RISK AND CAUSAL FACTORS OF SCHIZOPHRENIA
v BIOLOGICAL FACTORS
Ø Genetic Aspects
· Adoption Studies
Several studies have attempted to overcome the shortcoming of the twin method in achieving a true separation of heredity from environmental influences by using what is called the "adoption strategy". Here concordance rates for schizophrenia are compared for the biological and adoptive relatives of a persons who have been adopted out of their biological families at an early age (preferably at birth) and have subsequently developed schizophrenia. If concordance is greater among the patients' biological than adoptive relatives, a hereditary influence is strongly suggested; the reverse pattern would argue for environmental causation.
· Family Studies
As the genetic similarity to a person with schizophrenia decreases, an individual's risk of developing schizophrenia also decreases. Thus, a first degree relative of a person with schizophrenia, such as a non-twin sibling, who shares about 50 percent of genes with the person with schizophrenia, has about a 10 percent chance of developing the disorder. In contrast, a niece or nephew of a person with schizophrenia, who shares about 25 percent of genes with the person with schizophrenia, has only a 3 percent chance of developing the disorder.
The general population has a risk of about 1 to 2 percent. The relationship between an individual's degree of genetic similarity to a schizophrenia relative and the individual's own risk of developing schizophrenia strongly suggests that genes play role in the development of the disorder.
Having a biological relatives with schizophrenia increases an individual's risk for the disorder but does not mean that the individual will develop it. For example, of all children who have one parent with schizophrenia, 87 percent will not develop the disorder. Also, 63 percent of the people with schizophrenia have no first-or second-degree relative with the disorder (Gottesman & Erlenmeyer Kimling, 2001).
· Twin Studies
Even when a person carries a genetic risk for schizophrenia, however, many other biological and environmental factors may influence whether or how he or she manifests the disorder. The classic illustration of this point is found in the Genain quadruplets, who shared the same genes and family environment. While all developed schizophrenia, their specific symptoms, onset, course, and outcomes varied substantially (Mirsky et al., 2000).
Epigenetic (refers to the changes 'switching on or off' that occurs in genes) factors had been investigated by the researchers and said to be influence of the expression genes. DNA can be chemically modified by different environmental conditions, resulting in genes being turned on or off, thereby altering the development of cells, tissues, and organs. When MZ twins who were discordant for schizophrenia (i.e., one twin had schizophrenia but the other twin did not) were compared with MZ twins who both had schizophrenia, researchers found that the MZ twins discordant for schizophrenia showed numerous differences in the molecular structure of their DNA, particularly on genes regulating dopamine system. In contrast, the MZ twins concordant for schizophrenia showed many fewer molecular differences in their DNA (Petronis et. al., 2003).
The reasons for these epigenetic differences are unclear. However, a number of environmental events that could affect development in utero appear to increase the risk for schizophrenia. Some of these events might alter genes that guide brain development.
"GENAIN QUADRUPLETS"
Likely to be multiple genes "polygenic" with small individual effects; risk genes shared with other disorders like Autism, Depression, and Bipolar Disorder.
