(June 18, 2019) Researchers have been significantly over-estimating the involvement of genetics in schizophrenia, according to a provocative new paper written by Treatment Advocacy Center founder Dr. E. Fuller Torrey and board member Dr. Robert Yolken.
Combining heritability results from population-based twin studies, excluding those with methodological issues or highly-controlled sampling, Dr. Torrey and Dr. Yolken conclude that the heritability of schizophrenia is much lower than genetic researchers claim. Heritability is how much of the variability of a particular trait is due to genetics as compared to environmental factors.
For example, a 75% heritability in hair color suggests that 75% of the hair color of an individual is due to their genetic makeup. The authors calculate the heritability of schizophrenia to be 28%, much less than the 80-85% claimed by some genetic researchers.
Pseudo-genetic characteristics
Dr. Torrey and Dr. Yolken have long researched the role of infectious agents in schizophrenia. Through their work and others, an infection by Toxoplasma gondii, a parasite commonly found in cat feces, has been shown to be associated with schizophrenia. Research has also shown that increased exposure to the parasite is associated with recent-onset of psychosis.In addition to detailing the limitations of genetic research studies on schizophrenia, Dr. Torrey and Dr. Yolken suggest one of the reasons heritability has been overestimated in schizophrenia is due to pseudo-genetic characteristics of the illness, or non-genetic factors that produce results similar to genetic ones, that may mislead researchers.
Toxoplasma gondii infection, known as toxoplasmosis, also clusters in families because of shared environments such as infected water supplies or the sandbox in which children play. In addition, toxoplasmosis appears to be passed from parents to their children during pregnancy. In fact, in mice, a mother’s toxoplasma gondii infection has been shown to be transmitted up to ten generations. Thus, the characteristics of toxoplasma infection in schizophrenia may be mimicking genetic features in research settings.
“In the final picture, therefore, schizophrenia could be a disease in which environmental factors, infectious or otherwise, play the major etiological role but with genes determining susceptibility to the environmental factors,” the authors write. In fact, the 30% heritability of schizophrenia is a similar level to that of polio and tuberculosis, both diseases that are known to have infectious agent-gene interactions.
Although this may seem like an argument between researchers, there are significant implications of these issues in finding better ways to treat schizophrenia.
“The funds for research are finite,” the authors write. “When NIMH decides to spend $100 million a year on genetics research, other promising research areas, such as inflammation, infectious agents, and the microbiome, do not get adequately funded.”
Through the pioneering work of Dr. Torrey, Dr. Yolken and others, the Treatment Advocacy Center hopes to move beyond genetic theories of schizophrenia and to focus more attention is paid to other etiologies of psychiatric disorders.
References:
Elizabeth Sinclair
Director of Research
Treatment Advocacy Center