If a man has type 1 diabetes, there is a 1 in 17 chance that his child may also get it. The odds reduce to 1 in 25 if the woman has it, and the child is born before she is 25. Type 2 diabetes has a stronger link to genetics than type 1. Genes also play some role in conditions like hypertension and heart ailments. And just like these illnesses, mental health issues also have some degree of heritability. However, the stigma associated with mental health makes the heredity and its implications more difficult to navigate.
Recently, actor Rhea Chakraborty said in an interview that late actor Sushant Singh Rajput’s mother had lived with depression. Sushant, who was found dead in his Mumbai apartment in June, reportedly by suicide, was living with mental health issues as well. In the aftermath of the incident, there have been discussions on how Sushant’s mental health struggles could have been hereditary. But what do we really know about the link between genes and psychiatric disorders?
There have been numerous studies – in the past and ongoing – to study these links. A study by Consortium on the Genetics of Schizophrenia pinned the heredity of schizophrenia at 31% to 44% in nuclear and extended families respectively. When bipolar disorder was included in this, the heritability increased to 37% in nuclear families. And when it came to depression, in the same study, the heritability dropped to 34% in nuclear families and 20% in extended families.
Other studies say that the chances of a person developing schizophrenia if a biological parent has the condition are 13 in 100, and for bipolar disorder, it’s around 30 in 100. Studies have also been done on identical twins, and have found that if one sibling has schizophrenia, there’s a 40-50% chance the other will develop it, because they share the same genetic makeup. Alzheimer’s dementia is another disease known for high heritability.
Dr Sanjeev Jain, officer-in-charge of the Molecular Genetics laboratory and a professor of Psychiatry at the National Institute of Mental Health Sciences, (NIMHANS) in Bengaluru, points out that most things – including looks – are inherited from biological parents. “Mental illness is not very different that way. The only thing is that we have not been able to figure out the precise mechanism by which the causal genes get triggered. Besides, people tend to forget that if there is a 10% chance you will develop a mental health condition due to heredity, there is also a 90% chance that you won’t. Till date, we have not been able to predict mental illness, because it does not only depend on the genes,” he says.
Dr Sanjeev has been involved in many research and molecular diagnostics projects on neuropsychiatric disorders including Obsessive Compulsive Disorder (OCD), alcoholism, dementia and schizophrenia for several years. He, along with his colleague, Dr Biju Viswanath, Associate Professor of Psychiatry at NIMHANS, are conducting a study – Accelerator program for Discovery in Brain disorders using Stem cells (ADBS) – using “clinical investigations, modern human genetics, and stem cell technology" at NIMHANS. The study has been on since 2016.
Dr Biju says that while there's a 60-70% risk of developing a psychiatric illness contributed by genes, there are several other factors that cannot be discounted. Dr Hema Tharoor, a psychiatrist associated with Chennai-based Schizophrenia Research Foundation (SCARF), points out that it is ultimately the gene-environment interaction that determines whether a person could develop a mental health issue.
“There are studies, for instance, that look at siblings wherein one parent had schizophrenia but only one child developed it, and the other two did not. And it’s not just sociological factors that play a major role in determining one’s mental health, it is also things like the antenatal and postnatal care of the mother, low birth weight, infections during pregnancy, that determine the risk of developing these disorders in the person’s life too,” she explains.
One 2011 study, for instance, found that the risk for schizophrenia increased with certain obstetric complications, famines, migrant status, and advanced paternal age. Severe pregnancy and perinatal complications also accounted for environmental risks.
Further, Dr Sanjeev points out, it’s very difficult to separate genetics and culture. “At any point of time, you are expressing only a third of your genes. Different genes will express themselves at different points of time. Only when that expression crosses a threshold does it become a disease.”
Besides, we simply don’t know all the genes that contribute to the risk of developing a psychiatric disorder or illness. In the case of Alzheimer’s, studies have implicated 215 potential causative genes. However, the study by the Psychiatric Genomics Consortium said that these “only explain a small proportion of genetic variance”, meaning that yet undiscovered genes remain. This is the case with other mental health issues and psychiatric disorders as well.
Even so, experts say it is not necessary that everyone who carries the risk increasing genes will develop the illness – it’s just that the genes could increase the susceptibility of an individual to getting it, and the trigger threshold for the onset could be lower. “So, for instance, if someone is at a higher risk to develop OCD due to heredity, and they have also been through trauma in childhood, it could cause early onset. But it’s also possible that they do not develop it at all, depending on the other factors,” Dr Biju says.
Several illnesses like breast cancer and leukaemia have been proven to have a high heredity factor. In some cases, gene therapies have been devised to correct or replace certain genes that could cause some illnesses. However, this is not the case with mental illnesses. “The study of the brain and mind is harder because you can’t just do a biopsy of the brain,” says Dr Sanjeev. “However, the advantage to figuring out the genomic links to mental health is that once we know the cause, we can have better drugs to treat them. Most of the ones we use now are decades old.”
Dr Biju similarly says, “As a doctor, you want to see your patients’ health improve. Say, 50% of the patients get better with psychiatric medication, but the remaining struggle. But we do not know why the medication is not working. So, if we can understand how the illness or disorder originates by identifying the genes, you can identify the mechanisms; and then we can move to more effective treatments.”
Dr Abhishek ML, a psychiatrist and a research faculty at Bengaluru-based Indian Institute of Science (IISc), says that another hope in studying the heredity of mental health issues is early preventability. Dr Hema adds, “If we can establish the endophenotypes (the neurobehavioral traits that quantify genetic susceptibility to developing a psychiatric disorder), then it becomes a preventable risk. Special care can be taken then in, say, antenatal or postnatal care.”
However, Dr Abhishek cautions that this sort of technology should be accompanied by destigmatising mental health, lest it leads to profiling of persons living with certain mental health conditions and impinging of their rights.