A cellular map reveals how Down syndrome affects prenatal brain development

This study is excellent. It shows that in Down syndrome, the brain begins to develop differently very early in pregnancy.

Down syndrome does not merely involve having an extra copy of chromosome 21; rather, it triggers a massive and complex disruption of the activity of thousands of genes across the entire genome, in ways we cannot yet predict because cellular regulatory networks function like a black box filled with interactions we do not understand. That is why this study is important.

The study analyzes more than 100,000 cells over ten critical weeks of human fetal brain development, allowing researchers to reconstruct how neuronal differentiation progresses (and accelerates) in Down syndrome, rather than just observing the final outcome.

They show that changes begin very early, with accelerated neurogenesis, abnormal neuronal differentiation, problems with oligodendrocyte maturation, and early activation of the brain’s immune system. After birth, this inflammation persists and early signs of neurodegeneration appear, such as synapse loss and oxidative stress, which helps explain why people with Down syndrome have an extremely high risk of developing Alzheimer’s disease. These detailed maps of gene expression and chromatin accessibility in each cell type of the brain are essential for understanding how trisomy 21 affects neurological development, why there is such cognitive variability among individuals, and which processes could become future therapeutic targets.

One might think that the presence of an extra copy of chromosome 21 simply means that the genes on that chromosome are expressed more than normal. But the reality is far more complex: that third copy alters thousands of genes spread throughout the genome, and it does so differently depending on the cell type, tissue, and stage of development.

Trisomy 21 triggers such a broad and interconnected cascade of changes that, at present, it is impossible to theoretically predict how it will affect each cell. It is like a black box: we know what goes in (an extra chromosome), but we cannot precisely anticipate what outcome this produces in each tissue and cell type.

Furthermore, focusing solely on the final state of the brain does not indicate which mechanisms need to be corrected. The final state is the cumulative result of many processes that occurred earlier, but it does not reveal which step went wrong, what became dysregulated first, or which cellular program was activated at the wrong time. In contrast, studying gene expression throughout gestation allows us to identify which cellular decisions are made too early, which signals push cells to differentiate prematurely, and, therefore, where it would make sense to intervene: not to “fix” an already-formed brain, but to modulate developmental processes while they are still reversible.

The study analyzes each cell separately using multi-omics techniques. A multi-omics study analyzes multiple levels of information simultaneously in each cell. In this study, researchers simultaneously examine which genes are being expressed and which regions of DNA are accessible in each cell (the latter refers to how chromatin is organized, i.e., which parts of the DNA are open to regulate those genes). They look not only at what the cell is saying, but also at what enables it to say it.

This allows us to begin to understand how trisomy 21 leads to cognitive diversity, why some people are more severely affected than others, and which processes could be targets for future interventions. Furthermore, these data serve as a reference for evaluating experimental models (such as mice or cell cultures), which until now have not always accurately replicated what occurs in the human brain. The authors note that even earlier stages of development still need to be studied, where the initial triggers of these alterations might be found. But obtaining first-trimester human tissue is very difficult, so it will be necessary to develop more accurate models.

Thanks to this approach, we can understand not only what changes in Down syndrome, but also, for example, how and why development accelerates and ends up producing fewer early neurons, or why these neurons may be more susceptible to neurodegeneration, laying the groundwork for subsequent cognitive problems.

That is why studies analyzing which genes are active in each cell type of the human brain and how their chromatin (the structure that regulates which genes can be read) changes are so valuable. And that is exactly what they have done in this study.

This is an excellent study that presents a comprehensive molecular atlas of brain development in Down syndrome, a relatively common condition whose underlying brain mechanisms are not yet well understood. The study links trisomy 21 to changes in cell types and in the molecular programs that regulate their function, and demonstrates this directly in human tissue at high resolution—a first in the field of Down syndrome. Nevertheless, it is important to remember that this is a descriptive study and does not establish direct causal relationships with cognitive deficits.

One of its most robust conclusions is that neurons are generated at an inappropriate time and at an accelerated rate, leading to premature depletion of progenitor cells. As a result, although neuron production is brought forward, the final total number may be reduced, and furthermore, the appropriate neuronal types are not always generated at the correct time, disrupting the balance among the different neuronal types. This finding is particularly relevant because it offers a possible explanation for the smaller brain size observed in people with Down syndrome and aligns well with previous studies—including some from our laboratory—that already pointed to early neurogenesis associated with genes such as DYRK1A, but here it is directly confirmed in the human brain.

They also show a significant change in circuit architecture: there are fewer neurons involved in regulating information input and more neurons that could promote greater information flow within the cortex, which could result in less efficient and “noisier” networks.

Despite the significance of this work, caution is warranted. The number of samples is limited and does not capture the full variability of Down syndrome, and it spans a broad developmental period—between 13 and 26 weeks of gestation—which is critical for the formation of the cerebral cortex. The number of cases per developmental interval is small, making it difficult to precisely analyze the fine dynamics. In the short term, this type of study will be key to better understanding cellular mechanisms and validating experimental models. In the long term, it may help identify potential therapeutic targets, but we are still far from clinical applications, and we must not raise unrealistic expectations.

In any case, this work reflects a very positive shift in the field: an increasing number of groups are studying Down syndrome using advanced technologies, and it is beginning to be recognized as an area of great scientific interest—something that is undoubtedly very positive.

The study is excellent, and its findings are significant, representing a clear step forward in our understanding of the changes that occur during prenatal development in the brains of individuals with Down syndrome.

For decades, we have assumed that the brains of individuals with Down syndrome differ from those of neurotypical individuals: they have lower total volume, smaller cerebellum and hippocampus volumes, and lower neuronal density in the cerebral cortex. This article largely clarifies what is happening, including variations in neuronal types. These differences lead to functional changes such as mild-to-moderate cognitive impairment, difficulties with memory and speech, and a high predisposition to developing Alzheimer’s disease in later life, around age 60. This work lays the groundwork for a better understanding of the pathological mechanisms underlying these processes, particularly an increase in intratelencephalic neurons and a decrease in corticothalamic neurons.

It generates a molecular atlas of changes in neuronal expression during the weeks when the cerebral cortex forms, identifying cell types. It serves as a true roadmap for analyzing key processes in greater detail, such as proliferation (the generation of a sufficient population of neurons), migration (the movement of formed cells to their final destination), connection formation (synaptogenesis), cell survival, the development of activity patterns, and so on. The study details at the cellular level what occurs during the development of this brain and provides a robust explanation for the changes in these key processes that occur in Down syndrome. The number of cells analyzed is significant, but individual variability is always high. It opens the door to many other studies. It is also important for the future to examine changes in other time windows, both before and after the analyzed period. Furthermore, it will be necessary to analyze changes caused by prenatal, perinatal, and postnatal variables during nervous system development in people with Down syndrome (viral infections, microbiota, nutrition, birth complications, etc.).

In the short term, interest in prenatal neurodevelopment and the genes, proteins, and cells involved will increase. It is also important to remember that we are talking about human brains, not animal models where differences are always present. So to speak, this information should not be “translated” to humans; it is about humans.

In the long term, it will drive the connection between genetic, molecular, and cellular aspects, on the one hand, and the anatomy and physiology of the cerebral cortex, on the other, as well as the behavioral aspects resulting from these processes. An example would be the increased susceptibility to Alzheimer’s disease in people with Down syndrome: why it occurs, what changes during development, what happens in the cell types analyzed…

The authors propose a sensible approach, one that holds promise for the future, but without overestimating the possibilities. Gene lists provide clues to some new candidates, and the identification of processes and protein interactions may point to potential therapeutic targets. That said, we do not yet know how to correct most of the altered processes, and our methodological arsenal is limited. We must be especially careful with families where someone has Down syndrome, so as not to raise false expectations. We need to combine hope with caution: we are making progress and learning more, but clinical application is not on the immediate horizon.

Down syndrome (DS) is the most common genetic cause (1 in 700–1,000 births) of intellectual disability. Although it has been known for nearly 70 years that it results from the triplication of chromosome 21 (trisomy 21, TS21), its genetic complexity (approximately 250 coding genes and several hundred non-coding RNAs tripled in the first instance, with the regulation of thousands of them on other chromosomes consequently affected) has hindered progress in understanding the alterations in the cellular and molecular mechanisms that lead to the neurodevelopmental problems associated with DS.

Some neurohistological and imaging studies have shown significant changes in the cellular architecture of prenatal brains, particularly in the neocortex, of individuals with Ts21. This indicates that cellular processes of neurodevelopment are disrupted very early in DS, a finding also supported by research conducted using animal models (primarily mice). However, it should be noted that the limitations of these animal models—namely, their inability to reproduce human chromosomal organization, as well as some significant differences in the neurogenesis processes of the cerebral cortex (the region of the brain that is arguably the most evolved and developed in humans)—have prevented a precise determination of the cellular and molecular processes of early neurodevelopment altered by TS21.

Researchers at the University of California, Los Angeles (UCLA) have published a study in the journal Science that addresses precisely this problem. They have used genomic techniques that allow the identification, in individual cells, of both the genes that are expressed and the open chromatin regions—that is, areas of the genome available for activation. By applying these methods to dissociated cerebral cortex cells throughout the second trimester of fetal development (a crucial period for the generation of cortical neurons) in control brains and those with TS21, they were able to identify the DS-associated alterations occurring in the cell populations generated during that period.

The study clearly concludes that DS extensively affects the temporal sequence of cell population generation in the neocortex, leading to a decrease in the initial population of neural progenitor cells and premature neurogenesis, which could explain the well-known neuronal deficit in DS. More importantly, these changes result in an alteration of the spatiotemporal pattern of neuron production, which occur in successive layers from the inside out. This pattern is fundamental for the specification of neuronal populations that integrate into distinct cortical circuits depending on the timing of their birth and the position they occupy. Notably, in the neocortex with Ts21, there is a loss of balance between populations of excitatory neurons, with an increase in those occupying superficial layers—which participate in local circuits—at the expense of those located in deep layers, which connect to other brain regions. Furthermore, analysis of the collected genomic data allows for the prediction of potential gene regulatory networks altered by TS21 during this period of neurodevelopment.

Overall, the insights provided by this study are of immense value for advancing research into the fundamental causes of neurodevelopmental alterations caused by DS, and the possibility that it may help identify new therapeutic targets should not be overlooked. However, the possibility that this knowledge will translate into the development of therapeutic strategies to counteract early neurodevelopmental alterations in DS should be viewed with caution, at least in the short term, not only because of the various difficulties inherent in any intervention during the prenatal period, but also because the intricate networks of gene interactions altered by Trisomy 21—which the study itself confirms (2–10% of genes differentially expressed across all chromosomes)—make the hypothetical restoration of these processes to their normal course extremely difficult.