Earliest effects of chromosome 21 triplication in brain development discovered

By Charlene N. Rivera-Bonet | Waisman Science Writer

Human iPSC
Human iPSC being differentiated into dorsal forebrain neural progenitor cells

It is well established that brain development looks different for individuals with Down syndrome, but how early do these differences appear? New Waisman research using gene expression analysis shows that the triplication of the 21st chromosome, which causes Down syndrome, impacts brain development at its earliest stages. This is the first study to present the earliest timepoint yet studied at which the effects of trisomy 21 in the brain can be detected, expanding the understanding of brain development in individuals with Down syndrome.

Anita Bhattacharyya, PhD
Anita Bhattacharyya, PhD

The study from the lab of Waisman Center investigator Anita Bhattacharyya, PhD, associate professor of cell and regenerative biology revealed that triplication of chromosome 21, or trisomy 21, has effects on development as early as neural induction – the first step of embryonic cells becoming neurons. The study was published in Frontiers in Cellular Neuroscience.

“The overarching goal [of this study] was to try to understand how three copies of chromosome 21 impacted neurodevelopment, or neural induction from a transcriptomics point of view,” says José Martinez, PhD, a former graduate student in Bhattacharyya’s lab and first author of the study. Transcriptomics can tell researchers when and where each gene is turned on or off in cells.

Down syndrome, also known as trisomy 21, is caused by the triplication of the 21st chromosome. This triplication has effects on development, both on the brain and throughout the body. Previous studies have looked at the endpoint of neural development in trisomy 21 such as intellectual disability and reduced brain size. However, what happens in the early stages of the formation of the nervous system in individuals with Down syndrome was unknown.

“One of the first big takeaways is that as early as the time points we were looking at, which were day 6, 10 and 17, post neural induction, you’re able to see pathways that are affected,” Martinez says.

A key player in this discovery are induced pluripotent stem cells (iPSC). These cells are taken from an individual’s skin or blood, reprogrammed into an embryonic stem cell-like state, and converted into neurons. In this case, neurons of the dorsal forebrain, the upper part of the brain. “[This study] harnessed what I think is the major strength of human stem cells, they will recapitulate events in brain development. And even from the very earliest times,” Bhattacharyya says. “This is a period in time that you really can’t study any other way.”

Jose Martinez
José Martinez, PhD (Photo by Robin Davies)

The cells used in this study, both control and cells with trisomy 21, were acquired from the same individual, who has mosaic Down syndrome. Individuals with mosaic Down syndrome have a mixture of two different cell types. Some of the cells in their bodies contain an extra copy of the 21st chromosome and some of their cells have the typical set of two. The percentage of cells with trisomy 21 can vary by each individual with mosaic Down syndrome. This means that the genetic profile of the cells used for this study was exactly the same, with the sole exception of the chromosome 21 triplication. “What that does is it gets rid of all the other potential things that could impact our results,” Martinez says. “This allows us to ask what is this third copy of chromosome 21 really doing during neural induction.”

Their results showed that at these early timepoints, trisomy 21 dysregulates pathways involved in cell fate, inflammatory response, and oxidative stress. “We saw differences in molecular pathways that almost foreshadow things we know will happen later on,” Bhattacharyya says. Immune disorders, divergent metabolic states, physical differences, and premature aging are all commonly seen later on in life in individuals with Down syndrome, and may be explained by these very early changes in gene expression. For example, the gene TTC3, which can result in protein aggregation when overexpressed, was found to be consistently altered throughout the process of neural induction. This early upregulation of TTC3 may contribute to later protein aggregation associated with Alzheimer’s disease, which affects about 90% of individuals with Down syndrome over the age of 65.

While the dose of chromosome 21 is triple, the study showed that it is not just the genes in this chromosome that have a dysregulated expression in cells with trisomy 21.  In fact, there is a higher number of dysregulated genes in other chromosomes compared to the 21st. “Before this, people didn’t consider how much of an effect [the triplication] could have on other chromosomes,” Bhattacharyya says. It is possible that driver genes in chromosome 21 are starting the process and affecting the expression of genes in other chromosomes. “One of the beauties of science and biology is that nothing is isolated, right?” Martinez says. This applies to how chromosomes interact with each other to regulate the expression of genes.

The long-term effects of these very early changes in neuron development raise the question of whether interventions to address these changes can be introduced early on to promote healthy aging of individuals with Down syndrome. “Oxidative stress or mitochondrial deficits are two things that are strongly linked to age-related neural conditions, whether it’s neurodegeneration diseases or other disorders,” Martinez says. “So…is that something that we should be thinking about as early as birth?”

This study opens the door to future research to better understand development in individuals with Down syndrome. Bhattacharyya’s hope is to perform this analysis using cells from more individuals with Down syndrome, as well as look into the effects of the triplication in early formation of other organ systems outside the brain.

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