Retrotransposons in the development and progression of amyotrophic lateral sclerosis

Recent advances in the genetics of familial and sporadic ALS

ALS shows complex genetic inheritance patterns. In about 5% to 10% of cases, there is a family history of ALS or a related condition such as frontotemporal dementia in a first or second degree relative, and for about 80% of such people a pathogenic gene variant can be identified. Such variants are also seen in people with no family history because of factor influencing the expression of genes, such as age. Genetic susceptibility factors also contribute to risk, and the heritability of ALS is between 40% and 60%. The genetic variants influencing ALS risk include single base changes, repeat expansions, copy number variants, and others. Here we review what is known of the genetic landscape and architecture of ALS.

Transposable Elements: Emerging Therapeutic Targets in Neurodegenerative Diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS), are characterized by the progressive loss of neuronal function and structure. While several genetic and environmental factors have been implicated in the pathogenesis of these disorders, emerging evidence suggests that transposable elements (TEs), once considered "junk DNA," play a significant role in their development and progression. TEs are mobile genetic elements capable of moving within the genome, and their dysregulation has been associated with genomic instability, altered gene expression, and neuroinflammation. This review provides an overview of TEs, including long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and endogenous retroviruses (ERVs), mechanisms of repression and derepression, and their potential impact on neurodegeneration. The evidence linking TEs to AD, PD, and ALS by shedding light on the complex interactions between TEs and neurodegeneration has been discussed. Furthermore, the therapeutic potential of targeting TEs in neurodegenerative diseases has been explored. Understanding the role of TEs in neurodegeneration holds promise for developing novel therapeutic strategies aimed at mitigating disease progression and preserving neuronal health.

Transcriptomic profiling of cerebrospinal fluid identifies ALS pathway enrichment and RNA biomarkers in MND individuals

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder and the most common form of motor neurone disease (MND) which is characterized by the damage and death of motor neurons in the brain and spinal cord of affected individuals. Due to the heterogeneity of the disease, a better understanding of the interaction between genetics and biochemistry with the identification of biomarkers is crucial for therapy development. In this study, we used cerebrospinal fluid (CSF) RNA-sequencing data from the New York Genome Center (NYGC) ALS Consortium and analyzed differential gene expression between 47 MND individuals and 29 healthy controls. Pathway analysis showed that the affected genes are enriched in many pathways associated with ALS, including nucleocytoplasmic transport, autophagy, and apoptosis. Moreover, we assessed differential expression on both gene- and transcript-based levels and demonstrate that the expression of previously identified potential biomarkers, including CAPG, CCL3, and MAP2, was significantly higher in MND individuals. Ultimately, this study highlights the transcriptomic composition of CSF which enables insights into changes in the brain in ALS and therefore increases the confidence in the use of CSF for biomarker development.

Human Endogenous Retrovirus-K (HML-2)-Related Genetic Variation: Human Genome Diversity and Disease

Human endogenous retroviruses (HERVs) comprise a significant portion of the human genome, making up roughly 8%, a notable comparison to the 2-3% represented by coding sequences. Numerous studies have underscored the critical role and importance of HERVs, highlighting their diverse and extensive influence on the evolution of the human genome and establishing their complex correlation with various diseases. Among HERVs, the HERV-K (HML-2) subfamily has recently attracted significant attention, integrating into the human genome after the divergence between humans and chimpanzees. Its insertion in the human genome has received considerable attention due to its structural and functional characteristics and the time of insertion. Originating from ancient exogenous retroviruses, these elements succeeded in infecting germ cells, enabling vertical transmission and existing as proviruses within the genome. Remarkably, these sequences have retained the capacity to form complete viral sequences, exhibiting activity in transcription and translation. The HERV-K (HML-2) subfamily is the subject of active debate about its potential positive or negative effects on human genome evolution and various pathologies. This review summarizes the variation, regulation, and diseases in human genome evolution arising from the influence of HERV-K (HML-2).

A Genome-Wide Screen for the Exonisation of Reference SINE-VNTR-Alus and Their Expression in CNS Tissues of Individuals with Amyotrophic Lateral Sclerosis

The hominid-specific retrotransposon SINE-VNTR-Alu (SVA) is a composite element that has contributed to the genetic variation between individuals and influenced genomic structure and function. SVAs are involved in modulating gene expression and splicing patterns, altering mRNA levels and sequences, and have been associated with the development of disease. We evaluated the genome-wide effects of SVAs present in the reference genome on transcript sequence and expression in the CNS of individuals with and without the neurodegenerative disorder Amyotrophic Lateral Sclerosis (ALS). This study identified SVAs in the exons of 179 known transcripts, several of which were expressed in a tissue-specific manner, as well as 92 novel exonisation events occurring in the motor cortex. An analysis of 65 reference genome SVAs polymorphic for their presence/absence in the ALS consortium cohort did not identify any elements that were significantly associated with disease status, age at onset, and survival. However, there were transcripts, such as transferrin and HLA-A, that were differentially expressed between those with or without disease, and expression levels were associated with the genotype of proximal SVAs. This study demonstrates the functional consequences of several SVA elements altering mRNA splicing patterns and expression levels in tissues of the CNS.

Pathways to healing: Plants with therapeutic potential for neurodegenerative diseases

Some of the greatest challenges in medicine are the neurodegenerative diseases (NDs), which remain without a cure and mostly progress to death. A companion study employed a toolkit methodology to document 2001 plant species with ethnomedicinal uses for alleviating pathologies relevant to NDs, focusing on its relevance to Alzheimer's disease (AD). This study aimed to find plants with therapeutic bioactivities for a range of NDs. 1339 of the 2001 plant species were found to have a bioactivity from the literature of therapeutic relevance to NDs such as Parkinson's disease, Huntington's disease, AD, motor neurone diseases, multiple sclerosis, prion diseases, Neimann-Pick disease, glaucoma, Friedreich's ataxia and Batten disease. 43 types of bioactivities were found, such as reducing protein misfolding, neuroinflammation, oxidative stress and cell death, and promoting neurogenesis, mitochondrial biogenesis, autophagy, longevity, and anti-microbial activity. Ethno-led plant selection was more effective than random selection of plant species. Our findings indicate that ethnomedicinal plants provide a large resource of ND therapeutic potential. The extensive range of bioactivities validate the usefulness of the toolkit methodology in the mining of this data. We found that a number of the documented plants are able to modulate molecular mechanisms underlying various key ND pathologies, revealing a promising and even profound capacity to halt and reverse the processes of neurodegeneration.

Human leukocyte antigen super-locus: nexus of genomic supergenes, SNPs, indels, transcripts, and haplotypes

The human Major Histocompatibility Complex (MHC) or Human Leukocyte Antigen (HLA) super-locus is a highly polymorphic genomic region that encodes more than 140 coding genes including the transplantation and immune regulatory molecules. It receives special attention for genetic investigation because of its important role in the regulation of innate and adaptive immune responses and its strong association with numerous infectious and/or autoimmune diseases. In recent years, MHC genotyping and haplotyping using Sanger sequencing and next-generation sequencing (NGS) methods have produced many hundreds of genomic sequences of the HLA super-locus for comparative studies of the genetic architecture and diversity between the same and different haplotypes. In this special issue on 'The Current Landscape of HLA Genomics and Genetics', we provide a short review of some of the recent analytical developments used to investigate the SNP polymorphisms, structural variants (indels), transcription and haplotypes of the HLA super-locus. This review highlights the importance of using reference cell-lines, population studies, and NGS methods to improve and update our understanding of the mechanisms, architectural structures and combinations of human MHC genomic alleles (SNPs and indels) that better define and characterise haplotypes and their association with various phenotypes and diseases.

Characterisation of retrotransposon insertion polymorphisms in whole genome sequencing data from individuals with amyotrophic lateral sclerosis

The genetics of an individual is a crucial factor in understanding the risk of developing the neurodegenerative disease amyotrophic lateral sclerosis (ALS). There is still a large proportion of the heritability of ALS, particularly in sporadic cases, to be understood. Among others, active transposable elements drive inter-individual variability, and in humans long interspersed element 1 (LINE1, L1), Alu and SINE-VNTR-Alu (SVA) retrotransposons are a source of polymorphic insertions in the population. We undertook a pilot study to characterise the landscape of non-reference retrotransposon insertion polymorphisms (non-ref RIPs) in 15 control and 15 ALS individuals' whole genomes from Project MinE, an international project to identify potential genetic causes of ALS. The combination of two bioinformatics tools (mobile element locator tool (MELT) and TEBreak) identified on average 1250 Alu, 232 L1 and 77 SVA non-ref RIPs per genome across the 30 analysed. Further PCR validation of individual polymorphic retrotransposon insertions showed a similar level of accuracy for MELT and TEBreak. Our preliminary study did not identify a specific RIP or a significant difference in the total number of non-ref RIPs in ALS compared to control genomes. The use of multiple bioinformatic tools improved the accuracy of non-ref RIP detection and our study highlights the potential importance of studying these elements further in ALS.

Transcription start site signal profiling improves transposable element RNA expression analysis at locus-level

The transcriptional activity of Transposable Elements (TEs) has been involved in numerous pathological processes, including neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal lobar degeneration. The TE expression analysis from short-read sequencing technologies is, however, challenging due to the multitude of similar sequences derived from singular TEs subfamilies and the exaptation of TEs within longer coding or non-coding RNAs. Specialised tools have been developed to quantify the expression of TEs that either relies on probabilistic re-distribution of multimapper count fractions or allow for discarding multimappers altogether. Until now, the benchmarking across those tools was largely limited to aggregated expression estimates over whole TEs subfamilies. Here, we compared the performance of recently published tools (SQuIRE, TElocal, SalmonTE) with simplistic quantification strategies (featureCounts in unique, fraction and random modes) at the individual loci level. Using simulated datasets, we examined the false discovery rate and the primary driver of those false positive hits in the optimal quantification strategy. Our findings suggest a high false discovery number that exceeds the total number of correctly recovered active loci for all the quantification strategies, including the best performing tool TElocal. As a remedy, filtering based on the minimum number of read counts or baseMean expression improves the F1 score and decreases the number of false positives. Finally, we demonstrate that additional profiling of Transcription Start Site mapping statistics (using a k-means clustering approach) significantly improves the performance of TElocal while reporting a reliable set of detected and differentially expressed TEs in human simulated RNA-seq data.

Integration of TE Induces Cancer Specific Alternative Splicing Events

Alternative splicing of messenger RNA (mRNA) precursors contributes to genetic diversity by generating structurally and functionally distinct transcripts. In a disease state, alternative splicing promotes incidence and development of several cancer types through regulation of cancer-related biological processes. Transposable elements (TEs), having the genetic ability to jump to other regions of the genome, can bring about alternative splicing events in cancer. TEs can integrate into the genome, mostly in the intronic regions, and induce cancer-specific alternative splicing by adjusting various mechanisms, such as exonization, providing splicing donor/acceptor sites, alternative regulatory sequences or stop codons, and driving exon disruption or epigenetic regulation. Moreover, TEs can produce microRNAs (miRNAs) that control the proportion of transcripts by repressing translation or stimulating the degradation of transcripts at the post-transcriptional level. Notably, TE insertion creates a cancer-friendly environment by controlling the overall process of gene expression before and after transcription in cancer cells. This review emphasizes the correlative interaction between alternative splicing by TE integration and cancer-associated biological processes, suggesting a macroscopic mechanism controlling alternative splicing by TE insertion in cancer.

Locus specific reduction of L1 expression in the cortices of individuals with amyotrophic lateral sclerosis

The activation and dysregulation of retrotransposons has been identified in the CNS of individuals with the fatal neurodegenerative disorder Amyotrophic lateral sclerosis (ALS). This includes elements from multiple different families and subfamilies of retrotransposons, however there is limited knowledge of the specific loci from which this expression occurs in ALS. The long interspersed element-1 (L1) is the only autonomous retrotransposon in the human genome and members of this family of elements maintain the ability to mobilise. Despite L1s contributing to 17% of the human genome only 80-100 L1s encode the required proteins for mobilisation and are retrotransposition competent. Identifying the specific loci from which L1 expression occurs will inform on the potential functional consequences of their expression, such as the potential for somatic retrotransposition or DNA damage caused by the endonuclease activity of the ORF2 protein of the L1. Here we characterised L1 loci expression using the L1EM tool ( https://github.com/FenyoLab/L1EM ) in RNA sequencing data from 518 samples across four tissues (motor cortex, frontal cortex, cerebellum and cervical spinal cord) in the Target ALS cohort obtained from the New York Genome Center. There was a significant reduction in total intact L1 expression (those that encode functional proteins) in two brain regions of individuals with ALS compared to controls and clustering of the ALS brain regions occurred based on their intact L1 expression profile. Although overall the levels of L1 expression were reduced in ALS/ALS with other neurological disorder (ND) there were individuals in which L1s were expressed at much higher levels than the rest of the ALS/ALSND cohort. Expressed L1 loci were more frequently located in introns compared to those not expressed and the level of L1 expression positively correlated with the expression of the gene in which it was located. Significant differences were observed in the expression profiles of L1s in ALS and specific features of these elements, such as location in the genome and whether or not they are intact, were significantly associated with those that were expressed in the cohort.

Characterisation of the Function of a SINE-VNTR-Alu Retrotransposon to Modulate Isoform Expression at the MAPT Locus

SINE-VNTR-Alu retrotransposons represent one class of transposable elements which contribute to the regulation and evolution of the primate genome and have the potential to be involved in genetic instability and disease progression. However, these polymorphic elements have not been extensively analysed when addressing the missing heritability of neurodegenerative diseases, including Parkinson’s disease (PD) and amyotrophic lateral sclerosis (ALS). SVA_67, a retrotransposon insertion polymorphism, is located in a 1.8 Mb region of high linkage disequilibrium, called the MAPT locus, which is known to contribute to increased risk of developing PD, frontotemporal dementia and other tauopathies. To investigate the role of SVA_67 in directing differential gene expression at this locus, we characterised the impact of SVA_67 allele dosage on isoform expression of several genes in the MAPT locus using the datasets from both the Parkinson’s Progression Markers Initiative and New York Genome Center Consortium Target ALS cohort. The Parkinson’s data was from gene expression in the blood and the ALS data from a variety of CNS regions and allowed us to demonstrate that SVA_67 presence or absence correlated with both isoform- and tissue-specific expression of multiple genes at this locus. This study highlights the importance of addressing SVA polymorphism in disease genetics to gain insight into a better understanding of the role of these regulatory domains to a variety of neurodegenerative diseases.

Retrotransposons as a Source of DNA Damage in Neurodegeneration

The etiology of aging-associated neurodegenerative diseases (NDs), such as Parkinson’s disease (PD) and Alzheimer’s disease (AD), still remains elusive and no curative treatment is available. Age is the major risk factor for PD and AD, but the molecular link between aging and neurodegeneration is not fully understood. Aging is defined by several hallmarks, some of which partially overlap with pathways implicated in NDs. Recent evidence suggests that aging-associated epigenetic alterations can lead to the derepression of the LINE-1 (Long Interspersed Element-1) family of transposable elements (TEs) and that this derepression might have important implications in the pathogenesis of NDs. Almost half of the human DNA is composed of repetitive sequences derived from TEs and TE mobility participated in shaping the mammalian genomes during evolution. Although most TEs are mutated and no longer mobile, more than 100 LINE-1 elements have retained their full coding potential in humans and are thus retrotransposition competent. Uncontrolled activation of TEs has now been reported in various models of neurodegeneration and in diseased human brain tissues. We will discuss in this review the potential contribution of LINE-1 elements in inducing DNA damage and genomic instability, which are emerging pathological features in NDs. TEs might represent an important molecular link between aging and neurodegeneration, and a potential target for urgently needed novel therapeutic disease-modifying interventions.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Spatially Resolved Expression of Transposable Elements in Disease and Somatic Tissue with SpatialTE

Spatial transcriptomics (ST) is transforming the way we can study gene expression and its regulation through position-specific resolution within tissues. However, as in bulk RNA-Seq, transposable elements (TEs) are not being studied due to their highly repetitive nature. In recent years, TEs have been recognized as important regulators of gene expression, and thus, TE expression analysis in a spatially resolved manner could further help to understand their role in gene regulation within tissues. We present SpatialTE, a tool to analyze TE expression from ST datasets and show its application in somatic and diseased tissues. The results indicate that TEs have spatially regulated expression patterns and that their expression profiles are spatially altered in ALS disease, indicating that TEs might perform differential regulatory functions within tissue organs. We have made SpatialTE publicly available as open-source software under an MIT license.

Locus-specific analysis of Transposable Elements during the progression of ALS in the SOD1G93A mouse model

Transposable Elements (TEs) are ubiquitous genetic elements with the ability to move within a genome. TEs contribute to a large fraction of the repetitive elements of a genome, and because of their nature, they are not routinely analyzed in RNA-Seq gene expression studies. Amyotrophic Lateral Sclerosis (ALS) is a lethal neurodegenerative disease, and a well-accepted model for its study is the mouse harboring the human SOD1G93A mutant. In this model, landmark stages of the disease can be recapitulated at specific time points, making possible to understand changes in gene expression across time. While there are several works reporting TE activity in ALS models, they have not explored their activity through the disease progression. Moreover, they have done it at the expense of losing their locus of expression. Depending on their genomic location, TEs can regulate genes in cis and in trans, making locus-specific analysis of TEs of importance in order to understand their role in modulating gene expression. Particularly, the locus-specific role of TEs in ALS has not been fully elucidated. In this work, we analyzed publicly available RNA-Seq datasets of the SOD1G93A mouse model, to understand the locus-specific role of TEs. We show that TEs become up-regulated at the early stages of the disease, and via statistical associations, we speculate that they can regulate several genes, which in turn might be contributing to the genetic dysfunction observed in ALS.

A HML6 endogenous retrovirus on chromosome 3 is upregulated in amyotrophic lateral sclerosis motor cortex

There is increasing evidence that endogenous retroviruses (ERVs) play a significant role in central nervous system diseases, including amyotrophic lateral sclerosis (ALS). Studies of ALS have consistently identified retroviral enzyme reverse transcriptase activity in patients. Evidence indicates that ERVs are the cause of reverse transcriptase activity in ALS, but it is currently unclear whether this is due to a specific ERV locus or a family of ERVs. We employed a combination of bioinformatic methods to identify whether specific ERVs or ERV families are associated with ALS. Using the largest post-mortem RNA-sequence datasets available we selectively identified ERVs that closely resembled full-length proviruses. In the discovery dataset there was one ERV locus (HML6_3p21.31c) that showed significant increased expression in post-mortem motor cortex tissue after multiple-testing correction. Using six replication post-mortem datasets we found HML6_3p21.31c was consistently upregulated in ALS in motor cortex and cerebellum tissue. In addition, HML6_3p21.31c showed significant co-expression with cytokine binding and genes involved in EBV, HTLV-1 and HIV type-1 infections. There were no significant differences in ERV family expression between ALS and controls. Our results support the hypothesis that specific ERV loci are involved in ALS pathology.

CRISPR Deletion of a SVA Retrotransposon Demonstrates Function as a cis-Regulatory Element at the TRPV1/TRPV3 Intergenic Region

SINE-VNTR-Alu (SVA) retrotransposons are a subclass of transposable elements (TEs) that exist only in primate genomes. TE insertions can be co-opted as cis-regulatory elements (CREs); however, the regulatory potential of SVAs has predominantly been demonstrated using bioinformatic approaches and reporter gene assays. The objective of this study was to demonstrate SVA cis-regulatory activity by CRISPR (clustered regularly interspaced short palindromic repeats) deletion and subsequent measurement of direct effects on local gene expression. We identified a region on chromosome 17 that was enriched with human-specific SVAs. Comparative gene expression analysis at this region revealed co-expression of TRPV1 and TRPV3 in multiple human tissues, which was not observed in mouse, highlighting key regulatory differences between the two species. Furthermore, the intergenic region between TRPV1 and TRPV3 coding sequences contained a human specific SVA insertion located upstream of the TRPV3 promoter and downstream of the 3′ end of TRPV1, highlighting this SVA as a candidate to study its potential cis-regulatory activity on both genes. Firstly, we generated SVA reporter gene constructs and demonstrated their transcriptional regulatory activity in HEK293 cells. We then devised a dual-targeting CRISPR strategy to facilitate the deletion of this entire SVA sequence and generated edited HEK293 clonal cell lines containing homozygous and heterozygous SVA deletions. In edited homozygous ∆SVA clones, we observed a significant decrease in both TRPV1 and TRPV3 mRNA expression, compared to unedited HEK293. In addition, we also observed an increase in the variability of mRNA expression levels in heterozygous ∆SVA clones. Overall, in edited HEK293 with SVA deletions, we observed a disruption to the co-expression of TRPV1 and TRPV3. Here we provide an example of a human specific SVA with cis-regulatory activity in situ, supporting the role of SVA retrotransposons as contributors to species-specific gene expression.

ALS Genetics: Gains, Losses, and Implications for Future Therapies

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder caused by the loss of motor neurons from the brain and spinal cord. The ALS community has made remarkable strides over three decades by identifying novel familial mutations, generating animal models, elucidating molecular mechanisms, and ultimately developing promising new therapeutic approaches. Some of these approaches reduce the expression of mutant genes and are in human clinical trials, highlighting the need to carefully consider the normal functions of these genes and potential contribution of gene loss-of-function to ALS. Here, we highlight known loss-of-function mechanisms underlying ALS, potential consequences of lowering levels of gene products, and the need to consider both gain and loss of function to develop safe and effective therapeutic strategies.

Frequency and methylation status of selected retrotransposition competent L1 loci in amyotrophic lateral sclerosis

Long interspersed element-1 (LINE-1/L1) is the only autonomous transposable element in the human genome that currently mobilises in both germline and somatic tissues. Recent studies have identified correlations between altered retrotransposon expression and the fatal neurodegenerative disease amyotrophic lateral sclerosis (ALS) in a subset of patients. The risk of an individual developing ALS is dependent on an interaction of genetic variants and subsequent modifiers during life. These modifiers could include environmental factors, which can lead to epigenetic and genomic changes, such as somatic mutations, occurring in the neuronal cells that degenerate as the disease develops. There are more than 1 million L1 copies in the human genome today, but only 80-100 L1 loci in the reference genome are considered to be retrotransposition-competent (RC) and an even smaller number of these RC-L1s loci are highly active. We hypothesise that RC-L1s could affect normal cellular function through their mutagenic potential conferred by their ability to retrotranspose in neuronal cells and through DNA damage caused by the endonuclease activity of the L1-encoded ORF2 protein. To investigate whether either an increase in the genomic burden of RC-L1s or epigenetic changes to RC-L1s altering their expression, could play a role in disease development, we chose a set of seven well characterised genomic RC-L1 loci that were reported earlier to be highly active in a cellular L1 retrotransposition reporter assay or serve as major source elements for germline and/or somatic retrotransposition events. Analysis of the insertion allele frequency of five polymorphic RC-L1s, out of the set of seven, for their presence or absence, did not identify an increased number individually or when combined in individuals with the disease. However, we did identify reduced levels of methylation of RC-L1s in the motor cortex of those individuals with both familial and sporadic ALS compared to control brains. The changes to the regulation of the loci encompassing these RC-L1s demonstrated tissue specificity and could be related to the disease process.

An Increased Burden of Highly Active Retrotransposition Competent L1s Is Associated with Parkinson's Disease Risk and Progression in the PPMI Cohort

Long interspersed element-1 (LINE-1/L1s) contributes 17% of the human genome with more than 1 million elements present; however, fewer than 100 of these have evidence for being retrotransposition competent (RC). In addition to those RC-L1s present in the reference genome, there are a small number of known non-reference L1 insertions that are also retrotransposition competent. L1 activity, whether through the potentially detrimental effects of their mRNA or protein expression or somatic retrotransposition events, has been linked to several neurological conditions. The polymorphic nature of both reference and non-reference RC-L1s in terms of their presence or absence will result in individuals harboring a different combination of these elements and it is currently unknown if this type of germline variation contributes to the risk of neurological disease. Here, we utilized whole-genome sequencing data from 178 healthy controls and 372 Parkinson's disease (PD) subjects from the Parkinson's Progression Markers Initiative (PPMI) to investigate the role of RC-L1s in PD. In the PPMI cohort, we identified 22 reference and 50 non-reference polymorphic RC-L1 loci. Focusing on 16 highly active RC-L1 loci, an increased burden of these elements (≥9) was associated with PD (OR 1.25, 95% CI 1.03-1.51, p = 0.02). In addition, we identified significant associations of progression markers of PD and the burden of highly active RC-L1s. This study has identified a novel type of genetic element associated with PD risk and disease progression.

Human Endogenous Retrovirus K (HML-2) in Health and Disease

Human endogenous retroviruses (HERVs) are derived from exogenous retrovirus infections in the evolution of primates and account for about 8% of the human genome. They were considered as silent passengers within our genomes for a long time, however, reactivation of HERVs has been associated with tumors and autoimmune diseases, especially the HERV-K (HML-2) family, the most recent integration groups with the least number of mutations and the most biologically active to encode functional retroviral proteins and produce retrovirus-like particles. Increasing studies are committed to determining the potential role of HERV-K (HML-2) in pathogenicity. Although there is still no evidence for HERV-K (HML-2) as a direct cause of diseases, aberrant expression profiles of the HERV-K (HML-2) transcripts and their regulatory function to their proximal host-genes were identified in different diseases. In this review, we summarized the advances between HERV-K (HML-2) and diseases to provide basis for further studies on the causal relationship between HERV-K (HML-2) and diseases. We recommended more attention to polymorphic integrated HERV-K (HML-2) loci which could be genetic causative factors and be associated with inter-individual differences in tumorigenesis and autoimmune diseases.

TDP-43 regulates transcription at protein-coding genes and Alu retrotransposons

The 43-kDa transactive response DNA-binding protein (TDP-43) is an example of an RNA-binding protein that regulates RNA metabolism at multiple levels from transcription and splicing to translation. Its role in post-transcriptional RNA processing has been a primary focus of recent research, but its role in regulating transcription has been studied for only a few human genes. We characterized the effects of TDP-43 on transcription genome-wide and found that TDP-43 broadly affects transcription of protein-coding and noncoding RNA genes. Among protein-coding genes, the effects of TDP-43 were greatest for genes <30 thousand base pairs in length. Surprisingly, we found that the loss of TDP-43 resulted in increased evidence for transcription activity near repetitive Alu elements found within expressed genes. The highest densities of affected Alu elements were found in the shorter genes, whose transcription was most affected by TDP-43. Thus, in addition to its role in post-transcriptional RNA processing, TDP-43 plays a critical role in maintaining the transcriptional stability of protein-coding genes and transposable DNA elements.