Genetic Networks Identified

Photo: Graphic of schizophrenia

A new study provides insight into the molecular
causes of schizophrenia; © panthermedia.net/
Julien Tromeur

Although schizophrenia is highly genetic in origin, the genes involved in the disorder have been difficult to identify. In the past few years, researchers have implicated several genes, but it is unclear how they act to produce the disorder. A new study by researchers at Columbia University Medical Center identifies affected gene networks and provides insight into the molecular causes of the disease.

Using an unbiased collection of hundreds of mutations associated with schizophrenia, the Columbia researchers applied a sophisticated computational approach to uncover hidden relationships among seemingly unrelated genes. The analysis revealed that many of the genes mutated in schizophrenia are organized into two main networks, which take part in a few key processes, including axon guidance, synapse function, neuron mobility, and chromosomal modification.

The study also uncovered an intriguing connection between schizophrenia and autism. "If we hadn't known that these were two different diseases, and had put all the mutations into a single analysis, it would have come up with very similar networks," said the study's senior author, Dennis Vitkup, PhD. "It shows how closely the autism and schizophrenia genetic networks are intertwined," he added. Although it will take time to translate the findings into practical treatments, the study provides insight into the molecular causes of schizophrenia. It also suggests that mutations associated with schizophrenia, autism, and probably many other psychiatric disorders, are likely to converge on a set of interrelated molecular processes.

To discover potential connections among genes mutated in schizophrenia, Doctor Vitkup and colleagues developed a computational approach, called NETBAG+, to identify networks of genes likely to be responsible for the same genetic phenotype. They then gathered the strongest mutations that had been observed in schizophrenia by other researchers and fed them into the program. The program uncovered two genetic networks. Genes in the first network are involved primarily in axon guidance, synapse function, and cell migration. Genes in the second network are involved in chromosomal organization and remodeling. Parts of both networks are highly active during prenatal development, suggesting that changes in the brain that cause schizophrenia later in life are laid down very early in life.

Doctor Vitkup also compared his schizophrenia networks with networks found in neurodevelopmental disorders such as autism. One schizophrenia network is strongly related to an autism network he described in a study published last year. Both networks contain genes involved in axon guidance, synapse function, and cell migration. "Our recent mutational analysis showed that this overlap includes primarily genes that are important for early fetal development. This is not surprising, because some cases of schizophrenia and likely many cases of autism have neurodevelopmental origin," said Doctor Karayiorgou.

The close relationship between genetic networks involved in autism and schizophrenia raises an intriguing question: How can mutations in the same or related genes cause two different disorders? "I like to use the analogy of car brakes," said Dr. Vitkup. "Different mechanical malfunctions of the brake mechanism can have very different functional consequences, from rapid acceleration to stalling." Doctor Vitkup looked at large mutations called copy number variants (CNVs) that can lead to either schizophrenia or autism. CNVs involve long stretches of the genome containing several genes that have been either duplicated or deleted. Duplication of a region increases the "dosage" of its genes; deletion of a region decreases the dosage.

In CNVs involved in the growth of dendrites, or dendritic spines, at the ends of neurons, he found decrease in growth to be more common in schizophrenia and increase in growth more common in autism. "That's consistent with what's been found by postmortem brain studies," he said.

"Until a few years ago, people were looking for just a handful of genes responsible for autism and schizophrenia, so the idea that many hundreds of genes are involved is a big change in thinking," Doctor Vitkup said. "Our study and the studies of our collaborators suggest that in the search for the causes of complex genetic disorders, it will be more productive to look for common pathways and gene circuits than for a handful of causal genes. This type of network analysis gives us a way to begin to make sense of what's happening."

MEDICA.de; Source: Columbia University Medical Center