Summary of Advances
In Autism Spectrum Disorder Research
Neuron numbers increase in the human amygdala from birth to adulthood, but not in autism.
Avino TA, Barger N, Vargas MV, Carlson EL, Amaral DG, Bauman MD, Schumann CM. Proc Natl Acad Sci USA. 2018 Apr 3;115(14):3710-3715. [PMID: 29559529]
ASD is known to cause changes in global brain volume. However, little is known about the mechanisms underlying atypical brain volume in specific areas of the brain. One such area is the amygdala, a region of the brain that grows substantially in volume through adulthood during typical development. The amygdala plays a key role in regulating emotional behavior, and therefore is of particular interest to ASD researchers. Previous studies have suggested that individuals with ASD may have altered growth patterns in this region of the brain.
In this study, researchers compared the number of mature and immature neurons in the amygdala at different ages throughout life. Post-mortem brain samples were taken from adults and children with and without ASD who had died of other causes. The researchers found that the number of neurons in the amygdala increases with age in typically developing individuals but decreases over time in individuals with ASD. Specifically, children with ASD had more neurons in the amygdala as compared to neurotypical children. However, adults with ASD had fewer neurons in the amygdala than neurotypical adults.
Based on these results, the authors hypothesized that the age-related increase in mature neurons in neurotypical individuals is either the result of the maturation of immature neurons or the generation of new neurons after birth. In contrast, the researchers propose that the initially higher number of neurons in children with ASD could be caused by factors related to alterations in prenatal brain development. The subsequent decrease in mature neuron number over time in adults with ASD may be caused by dysregulation of the neuronal maturation process. Another possibility is that the increased number of neurons in the young amygdala leads to a hyperactivity that subsequently damages neurons. This may account for the lower number of mature amygdala neurons in adults with ASD compared to typically developed adults.
The results from this study help researchers understand the typical development of the amygdala, as well as identify developmental differences in children and adults with ASD. Studies such as these will continue to shed light on the biological mechanisms underlying ASD and some of its co-morbid conditions such as anxiety.
Complete Disruption of Autism-Susceptibility Genes by Gene Editing Predominantly Reduces Functional Connectivity of Isogenic Human Neurons.
Deneault E, White SH, Rodrigues DC, Ross PJ, Faheem M, Zaslavsky K, Wang Z, Alexandrova R, Pellecchia G, Wei W, Piekna A, Kaur G, Howe JL, Kwan V, Thiruvahindrapuram B, Walker S, Lionel AC, Pasceri P, Merico D, Yuen RKC, Singh KK, Ellis J, Scherer SW. Stem Cell Reports. Nov 13;11(5):1211-1225. [PMID: 30392976]
Development of ASD is due in part to genetic factors. Currently, researchers are working to identify specific genes that increase the risk for ASD and to determine how each gene affects neurodevelopment. New gene-editing technology allows researchers to alter genes in induced pluripotent stem cells (iPSC), cultured cells taken from skin or blood that are reprogrammed into stem cells. By converting these edited iPSCs into neurons, researchers can better understand the relationship of these genes to ASD symptoms.
In this study, researchers used a gene-editing technique called CRISPR, which allows them to inactivate genes associated with ASD in cells and study the cells’ activity. They selected 14 genes that are associated with an increased risk of ASD but whose functions were not well-understood. These genes are involved in various biological processes, including those that regulate cellular activity, growth, and communication. The researchers created 14 iPSC lines that each had one gene inactivated, then converted cells from each line into neurons. This allowed the researchers to study the changes in the behavior of neurons that lacked the function of the genes of interest.
The researchers found that inactivating some of the ASD-associated genes caused decreases in electrical activity in individual neurons. Neurons use electrical activity to send signals associated with biological processes, and this result suggests that the alteration of these genes changes behavior in individuals with ASD by affecting synaptic communication, which can limit the functionality of brain cells. When the researchers focused on the genes that resulted in the largest decrease in neuronal activity, they found that the loss of these genes reduced electrical activity not just in individual cells, but in populations of cells as well. They also found that some of the genes affected the way the neurons developed and formed connections with other neurons. Importantly, the researchers observed that, although these genes belonged to different functional groups, they all exhibited similarly reduced synaptic activity. These results support the idea that communication between neurons is altered in individuals with ASD, and that this change in cellular communication affects how these cells function.
This study presented a novel CRISPR strategy to isolate and analyze the expression of ASD-related genes. The researchers suggest that the next steps to understanding the relationships among these genes to ASD development should be to remove the genes in mouse models and observe behavioral changes in the animals.
Social deficits in Shank3-deficient mouse models of autism are rescued by histone deacetylase (HDAC) inhibition.
Qin L, Ma K, Wang ZJ, Hu Z, Matas E, Wei J, Yan Z. Nat Neurosci. 2018 Apr;21(4):564-575. [PMID: 29531362]
Recent research demonstrates that some genes are expressed differently in individuals with ASD as compared to typically-developing individuals. In some individuals with ASD, the Shank3 gene, which forms proteins that build and maintain the structure of synaptic connections between brain cells, is only expressed in one copy instead of two, or may be present in a mutated form. Mouse models that contain Shank3 mutations display social deficits and repetitive behaviors similar to many individuals with ASD.
The researchers in this study sought to determine if they could correct the mutation of the Shank3 gene by targeting enzymes — proteins that regulate cellular activities — that affect gene expression. They focused on histone deacetylase (HDAC), an enzyme that is known to suppress gene expression. The research team hoped that inhibiting HDAC could increase the expression of Shank3.
They compared mice with the Shank3 mutation (a mouse model for ASD) to wildtype (normal) mice. They found that the Shank3-mutated mice had altered expression of several genes. They injected Shank3 mutated mice with romidepsin, an HDAC inhibitor, and found this restored gene expression to levels similar to wildtype mice.
To understand whether correcting gene expression with romidepsin affected ASD-related behavior, the researchers observed the social behaviors of these mice. As previously demonstrated, Shank3-mutant mice prefer a non-social stimulus in a social interaction test, while wildtype mice prefer a social stimulus. The researchers found that romidepsin-treated mice spent more time exploring the social aspects of their environment and now behaved similarly to wildtype mice. Surprisingly, they found that this effect lasted for 21 days following the injection, suggesting the drug treatment had long-lasting effects. The researchers tested whether romidepsin had any effect on other ASD-related behaviors, but they did not observe an effect on motor function, anxiety-related behaviors, or repetitive behaviors.
In these experiments, the researchers manipulated HDAC activity using a drug, but they also wanted to better understand how HDAC activity is typically regulated without chemical intervention. They focused on a protein called ß-catenin, a protein that is involved in regulating HDAC activity. In wildtype mice, ß-catenin moves from the cell nucleus to the cell periphery, where it plays a role in transmitting messages from neighboring cells. The researchers determined that, in Shank3-mutated mice, ß-catenin primarily remains in the nucleus. To better understand the role of this protein, the researchers reduced the levels of ß-catenin in Shank3-mutant mice. They found that these mice had lower levels of HDAC and displayed higher levels of social interaction. Furthermore, the researchers identified several targets of HDAC activity that were altered in Shank3-mutant mice, and they were able to restore normal expression and/or activity of these targets with romidepsin treatment.
Identification of and characterization of this molecular pathway provides insight into the underlying biology of ASD symptoms. The researchers did not observe any unintended affects as a result of treatment with romidepsin. This drug is already FDA-approved for use in cancer treatment, and the dose required for the effect seen in ASD behaviors is equivalent to about 5% of the dose used to treat cancer in humans. The researchers suggest that romidepsin may be a promising therapeutic intervention for ASD patients who have Shank3 mutations.