Neurobiologic Causes of Mental Illness

NEUROBIOLOGIC CAUSES OF MENTAL ILLNESS

Genetics and Heredity

Unlike many physical illnesses that have been found to be hereditary, such as cystic fibrosis, Huntington’s disease, and Duchenne’s muscular dystrophy, the origins of mental disorders do not seem to be that simple. Current theories and studies indicate that several mental disorders may be linked to a specific gene or combination of genes but that the source is not solely genetic; nongenetic factors also play important roles.

To date, one of the most promising discoveries is the identification in 2007 of variations in the gene SORL1 that may be a factor in late-onset Alzheimer’s disease. Research is continuing in an attempt to find genetic links to other diseases such as schizophrenia and mood disorders. This is the focus of the ongoing research in the Human Genome Project, funded by the National Institutes of Health (NIH) and the U.S. Department of Energy. This international research project, started in 1988, is the largest of its kind. It has identified all human DNA and continues with research to discover the human characteristics and diseases each gene is related to (encoding). In addition, the project also addresses the ethical, legal, and social implications of human genetics research. This program (known as ELSI) focuses on privacy and fairness in the use and interpreta-tion of genetic information, clinical integration of new genetic technologies, issues surrounding genetics research, and professional and public education (NIH, 2007). The researchers publish their results in the journal Science; fur-ther information can be obtained at www.genome.gov.

Three types of studies are commonly conducted to investigate the genetic basis of mental illness:

·    Twin studies are used to compare the rates of certain mental illnesses or traits in monozygotic (identical) twins, who have an identical genetic makeup, and dizy-gotic (fraternal) twins, who have a different genetic makeup. Fraternal twins have the same genetic simi-larities and differences as nontwin siblings.

·    Adoption studies are used to determine a trait among biologic versus adoptive family members.

·    Family studies are used to compare whether a trait is more common among first-degree relatives (parents, siblings, and children) than among more distant rela-tives or the general population.

Although some genetic links have been found in certain mental disorders, studies have not shown that these illnesses are solely genetically linked. Investigation continues about the influence of inherited traits versus the influence of the environment—the “nature versus nurture” debate.

Stress and the Immune System (Psychoimmunology)

Researchers are following many avenues to discover pos-sible causes of mental illness. Psychoimmunology, a rel-atively new field of study, examines the effect of psycho-social stressors on the body’s immune system. A compromised immune system could contribute to the development of a variety of illnesses, particularly in pop-ulations already genetically at risk. So far, efforts to link a specific stressor with a specific disease have been unsuccessful.

Infection as a Possible Cause

Some researchers are focusing on infection as a cause of mental illness. Most studies involving viral theories have focused on schizophrenia, but so far none has provided specific or conclusive evidence. Theories that are being developed and tested include the existence of a virus that has an affinity for tissues of the CNS, the possibility that a virus may actually alter human genes, and maternal expo-sure to a virus during critical fetal development of the ner-vous system.

Swedo and Grant (2005) studied the relation of strep-tococcal bacteria and obsessive–compulsive disorder (OCD) and tics. They found enlarged basal ganglia, indi-cating a possible autoimmune response to streptococcal infection. When blood plasma (high in streptococcal anti-bodies) was replaced by transfusion with healthy donor plasma, the incidence of tics decreased by 50%, and other OCD symptoms were reduced by 60%. Studies such as this are promising in discovering a link between infection and mental illness.