Gene Links Schizophrenia to Impaired Decision-Making

New research has uncovered a genetic mutation that appears to contribute significantly to the impaired decision-making and detachment from reality often observed in individuals with schizophrenia. This mutation, located in the GRIN2A gene, disrupts a specific brain circuit responsible for integrating new sensory information with existing beliefs, a crucial process for maintaining a coherent understanding of reality. This discovery provides a novel molecular target for developing treatments aimed at alleviating the cognitive challenges associated with schizophrenia.

Schizophrenia is a complex disorder characterized by a range of symptoms, including cognitive impairments that hinder an individual's ability to process new information and make sound decisions. This cognitive inflexibility can ultimately lead to a disconnect from reality, manifesting as delusions and hallucinations. The GRIN2A gene, identified through large-scale genetic screening, codes for a protein component of the NMDA receptor, which plays a vital role in neuronal communication. Mutations in this gene have been strongly linked to an elevated risk of developing schizophrenia.

A team of neuroscientists from MIT, led by Guoping Feng and Michael Halassa, conducted a study using mouse models engineered with the GRIN2A mutation. Their experiments focused on understanding how this mutation affects the brain's capacity for 'belief updating.' In neurotypical brains, new sensory data constantly refines prior beliefs, ensuring an accurate perception of the world. However, in schizophrenia, this updating mechanism is compromised, leading to an over-reliance on outdated beliefs and a diminished ability to adapt to changing circumstances.

To investigate this phenomenon, the researchers devised a task where mice had to choose between two levers offering different reward structures. Initially, healthy mice and those with the GRIN2A mutation both favored the high-reward lever. As the experiment progressed, the effort required for the high-reward lever increased, while the low-reward option remained constant. Healthy mice demonstrated adaptive behavior, gradually shifting their preference to the low-reward lever as the value proposition changed. In contrast, mice with the GRIN2A mutation were significantly slower to adapt, exhibiting prolonged switching between levers and delaying their permanent shift to the more advantageous low-reward option. This indicated a clear impairment in their adaptive decision-making processes.

Further investigation using functional ultrasound imaging and electrical recordings revealed that the mediodorsal thalamus, a brain region intimately connected with the prefrontal cortex, was most significantly affected by the GRIN2A mutation. This thalamocortical circuit is known to regulate executive functions and decision-making. The study found that neuronal activity in the mediodorsal thalamus normally tracks changes in reward values and cognitive states. However, in mice with the mutation, this tracking mechanism was disrupted.

Crucially, the researchers were able to reverse the behavioral deficits in the mutated mice using optogenetics, a technique that employs light to control genetically modified neurons. By artificially activating neurons in the mediodorsal thalamus, they restored the mice's ability to make normal, adaptive decisions. This remarkable finding suggests that the circuit itself is not permanently damaged but rather underactive, opening avenues for therapeutic interventions that could reactivate or modulate this crucial brain pathway.

While the GRIN2A mutation is present in only a small percentage of schizophrenia patients, the circuit dysfunction identified in this study may represent a common underlying mechanism for cognitive impairment across various forms of the disorder. The ability to precisely target and influence this circuit offers a promising direction for developing new pharmacological or non-invasive brain stimulation therapies to help individuals with schizophrenia better integrate new information and reconnect with reality.