For decades, researchers have known that schizophrenia and bipolar disorder run strongly in families, yet the common genetic variants found so far account for only about a quarter of that inheritance, leaving a stubborn gap. A team in Pune now proposes that the missing piece may lie in short, stuttering runs of DNA that standard methods have largely passed over, long familiar from rare diseases like Huntington's. Their model remains an idea rather than a finding, and it asks a patient question of the field: whether a multitude of modestly expanded repeats, each harmless alone, might together w
A Proposed Repeat-Expansion Mechanism Could Help Explain Missing Heritability in Schizophrenia and Bipolar Disorder
A map that can be proven wrong is worth having.
Mark
Why does a quarter versus 81 percent matter so much?
Mimi
Because it's a huge gap. Families clearly pass something on, yet the big genetic surveys explain only about a quarter of the risk. This paper says to look at repeats those surveys largely skipped.
Mark
What's special about CAG?
Mimi
It codes for glutamine, and in single genes long expansions cause Huntington disease and ataxias, where psychosis shows up often. The authors propose 45 candidate genes, most of them moderately expanding.
Luke
Is there any measurement showing these genes explain the gap?
Mimi
No. The authors say the repeat-length data in schizophrenia and bipolar populations don't exist yet, so the size of the contribution is unknown.
Mark
What about the astrocyte mechanism?
Mimi
Astrocytes would hoard glutamine, starving neurons of glutamate, GABA and glutathione, and turn reactive. A meta-analysis of 28 studies at 7 tesla found glutamine up and glutamate down in the medial frontal region.
Luke
But that doesn't show the astrocyte part, right?
Mimi
Right. Hegde himself says spectroscopy gives whole-region totals, so assigning those changes to astrocytes and neurons is inference. The neuroinflammation link is borrowed from hepatic encephalopathy.
Luke
And the one best-studied repeat, KCNN3?
Mimi
It replicated in some cohorts and not many others. The authors read that as evidence it can't act alone, which is an interpretation, not a demonstration.
Mark
So what would settle it?
Mimi
Repeat-aware genomic profiling in patient cohorts and single-cell mapping of expression. The authors accept it may be proven wrong.
Le Pouls
- Twin studies point to 81 percent heritability, yet common variants explain only about a quarter, and the rest sits unexplained in the genome.
- A proposed hidden cause, 45 CAG-repeat genes scattered across the brain, has been overlooked by the standard tools that scan for common variation.
- Astrocytes, the support cells feeding neurons glutamine, may be strained into hoarding it, leaving synapses starved of glutamate, GABA and glutathione.
- Imaging at 7 tesla hints that glutamine rises while glutamate falls in the frontal cortex, a fit the authors call encouraging but not proof.
- The hypothesis is untested: direct repeat-length measurements in patient cohorts must come before anyone can say whether the idea holds.
For decades, researchers have known that schizophrenia and bipolar disorder run strongly in families, yet the common genetic variants found so far account for only about a quarter of that inheritance, leaving a stubborn gap. A team in Pune now proposes that the missing piece may lie in short, stuttering runs of DNA that standard methods have largely passed over, long familiar from rare diseases like Huntington's. Their model remains an idea rather than a finding, and it asks a patient question of the field: whether a multitude of modestly expanded repeats, each harmless alone, might together weave into a chronic illness of the mind.
Twin studies have long placed the heritability of schizophrenia and bipolar disorder near 81 percent, but the large genetic surveys of recent decades have accounted for only about a quarter of the risk. A Perspective in Genomic Psychiatry from researchers at Bharati Vidyapeeth Deemed University in Pune asks where the rest might be hiding, and points to stretches of DNA where the three-letter code CAG repeats over and over. When such repeats expand far enough in a single gene, they cause Huntington disease and several ataxias, and those illnesses often bring psychosis with them.
The authors, led by Mahabaleshwar Hegde, first raised the idea in 2011 and now list 45 candidate genes. Their central claim is that very long expansions kill neurons, while moderate ones would not kill anything but might accumulate across the brain into psychosis. In their model, astrocytes, which supply neurons with glutamine, are overwhelmed by the glutamine these proteins carry. Neurons then run short of the chemicals they need to communicate and protect themselves, and the circuits begin to fail.
Some supporting observations fit the picture. A meta-analysis of 28 studies found glutamine rising and glutamate falling in the frontal region in schizophrenia, a pattern visible only at high-field scanners. Yet Hegde is careful to note that scans measure whole regions, so linking the signals to astrocytes and neurons remains an inference. The authors are equally candid that the key data on repeat lengths in these patient populations do not yet exist, and that the best-studied candidate has replicated unevenly. If the idea holds, repeat profiles might one day serve as biomarkers, but for now it stands as a hypothesis awaiting its test.
Twin studies put the heritability of schizophrenia and bipolar disorder at 81 percent. The large genome-wide surveys of recent decades have accounted for only part of that: common variants explain about a quarter of the risk. A peer-reviewed Perspective in Genomic Psychiatry, from a team at Bharati Vidyapeeth Deemed University in Pune, India, proposes where some of the remainder might be hiding, in short, stuttering stretches of DNA that standard methods largely overlooked.
The stutter is CAG, the three-letter code for the amino acid glutamine, repeated many times in a row. When a repeat in a single gene expands far enough, it causes Huntington disease and several spinocerebellar ataxias. Psychosis of the schizophrenia and bipolar type is reported in those illnesses far more often than in the general population. The corresponding author, Mahabaleshwar V. Hegde, who directs the Center for Innovation in Nutrition Health and Disease at Interactive Research School for Health Affairs, wrote with Asavari A. Joshi, Yogesh S. Badhe and Anvita A. Kale. They ask whether many modestly expanded repeat genes, scattered across the brain, could add up to a severe chronic mental illness.
The authors first floated the idea in 2011 under the title "Unstable Genes Unstable Mind". Using a published catalogue of human CAG-repeat genes, they now list 45 candidates. Ten can expand into classic neurodegenerative diseases, three carry repeats outside the coding regions that also cause such diseases, and 32 expand moderately. One recent estimate attributes about 4 percent of schizophrenia risk to tandem repeats in exons. Their argument is that repeats beyond roughly 30 to 35 kill neurons and cause neurodegeneration, while moderate expansions would not kill neurons but would cumulatively produce psychosis.
The proposed chain runs through astrocytes, the support cells that supply neurons with glutamine, the raw material for glutamate and GABA. If many genes make proteins with enlarged glutamine stretches, astrocytes would face outsized demand. In the model, they respond by activating a metabolic side route to make more alpha-ketoglutarate, scavenging ammonia from sources including D-serine, glycine and adenosine, and holding glutamine back rather than exporting it. Neurons then run short of glutamate, GABA and the antioxidant glutathione.
Glutamine accumulating inside astrocytes is likened to a Trojan horse, a comparison borrowed from hepatic encephalopathy, where trapped glutamine swells astrocytes and makes them reactive. Reactive astrocytes can release inflammatory signals, recruit complement and eliminate synapses. Circuits break and, in this model, psychosis follows. Why one root would yield two illnesses, the authors suggest, depends on which brain regions and receptors are involved: NMDA receptor hypofunction favoring schizophrenia-type symptoms, and metabotropic glutamate receptor 5 implicated in bipolar disorder.
Part of the model can be checked against imaging. At 3 tesla and below, glutamate and glutamine signals overlap and were long reported together. At 7 tesla, magnetic resonance spectroscopy separates them. A meta-analysis of 28 studies, with 647 patients and 608 controls, found glutamine rising and glutamate falling in the medial frontal region in schizophrenia. Hegde calls this the most encouraging fit for the model but notes a limit: spectroscopy reports totals for a whole region, so assigning the glutamine to astrocytes and the deficits to neurons is an inference, not something the scans prove.
The framework also ties together long-loose observations. Genetic anticipation, with illness arriving earlier or more severely in later generations, is a hallmark of repeat diseases and has been reported in both disorders, though earlier researchers blamed ascertainment bias. In Huntington disease and spinocerebellar ataxia 17, longer repeats track with earlier onset of psychosis as well as neurological illness. Men develop schizophrenia three to four years earlier than women, and the authors speculate that the unstable repeat in the X-linked androgen receptor gene could be involved. Because the mechanism involves glutamate deficits rather than excitotoxicity, Hegde says, it predicts no neuronal death, fitting a neurodevelopmental course in which metabolic burden builds until it crosses a threshold.
The authors state plainly that this is a hypothesis. The repeat-length data needed to estimate how much heritability these genes explain do not yet exist for schizophrenia and bipolar populations. The best-studied candidate, a repeat in the KCNN3 potassium channel gene, has replicated in some cohorts and not in many others, which they read as a sign it cannot act alone. Evidence linking astrocyte glutamine buildup to neuroinflammation comes from hepatic encephalopathy and must be checked in these disorders. Computational tools for profiling tandem repeats and single-cell transcriptomics now exist to do that. If the model holds, repeat profiles could become biomarkers for illnesses still diagnosed subjectively, and drugs aimed at the earliest metabolic steps might treat both.
Citations marquantes
Spectroscopy reports totals for a whole region, so assigning higher glutamine to astrocytes is a model-based inference, not something the scans prove directly.— Mahabaleshwar V. Hegde, corresponding author
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