Alu elements are short stretches of DNA, roughly 300 base pairs long, that belong to a family of mobile genetic sequences known as retrotransposons. Over tens of millions of years of primate evolution, they have copied and pasted themselves throughout the genome with extraordinary success. As the classic review by Mark Batzer and Prescott Deininger noted, Alus now account for roughly one tenth of the human genome, numbering well over a million copies. For decades, they were largely written off as genomic parasites or, at best, molecular fossils. That view has been steadily revised. Work summarized by Rie Fueyo, Jared Judd, Cédric Feschotte and Joanna Wysocka has shown that transposable elements have repeatedly been co-opted by their hosts as raw material for new gene-regulatory circuits, supplying binding sites for transcription factors and contributing to the evolution of gene networks in ways that can shape tissue identity and development.

The new research, a study by Matoba and colleagues published in the same issue of Nature Neuroscience, takes this idea into the territory of human cortical biology. Writing in a News and Views commentary, Sinniah and Pollen explain that Matoba and colleagues identified genetic variants residing inside Alu elements that are linked to measurable differences in human cortical structure. In other words, the repetitive DNA that so often gets excluded from genomic analyses because it is difficult to map and annotate may actually carry functional variants that help explain why different people, and by extension different lineages, build cortices of different sizes and shapes.

The significance of this finding becomes clearer when one considers how the cortex is built. Cortical size depends fundamentally on the number of neural progenitor cells produced during development and on how many rounds of division they undergo before differentiating into neurons. As early as 2002, Anjen Chenn and Christopher Walsh demonstrated in Science that modulating a key cell-signaling pathway in the developing mouse brain could expand the progenitor pool and produce a dramatically enlarged, folded cortical surface. That work established a principle that has guided the field ever since: relatively subtle changes in developmental regulation can produce large changes in cortical architecture. The question has always been which genetic variants in the human lineage perform this kind of regulatory tuning, and how their effects can be tested.

This is where the technical challenge lies. Most studies of human brain evolution have focused on protein-coding changes, which are comparatively easy to identify and to model in experimental systems. But a large fraction of the genome’s functional variation lies in regulatory DNA, the non-coding sequences that determine when, where and how strongly genes are expressed. As A. Marie J. Kim and Joanna Wysocki reviewed in Molecular Cell, mapping regulatory variation in the human brain, and distinguishing functional variants from the vast background of neutral sequence differences, remains one of the hardest problems in functional genomics. Variants buried inside repetitive elements compound the difficulty, because standard genome-sequencing pipelines often cannot uniquely place short reads within repeated Alu copies, meaning that variation in these elements is systematically under-detected and under-studied.

What makes Alu elements especially interesting as carriers of regulatory variation is their internal architecture. Alus contain their own built-in regulatory signals, including internal RNA polymerase III promoter elements that originally allowed them to be transcribed by the retrotransposition machinery. Work dating back to Greg Vansant and William Reynolds in 1995 showed that Alu sequences can also interact with the transcriptional machinery of RNA polymerase II, the polymerase responsible for protein-coding genes, and can influence the expression of nearby genes. When a variant arises within an Alu element sitting near a developmental gene, it can therefore perturb an existing regulatory module rather than needing to build one from scratch. Sinniah and Pollen emphasize that Matoba and colleagues show precisely this: variants that shape human cortical structure co-opt the inbuilt machinery of Alu elements to rewire regulatory networks in the developing brain.

The broader context is the comparative genomics of cortical development, a field comprehensively surveyed by André Sousa, Kenneth Meyer, Gabriel Santpere, Forrest Gulden and Nenad Sestan in Cell. Their synthesis documented how the human cortex differs from that of other mammals in its progenitor cell types, its developmental timing and its gene-expression programs, and how many of these differences are likely to be regulatory rather than protein-coding in origin. Because Alu elements expanded explosively in the primate lineage after splitting from rodents, and continued to expand within the ape and human lineages, they represent a plausible substrate for primate-specific and even human-specific regulatory innovation. A variant in an Alu element near a cortical progenitor gene is, in evolutionary terms, a candidate for a change that is both recent enough to be human-relevant and positioned exactly where a regulatory effect could influence cortical expansion.

The approach taken by Matoba and colleagues, as framed by the commentary, also illustrates a shift in how such questions can now be asked. Rather than examining one candidate gene at a time, the study connects genetic variants linked to cortical traits with functional assays of gene regulation in the developing brain, closing the gap between population-level association and mechanistic biology. This matters because the genetic changes associated with the development of the human cortex, as Sinniah and Pollen put it, are difficult to test functionally, especially in the context of gene regulation. Demonstrating that Alu-harbored variants can be tied both to measurable cortical differences and to concrete regulatory mechanisms provides a template for future work on other repetitive elements and other aspects of brain development.

There are also implications beyond evolutionary biology. Variation in cortical size and structure is associated with a range of neurological and psychiatric conditions, and understanding which variants influence cortical development could sharpen the interpretation of genetic risk factors. If a meaningful portion of that variation resides in Alu elements, then the routine exclusion of repetitive regions from genome-wide analyses may be causing researchers to miss relevant variants. The commentary’s central message is effectively a call to stop treating repetitive DNA as unreadable noise. The tools for resolving variation within repeated elements are improving, and studies such as that of Matoba and colleagues demonstrate that the effort is repaid with biological insight.

The image that emerges is one of the genome as a layered archive rather than a finished blueprint. Alu elements, once viewed purely as selfish molecular hitchhikers, have been conscripted over evolutionary time into the service of gene regulation, and now appear to carry variants that contribute to one of the most distinctive features of our species: the size and structure of the cerebral cortex. As Sinniah and Pollen conclude in their commentary, the variants that shape human cortical structure are hiding inside Alu transposable elements, co-opting their inbuilt machinery to rewire regulatory networks in the developing brain. For a field that has spent decades searching the genome for what makes the human cortex human, the answer, at least in part, appears to have been written in the repeats.

Subject of Research: The role of Alu transposable element variants in human cortical development and gene regulation

Article Title: Alus harbor genetic variants that shape the human cortex

Article References: Alus harbor genetic variants that shape the human cortex. (n.d.). https://doi.org/10.1038/s41593-026-02492-w

Image Credits: AI Generated

DOI: 10.1038/s41593-026-02492-w

Keywords: Alu elements, transposable elements, human cortex, cortical development, gene regulation, functional genomics, retrotransposons, neural progenitors, brain evolution, regulatory variants, primate genomics, Nature Neuroscience

The investigation emerged from a long-standing question about why males appear more vulnerable to autism. Corresponding author Dr. Huda Zoghbi, Distinguished Service Professor at Baylor, director of the Duncan NRI and a Howard Hughes Medical Institute investigator, describes autism as a common, heritable neurodevelopmental trait that affects how a person communicates, interacts with others and experiences the world, characterized by altered social interactions and restricted, repetitive behaviors or interests. The pronounced sex skew in diagnosis has long hinted that sex-specific factors may raise male susceptibility, but identifying those factors has proven difficult. The new study offers a concrete mechanistic hypothesis rooted in the peculiar genetics of the sex chromosomes.

The logic behind the hypothesis rests on a fundamental difference between male and female genomes. Females carry two X chromosomes, while males carry only one. First author Dr. Rebecca Meyer-Schuman, a postdoctoral associate in the Zoghbi lab, explained the team’s reasoning: mild mutations in the regulatory regions of an X chromosome gene might be tolerated in females, whose second chromosome can buffer the effect, but hit males with full force because they possess only a single copy. Regulatory regions act like dials that control whether a gene is expressed and by how much. If one of those dials is slightly miscalibrated on a male’s only X chromosome, there is no backup copy to compensate.

MECP2 was an obvious starting point for testing this idea. Decades of research, including earlier mouse studies from the Zoghbi lab and other groups, established that the protein produced by MECP2 behaves in what scientists call a Goldilocks fashion: the brain requires a just-right concentration or activity level. Too little of the protein, roughly half of normal levels or less, causes Rett syndrome, a profound neurodevelopmental disorder. Too much, about twice the normal amount, causes MECP2 duplication syndrome, a different condition with its own severe features. The gene sits in an unusually narrow therapeutic and biological window, which makes its regulatory regions especially consequential.

Crucially, the mouse studies also showed that milder shifts in MECP2 dosage fall into a different clinical territory. Reducing expression by about 30 percent or increasing it by 50 percent does not produce the defining characteristics of Rett syndrome or MECP2 duplication syndrome, such as seizures or motor problems. Instead, these moderate changes lead to autism-like behaviors in the animals. That dose-response relationship gave the researchers a precise prediction to test in human genetics: regulatory variants that modestly reduce MECP2 expression should be found in autistic individuals, particularly males, and should not be accompanied by the severe symptoms of the better-known MECP2 disorders.

To interrogate the regulatory regions of MECP2 systematically, the team employed a laboratory technique called a Massively Parallel Reporter Assay, or MPRA. This method allows researchers to test thousands of DNA sequences simultaneously by placing them upstream of a reporter gene and measuring how each sequence drives expression. In this way the team could identify and manipulate the regulatory regions of MECP2 and determine what kinds of sequence changes would alter their function. The assay served as a functional screening platform, converting raw genetic variation into measurable effects on gene activity rather than relying on computational predictions alone.

The researchers then bridged from the bench to human patients. They screened the MECP2 regulatory regions in autistic individuals and took the genetic variants they identified back into the MPRA assays to test their functional consequences. Two regulatory regions emerged where male autistic individuals had inherited a variant from their unaffected mother that altered regulatory activity. In one case, the variant reduced MECP2 expression by approximately 30 percent in human neurons, a magnitude that in mice produces social deficits, hyperactivity and anxiety-like characteristics. Consistent with the mouse model prediction, that individual was diagnosed with autism and ADHD but displayed none of the defining characteristics of Rett syndrome.

The pattern of inheritance is itself informative. Because the variants were carried by unaffected mothers, they fit the expectation that females are buffered against mild MECP2 regulatory changes, likely through the contribution of their second X chromosome, while their sons, receiving a single X chromosome, express the altered dosage. This transmission pattern mirrors the male bias in autism diagnosis and provides a plausible genetic route by which a variant can persist silently in the female population while producing neurodevelopmental effects in males. It also illustrates why X-linked regulatory variation deserves closer attention in studies of autism’s sex skew.

Zoghbi characterized the findings as the result of integrating human and animal studies, suggesting that MECP2 regulatory variants can contribute to male-biased autism while revealing just the tip of the iceberg. In her view, the study provides a framework for uncovering regulatory variants in other X chromosome neurodevelopmental genes that may contribute to autism’s missing heritability, the substantial portion of autism risk that twin and family studies attribute to genetics but that sequencing of protein-coding regions has not yet fully explained. Regulatory regions, which do not encode proteins but control gene activity, are increasingly recognized as a rich hunting ground for such unaccounted risk.

Technically, the study demonstrates the power of pairing high-throughput functional assays with patient genetics. Rather than treating every noncoding variant as a candidate, the MPRA approach assigns functional weight to specific sequences, and the mouse dosage data supply a quantitative bridge between a measured percent change in expression and an expected behavioral outcome. The convergence of all three lines of evidence, functional assay, human variant and animal dose-response, is what elevates these two regulatory regions from statistical associations to mechanistic candidates. Other contributors to the work include Fisher Cherry, Yang Sui, Athanasios Papastathopoulos-Katsaros, Yi Zhong, Yidan Li, Tianyun Wang, Kelsey Hennick, Druha Karunakaran, Hanna Berk-Rauch, Zhandong Liu, Aravinda Chakravarti, Tomasz Nowakowski and Evan E. Eichler. The findings open a path toward systematically scanning the X chromosome for regulatory variants that quietly shape who develops autism, and why boys bear the greater burden.

Subject of Research: X-linked MECP2 regulatory variants and male-biased autism susceptibility

Article Title: New genetic candidates help explain why autism affects males more often than females

Article References: New genetic candidates help explain why autism affects males more often than females. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: autism, MECP2, X chromosome, regulatory variants, male bias, Rett syndrome, MPRA, genetics, neurodevelopment, Baylor College of Medicine, gene expression, missing heritability

The central challenge the researchers set out to address is a familiar one in human genetics. Most of the thousands of genetic variants identified by genome-wide association studies as risk factors for disease do not lie within protein-coding genes. Instead, they sit in regulatory regions of the genome, the stretches of DNA that control when, where, and how strongly genes are switched on. Decades of work have established this pattern, but translating a regulatory variant into a concrete molecular mechanism has remained difficult, because the field lacked an empirical framework for characterizing the downstream consequences of regulatory variation at population scale.

Single-cell molecular quantitative trait locus mapping, a technique that links genetic variants to gene regulation measured in individual cells, has emerged as a promising approach. However, previous efforts suffered from limited statistical power and, crucially, from the inability to measure chromatin state and gene expression simultaneously in the same cells. Without both layers of information, researchers could observe that a variant was associated with changes in gene expression or with changes in chromatin accessibility, but they could not reliably connect the two steps into a single causal chain running from DNA accessibility through to transcript output.

To overcome these limitations, the consortium performed paired single-nucleus RNA sequencing and single-nucleus assay for transposase-accessible chromatin sequencing on peripheral blood mononuclear cells from FinnGen donors, capturing roughly ten million nuclei. The resulting atlas identified 51,083 cis-expression quantitative trait loci affecting 20,829 genes, 338,100 cis-chromatin accessibility quantitative trait loci affecting 210,584 open chromatin peaks, 119,094 putative causal variants, and 593,765 statistical links between regulatory peaks and the genes they control. These numbers represent a resource of extraordinary depth, spanning the major classes of immune cells found in blood and providing cell-type-resolved maps of genetic regulation.

A key analytical insight came from classifying variants according to whether they completed the full regulatory cascade. Variants that altered chromatin accessibility and were also linked, through a peak-gene connection, to altered gene expression showed twice the rate of colocalization with disease associations compared with variants that affected chromatin alone. In other words, the variants most likely to be genuinely causal for disease are those whose effects can be traced through the entire chain, from open chromatin to enhancer activity to target gene expression. This finding provides a practical filter for prioritizing candidate causal variants in complex disease loci.

To validate these statistical findings experimentally, the team turned to massively parallel reporter assays, a high-throughput technique that tests the regulatory activity of thousands of DNA sequences simultaneously. The assays confirmed the regulatory effects of 10,428 fine-mapped molecular quantitative trait loci, providing direct experimental support for a substantial fraction of the atlas’s predictions and demonstrating that the computational framework captures biologically real regulatory variation rather than statistical artifacts.

Perhaps the most conceptually significant discovery concerns genes under strong evolutionary constraint, those whose sequences change rarely because mutations are harmful. Previous studies had noted a paradox: disease variants preferentially target constrained genes, yet constrained genes appear depleted of detectable expression quantitative trait loci. The new atlas resolves this contradiction by revealing a phenomenon the authors call multilayered regulatory buffering. At constrained genes, chromatin accessibility changes caused by variants occur with normal effect sizes, but their transmission to gene expression is attenuated because these genes are regulated through weaker and more numerous enhancer-gene links. The redundancy of many weak regulatory connections dampens the expression consequence of any single regulatory perturbation.

Crucially, the reporter assay experiments confirmed that this buffering operates downstream of the regulatory element itself. Constraint acts at the interface between chromatin and expression rather than on the intrinsic activity of the cis-regulatory DNA. This distinction matters because it explains why the paradox exists in the first place: standard expression quantitative trait locus studies measure only the final output, and buffering at the chromatin-to-expression step suppresses that output signal even when the underlying chromatin effects are intact. The finding reconciles apparently conflicting observations across the field and suggests that disease risk at constrained genes may accumulate through subtle, distributed effects that individual expression studies systematically miss.

The atlas also delivers concrete mechanistic hypotheses for specific diseases. At autoimmune loci, the researchers traced complete regulatory cascades at TICAM1, an adaptor protein in Toll-like receptor signaling, and RHOH, a small GTPase involved in T cell receptor signaling, both linked to autoimmune hypothyroidism. They similarly dissected Finnish-enriched variants at TNRC18 and IL21R, the latter a receptor for an interleukin with established roles in T cell biology. Beyond blood-related traits, the framework explained associations in other tissues, including loci connected to Alzheimer’s disease such as PILRB and TYROBP, genes involved in microglial signaling, illustrating that regulatory mechanisms mapped in immune cells can illuminate neurological and other complex diseases.

The resource is being made broadly available to the research community. Summary statistics for chromatin accessibility quantitative trait loci, expression quantitative trait loci, and peak-gene links are publicly accessible and browsable through an interactive web interface, and the analysis pipelines, including the CASCADE classification framework and supporting software packages, have been released on GitHub. Individual-level data remain protected under Finnish and European data regulations but can be accessed by approved researchers through established application channels. As the field moves toward functional interpretation of the ever-growing catalog of disease-associated variants, atlases of this kind, which connect genetic variation to regulatory architecture across millions of cells and hundreds of donors, are likely to become foundational references for drug target discovery and precision medicine in immunology and beyond.

Subject of Research: Population-scale single-cell multiomic mapping of regulatory genetic variation in human immune cells

Article Title: Population-scale immune multiome atlas reveals regulatory disease mechanisms

Article References: Kanai, M., Delorey, T. M., Honkanen, J., Rodosthenous, R. S., Juvila, J., Murphy, S., Teixeira-Soldano, I., Hwang, H. S., Karjalainen, J., Halonen, J., Panagiotaropoulou, G., Zhang, Y., McCabe, C., Chen, E., Nanki, K., Yoshida, T., Liu, K., Glean, M., Mehrotra, N., … Xavier, R. J. (2026). Population-scale immune multiome atlas reveals regulatory disease mechanisms. Nature. https://doi.org/10.1038/s41586-026-11078-2

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11078-2

Keywords: single-cell multiomics, chromatin accessibility, gene expression, quantitative trait loci, FinnGen, immune cells, regulatory variants, enhancer-gene links, autoimmune disease, genetic fine-mapping, reporter assays, evolutionary constraint