Genetic Evidence for Autism in the Cobb Collection

By Jayla Harvey, Department of Biology, Howard University '16


Abstract

1 in 88 children in the United States are diagnosed with autism; this number has grown exponentially over the past couple of years, primarily due to raised awareness of this disorder. The definition of autism as we know it today didn’t come about until 1980 (CT Education Advocates, LLC, 2013).  Prior to that, autism was defined as schizoid personality disorder attributed fundamentally to a lack of maternal warmth.  Mental health issues, in general, in African Americans have not been extensively studied and are frequently under-diagnosed. Differential expression of mental disease likely stems from the multi-generational inhumane effects of the transatlantic slave trade, segregation, racism, and discrimination. As a result, the literature is scant on explicit descriptions of autism for the African American population. However, new scientific advances are suggesting that autism is genetically linked by clusters of DNA markers (Sarah R. Gilman, 2011). Since African American’s rich diversity is not widely represented in either genetic or behavioral studies, this research will look for evidence of mental disease in specific individuals in the Cobb Collection try to develop a bridge between the behavioral expression of mental disease and the presence of genetic susceptibility genes for autism. This study will focus on African American adults from the District of Columbia, Maryland, and Virginia area who died in either the 1930s, 40s and 50s and for whom clinical reports and other clinical and demographic clues suggest evidence of mental disease. Advanced bioinformatic approaches will be used to identify likely autism gene clusters in the targets of study.

Introduction: 

Many of the patients confined at St. Elizabeth’s Hospital in Washington, D.C between the decades of 1930s through 1950s were institutionalized because they were unable to function in society (Barak Goodman, 2010). This inability to assimilate into the general public may be a consequence of an undiagnosed mental illness.  Autism Spectrum Disorders (ASD) were not characterized separately from schizophrenia at this time, therefore it is possible that some patients that were being held at St. Elizabeth’s Hospital because they didn’t exhibit ‘normal’ behavior and suffered from an ASD.  Latest genetic research shows that ASD can be shown through a connection of many mutated genes (Geschwind, 2008).   These mutations can be etiologically effected by the environment the patient was subjected to. African-Americans of this time period would likely be more susceptible to these mutations due to the immense stressors of living in a time of segregation, Jim Crow Laws, post slavery traumatic syndrome, and poverty.

Methods and Results: 

The genes that will be studied in this experiment are various genes that have been linked to autism over the past couple years.  The genes have been grouped into three categories: mutated genes, deleted or copied genes, or genes with methylation changes. 

Mutated genes either show single nucleotide polymorphisms (SNPs) or frameshift mutations.  The mutated genes being studied are Dopamine Receptor D2 (DRD2: Gene ID1813), Protein phosphate1, regulatory inhibitor subunit 1B (PPP1R1B: Gene ID 84152), and Neuroligin4, X-linked (NLGN4X: Gene ID57502).  DRD2 encodes for the D2 subtype of a dopamine receptor which has roles in postsynaptic neurons and autorecpetor mediating dopamine synthesis and neurotransmission. The receptor inhibits adenylyl cyclase activity, which catalyzes the conversion of ATP and cyclic adenosine monophosphate (cAMP) and pyrophosphate (NCBI, 2015).  cAMP is important because it is used in the intracellular signal transduction.  In ASD, the DRD2 gene shows an over transmission of the T allele, which causes the gene to be incorrectly, negatively effecting how cells send signals to each other  (J. Reece, 2002).  The PPP1R1B gene encodes for DARPP-32, a bifunctional signal transduction molecule, expressed in dopaminoceptive neurons and mediates the effects of D1 and D2 dopamine receptors.  In ASDs, this gene shows an over transmission of the C allele, which can show decreased release of dopamine.  Altered levels of the DARPP-32 molecule displays impaired reversal learning.  Mutated DRD2 and PPP1R1B genes additively predispose ASDs and are only found in male patients.  The decreased dopamine activity in the medial prefrontal cortex is also a sign of ASD (Joe A Hettinger, 2012)  The NLGN4X gene effects cell adhesion molecules localized at the CNS synapse.  It is still being studied, about the exact variation of this gene sequence that predisposes ASD, but it has been suggested that the defect in this gene negatively affects synaptogenesis (Jamain, 2003).  Therefore, when this gene is mutated, neurons in the CNS have trouble communicating with each other due to the fact that they are unable to create proper synapses. 

Certain genes have deleted or copied sequences that cause either a lack of or overproduction of specific proteins.  The deleterious or copied genes in this study are Fragile X mental Retardation 1 (FMR1: Gene ID2332), CD38 molecule (CD38: Gene ID952), Ca2+-dependent secretion activator 2 (CADPS2), and protocadherin alpha cluster 10, complex locus (PCDHA10: Gene ID56139).  FMR1 is a very well studied gene that encodes for the Fragile X mental retardation 1 protein.  In the brain, the protein may play a role in the development of the connections between nerve cells at the synapse (U.S National Library of Medicine, 2012).  The protein also regulates synaptic plasticity.  Synaptic plasticity is the ability of synapse to adapt over time in response to experience. Synaptic plasticity plays a role in learning and memory.  In Fragile X syndrome, which has features of ASD, 200+ CGG repeats causes the gene to be unstable and in consequence to be silenced, making little to no protein.  Fragile X syndrome is a precursor for Autism.  The CD38 gene encodes for a transmembrane protein, CD38, which regulates oxytocin secretion. Oxytocin is described as the ‘bonding hormone.’ (Heon-Jin Lee, 2009) It also promotes ethnocentric behavior, incorporating the trust and empathy of in-groups with their suspicion and rejection of outsiders.  In ASDs, there is reduced to no expression of the CD38 protein due the fact that the gene is mutated or deleted.  The CADPS2 gene encodes for calcium binding proteins that play a major role in exocytosis of neurotransmitters and neuropeptides into the synapse and of dense core vesicles in neuroendocrine cells (Declan J. James, 2013).  This gene also regulates neurotrophin release from granule cells leading to regulate cell differentiation and survival during cerebellar development.  CADPS2 is deleted in ASD.  The gene PCDH10 deals with the establishment and function of cell to cell connections in the brain at the synapse.  This is also one of the largest deletions in ASD (Eric M. Morrow, 2008).  The absence of this gene negatively affects the cells to create synapses and communicate with each other.

Methylation changes on a DNA sequence alters how the gene or protein is expressed without changing the actual sequence.  The genes with epigenetic changes that may predispose autism that are being studied in this research are Nuclear Receptor Subfamily 3, group C, Member 1 (NR3C1: Gene ID2908) and Methyl CpG Binding Protein 2 (MECP2: Gene ID4204).   NR3C1 gene encodes for a glucocorticoid receptor that regulates transcription factors.  Glucocorticoids are involved in inflammatory responses, cellular proliferation and differentiation in target tissues.  In ASD, it has been found that suppressed methylation on hippocampal DNA may be due to deprived maternal care during infancy (L J van der Knaap, 2014).  This methylation suppression at the NR3C1 gene leads to a decreased number of the glucocorticoid receptors needed to regulate transcription. The MECP2 gene encodes for the methyl-CpG binding protein 2.  This gene is located on the X chromosome at Xq28; it is an X-linked dominant mutation that is only found in females, due to the fact that it is lethal in males.  MECP2 is identified with Retts Syndrome, a precursor to autism (Lam CW, 2000).  De novo mutations at CpG dinucleotide causes epigenetic change of deacylation of core histones, which changes the chromatin architecture and leads to transcriptional repression.  This repression show low levels of gene transcription. 

Conclusion 

Autism is a heterogeneous neurodevelopmental syndrome with a complex genetic etiology. Unifying principles among cases of autism are likely to be at the level of brain circuitry at the synapse.  All of the genes in this study effect the synaptic transmission of information in some way.  The idea that ASD can be affected or caused by environmental factors is reinforced by the characteristic called synaptic plasticity, in which a synapse can change and adapt by either strengthening of weakening over time in response to increases or decreases in activity or because of an alternation in the number of neurotransmitter receptors on the synapse.  Conclusively,  ASD is likely to be a synaptic disorder.

In the future we plan to extract DNA from the selected specimen andfirst look for the group of genes that would have been deleted or copied, then mutated genes . Lastly, we will look for possible methylation changes on the previously highlighted genes.  If a patient exhibits more than 3 mutated genetic factors then we will pull their archived records and compare their psychiatricrecords with genetic evidence found on their genome and possibly, tentatively change their diagnoses to an ASD

References

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