Posts tagged 22q11 deletion syndrome

St. Jude Children’s Research Hospital scientists have identified problems in a connection between brain structures that may predispose individuals to hearing the “voices” that are a common symptom of schizophrenia. The work appears in the June 6 issue of the journal Science.

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Researchers linked the problem to a gene deletion. This leads to changes in brain chemistry that reduce the flow of information between two brain structures involved in processing auditory information.

The research marks the first time that a specific circuit in the brain has been linked to the auditory hallucinations, delusions and other psychotic symptoms of schizophrenia. The disease is a chronic, devastating brain disorder that affects about 1 percent of Americans and causes them to struggle with a variety of problems, including thinking, learning and memory.

The disrupted circuit identified in this study solves the mystery of how current antipsychotic drugs ease symptoms and provides a new focus for efforts to develop medications that quiet “voices” but cause fewer side effects.

“We think that reducing the flow of information between these two brain structures that play a central role in processing auditory information sets the stage for stress or other factors to come along and trigger the ‘voices’ that are the most common psychotic symptom of schizophrenia,” said the study’s corresponding author Stanislav Zakharenko, M.D., Ph.D., an associate member of the St. Jude Department of Developmental Neurobiology. “These findings also integrate several competing models regarding changes in the brain that lead to this complex disorder.”

The work was done in a mouse model of the human genetic disorder 22q11 deletion syndrome. The syndrome occurs when part of chromosome 22 is deleted and individuals are left with one rather than the usual two copies of about 25 genes. About 30 percent of individuals with the deletion syndrome develop schizophrenia, making it one of the strongest risk factors for the disorder. DNA is the blueprint for life. Human DNA is organized into 23 pairs of chromosomes that are found in nearly every cell.

Earlier work from Zakharenko’s laboratory linked one of the lost genes, Dgcr8, to brain changes in mice with the deletion syndrome that affect a structure important for learning and memory. They found evidence that the same mechanism was at work in patients with schizophrenia. Dgcr8 carries instructions for making small molecules called microRNAs that help regulate production of different proteins.

For this study, researchers used state-of-the-art tools to link the loss of Dgcr8 to changes that affect a different brain structure, the auditory thalamus. For decades antipsychotic drugs have been known to work by binding to a protein named the D2 dopamine receptor (Drd2). The binding blocks activity of the chemical messenger dopamine. Until now, however, how that quieted the “voices” of schizophrenia was unclear.

Working in mice with and without the 22q11 deletion, researchers showed that the strength of the nerve impulse from neurons in the auditory thalamus was reduced in mice with the deletion compared to normal mice. Electrical activity in other brain regions was not different.

Investigators showed that Drd2 levels were elevated in the auditory thalamus of mice with the deletion, but not in other brain regions. When researchers checked Drd2 levels in tissue from the same structure collected from 26 individuals with and without schizophrenia, scientists reported that protein levels were higher in patients with the disease.

As further evidence of Drd2’s role in disrupting signals from the auditory thalamus, researchers tested neurons in the laboratory from different brain regions of mutant and normal mice by adding antipsychotic drugs haloperidol and clozapine. Those drugs work by targeting Drd2. Originally nerve impulses in the mutant neurons were reduced compared to normal mice. But the nerve impulses were almost universally enhanced by antipsychotics in neurons from mutant mice, but only in neurons from the auditory thalamus.

When researchers looked more closely at the missing 22q11 genes, they found that mice that lacked the Dgcr8 responded to a loud noise in a similar manner as schizophrenia patients. Treatment with haloperidol restored the normal startle response in the mice, just as the drug does in patients.

Studying schizophrenia and other brain disorders advances understanding of normal brain development and the missteps that lead to various catastrophic diseases, including pediatric brain tumors and other problems.

(Source: stjude.org)

Scientists studying a rare genetic disorder have identified a molecular pathway that may play a role in schizophrenia, according to new research in the Oct. 10 issue of The Journal of Neuroscience. The findings may one day guide researchers to new treatment options for people with schizophrenia — a devastating disease that affects approximately 1 percent of the world’s population.

Schizophrenia is characterized by a multitude of symptoms, including hallucinations, social withdrawal, and learning and memory deficits, which usually appear during late adolescence or early adulthood. Efforts to identify disease causes have been complicated by the fact that no single genetic mutation is strongly associated with the disease. By studying a rare genetic disorder that increases the risk of schizophrenia, Laurie Earls, PhD, and colleagues in the laboratory of Stanislav Zakharenko, MD, PhD, at St. Jude Children’s Research Hospital identified molecular changes that affect memory and are also present in people with schizophrenia.

Approximately 30 percent of people with a genetic disorder known as 22q11 deletion syndrome develop schizophrenia, making it one of the strongest risk factors for the disease. In previous studies of mice with the 22q11 deletion, Zakharenko’s group identified changes in nerve cells leading to deficits in the hippocampus — the brain’s learning and memory center — that appear with age. In the current study, the group confirmed similar molecular changes occur in people with schizophrenia. They also zeroed in on the gene contributing to the nerve cell changes.

"This study makes some very important discoveries about the precise mechanisms underlying the learning and memory deficits seen in the genetic mouse model — problems that are a central part of the human disease," said Carrie Bearden, PhD, an expert on 22q11 deletion syndrome at the University of California, Los Angeles, who was not involved in the study. "Pinpointing the specific gene involved is the first step toward developing targeted therapies that could reverse the cognitive deficits associated with schizophrenia, both in the context of this genetic mutation and the broader population," she added.

In previous studies, Zakharenko’s group found that abnormal nerve cell communication and cognitive dysfunction was associated with elevated levels of a protein that regulates calcium in certain nerve cells known as Serca2. These abnormalities are only detectable with age in mice with the 22q11 deletion.

In the current study, the researchers identified the gene Dgcr8 as the source of the changes.It produces molecules called microRNAs that normally keep Serca2 in check. Without them, the protein becomes elevated.By adding these molecules back into the hippocampus of animals with the 22q11 deletion, the researchers were able to reduce elevated Serca2 levels and reduce the cellular deficits associated with this genetic defect.

To assess whether the findings from these genetic mouse studies might translate to schizophrenia, the authors analyzed post-mortem brain tissue from people with schizophrenia. The researchers discovered that Serca2 was elevated even in patients with schizophrenia who did not have the 22q11 deletion.

"These data suggest a link between the nerve cell changes in patients with the 22q11 deletion syndrome and those that occur in patients with schizophrenia," Zakharenko said. "Serca2 regulation represents a novel therapeutic target for schizophrenia."

(Source: sott.net)