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Schizophrenia Is 80% Genetic, According to This Massive New Study on Twins

Most accurate results so far.

MIKE MCRAE
6 OCT 2017
 

As many as four out of five cases of schizophrenia can be traced back to genes inherited from the child's parents.

By applying a new statistical approach to data collected on more than 30,000 pairs of twins, researchers have produced the most accurate figures to date on risk factors for the condition, potentially helping us identify the genes responsible for its symptoms.

 

Scientists from the University of Copenhagen, Denmark, dipped into a pool of information collected through their national Danish Twin Register and combined it with data from the Danish Psychiatric Central Research Register to come up with a whopping sample of 31,524 twin pairs, all born between 1951 and 2000.

Twins are a fairly solid way to determine whether a condition was inherited at conception or is the result of other environmental factors.

So-called identical twins – or monozygotic twin pairs – inherited the same sets of genes from their parents.

Comparing characteristics found among them with those in dizygotic twin pairs (or non-identical twins) can provide a strong indication of whether it was caused by genes or something in the environment as they developed.

While this is good in theory, biology is a messy affair where lots of numbers are needed to come to a reliable conclusion. It can be challenging finding enough twins with the condition being studied to participate in a study.

In the case of schizophrenia, the neurological condition affects just under 5 out of every 1,000 individuals at any given point in time, making it especially difficult in collecting enough data on twins.

 

Thus, Denmark's big national register combined with sharp statistical tools has turned out to be an excellent way to go.

A similar twins study conducted in Finland in 1998 using a smaller sample from their Finnish National Population Register concluded heritability of schizophrenia to be 83 percent.

Another analysis conducted in Sweden in 2007 broke the risk down between the sexes, finding genes to be the cause of schizophrenia in 67 percent of female cases and 41 percent in male.

These numbers aren't helped at all by the fact that schizophrenia itself is a disputed condition. Like autism in days past, the word attempts to cover a broad spectrum of causes and symptoms that needs to be better labelled or split up altogether.

In an attempt to get a better grip on the statistics, the researchers in this latest study calculated two estimates, on both a narrow definition and a broader schizophrenia 'spectrum' disorder.

For the more narrow definition they estimated genes determined the condition's diagnosis in 79 percent of total cases.

When expanded to include those with a schizophrenia spectrum disorder, the number dropped to 73 percent.

 

"This study is now the most comprehensive and thorough estimate of the heritability of schizophrenia and its diagnostic diversity," says researcher Rikke Hilker from the University of Copenhagen.

"It is interesting since it indicates that the genetic risk for disease seems to be of almost equal importance across the spectrum of schizophrenia."

The research also provided an average age of 28.9 at which the symptoms of schizophrenia become significant enough for a diagnosis.

Twin studies are useful tools, but are based on the fair assumption that twins reflect the same inheritance patterns of the general population.

There is also the question of how much each national database can be generalised to other parts of the world.

The tug-of-war debate of nature versus nurture often hides the complexity of disease and disability.

Even the inheritance of a gene can be complicated by the editing effects of epigenetics, or so-called post-zygotic mosaic mutations that take place shortly after conception.

Individual genes have been linked with schizophrenia in the past, and based on this study's results there are almost certainly more to be discovered in the future.

The boundaries and definitions of this serious mental condition might shift around, but no matter what we call it, those who suffer from schizophrenia's debilitating effects will benefit from knowing more about its underlying causes. 

This research was published in Biological Psychiatry.

 

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