With more than 20% of Americans living with at least one diagnosed mental illness, the question begs: Can mental illnesses run through the family? And if the answer is yes, what is the likelihood that someone will pass their mental illness down to their children? If you yourself have a mental illness, you may be wondering how you got it in the first place, or how you might prevent your future children from inheriting the same disorders you have. This is understandable that you should want to know, and it may provide some comfort to know why or how you might have developed a certain mental disorder.
The simple answer is this — currently, we do not fully understand what causes mental illness or the nuances associated with how it is passed on through families. Research has determined mental illnesses contain both genetic and environmental components, with some illnesses more closely tied to genetics than others. However, there is no concrete data to predict who will get what mental illnesses, and there is not a test that individuals can take to see if their genes will make them more likely to develop mental illness. Having family members with a certain mental illness may increase your risk for developing it, but take comfort in the fact that a risk is not a confirmation that you will ever develop it. In any case, it is probably for the best that we all are diligent and aware of what mental illnesses we may be at higher genetic risk for, since it allows us to take a proactive approach toward our collective mental health!
Are some mental illnesses more heritable than others?
In more recent times, it is widely accepted that illnesses of any kind (including mental) are a culmination of both genetic and environmental factors. Twin studies have shown that two individuals who share virtually the exact same DNA can have two completely different medical conditions even with the same genetic makeup. With that being said, there are some mental illnesses that research shows to have greater genetic components than others. In this blog, we will be talking about the common genetic factors that are found in four very common mental disorders. These mental disorders that have higher genetic inheritability include:
- Attention deficit hyperactivity disorder (ADHD)
- Autism
- Bipolar disorder
- Major depression
According to the National Institute of Health, there are some genetic variations that are found across every one of these mental disorders. Variations in the CACNA1C gene have been found to affect circuitry within the brain, especially the areas that specialize in emotion, attention, thinking and memory (which all may be inhibited or affected by mental disorders). In addition, variations in the CACNB2 gene have also been found to affect each one of the listed mental illnesses. Both CACNA1C and CACNB2 play a role in regulating the effects of calcium on neurons. Additionally, researchers found that variation within certain regions of chromosome 3 and 10 may contribute to the development of mental illness. They have not yet determined what factors within these regions specifically affect the progression of mental disorders, but a region along chromosome 3 has especially strong links to the five illnesses listed above. Variations within this region have also been linked closely to bipolar disorder and schizophrenia in the past. Regardless, genetic variability plays only a minor role in the development of mental illness, and can only account for some of the risk that individuals may have of developing a disorder.
ADHD and Genetics
Attention deficit hyperactivity disorder, or ADHD, is one of the most common mental illness diagnoses in the United States. According to national statistics, about 6 million children have ADHD, and about two-thirds of those children who are diagnosed will continue to have symptoms of this disorder throughout adulthood. In a past blog post, we explored in detail what ADHD symptoms present as in our youth, and we also discussed a new type of treatment to combat symptoms of this illness. Children and adults who live with ADHD may struggle with paying attention, controlling their behaviors, and may be overly active.
Numerous studies have explored the way ADHD presents in twins, adopted children, and among control siblings to determine how much of this condition can be attributed to genetics or the environment. One study demonstrated that among 894 ADHD-diagnosed children and 1,135 of their siblings aged 5-17 years old, there was a nine-fold increased risk of a sibling developing ADHD as compared with control siblings who both did not have ADHD. In addition, studies on adopted children reveal that rates of ADHD are greater among biological relatives of non-adopted children with ADHD compared with adoptive relatives that had adopted children with ADHD. This suggests that biological, rather than environmental factors, are more attributable to the development of ADHD. With all of this data, researchers have determined that the heritability of ADHD may fall anywhere between 77%-88%. This convincing number concludes that DNA variations that run in families can increase the risk of developing ADHD. This isn’t to say that the environment in which an individual grows up may also propagate gene-gene or gene-environment factors that can also affect how the condition presents itself.
Autism and Genetics
Autism spectrum disorder, or ASD, is a developmental disorder that appears very early on in a person’s childhood. ASD impairs the ways that children are able to function in different environments, and those with this condition have an impaired ability to interact with others, understand non-verbal social cues, and communicate their own feelings in an appropriate manner. CDC data on ASD show that about 1 in 36 children in the US have this disorder, and it is 4 times more common in boys than in girls. In addition, about 1 in 3 individuals who have ASD are also diagnosed with an intellectual disability.
As with the rest of the disorders we will discuss, the development of autism is not influenced by just one thing— it is influenced by a combination of biology, genetics, and the environment. What may cause one person’s ASD may not be the reason for another’s. Research has shown that genetics play a role in the development of autism since the 1970s, when it was discovered that identical twins are more likely to share the same ASD diagnosis. For example, when one identical twin has ASD, there is an 80% chance that the other twin also has it. This statistic drops to 40% when discussing fraternal twins. While the development of ASD is incredibly heritable, there is no single ‘autism gene’ that is found in each and every person who has this disorder. Research into this topic has found that about 100 genes may be strongly linked to the development of ASD. This could mean that any number of interactions between mutations in these genes and other environmental factors may contribute to the manifestation of ASD. How do people acquire these mutations within these genes? Researchers have determined that most mutations are inherited from parents, and sometimes they can also arise spontaneously in an egg or sperm. It is also important to note that just because someone has a mutation does not mean that they will be diagnosed with ASD. Rather, it may increase their risk.
Bipolar Disorder and Genetics
According to research, bipolar disorder is one of the most heritable psychiatric disorders. With about 1-4% of the population being diagnosed with this condition, many individuals are probably aware of bipolar disorder and how it is characterized by episodes of mania (abnormally elevated moods) and depression (abnormally depressed moods). Research has shown that up to 80% of the cause of this condition can be attributed to genetic factors. For example, if one parent has bipolar disorder, there is a 1 in 10 chance that their child will also develop the illness. Additionally, if both parents have this disorder, the chances that their child will be diagnosed with the illness rises to a 4 in 10 chance.
Within our genomes, all of us may have various SNFs, or single nucleotide polymorphisms. These substitutions account for most of the variation that we have in our genes. For those with bipolar disorder, SNPs within the CACNA1C, ODZ4, TRANK1, GNG2, ANK3, TPH2, ITPR2, SHANK2, & NCAN genes have shown to be important contenders for increasing the chances of developing bipolar disorder. The way that people develop these SNPs can occur through inheritance directly from their parents or from random chance during development.
Major Depression and Genetics
Major depression is a mood disorder that causes an individual to experience a persistent feeling of sadness and affects how someone thinks, behaves, and feels about themselves or those around them. These individuals may feel like they have nothing to live for and have trouble getting through everyday tasks. Research shows that nearly 10% of people will experience major depression at some point in their lives. According to twin studies, heritability of major depression is quite high, at around 40-50%. In other words, the development of major depression may be attributed to 50% genetics and 50% to other factors. Additionally, adoption studies show that an adopted individual is more likely to develop major depression if their biological parent also had the diagnosis rather than their adopted parent.
If an individual has a parent or sibling with major depression, that individual has a 2-3 times higher risk of developing depression as compared with an average individual. Again, just because someone is at high risk for a disease does not mean that they will definitely become diagnosed with it. People inherit a mixture of genes from both their father and mother, and these different combinations interact with each other and the environment to potentially predispose a person to an illness.
