Potential causes
 
 
 
                                                                                   Heredity
 
    
In recent years it has become clear that heredity plays a significant role in the development of schizophrenia. However, there is also mounting evidence that environmental factors play a role that may be equally important. Let’s look at the numbers (Gottesman, I.I. (1991) Schizophrenia genesis: The origins of madness. New York: Freeman). People in the general population have only about a 1-1 1/2 % risk of developing this illness. On the other end of the spectrum, a person with an identical twin with schizophrenia has a much greater risk, from 50-60%. For a fraternal (non-identical) twin the risk is about 17%. For children, it is about 13%. It is easiest to understand these statistics if we look at how much genetic information is shared between the people at risk (what percentage of their genes do they have in common). Identical twins share all (100%) of their genes. They came from one egg that divided and formed two embryos, and ultimately two identical babies. Even though they have exactly the same genes, the risk is only about 50%. First degree relatives share half of their genes. They include siblings (brothers and sisters) and children. The children of people with schizophrenia have a 13% risk, siblings have a 9% risk, and a special type of siblings (fraternal, or non-identical twins) have a 17% risk. Why would a sibling who happens to develop alongside a person who will later develop schizophrenia in the womb have any higher risk than one who was born earlier or later? This is probably because, even in the very beginning, inside the womb, environment is already affecting risk. Second degree relatives share one quarter of their genes. They include half siblings, whose risk is 6%, grandchildren, whose risk is 5%, nieces and nephews, whose risk is 4%, as well as aunts and uncles, whose risk is about 2%. Why the difference, when the number of genes that are shared is the same? This is likely due to something called “anticipation”. This simply means that some illnesses may show an increased severity or an earlier onset in successive (following) generations. Schizophrenia seems to be one of those illnesses. Why? Possibly because there is a buildup of unstable or defective genetic material with each generation. Therefore, your aunts and uncles will have a risk that is more like your parents, but grandchildren, nieces and nephews, who come along in later generations, have somewhat higher risks. Third degree relative, like first cousins, share only one eighth of their genes. Their risk is only about 2%, similar to the rest of the population. When we look at these numbers, it becomes clear that the more genes we have in common with someone who develops schizophrenia, the more likely we are to develop the illness too. It is also clear that it is not that simple. Even when we have all of the genes that they do, we only develop the illness half the time. Environment seems to be just as important. Information related to the risk we acquire if our parents have the disease and comparisons between being raised with them or away from them confirms this conclusion. If one parent has schizophrenia, our risk is between 8% and 18%, even if we were adopted and never lived with them. If both parents had the disease, our risk is higher, between 15% and 50%, regardless of whether we lived with them or not. Children whose biological (natural) parents were healthy, but who were raised by two parents who have schizophrenia, have only a 1% risk. This is about the same as the general population. This is consistent with the rest of the information about heredity. The more genes we share with someone with schizophrenia, the more likely we are to develop the disease. It doesn’t suggest that environment isn’t important. To summarize those numbers: third degree relatives are twice as likely to acquire schizophrenia than people who are not related, second degree relatives are 4-6 times as likely, and first degree relatives are 6-17 times as likely. To the extent that genes do influence how likely we are to acquire schizophrenia, it is also clear that there isn’t a single gene that determines this susceptibility or predisposition. Current scientific evidence suggests that several genes may be important. In 2004, Dr. Daniel Weinberger, Director of the Genes, Cognition, and Psychosis Program of NIMH (National Institute of Mental Health) proposed that there may be ten gene variations linked to schizophrenia. In 2002, he revealed that the presence of a gene on chromosome 22, called COMT, almost doubled the risk that an individual would develop schizophrenia. His thoughts about COMT and other “putative susceptibility genes” (neuregulin,, dysbindin, DISC1, RGS4, GRM3 and G72), that might be playing a role are summarized in: Schizophrenia genes, gene expression, and neuropathology: on the matter of their convergence. Mol Psychiatry. 2005 Jan; 10(1):40-68. (Sources: The author’s knowledge base, unless otherwise noted.) _________________________________________________________________________________ Related Articles: Environmental Factors environment.htmlshapeimage_2_link_0