Introduction
Several psychiatric disorders are highly heritable, although major genes have yet to be found. Furthermore, psychiatry is based on a complex classification system that is widely accepted and used. Therefore, it should offer the precise clinical phenotyping necessary for the success of genetic investigations.
The field of psychiatric genetics has yet to meet these high goals. However, progress has been made by incorporating intermediate qualities (endophenotypes) and environmental factors into genetic analysis and identifying rare inherited variants and novel structural changes. Nevertheless, it is essential to recognize that genetics for complex disorders has limitations and may never provide a binary diagnosis for a condition like schizophrenia. Nonetheless, a few genes, mostly from monoamine pathways, have been shown to have genetic effects. To learn more about psychiatric genetics, keep reading this article.
What Is Psychiatry Genetics?
Psychiatric genetics is a branch of behavioral genetics and neurogenetics that investigates the impact of genetics on the development of mental disorders. The core idea behind psychiatric genetics is that genetic polymorphisms (as demonstrated by linkage to a single nucleotide polymorphism) lead to the development of psychiatric conditions. The purpose of psychiatric genetics is to understand better the causes of mental illnesses to improve treatment procedures and develop personalized treatments based on genetic profiles. Unfortunately, several studies suggest that genetic variations increase the chance of developing psychiatric diseases.
What Are Genetic Psychiatric Disorders?
Scientists have recognized many psychiatric diseases in families, indicating possible hereditary roots. Bipolar disorder (disorder with episodes of mood swings), attention deficit hyperactivity disorder (neurodevelopmental childhood disorder), autism (impaired ability to communicate and interact), depression, and schizophrenia (impaired ability to think, feel, and behave) are a few examples of these disorders. Like other common medical conditions such as diabetes and hypertension, major psychiatric disorders are common with complex genetics. Linkage studies in families with numerous cases have found chromosomal regions containing susceptibility genes for most illnesses.
What Is the Complexity of Psychiatric Disorders?
Many genes are involved in mental diseases and may operate additively, making them genetically complicated conditions. According to epidemiology and twin studies, environmental factors can modify a person's genetic susceptibility to developing a mental disease. Based on genetic studies in neurodegenerative disorders such as Alzheimer's (progressive memory loss disorder), Parkinson's (progressive disorder affecting nervous system), or motor neuron disease, dozens of genes are involved in psychiatric disorders. A large sample of patients and controls are necessary to detect a significant effect of a minor gene and the interaction between numerous genes. Even with modern technologies, managing this task is still challenging. The high prevalence of findings that have not been confirmed in other studies might be attributed partly to factors such as sample size, differences in genetic structure (population stratification), environmental exposures (e.g., culture), and diagnostic criteria.
Certain reports show that mental illness runs in families, and the gene has been found to cause disease as a traditional monogenic trait (e.g., autosomal dominant). An exception to this is that the first gene for Tourette's syndrome (disorder with repetetive movements and unwanted sounds) and obsessive-compulsive disorder may have recently been identified. Even though these genes are likely only a minor contributor to the causes of mental illnesses, discovering new molecular pathways could be extremely beneficial. For example, in Alzheimer's disease, early-onset variants are mostly caused by mutations in three separate genes: presenilins 1 and 2 and amyloid beta precursor protein. Animal models based on these genes have now been produced, which have aided in studying late-onset Alzheimer's disease.
What Methods Are Used in Psychiatric Genetic Research?
Traditional and newer approaches are used for genetic research that includes the following:
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Traditional Methods - Family Study, Twin Study, and Adoption Study.
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Newer Approaches - Linkage studies, Association studies, etc.
Family Study - The Family Study demonstrates that genes are engaged in priming an illness differently; it can detect which clinical entities are transferred together compared to a random transmission pattern. According to the theory behind these investigations, the disease should be more common in the families of affected members than in the control population.
Twin Study - The Twin Study aims to distinguish the effect of the gene from that of the environment. This study is based on the following assumptions: Monozygotic (MZ) twins share 100 % of their genomes, while Dizygotic (DZ) twins share roughly 50 %. The environment can be controlled as a confounding variable because monozygotic and Dizygotic twins live in the same environment.
Adoption Study - It clarifies the importance of genetic vs. environmental variables in disease by investigating people who are genetically related but do not share family environmental factors and people who share familial environmental factors but are not genetically related.
When adopted children are separated from their original parents at birth, it is easiest to make inferences from adoption studies. Therefore, it is conducted as follows:
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Parent-as-proband - The sickness rate in adopted children of sick and healthy people is compared in this study. When the former has a higher rate of sickness, support for a hereditary component is gained.
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Adoptee-as-proband - The risks associated with adoptive relatives versus biological relatives of unwell probands are evaluated.
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Cross-fostering strategy - The cross-fostering strategy compares disease rates between two groups of adoptees: those with sick biological parents who were fostered by healthy adoptive parents and those with healthy biological parents who poor adoptive parents fostered. These strategies are only available to countries with meticulously maintained adoption registers.
Linkage Analysis - It is the traditional method for identifying a disease gene in humans. It looks at the link between family-affected status and DNA (deoxyribonucleic acid) polymorphism markers. It is believed that markers closest to the disease locus will have the largest connection with a disease in families. Markers in the candidate region identified by linkage analysis can be used to narrow the location of the disease gene. Linkage disequilibrium is a population association between two alleles at different loci. Usually, the closer the marker is to the disease locus, the greater the proportion of affected subjects who carry the identical allele at the marker.
Markers in the candidate region discovered by linkage analysis can be utilized to narrow down the location of the disease gene. A population relationship between two alleles at various loci is known as linkage disequilibrium. The proportion of affected people who possess the same allele at the marker often increases directly to the distance between the marker and the disease locus.
Association Studies - Association studies are similar to standard epidemiologic research since they look for a connection between an exposure and an outcome. Due to the complexity of psychiatric diseases, association studies are often used in these cases. Researchers have effectively identified the genes involved in Mendelian diseases with significant relative risks in families. A typical association research design compares the frequency of marker genotypes in cases with an adequate control group. Case-control and family-based designs are the two most popular methods in association studies. For traits with a late onset (such as Alzheimer's disease), case-control samples may be the only viable design.
Conclusion
As we discussed above, psychiatric genetics helps determine the underlying genetic causes of psychiatric disorders. No known genes are responsible for most of a given psychiatric disease. Genes with a substantial influence may not exist; rather, the higher risk may result from many modest gene effects. Soon, genetics in psychiatric studies will assist in more predictable and effective treatment of psychiatric disorders. In the next 20 years, psychiatric genetics will make unimaginable advances.
