Unborn Secrets: The Prenatal Development of Fragile X Syndrome

Study results fundamentally change how scientists understand the developmental origins of Fragile X syndrome and suggest a potential treatment for brain cells damaged by the dysfunction.

Fragile X syndrome, the most common form of inherited intellectual disability, may be unfolding in brain cells even before birth, despite typically going undiagnosed until age 3 or later.

A new study published on October 10 in the journal Neuron by researchers at the University of Wisconsin–Madison showed that FMRP, a protein deficient in individuals with Fragile X syndrome, has a role in the function of mitochondria, part of a cell that produces energy, during prenatal development. Their results fundamentally change how scientists understand the developmental origins of Fragile X syndrome and suggest a potential treatment for brain cells damaged by the dysfunction.

The energy-making organelles called mitochondria (shown in green) that work inside cells to make energy, aren’t working as they should in the neurons (shown in red) of people with fragile X syndrome. University of Wisconsin–Madison researchers have identified a protein and gene involved in this mitochondrial dysfunction, as well as a potential treatment. Credit: Minjie Shen, University of Wisconsin–Madison

The Central Role of FMRP and the Implications

The study, led by four postdoctoral fellows — Minjie Shen, Carissa Sirois, Yu (Kristy) Guo, and Meng Li — working in the lab of the lab of Xinyu Zhao, neuroscience professor and neurodevelopmental diseases researcher a UW–Madison’s Waisman Center, found FMRP regulating a gene called RACK1 to promote mitochondrial function. Using a drug to enhance mitochondrial function, they were able to rescue brain cells damaged by lack of FMRP.

Individuals with FXS may present developmental delays — not sitting, walking, or talking at expected ages — as well as mild to severe intellectual disability, learning disabilities, and social and behavioral problems. About half are also diagnosed with autism spectrum disorderAutism Spectrum Disorder (ASD) is a complex developmental disorder that affects how a person communicates and interacts with others. It is characterized by difficulty with social communication and interaction, as well as repetitive behaviors and interests. ASD can range from mild to severe, and individuals with ASD may have a wide range of abilities and challenges. It is a spectrum disorder because the symptoms and characteristics of ASD can vary widely from person to person. Some people with ASD are highly skilled in certain areas, such as music or math, while others may have significant learning disabilities.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>autism spectrum disorder.

In previous research, Zhao found that mitochondria in mice with an FMRP deficiency that imitates FXS were smaller and unhealthy. Diving deeper, they also discovered that FMRP regulates genes involved in mitochondria fission-fusion, a process into which mitochondria fuse into a bigger shape in order to produce more energy for the cell.

Xinyu Zhao is a neuroscience professor and neurodevelopmental diseases researcher a UW–Madison’s Waisman Center. Four postdoctoral fellows in her lab led the study. Credit: University of Wisconsin–Madison

From Mice to Human Neurons: A Comparative Analysis

For the study, researchers grew brain cells called neurons grown from induced pluripotent stem cells. Because the stem cells came from people with FXS, the researchers could study the development of the disorder at a cellular level, determining whether mitochondria in human cells experienced issues similar to those in mice.

“And indeed, we found that human neurons also have fragmented (smaller) mitochondria,” Zhao says. They also found fewer mitochondria in neurons derived from FXS patients, which they did not see in the neurons of the mice modeling FXS.

“In human neurons, it’s a deficit in twofold. Not just fission-fusion, but also likely in the production of mitochondria,” Zhao says.

Although it has been long known that FMRP is deeply involved in FXS, the new discovery pinpoints a role for the protein in early development of the condition.

Symptoms of FXS present long after the baby is born. Many babies appear to be developing typically before showing slower development, autistic features or developmental deficits. Children with FXS are typically diagnosed at three years of age or older.

“Which means many scientists have been thinking that FMRP is more important for the postnatal maturation state,” Zhao says.

Link Between FMRP, FXS, and Autism

FMRP is protein that regulates the use of messenger RNARibonucleic acid (RNA) is a polymeric molecule similar to DNA that is essential in various biological roles in coding, decoding, regulation and expression of genes. Both are nucleic acids, but unlike DNA, RNA is single-stranded. An RNA strand has a backbone made of alternating sugar (ribose) and phosphate groups. Attached to each sugar is one of four bases—adenine (A), uracil (U), cytosine (C), or guanine (G). Different types of RNA exist in the cell: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>RNA, sort of a working copy of DNADNA, or deoxyribonucleic acid, is a molecule composed of two long strands of nucleotides that coil around each other to form a double helix. It is the hereditary material in humans and almost all other organisms that carries genetic instructions for development, functioning, growth, and reproduction. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>DNA used to produce the proteins that make things happen in cells. The researchers found that many of the mRNA strands that interact with FMRP are implicated in autism, providing a molecular link between FXS and autism spectrum disorder. Unexpectedly, many FMRP-bound mRNAs are expressed by genes classified as essential — genes that are very busy during prenatal development but less active after birth.

“This means that FMRP has a function in prenatal development that we have not really thought about before,” Zhao says. “The fact that we found that FMRP also regulates prenatal development is really interesting and is actually indicating that what we see in Fragile X syndrome, some of the effects already happened within the prenatal development.”

Potential Treatment and Future Research

One of those essential genes is RACK1, identified for the first time as playing a role in FXS.

“When RACK1 is lower in Fragile X neurons, the mitochondria are suffering and the neurons exhibit mitochondrial deficit and hyperexcitability, like immature neurons. But when we reintroduce RACK1, we can rescue this,” Zhao says.

Using cultured neurons derived from individuals with FXS to screen for drugs, the researchers found a drug called leflunomide that corrected mitochondrial deficits. The treatment improved mitochondrial function and reduced the neurons’ hyperexcitability.

Next, Zhao wants to do a detailed biochemical analysis of mitochondrial dysfunction and figure out which key proteins are less present in FXS-affected neurons. She is also working on better understanding how RACK1 and leflunomide work to rescue mitochondrial function.

Reference: “Species-specific FMRP regulation of RACK1 is critical for prenatal cortical development” by Minjie Shen, Carissa L. Sirois, Yu Guo, Meng Li, Qiping Dong, Natasha M. Méndez-Albelo, Yu Gao, Saniya Khullar, Lee Kissel, Soraya O. Sandoval, Natalie E. Wolkoff, Sabrina X. Huang, Zhiyan Xu, Jonathan E. Bryan, Amaya M. Contractor, Tomer Korabelnikov, Ian A. Glass, Dan Doherty, Jon E. Levine, André M.M. Sousa, Qiang Chang, Anita Bhattacharyya, Daifeng Wang, Donna M. Werling and Xinyu Zhao, 10 October 2023, Neuron.
DOI: 10.1016/j.neuron.2023.09.014

Other collaborators on the study include Waisman Center investigators Qiang Chang, Anita Bhattacharyya, Andre Sousa, Daifeng Wang, Donna Werling and UW–Madison neuroscience professor Jon Levine.

This research was supported by grants from the National Institutes of HealthThe National Institutes of Health (NIH) is the primary agency of the United States government responsible for biomedical and public health research. Founded in 1887, it is a part of the U.S. Department of Health and Human Services. The NIH conducts its own scientific research through its Intramural Research Program (IRP) and provides major biomedical research funding to non-NIH research facilities through its Extramural Research Program. With 27 different institutes and centers under its umbrella, the NIH covers a broad spectrum of health-related research, including specific diseases, population health, clinical research, and fundamental biological processes. Its mission is to seek fundamental knowledge about the nature and behavior of living systems and the application of that knowledge to enhance health, lengthen life, and reduce illness and disability.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>National Institutes of Health (R01MH118827, R01NS105200, R01MH116582, R01MH118827, R01HD064743, R01NS064025, R01AG067025, U01MH116492, P51 OD011106, U54HD090256, P50HD105353, R24HD000836 and T32 GM141013) and the Department of Defense (W81XWH-22-1-0621).