Have you ever noticed that certain medical conditions get passed down along generations. Usually, we just associate physical traits with inheritance but in recent years the relationship between genetics and mental illness has sparked more attention. Scientific advances have allowed us to better predict the risk of developing a mental health condition based on insight from our genetic makeup. Research indicates that genetics accounts for approximately 40-60% of the increased risk for developing mental health disorders. In this article, we’ll discuss the relationship between mental illness and genetics.
What is Mental Health and Mental Illness?
Let’s start by defining mental health. Mental health is a state of mind that allows an individual to process information and make choices in a stable and thoughtful manner. Declining mental health not only affects the individual, but their family, colleagues, neighbors and anyone in their support system. Mental illness refers to a variety of health conditions that affect mood, thinking and behavior which can impact a person’s quality of life depending on the severity of the condition. Mental illness is influenced by many factors such as the environment and life experiences, such as trauma and exposures. The key contributing factor is genetics, which plays a major role in susceptibility to these conditions. Anxiety, depression, obsessive-compulsive disorder (OCD), schizophrenia, and bipolar disorder are some common mental illnesses that you might be familiar with. There is a huge proportion of the population who suffer from at least one category of mental illness in their lifetime. Depression alone affects 350 million people globally.
How Does Gene Inheritance Work?
Now that we know what mental illness is, let’s discuss genetics. The basis of genetics lies in our genetic material, also known as DNA. DNA is a series of genes which together make-up a person. DNA is different among every individual and the genes originate from the combination of the parental DNA. This combination of DNA is what gives us our physical features, but it also contributes to our susceptibility for medical conditions. Scientists have found that the risk for mental illness is influenced by a combination of genes working together. This means that while it’s not guaranteed that a gene will result in a mental illness, the combination of genes with an environmental exposure might lead to expression of the condition. Conditions such as schizophrenia, bipolar disorder and major depression have shown a correlation between increased risk of disease development and family history.
Earlier we mentioned that DNA is different among every individual, the only exception to this rule is in identical twins. Identical twins originate from one egg and sperm joining together before splitting into two different babies. Since the combination of DNA occurred prior to the splitting, these babies will share the exact same genetic material and in theory the exact same genetic susceptibility to illnesses. A regular sibling’sDNA is also derived from a combination of both parents’ DNA, but there are slight variations among genes during the combination phase which leads to approximately 50% of shared genes instead of 100% like in identical twins.
How Is This Studied?
Now that we have all the basic information, how can we break down research? Twin studies and family studies are best used to show a proportional relationship between the role of genetics and mental illness. Family studies trace the incidence of conditions throughout a family tree to determine if there is an increased risk proving disease inheritance. Twin studies can be used to further stratify the relationship between mental illness and genetics because they share the same DNA. An example of this is in schizophrenia. The risk of schizophrenia in a general population is between 0.5-1%. When there is a third-degree relative with the condition, the risk increases to 2%. When there is a first degree relative with the condition, the risk increases to 9%. If both parents have schizophrenia, there is a 27% chance of the child developing the condition. In identical twins, if twin A develops schizophrenia, twin B has a 40-50% chance of also developing schizophrenia. In comparison, if one sibling has schizophrenia, there is a 10-15% chance of the other sibling developing schizophrenia.
With these studies, we can see that there is a relationship between genetics and the expression of mental illness. Identifying the specific genes involved would be the next step. This can be quite difficult because most mental illnesses are influenced by many genes. The complexity of these interactions makes it difficult to identify the specific set and combination of genes associated with an increased susceptibility for a specific condition. For example, a gene study identified seven genes associated with various depressive disorders. The presence of any of these genes does not mean you are guaranteed to develop a depressive disorder, it just means there is a slightly increased risk. Depression can be affected by fluctuations in hormone levels such as dopamine, norepinephrine and serotonin. There are several other genes that affect the levels of these hormones, which in turn can also affect the risk of developing a depressive disorder.
How Do We Quantify an Increased Risk?
Can a gene increase a person’s risk? Polygenic risk scores are a way researchers can infer the combined risk of developing a mental illness based on the combination of genes. Polygenic means ‘many genes’, so this risk score is used to calculate the overall risk from the presence of multiple ‘risky’ genes. While a patient with one high risk gene might not develop a condition, a patient with 10 combined moderate-riskgenes will be more likely to develop a condition and thus they will have a higher polygenic risk score. These scores can provide some information on genetic predisposition but as of now they are not precise enough to determine with certainty whether someone will develop a condition.
This certainty may also be skewed by the influence of the environment. This concept is known as gene-environment interaction, which states that genetics act in combination with the environment and together they determine the susceptibility of a mental illness. This concept is like the ‘nature versusnurture’ theory we have all heard of, but instead of it being one or the other, it’s a combination of that affects the outcome. Environmental exposures such as life stressors, trauma, loss, chronic stress, drug exposure and birth complications can all play a role in the predisposition to certain mental illnesses.
The major advance in genetic research originates from the use of genome-wide association studies (GWAS). In these studies, an entire genome is scanned in populations and genetic variations are identified and marked. When exploring the data, certain variations can be associated with mental illness. These studies have shown genetic variants associated with schizophrenia, bipolar disorder and depression. For example, in bipolar disorder, the presence of an individual GWAS marker will show a slightly increased risk but common variants combined can account for approximately 25% of heritability.
Final Thoughts
The origin of mental illness is complex and dependent on many variables. By identifying the genetic components of mental illness, we can see a clear correlation between genetics and mental illness. In the future, further genetic research could help with the treatment and management of certain conditions as well as better predicting an individual’s susceptibility to a certain mental illness. These further advances could improve the mental health care field as a whole.
My name is Natasha Zand, I am currently an Internal Medicine resident. I attended medical school at Saint James School of Medicine where I completed my clinical rotations in McAllen Texas. I had the pleasure of rotating at Tropical Texas Behavioral Health where I was able to work with patients and learn more about the community I was living in. In this rotation I noticed a correlation between mental illness and family history, so I wrote this article to better understand the relationship between mental illness and genetics.
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