HD is commonly categorized as a late-onset neurodegenerative disease, as its defining symptoms typically appear late in life. However, HD is caused by mutated huntingtin protein (mHTT), which is present in cells long before the onset of symptoms, and is necessary for fetal development, as shown in a study with mice embryos.1 This led researchers Barnat et. al. to investigate the effects HD can have on cells and brain development in both human and mouse fetuses.
Existing Mice Research
There is some pre-existing research on wild-type, or unmutated, huntingtin protein (HTT) and mHTT’s effects on fetal and early brain development. Reiner et. al. used a mouse model in which the wild-type huntingtin gene was completely deleted in mice embryos to determine the function of HTT in early development. Deletion of HTT resulted in the death of mice embryos after 8 to 10 days, because it plays a critical role in embryo development, at least with mice.2
Further mice models of HD reveal that mHTT hinders various processes that are necessary for neuron development. These processes include brain cell differentiation, the migration of neurons to their proper locations in the brain, and maturity of neuronal cells.34 These studies—that show the effects of mHTT and HTT on early brain development in life—suggest that HD could be involved in human brain development as well.
HD in the Pre-symptomatic Brain
Beyond the mice models, there are also studies that show the effects of the mHTT present in HD on the human brain before symptom onset. One study by Nopoulos et. al. compared the intracranial volume, which is essentially brain size, of children with the HD gene expansion to children without the gene expansion. They found significantly smaller intracranial volumes in gene-expanded children, which means that mHTT may impede brain development long before symptom onset.5
Additional studies show that mHTT more specifically causes defects in the corticostriatal pathway, which is a brain pathway involved with physical movement and learning, and even alters gene expression in stem cells. When HD changes gene expression in fetal stem cells, growing fetuses will receive different instructions for brain and body development, and may develop abnormally.6
Huntingtin Location in the Fetal Brain
To investigate the impacts of mHTT on human fetal development, Barnat et. al. analyzed brain tissues from four fetuses with the HD mutation and four healthy fetuses.7 These tissues were collected at gestational week 13, which means 13 weeks from the parent’s last menstrual cycle, not from conception. At this point of fetal development, cortical neurons, which are brain cells located in the outer layer of the brain, begin to form. In fetal development, these cortical neurons come from the ventricular zone, which is a layer of tissue made of stem cells that will eventually become neurons.
The researchers wanted to examine where HTT and mHTT are present in the ventricular zone of fetuses. To do so, they used an antibody, which is a natural part of the human immune system that recognizes proteins on disease-causing viruses or bacteria. Instead of identifying those types of proteins, however, the antibody used in this study recognizes both HTT and mHTT. This antibody acts as a stain, as it binds to the proteins and makes them visible to researchers.
After this staining process, researchers could see a difference between wild-type fetuses and fetuses with the HD mutation. In wild-type tissues, HTT was present in the apical surface, or the innermost end, of the ventricular zone, and was also spread throughout the basal region, which is the area opposite to the apical end. However, the protein patterns in brain tissue with the HD mutation are different: HTT and mHTT are mainly located at the apical surface, and are less present in the basal region than the wild-type. Because the location of proteins is different in the ventricular zone for HD fetuses, this could mean that HD causes brain cells to develop differently before birth.
Because brain tissue for fetuses with HD mutations is rare, the researchers also repeated the same process with a mice model. The mice model gave similar results, that mHTT is present in the apical surface and decreased in the basal region as compared to the wild-type.
Huntingtin Interactions with Protein Pathways
Proteins play a very important role in fetal development, partly because they are a way for cells to communicate with each other and can influence a cell’s identity. For instance, a certain mixture and amount of proteins can tell a cell to divide and eventually become a different kind of cell. Because of the importance of proteins to fetal development, Barnat et. al. also looked at HTT and mHTT’s relationship to cell processes that create and move proteins.
To do so, researchers also used staining to see if HTT and mHTT are present at the same locations as other proteins associated with protein assembly, modification, or transport processes. In the wild-type samples, HTT colocalized partially with these proteins, which means that HTT was present in the same place but not in very high amounts. In the human and mouse HD samples, HTT and mHTT strongly colocalized with some of the proteins, which means they were present in much higher amounts compared to unmutated tissue. Researchers interpreted this to suggest that mHTT changes the way cells transport and secrete proteins very early in human development
Huntingtin Interactions with Junction Proteins
Further, the researchers examined HTT’s interactions with junction proteins in the apical ventricular zone. These junction proteins connect the cells in this region, stabilize the tissue, regulate what goes in and out of cells, and also play a role in cell growth and division. So, if junction proteins are not present in the correct places or in the right amounts, these processes will be affected. To analyze these important junction proteins, the researchers looked at fetuses at gestational week 16, because at this age the junction structures would be more developed. In the images of mutant tissues, there was a bright line of HTT along the apical surface of cells that was not present in the wild-type.
Researchers also looked at junction proteins Z01, NCAD, β-catenin, and PAR3 in the mouse model. In wild-type fetuses, Z01, NCAD, and β-catenin are found towards the sides of cells they join together, while PAR3 is found closer toward the center. In mice with the HD mutation, Z01, NCAD, and β-catenin are not concentrated on the sides but spread throughout, and PAR3 was less visible. These differences in junction proteins between wild-type and HD mice may have implications for brain tissue stability and cell division.
mHTT and the Cell Cycle
The cell cycle describes a process in which cells grow, prepare to divide, and then divide into two cells, called daughter cells. In the region that researchers mainly focused on, the apical ventricular zone, the junction proteins influence the cell cycle and are improperly arranged. Because of this, phases of the cell cycle last different amounts of time in the HD mutant mice, compared to the wild-type. The G1 and G2 phases, during which cells normally grow and prepare for division, take longer in HD mutated cells. The transition from G1 to S phase, S phase being the portion in which genetic material is replicated, takes less time. Researchers also looked for a substance called PH3, which is present when cells divide in two to create more cells. The mutant mice had about half the amount of PH3 compared to wild-type mice, which means that there are less dividing cells in the brain tissue.
The study conducted by Barnat et. al. shows that HD has impacts on brain development in fetuses with the HD mutation. Their findings point out differences between the ventricular zones of wild-type and mutant tissues, which set the stage for further brain development later on in life. While the specific differences that researchers found between wild-type and mutants have not been tied to differences in brain function throughout the life of a pre-symptomatic HD patient, the study sets the stage for further research on HD and development. It also invites future studies about HD treatments delivered early in life, because treatments in adulthood might be too late to reverse the impact mHTT has already had on brain circuitry.
- Duyao M, Auerbach A, et. al. (1995). Inactivation of the mouse Huntington’s disease gene homolog Hdh. Science, 1095-9203.
- Reiner A, Dragatsis I, et. al. (2003). Wild-type huntingtin plays a role in brain development and neuronal survival. Molecular Neurobiology, 28, 259–275.
- Barnat M, Le Friec J, et. al. (2017) Huntingtin-Mediated Multipolar-Bipolar Transition of Newborn Cortical Neurons Is Critical for Their Postnatal Neuronal Morphology. Neuron. 93, 99–114.
- Arteago-Bracho E, Gulinello M, et. al. (2016) Postnatal and adult consequences of loss of huntingtin during development: Implications for Huntington’s disease. Neurobiology of Disease. 96, 144-155.
- Nopoulos P, Aylward E, et. al. (2011) Smaller intracranial volume in prodromal Huntington’s disease: evidence for abnormal neurodevelopment. Brain, 20923788.
- Ring K, An M, et. al. (2015) Genomic Analysis Reveals Disruption of Striatal Neuronal Development and Therapeutic Targets in Human Huntington’s Disease Neural Stem Cells. Stem Cell Reports, 5(6):1023-1038.
- Barnat M, Capizzi M, et. al. (2020) Huntington’s disease alters human neurodevelopment. Science, 1095-9203.