retrotransposons, which account for approximately 42% of the human genome, have been increasingly recognized as "non-self" pathogen-associated molecular patterns (PAMPs) due to their virus-like sequences. In abnormal conditions such as cancer and viral infections, retrotransposons that are aberrantly expressed due to impaired epigenetic suppression display PAMPs, leading to their recognition by pattern recognition receptors (PRRs) of the innate immune system and triggering inflammation. This viral mimicry mechanism has been observed in various human diseases, including aging and autoimmune disorders. However, recent evidence suggests that retrotransposons possess highly regulated immune reactivity and play important roles in the development and function of the immune system. In this review, I discuss a wide range of retrotransposon-derived transcripts, their role as targets in immune recognition, and the diseases associated with retrotransposon activity. Furthermore, I explore the implications of chimeric transcripts formed between retrotransposons and known gene mRNAs, which have been previously underestimated, for the increase of immune-related gene isoforms and their influence on immune function. Retrotransposon-derived transcripts have profound and multifaceted effects on immune system function. The aim of this comprehensive review is to provide a better understanding of the complex relationship between retrotransposon transcripts and immune defense.

Endogenous retroviruses Suppressyn and Syncytin-2 as innovative prognostic biomarkers in Acute Myeloid Leukemia

Introduction

Emerging evidence has proven that human endogenous retroviruses (HERVs) play a critical role in the pathogenesis of Acute Myeloid Leukemia (AML), whereas the specific HERVs influencing the prognosis of AML patients have yet to be fully understood.

Methods

In this study, a systematic exploration was achieved to identify potential prognostic HERVs for AML, sourced from TCGA and GTEx database. Differential analysis and functional enrichment studies were conducted using GO, KEGG, GSEA, and GSVA. The ESTIMATE algorithm was applied to explore the immune infiltration of HERVs in AML. A prognostic risk-score model was evaluated with predicted yearly accuracy using ROC analysis.

Results

Two HERVs Suppressyn and Syncytin-2, were identified as promising prognostic biomarkers, with high discrimination ability based on ROC analysis between AML and healthy cohorts from TCGA. Their expression was notably higher in AML patients compared to those in healthy individuals but correlates with favorable clinical outcomes in sub-groups such as white race, lower WBC counts, favorable and intermediate risks, and NPM1 or IDH1 mutation. Suppressyn and Syncytin-2 participated in immune-related pathways and exhibited correlations with multiple immune infiltration cells, such as T cells, mast cells, and tumor-associated macrophages. Finally, we developed a prognostic risk-scoring model combining Suppressyn and Syncytin-2, where a high risk-score is associated with better prognosis.

Discussion

Collectively, our findings revealed that Suppressyn and Syncytin-2 may act as valuable diagnostic and prognostic biomarkers for individuals with AML, while highlighting links between HERV activation, immunogenicity, and future therapeutic targets.

Transposable elements may enhance antiviral resistance in HIV-1 elite controllers

Less than 0.5% of people living with HIV-1 are elite controllers (ECs) - individuals who have a replication-competent viral reservoir in their CD4+ T cells but maintain undetectable plasma viremia without the help of antiretroviral therapy. While the EC CD4+ T cell transcriptome has been investigated for gene expression signatures associated with disease progression (or, in this case, a lack thereof), the expression and regulatory activity of transposable elements (TEs) in ECs has not been explored. Yet previous studies have established that TEs can directly impact the immune response to pathogens, including HIV-1. Thus, we hypothesize that the regulatory activities of TEs could contribute to the natural resistance of ECs against HIV-1. We perform a TE-centric analysis of previously published multi-omics data derived from EC individuals and other populations. We find that the CD4+ T cell transcriptome and retrotranscriptome of ECs are distinct from healthy controls, treated patients, and viremic progressors. However, there is a substantial level of transcriptomic heterogeneity among ECs. We categorize individuals with distinct chromatin accessibility and expression profiles into four clusters within the EC group, each possessing unique repertoires of TEs and antiviral factors. Notably, several TE families with known immuno-regulatory activity are differentially expressed among ECs. Their transcript levels in ECs positively correlate with their chromatin accessibility and negatively correlate with the expression of their KRAB zinc-finger (KZNF) repressors. This coordinated variation is seen at the level of individual TE loci likely acting or, in some cases, known to act as cis-regulatory elements for nearby genes involved in the immune response and HIV-1 restriction. Based on these results, we propose that the EC phenotype is driven in part by the reduced availability of specific KZNF proteins to repress TE-derived cis-regulatory elements for antiviral genes, thereby heightening their basal level of resistance to HIV-1 infection. Our study reveals considerable heterogeneity in the CD4+ T cell transcriptome of ECs, including variable expression of TEs and their KZNF controllers, that must be taken into consideration to decipher the mechanisms enabling HIV-1 control.

Exploring HERV-K (HML-2) Influence in Cancer and Prospects for Therapeutic Interventions

This review investigates the intricate role of human endogenous retroviruses (HERVs) in cancer development and progression, explicitly focusing on HERV-K (HML-2). This paper sheds light on the latest research advancements and potential treatment strategies by examining the historical context of HERVs and their involvement in critical biological processes such as embryonic development, immune response, and disease progression. This review covers computational modeling for drug-target binding assessment, systems biology modeling for simulating HERV-K viral cargo dynamics, and using antiviral drugs to combat HERV-induced diseases. The findings presented in this review contribute to our understanding of HERV-mediated disease mechanisms and provide insights into future therapeutic approaches. They emphasize why HERV-K holds significant promise as a biomarker and a target.

Recent Updates on Viral Oncogenesis: Available Preventive and Therapeutic Entities

Human viral oncogenesis is a complex phenomenon and a major contributor to the global cancer burden. Several recent findings revealed cellular and molecular pathways that promote the development and initiation of malignancy when viruses cause an infection. Even, antiviral treatment has become an approach to eliminate the viral infections and prevent the activation of oncogenesis. Therefore, for a better understanding, the molecular pathogenesis of various oncogenic viruses like, hepatitis virus, human immunodeficiency viral (HIV), human papillomavirus (HPV), herpes simplex virus (HSV), and Epstein-Barr virus (EBV), could be explored, especially, to expand many potent antivirals that may escalate the apoptosis of infected malignant cells while sparing normal and healthy ones. Moreover, contemporary therapies, such as engineered antibodies antiviral agents targeting signaling pathways and cell biomarkers, could inhibit viral oncogenesis. This review elaborates the recent advancements in both natural and synthetic antivirals to control viral oncogenesis. The study also highlights the challenges and future perspectives of using antivirals in viral oncogenesis.

The Evaluation of HERV-K np9, rec, gag Expression in Isolated Human Peripheral Blood Mononuclear Cell (PBMC) of Gastric and Colon Cancer

Background

In the current age of diagnostic approaches in cancer, countless efforts have been allocated to identify novel and efficient biomarkers to detect cancer in its early stages. We focused on evaluating the correlation between the progression of gastrointestinal cancer, a leading cause of cancer death worldwide, and human endogenous retrovirus (HERV).

Materials and Methods

In this study, we conducted a study on the peripheral blood mononuclear cells (PBMC) gathered from gastric and colon cancer patients. We focused on HERV-K rec, np9, gag expression analysis by quantitative real-time PCR, after extraction of RNA and synthesizing cDNA.

Results

Unlike np9 whose expression increased significantly in the colon and gastric cancers, the mRNA level of the rec gene declined in both cancers. Moreover, our data illustrated that the over-expression of the gag gene was only observed in colon cancerous cells rather than gastric malignancy.

Conclusions

Overall, given the correlation between the expression level of HERV-associated genes and gastrointestinal cancer, our study suggests that these genes could be considered beneficial markers for cancer diagnosis. However, researchers should conduct studies in future articles on whether these genes can be employed as biomarkers in gastrointestinal cancer.

The role of transposable elements in aging and cancer

Transposable elements (TEs) constitute a large portion of the human genome. Various mechanisms at the transcription and post-transcription levels developed to suppress TE activity in healthy conditions. However, a growing body of evidence suggests that TE dysregulation is involved in various human diseases, including age-related diseases and cancer. In this review, we explained how sensing TEs by the immune system could induce innate immune responses, chronic inflammation, and following age-related diseases. We also noted that inflammageing and exogenous carcinogens could trigger the upregulation of TEs in precancerous cells. Increased inflammation could enhance epigenetic plasticity and upregulation of early developmental TEs, which rewires the transcriptional networks and gift the survival advantage to the precancerous cells. In addition, upregulated TEs could induce genome instability, activation of oncogenes, or inhibition of tumor suppressors and consequent cancer initiation and progression. So, we suggest that TEs could be considered therapeutic targets in aging and cancer.

Human Endogenous Retrovirus (HERV) Transcriptome Is Dynamically Modulated during SARS-CoV-2 Infection and Allows Discrimination of COVID-19 Clinical Stages

SARS-CoV-2 infection is known to trigger an important inflammatory response, which has a major role in COVID-19 pathogenesis. In infectious and inflammatory contexts, the modulation of human endogenous retroviruses (HERV) has been broadly reported, being able to further sustain innate immune responses due to the expression of immunogenic viral transcripts, including double-stranded DNA (dsRNA), and eventually, immunogenic proteins. To gain insights on this poorly characterized interplay, we performed a high-throughput expression analysis of ~3,300 specific HERV loci in the peripheral blood mononuclear cells (PBMCs) of 10 healthy controls and 16 individuals being either convalescent after the infection (6) or retesting positive after convalescence (10) (Gene Expression Onmibus [GEO] data set GSE166253). Results showed that the exposure to SARS-CoV-2 infection modulates HERV expression according to the disease stage and reflecting COVID-19 immune signatures. The differential expression analysis between healthy control (HC) and COVID-19 patients allowed us to identify a total of 282 differentially expressed HERV loci (deHERV) in the individuals exposed to SARS-CoV-2 infection, independently from the clinical form. In addition, 278 and 60 deHERV loci that were specifically modulated in individuals convalescent after COVID19 infection (C) and patients that retested positive to SARS-CoV-2 after convalescence (RTP) as individually compared to HC, respectively, as well as 164 deHERV loci between C and RTP patients were identified. The identified HERV loci belonged to 36 different HERV groups, including members of all three classes. The present study provides an exhaustive picture of the HERV transcriptome in PBMCs and its dynamic variation in the presence of COVID-19, revealing specific modulation patterns according to the infection stage that can be relevant to the disease clinical manifestation and outcome. IMPORTANCE We report here the first high-throughput analysis of HERV loci expression along SARS-CoV-2 infection, as performed with peripheral blood mononuclear cells (PBMCs). Such cells are not directly infected by the virus but have a crucial role in the plethora of inflammatory and immune events that constitute a major hallmark of COVID-19 pathogenesis. Results provide a novel and exhaustive picture of HERV expression in PBMCs, revealing specific modulation patterns according to the disease condition and the concomitant immune activation. To our knowledge, this is the first set of deHERVs whose expression is dynamically modulated across COVID-19 stages, confirming a tight interplay between HERV and cellular immunity and revealing specific transcriptional signatures that can have an impact on the disease clinical manifestation and outcome.

Identification, comprehensive characterization, and comparative genomics of the HERV-K(HML8) integrations in the human genome

Around 8% of the human genome is composed by Human Endogenous Retroviruses (HERVs), ancient viral sequences inherited from the primate germ line after their infection by now extinct retroviruses. Given the still underexplored physiological and pathological roles of HERVs, it is fundamental to increase our information about the genomic composition of the different groups, to lay reliable foundation for functional studies. Among HERVs, the most characterized elements belong to the beta-like superfamily HERV-K, comprising 10 groups (HML1-10) with HML2 being the most recent and studied one. Among HMLs, the HML8 group is the only one still lacking a comprehensive genomic description. In the present work, we investigated HML8 sequences' distribution in the human genome (GRCh38/hg38), identifying 23 novel proviruses and characterizing the overall 78 HML8 proviruses in terms of genome structure, phylogeny, and integration pattern. HML8 elements were significantly enriched in human chromosomes 8 and X (p<0.005) while chromosomes 17 and 20 showed fewer integrations than expected (p<0.025 and p<0.005, respectively). Phylogenetic analyses classified HML8 members into 3 clusters, corresponding to the three LTR types MER11A, MER11B and MER11C. Besides different LTR types, common signatures in the internal structure suggested the potential existence of three different ancestral HML8 variants. Accordingly, time of integration estimation coupled with comparative genomics revealed that these three clusters have a different time of integration in the primates' genome, with MER11C elements being significantly younger than MER11A- and MER11B associated proviruses (p<0.005 and p<0.05, respectively). Approximately 30% of the HML8 elements were found co-localized within human genes, sometimes in exonic portions and with the same orientation, deserving further studies for their possible effects on gene expression. Overall, we provide the first detailed picture of the HML8 group distribution and variety among the genome, creating the backbone for the specific analysis of their transcriptional activity in healthy and diseased conditions.

Prolonged activity of HERV-K(HML2) in Old World Monkeys accounts for recent integrations and novel recombinant variants

Around 8% of the human genome comprises Human Endogenous Retroviruses (HERVs) acquired over primate evolution. Some are specific to primates such as HERV-K, consisting of 10 HML subtypes and including the most recently acquired elements. Particularly, HML2 is the youngest clade, having some human-specific integrations, and while it has been widely described in humans its presence and distribution in non-human primates remain poorly characterized. To investigate HML2 distribution in non-human primates, the present study focused on the characterization of HML2 integrations in Macaca fascicularis and Macaca mulatta which are the most evolutionarily distant species related to humans in the Catarrhini parvorder. We identified overall 208 HML2 proviruses for M. fascicularis (77) and M. mulatta (131). Among them, 46 proviruses are shared by the two species while the others are species specific. Only 12 proviruses were shared with humans, confirming that the major wave of HML2 diffusion in humans occurred after macaques' divergence. Phylogenetic analysis confirmed structural variations between HML2 macaques' species-specific proviruses, and the ones shared between macaques and humans. The HML2 loci were characterized in terms of structure, focusing on potential residual open reading frames (ORFs) for gag, pol, and env genes for the latter being reported to be expressed in human pathological conditions. The analysis identified highly conserved gag and pol genes, while the env genes had a very divergent nature. Of the 208 HML2 proviral sequences present in Macaca species, 81 sequences form a cluster having a MER11A, a characteristic HML8 LTR sequence, insertion in the env region indicating a recombination event that occurred between the HML2 env gene and the HML8 LTR. This recombination event, which was shown to be present only in a subset of macaques' shared sequences and species-specific sequences, highlights a recent viral activity leading to the emergence of an env variant specific to the Old World Monkeys (OWMs). We performed an exhaustive analysis of HML2 in two species of OWMs, in terms of its evolutionary history, structural features, and potential residual coding capacity highlighting recent activity of HML2 in macaques that occurred after its split from the Catarrhini parvorder, leading to the emergence of viral variants, hence providing a better understanding of the endogenization and diffusion of HML2 along primate evolution.

Alternative and aberrant splicing of human endogenous retroviruses in cancer. What about head and neck? —A mini review

Human endogenous retroviruses (HERVs) are transcribed in many cancer types, including head and neck cancer. Because of accumulating mutations at proviral loci over evolutionary time, HERVs are functionally defective and cannot complete their viral life cycle. Despite that, HERV transcripts, including full-length viral RNAs and viral RNAs spliced as expected at the conventional viral splice sites, can be detected in particular conditions, such as cancer. Interestingly, non-viral–related transcription, including aberrant, non-conventionally spliced RNAs, has been reported as well. The role of HERV transcription in cancer and its contribution to oncogenesis or progression are still debated. Nonetheless, HERVs may constitute a suitable cancer biomarker or a target for therapy. Thus, ongoing research aims both to clarify the basic mechanisms underlying HERV transcription in cancer and to exploit its potential toward clinical application. In this mini-review, we summarize the current knowledge, the most recent findings, and the future perspectives of research on HERV transcription and splicing, with particular focus on head and neck cancer.

Human endogenous retrovirus regulates the initiation and progression of cancers (Review)

The expression of genes is altered in various diseases and is responsible for the disease's initiation, progression and pathology. Several other genes, predominantly inactivated, may become activated in a given condition and contribute to the initiation and progression of the disease. Similarly, human endogenous viruses (HERVs) are an incomplete, non-productive and inactive viral sequence present in the heterochromatin of the human genome, and are often referred to as junk DNA. HERVs were inserted into the host genome millions of years ago. However, they were silenced due to multiple mutations and recombination that occurred over time. However, their expression is increased in cancers due to either epigenetic or transcriptional dysregulation. Some of the HERVs having intact open reading frames have been reported to express virus-like particles, functional peptides and proteins involved in tumorigenesis. To summarize, there is involvement of different HERVs in the initiation and progression of several cancers. The present review aims to provide concise information on HERV and its involvement in the initiation and progression of multiple types of cancer.

Specific human endogenous retroviruses predict metastatic potential in uveal melanoma

Uveal melanoma (UM) is a unique disease in that patients with primary UM are well stratified based on their risk of developing metastasis, yet there are limited effective treatments once metastases occur. There is an urgent need to better understand the distinct molecular pathogenesis of UM and the characteristics of patients at high risk for metastasis to identify neoantigenic targets that can be used in immunotherapy and to develop novel therapeutic strategies that may effectively target this lethal transition. An important and overlooked area of molecular pathogenesis and neoantigenic targets in UM comes from human endogenous retroviruses (HERVs). We investigated the HERV expression landscape in primary UM and found that tumors were stratified into 4 HERV-based subsets that provide clear delineation of risk outcome and support subtypes identified by other molecular indicators. Specific HERV loci are associated with the risk of uveal melanoma metastasis and may offer mechanistic insights into this process, including dysregulation of HERVs on chromosomes 3 and 8. A HERV signature composed of 17 loci was sufficient to classify tumors according to subtype with greater than 95% accuracy, including at least 1 intergenic HERV with coding potential (HERVE_Xp11.23) that could represent a potential HERV E target for immunotherapy.

Exploring the application of immunotherapy against HIV infection in the setting of malignancy: A detailed review article

According to the Joint United Nations Programme on HIV/AIDS (UNAIDS), as of 2019, approximately 42.2 million people have died from acquired immunodeficiency syndrome (AIDS)-related illnesses since the start of the epidemic. Antiretroviral therapy (ART) has significantly reduced mortality, morbidity, and incidence of the human immunodeficiency virus (HIV)/AIDS-defining cancers, taming once-dreaded disease into a benign chronic infection. Although the treatment has prolonged the patients' survival, general HIV prevalence has increased and this increase has dovetailed with an increasing incidence of Non-AIDS-defining cancers (NADCs) among people living with HIV (PLWH). This is happening when new promising approaches in both oncology and HIV infection are being developed. This review focuses on recent progress witnessed in immunotherapy approaches against HIV-related, Non-AIDS-defining cancers (NADCs), and HIV infection.

HERV-K(HML7) Integrations in the Human Genome: Comprehensive Characterization and Comparative Analysis in Non-Human Primates

Simple Summary

The human genome is not human at all, but it includes a multitude of sequences inherited from ancient viral infections that affected primates’ germ line. These elements can be seen as the fossils of now-extinct retroviruses, and are called Human Endogenous Retroviruses (HERVs). View as “junk DNA” for a long time, HERVs constitute 4 times the amount of DNA needed to produce all cellular proteins, and growing evidence indicates their crucial role in primate brain evolution, placenta development, and innate immunity shaping. HERVs are also intensively studied for a pathological role, even if the incomplete knowledge about their exact number and genomic position has thus far prevented any causal association. Among possible relevant HERVs, the HERV-K supergroup is of particular interest, including some of the oldest (HML5) as well as youngest (HML2) integrations. Among HERV-Ks, the HML7 group still lack a detailed description, and the present work thus aimed to identify and characterize all HML7 elements in the human genome. Results showed that the HML7 group includes 23 elements and an additional 160 “scars” of past infection that invaded in primates mostly between 20 and 30 million years ago, providing an exhaustive background to study their impact on human pathophysiology.

Abstract

Endogenous Retroviruses (ERVs) are ancient relics of infections that affected the primate germ line and constitute about 8% of our genome. Growing evidence indicates that ERVs had a major role in vertebrate evolution, being occasionally domesticated by the host physiology. In addition, human ERV (HERV) expression is highly investigated for a possible pathological role, even if no clear associations have been reported yet. In fact, on the one side, the study of HERV expression in high-throughput data is a powerful and promising tool to assess their actual dysregulation in diseased conditions; but, on the other side, the poor knowledge about the various HERV group genomic diversity and individual members somehow prevented the association between specific HERV loci and a given molecular mechanism of pathogenesis. The present study is focused on the HERV-K(HML7) group that—differently from the other HERV-K members—still remains poorly characterized. Starting from an initial identification performed with the software RetroTector, we collected 23 HML7 proviral insertions and about 160 HML7 solitary LTRs that were analyzed in terms of genomic distribution, revealing a significant enrichment in chromosome X and the frequent localization within human gene introns as well as in pericentromeric and centromeric regions. Phylogenetic analyses showed that HML7 members form a monophyletic group, which based on age estimation and comparative localization in non-human primates had its major diffusion between 20 and 30 million years ago. Structural characterization revealed that besides 3 complete HML7 proviruses, the other group members shared a highly defective structure that, however, still presents recognizable functional domains, making it worth further investigation in the human population to assess the presence of residual coding potential.

Retrotransposons as Drivers of Mammalian Brain Evolution

Retrotransposons, a large and diverse class of transposable elements that are still active in humans, represent a remarkable force of genomic innovation underlying mammalian evolution. Among the features distinguishing mammals from all other vertebrates, the presence of a neocortex with a peculiar neuronal organization, composition and connectivity is perhaps the one that, by affecting the cognitive abilities of mammals, contributed mostly to their evolutionary success. Among mammals, hominids and especially humans display an extraordinarily expanded cortical volume, an enrichment of the repertoire of neural cell types and more elaborate patterns of neuronal connectivity. Retrotransposon-derived sequences have recently been implicated in multiple layers of gene regulation in the brain, from transcriptional and post-transcriptional control to both local and large-scale three-dimensional chromatin organization. Accordingly, an increasing variety of neurodevelopmental and neurodegenerative conditions are being recognized to be associated with retrotransposon dysregulation. We review here a large body of recent studies lending support to the idea that retrotransposon-dependent evolutionary novelties were crucial for the emergence of mammalian, primate and human peculiarities of brain morphology and function.