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MELBOURNE (Reuters) - An Australian research team said on Tuesday it had identified 153 genes affected by schizophrenia in a step toward discovering the causes of the illness.

The 153 affected genes were identified after completing high-throughput screening of 12,000 genes in post-mortem studies of brain tissue from schizophrenia sufferers.

"Within that affected group will be the core genes that actually cause the symptoms of the illness, and clearly that is what we are trying to get at," Mental Health Research Institute Associate Professor Brian Dean told Reuters.

The research team at the institute's Rebecca Cooper Laboratories outlined its plans to identify the core genes to the Australian Health and Medical Research Congress.

"I think what is important about this is up until now schizophrenia is a psychiatric illness that is defined on symptoms alone," Dean said.

"What this gene screen has allowed us to do with our research into the future, is now base our research on genes that we now know are affected in brain tissue from someone with the illness."

Dean said general current thinking is that genetics plays a major role in the development of schizophrenia, but studies with the identical twins of sufferers suggest there must also be another factor that brings on the disease.

"They have got identical genetic material essentially, but the likelihood of them getting the illness is only actually 46%," he said.

"There has to be something else other than genes involved, but if we can get at the core genes, then we can start thinking about what the second hit might be that affects the susceptible individual."

The research has implications for improving treatment of the disease without some of the side effects of existing medication. The next stage involves more advanced screening of protein and genetic material, known as mRNA, to try and narrow down the number of genes that actually cause schizophrenia to possibly five or 10.

"The core of the problem rests in those genes," Dean said.