Gene Therapy as a Promising Approach for the Underlying Causes of Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a range of developmental disorders characterized by impaired traits associated with three distinct domains: communication, social interaction, and stereotypic repetitive behavior. Although the etiology of ASD depends on various components, current research mainly focuses on the genetic factors that contribute to the development of ASD and its effects. A big treatment consideration is gene therapy, which has disease-modifying potential. This article provides insight into the foundation of ASD and the leading gene therapies that aim to address impaired neurological behavior. We will discuss how certain genetic factors can have large contributions to ASD development and how scientists can go about targeting these factors for potential remedies.


Introduction
Autism spectrum disorder (ASD) is a neurodevelopmental condition that a ects communication and social interaction. It is a complex condition with a range of symptoms and severity levels, and the exact causes are still not fully understood. 1 However, genetic factors are believed to play a signi cant role in the development of ASD. 2 Symptoms of ASD, which typically appear in early childhood, include impaired social interaction and communication, repetitive behaviors and interests, and sensory processing issues. These symptoms range from mild to severe and may a ect an individual's ability to function in daily life. 1,3 At a brain developmental level, ASD is thought to result from abnormal development, particularly in regions of the brain involved in social interaction and communication. Some studies have suggested that individuals with ASD may have di erences in brain structure and function, such as altered connectivity between di erent regions. 4 At a genetic level, ASD is believed to be caused by a combination of genetic and environmental factors. While the exact causes of ASD are still not fully understood, research has identi ed a number of genetic variations that are associated with an increased risk of developing the condition. These genetic factors may interact with environmental factors to in uence brain development, ultimately resulting in ASD. 1 Autism is one of the most heritable neurodevelopmental disorders, a ecting 78 million people or 1.5 % of the world's population. 5 Despite its ubiquitous existence, there are aspects of the disorder that science has yet to unveil. With the etiology of autism still in question, scientists have been able to conclude that at least 40% of neurobehavioral disorders within the autism spectrum are the result of genetic abnormalities. 6 The current treatment for ASD falls into two categories: those that target core symptoms which include impaired communication, social interaction, and repetitive behaviors; and those that target secondary or consequential symptoms such as ADHD and irritability. Gene therapy, a treatment that targets core symptoms, focuses on mending genetic building blocks. It aims to alter abnormalities in hopes of preventing domino-e ects in the body that result in the neurobehavioral de cits which characterize ASD. Using viral vectors such as recombinant adeno-associated viruses, gene therapy is able to introduce new genetic material to counteract the establishment of ASD symptoms. 7 This paper will be discussing studies and their adaptation of gene therapy to varying pathophysiologies of ASD in order to o er e ective treatment methods. This paper will cover the underdevelopment of neurons, impaired neural migration, impaired synaptogenesis, and dendritic morphogenesis. 8

Etiology of Autism Spectrum Disorders
Although the details of the origin of ASD have not yet been discovered, scientists have come up with multiple theories answering the question of "how" ASD comes to be in individuals. One such theory on the pathophysiology of ASD is linked to neural connectivity. In a typical human body, an individual develops a surplus of neurons where over time, non-functional and unnecessary neurons are removed through various mechanisms. In patients who have ASD, the mechanism that targets the elimination of underdeveloped neurons is damaged. As a result, the excess neurons impair the shaping and ne-tuning of neural circuits. 9 Another theory focuses on neural migration. Similar to neural connectivity, neural migration also contributes to well-formed neural circuits which aid in proper communication and other neural behaviors. In ASD patients, the misplacement of neurons as a result of faulty migration during development increases the thickness of the cortex and "smudges" the boundaries of white matter. This mostly a ects the frontal and temporal lobes: key elements in processing language and emotion-functions commonly found to be impaired in ASD patients. 9 An additional theory that de nes the mechanism by which ASD is established focuses on impaired synaptogenesis and dendritic morphogenesis. The normal development of synapses and dendrites entails excess formation followed by the purging of faulty expression. In patients with ASD, the suppression mechanism responsible for the expulsion of faulty signaling factors is impaired. The overwhelming number of defective synapses and dendrites damages the pathway of signals within the body, resulting in the core symptoms associated with ASD. 9 However, the use of two viral vectors created another problem: a decrease in cellular aggregation in the brain organoids. The scientists attempted another method called virus-mediated overexpression in which a wild-type copy of TCF4 was overexpressed in hopes of overriding ectopic expression of the mutation. PTSH organoids exposed to overexpression-type gene therapy showed improvements in two key regions that marked the downstream corrected function of the TCF4 gene: increased ring rates and number of network electrical bursts. The combination of these two methods, provided that the impaired proliferation of neurons from a mutated TCF4 can be genetically corrected, could prove to lessen the symptoms of other genetic autism disorders. 11

RELN Study
RELN is a gene that encodes for a RELN glycoprotein in the extracellular matrix of GABAergic (Gamma-Aminobutyric Acid) neurons. These cells perform important functions in neural migration and cortical lamination. 13 Neural migration is an important process that occurs in mammalian nervous system development. 14  In addition, sex hormones can also play a role in methylation of the RELN promoter, which has shown to increase ASD-associated behaviors. After postmortem cerebellar studies of ASD patients, it was found that lower RELN mRNA levels are associated with higher MECP2 (another essential protein for nerve cells) binding, increasing gene regulator 5-hmC at the RELN promoter and in turn decreasing transcription and protein levels.
Such RELN mutations that cause disruptions in signaling pathways are connected to ASD disorders. One example is the loss of Purkinje cells, regulated by RELN, which increases the risk of cognitive delay and epilepsy.
This is a phenomenon also seen in ASD patients. 15 Investigating the role of RELN on neuronal signaling and ASD development can produce promising leads for potential treatments of ASD.
Increasing levels of RELN protein can alleviate behavioral symptoms of RELN that ASD patients also experience. However, although there is much evidence for RELN's in uence on ASD, it is not the sole factor of ASD development. For the diagnosis of ASD, there are usually secondary genetic or environmental factors that contribute to the disorder's development. 16

MeCP2 Gene Study
Many studies have shown MeCP2 protein's role in brain development and regulation, such as expression of the brain-derived neurotrophic factor    sequencing showed that p.P152L and p.R294X were de novo mutations, but p.P376S was inherited maternally. They did not nd any of these mutations in GnomAD, indicating that they were rare mutations. 18 In terms of clinical features, poor social interaction and functional impairment were major symptoms for all three mutations, but language use and repetitive behavior di ered greatly between them. Also noted were abnormalities of dendritic and axonal growth found after autism-related MeCP2 mutants were expressed in mouse cortical neurons, suggesting that autism-related MECP2 mutations impair proper development of neurons. 18 The results strongly suggest that MeCP2-P152L, MeCP2-R294X, and MeCP2-P376S a ect the proper physiological function of the MeCP2 protein and may contribute to the pathogenesis of autism. 18

MECP2 Gene Therapy Methods
In addition to these studies, there have been a

Conclusion
While there is currently no cure for ASD, there are various approaches to managing its symptoms and improving quality of life. These include behavioral therapy, medication, and support services. Research into the causes and potential treatments of ASD is ongoing, and new developments are constantly emerging. 2 Gene therapy is one promising approach that holds signi cant potential for treating the underlying causes of ASD, rather than just managing its