Fatal Familial Insomnia

Abstract

Fatal familial insomnia is a rare inherited prion disease in which a single mutation in the prion protein gene destroys the parts of the thalamus that regulate sleep. Carriers appear healthy until midlife, when they develop a progressive, untreatable loss of sleep alongside a racing heart, sweating, and other signs of an autonomic nervous system out of control. Over months the capacity for deep and dream sleep collapses, attention and memory fray, and the illness proves uniformly fatal. Its cause is a misfolded protein rather than a virus or bacterium, and its course depends on a second site in the same gene. This article covers the molecular cause, the clinical course, and the thalamic pathology, and what the disease reveals about how the brain builds sleep, with a worked example and three demonstrations.

Keywords: prion disease, fatal familial insomnia, thalamus

What Fatal Familial Insomnia Is

*Fatal familial insomnia* is a rare, inherited disease of the brain in which sleep itself is progressively destroyed. It belongs to a small group of disorders called *prion* diseases, in which a normal protein of the nervous system misfolds into an abnormal shape and then coaxes its healthy neighbors to do the same, so that the damage spreads through the tissue like a chemical chain reaction rather than an infection. The disease was first recognized in 1986, when Lugaresi and his colleagues described a man who lost the ability to sleep, developed a storm of autonomic symptoms, and died within months, and who at autopsy showed a striking and selective loss of neurons in the *thalamus* (Lugaresi et al., 1986).

What makes the condition singular is the precision of the damage. Most diseases that disturb sleep do so as a side effect of injury elsewhere; here the illness attacks, almost surgically, the very nuclei of the thalamus that gate the brain into sleep. The result is not the ordinary difficulty falling asleep that the word insomnia usually calls to mind, but the near-total abolition of the states of sleep, a condition the brain cannot survive. Because a misfolded protein is the culprit, no virus or bacterium is ever found, and the mechanism of spread is the conformational conversion of one protein by another that won Prusiner a Nobel Prize and reshaped the understanding of neurodegeneration (Prusiner, 1998).

The Molecular Cause

Every case of fatal familial insomnia traces to a single change in the *PRNP gene* on chromosome 20, the gene that encodes the *prion protein (PrP)*. The change is the *D178N mutation*, a substitution that swaps one amino acid, asparagine, for another, aspartate, at position 178 of the protein. Medori and colleagues identified this mutation as the cause of the disease in 1992, establishing fatal familial insomnia as a genetic prion disease rather than an infectious or purely sporadic one (Medori et al., 1992). The mutation is inherited in an *autosomal dominant* pattern: a single copy is enough to cause the disease, so on average half the children of an affected parent inherit it.

The mutation alone, however, does not fix which disease results. A second position in the same gene, the *codon 129 polymorphism*, acts as a switch. Codon 129 normally carries either methionine (Met) or valine (Val) in the healthy population, and the outcome depends on which sits on the same chromosome as the D178N mutation. When the mutant allele carries methionine, the illness is fatal familial insomnia; when it carries valine, the very same D178N mutation instead produces a familial form of Creutzfeldt-Jakob disease, a distinct condition with different symptoms and pathology (Goldfarb et al., 1992). Codon 129 on the person's other, normal allele then tunes the severity: patients who are methionine on both alleles tend to run a shorter, more explosive course, while methionine-valine heterozygotes survive longer and show more widespread brain involvement (Montagna et al., 2003). The molecular basis of these differences is reflected in the regional pattern of abnormal, protease-resistant prion protein that accumulates in the brain (Parchi et al., 1995).

The distinctions matter enough to set out explicitly. Table 1 summarizes how the two positions in the gene combine to determine both which disease appears and how it runs its course.

The Clinical Course

Fatal familial insomnia typically begins in midlife, most often between the ages of forty and sixty, after the carrier has lived an ordinary and healthy life. The first sign is usually a mounting inability to sleep that no sedative relieves. In the detailed studies of affected families, the illness then unfolds in overlapping phases: a period of worsening insomnia laced with panic and phobias, a stage of hallucinations and agitation, a phase in which the loss of sleep and weight becomes profound, and a final stage of mutism and rapid decline (Manetto et al., 1992). Throughout, the autonomic nervous system runs unchecked, producing a fast heart rate, high blood pressure, fever, sweating, and irregular breathing, a syndrome of *dysautonomia* that is as characteristic of the disease as the sleeplessness itself.

The course is relentlessly progressive and always fatal, but its length varies with the codon 129 genotype described above, ranging from a matter of months to a few years (Montagna et al., 2003). Recognizing the disease early is difficult, because the initial insomnia and mood changes can be mistaken for a psychiatric illness; clinical series have worked to define the features that flag it sooner, including the combination of sleep loss, dysautonomia, and rapidly progressive cognitive decline (Krasnianski et al., 2008). Formal diagnostic criteria now combine this clinical picture with genetic testing for the PRNP mutation and, where available, sleep studies and imaging, so that the diagnosis no longer depends on autopsy (Wu et al., 2018).

Sleep, the Thalamus, and Cognition

The reason fatal familial insomnia abolishes sleep so completely lies in where it strikes. The disease falls with unusual selectivity on the anterior and dorsomedial nuclei of the *thalamus*, the deep relay structures that, in the healthy brain, help orchestrate the transition from waking into sleep. As these neurons die and the surrounding tissue fills with the reactive scarring called *gliosis*, the machinery that generates sleep is dismantled. Recordings of the sleep-wake cycle in patients show that the normal architecture of sleep does not merely shrink but disintegrates: the orderly progression through stages is lost, and the electrical rhythms that mark deep sleep fail to appear (Sforza et al., 1995). Imaging of brain metabolism confirms the anatomy, revealing profound thalamic hypometabolism that tracks the clinical severity and the distribution of the disease (Cortelli et al., 1997).

Two features of normal sleep are lost early and conspicuously. The *sleep spindle*, the brief burst of rhythmic activity generated by the thalamus that punctuates light sleep, fades as the thalamic circuits degrade, and *slow-wave sleep*, the deep, restorative stage marked by large slow brain waves, becomes impossible to enter. In their place the patient enters a peculiar state of *agrypnia excitata*, a combination of the inability to sleep with a persistent overactivation of both the motor and autonomic systems, so that the person is neither properly awake nor able to rest (Baldelli & Provini, 2019). The abolition of restorative sleep has direct cognitive consequences, eroding attention and disrupting the memory consolidation that normally depends on the deep and spindle-rich stages of sleep. The disease thus offers a stark natural demonstration that sleep is not a passive shutdown but an actively generated state, one for which the thalamus is indispensable.

Worked Example: The Odds Facing an Affected Family

Consider how the two features of the genetics combine to shape the risk within a family. Fatal familial insomnia is inherited as an autosomal dominant trait, so a parent who carries the D178N mutation on one of their two copies of PRNP passes that mutant copy to each child with a probability of one-half. A child who does not inherit it cannot develop the disease.

Now add the codon 129 switch, which sets the severity. In fatal familial insomnia the mutant allele always carries methionine, so every affected child inherits at least one methionine at codon 129 from the affected parent. Whether that child is a methionine homozygote, and so faces the shorter and more rapid course, or a methionine-valine heterozygote, and so faces the longer course, depends on the codon 129 that comes from the other, unaffected parent. Suppose that parent is a methionine-valine heterozygote, the commonest case in many populations. Then the copy they contribute is methionine half the time and valine half the time.

Multiplying these independent chances gives the outlook for any given child. The probability of inheriting the disease at all is one-half. Given inheritance, the probability of the methionine-homozygous, short-course form is one-half, and the probability of the heterozygous, longer-course form is the other one-half. So across all children of this couple, one-quarter are expected to develop the rapid form of the disease, one-quarter the slower form, and the remaining half to be unaffected. The arithmetic is simple, but it captures the cruelty of the condition: the same coin flip that decides whether a child is spared decides, if they are not, how long they will have.

Discussion

Fatal familial insomnia occupies an unusual place at the intersection of three fields. As a prion disease it belongs with Creutzfeldt-Jakob disease and kuru, sharing their mechanism of protein misfolding and their fatal, untreatable trajectory; indeed the classification of the familial prion diseases treats fatal familial insomnia and the D178N-valine form of Creutzfeldt-Jakob disease as two faces of one mutation, separated only by codon 129 (Gambetti et al., 2003). As a genetic disease it is a textbook case of how a single nucleotide, and even a silent-seeming polymorphism nearby, can determine not just whether illness strikes but which illness it is. And as a disease of sleep it has become an involuntary experiment that no ethical researcher could design.

That last role is where the condition speaks most directly to cognitive science. Because the damage is so localized to the thalamus, the disease isolates the contribution of a single structure to the generation of sleep, and it does so in humans rather than in an animal model. The finding that destroying these thalamic nuclei abolishes sleep spindles and slow-wave sleep, while leaving the person trapped in agrypnia excitata, is among the strongest evidence that the thalamus is not merely a relay but an active pacemaker of sleep (Sforza et al., 1995). The disease also underscores how tightly the regulation of sleep is bound to the regulation of the autonomic nervous system, since the same lesion that abolishes sleep unleashes the storm of dysautonomia (Cracco et al., 2018).

Current Directions

Research on fatal familial insomnia now moves along two fronts. The first is diagnostic and mechanistic. Ultrasensitive assays that detect trace amounts of misfolded prion protein in cerebrospinal fluid, and refined imaging and sleep criteria, are pushing recognition of the disease earlier and making it possible to study its progression before the brain is devastated; the recent consensus criteria formalize this shift toward antemortem diagnosis (Wu et al., 2018). Careful characterization of the autonomic and sleep phenotype continues to sharpen the picture of how the thalamic lesion produces agrypnia excitata, linking the anatomy to the physiology in ever finer detail (Baldelli & Provini, 2019).

The second front is therapeutic, and it is where the greatest hope and the greatest uncertainty lie. Because carriers of the mutation can be identified decades before symptoms, the disease is a leading candidate for preventive treatment: strategies that lower the amount of prion protein the brain produces, using antisense oligonucleotides or related tools, aim to intervene before misfolding begins rather than after neurons are lost. Whether such approaches can safely delay or prevent a disease that has been uniformly fatal remains an open and actively pursued question, and reviews of the fatal insomnias frame this preclinical window as the critical opportunity (Cracco et al., 2018).

Common Misconceptions

Fatal familial insomnia is just very severe ordinary insomnia.

It is a distinct neurodegenerative prion disease that destroys the brain structures that generate sleep; ordinary insomnia involves no such damage and is not fatal (Lugaresi et al., 1986).

The disease is caused by an infection that spreads between people.

The familial form is inherited through a mutation in the PRNP gene, not transmitted like a cold; the abnormal protein spreads within the brain by converting normal protein, not between people in daily contact (Medori et al., 1992).

Anyone with the D178N mutation gets fatal familial insomnia.

The same mutation produces a familial form of Creutzfeldt-Jakob disease when codon 129 on the mutant allele carries valine instead of methionine; the second site decides which disease results (Goldfarb et al., 1992).

Patients simply stay awake but are otherwise fine.

The loss of sleep comes bound to severe dysautonomia and to a state of restless overactivation, agrypnia excitata, along with progressive cognitive and motor decline (Baldelli & Provini, 2019).

Glossary

Agrypnia excitata.

A state combining the inability to sleep with persistent overactivation of the motor and autonomic systems, characteristic of fatal familial insomnia.

Autosomal dominant.

An inheritance pattern in which a single copy of a mutated gene is enough to cause the disease, so each child of a carrier has a one-half chance of inheriting it.

Codon 129 polymorphism.

A common variation at position 129 of the prion protein gene, carrying either methionine or valine, that determines which prion disease a given mutation produces and how severe it is.

Creutzfeldt-Jakob disease.

The most common human prion disease, which the D178N mutation produces in its familial form when codon 129 on the mutant allele carries valine rather than methionine.

D178N mutation.

The change at codon 178 of the PRNP gene, substituting asparagine for aspartate, that underlies fatal familial insomnia and one familial form of Creutzfeldt-Jakob disease.

Dysautonomia.

A malfunction of the autonomic nervous system producing signs such as a fast heart rate, high blood pressure, fever, and sweating, prominent throughout the disease.

Gliosis.

The reactive proliferation of glial cells that follows the loss of neurons, seen densely in the affected thalamic nuclei.

Prion protein (PrP).

The normal cellular protein encoded by PRNP that, when misfolded, causes prion disease; its healthy function is still incompletely understood.

Prion.

A misfolded protein that acts as an infectious agent by inducing normal copies of the same protein to adopt its abnormal shape.

PRNP gene.

The gene on chromosome 20 encoding the prion protein, in which the D178N mutation and the codon 129 polymorphism both lie.

Sleep spindle.

A short burst of rhythmic brain activity generated by the thalamus during light sleep, one of the first features of normal sleep to disappear in the disease.

Slow-wave sleep.

The deep, restorative stage of sleep marked by large slow brain waves, which patients become progressively unable to enter.

Sporadic fatal insomnia.

A very rare form with the same clinical and pathological picture but no PRNP mutation and no family history, arising spontaneously.

Thalamus.

The deep brain structure whose anterior and dorsomedial nuclei help generate sleep and are selectively destroyed in fatal familial insomnia.

Key Researchers

Pietro Cortelli

A neurologist at the University of Bologna who studies the autonomic nervous system and sleep and who used brain imaging to map the thalamic hypometabolism of fatal familial insomnia.
ORCID - Wikidata

Pierluigi Gambetti

A neuropathologist at Case Western Reserve University who co-identified fatal familial insomnia as a prion disease and founded the United States National Prion Disease Pathology Surveillance Center.
Faculty page

Elio Lugaresi

(1926-2015). The neurologist at the University of Bologna who described the first case of fatal familial insomnia in 1986, recognizing the link between untreatable insomnia, dysautonomia, and selective thalamic degeneration.
Wikipedia - Wikidata

Pasquale Montagna

(1950-2010). A sleep neurologist at the University of Bologna who characterized the collapse of sleep architecture and the clinical course of the disease and co-authored its defining reviews.
Obituary

Piero Parchi

A neuropathologist at the University of Bologna who mapped the regional distribution of protease-resistant prion protein in fatal familial insomnia and helped build the molecular classification of human prion diseases.
ORCID - Faculty page

Federica Provini

A sleep-medicine neurologist at the University of Bologna who analyzed the autonomic dysfunction and the agrypnia excitata of fatal familial insomnia.
ORCID - Faculty page

Stanley B. Prusiner

A neurologist at the University of California, San Francisco who won the 1997 Nobel Prize in Physiology or Medicine for the discovery of prions, the misfolded proteins that cause fatal familial insomnia and the other transmissible spongiform encephalopathies.
Wikipedia - Wikidata - Faculty page

Frequently Asked Questions

What is fatal familial insomnia?

It is a rare inherited prion disease in which a mutation in the prion protein gene causes the progressive destruction of the sleep-regulating nuclei of the thalamus, leading to an untreatable loss of sleep, severe autonomic dysfunction, cognitive decline, and death (Lugaresi et al., 1986).

What causes the disease?

It is caused by the D178N mutation in the PRNP gene, but only when the mutant allele also carries methionine at codon 129. The mutation makes the prion protein misfold, and the abnormal protein then spreads through the brain by converting normal protein (Medori et al., 1992).

Why does the same mutation sometimes cause a different disease?

The codon 129 polymorphism acts as a switch. With methionine on the mutant allele the D178N mutation produces fatal familial insomnia; with valine it produces a familial form of Creutzfeldt-Jakob disease instead (Goldfarb et al., 1992).

Why does it specifically destroy sleep?

The disease damages the anterior and dorsomedial nuclei of the thalamus, the structures that help generate sleep. As they are lost, sleep spindles and slow-wave sleep can no longer be produced, and normal sleep architecture disintegrates (Sforza et al., 1995).

How long do patients live after symptoms begin?

The course is always fatal but varies with the codon 129 genotype, from a matter of months in methionine homozygotes to a year or more in methionine-valine heterozygotes (Montagna et al., 2003).

Is the disease inherited, and what are the odds for children?

It is inherited in an autosomal dominant pattern, so each child of an affected parent has a one-half chance of inheriting the mutation. A rare sporadic form with the same features but no mutation also exists (Manetto et al., 1992).

How is it diagnosed?

Diagnosis combines the clinical picture of progressive insomnia, dysautonomia, and cognitive decline with genetic testing for the PRNP mutation and, where available, sleep studies and brain imaging, following formal consensus criteria (Wu et al., 2018).

Is there any treatment?

No treatment yet halts the disease, but because carriers can be identified before symptoms, strategies that lower prion protein production are being explored as preventive therapies, making the presymptomatic window the focus of current research (Cracco et al., 2018).

References

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Cortelli, P., Perani, D., Parchi, P., Grassi, F., Montagna, P., De Martin, M., Castellani, R., Tinuper, P., Gambetti, P., Lugaresi, E., & Fazio, F. (1997). Cerebral metabolism in fatal familial insomnia: Relation to duration, neuropathology, and distribution of protease-resistant prion protein. Neurology, 49(1), 126-133. https://doi.org/10.1212/WNL.49.1.126

Prusiner, S. B. (1998). Prions. Proceedings of the National Academy of Sciences of the United States of America, 95(23), 13363-13383. https://doi.org/10.1073/pnas.95.23.13363

Gambetti, P., Kong, Q., Zou, W., Parchi, P., & Chen, S. G. (2003). Sporadic and familial CJD: Classification and characterisation. British Medical Bulletin, 66(1), 213-239. https://doi.org/10.1093/bmb/66.1.213

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Wu, L. Y., Zhan, S. Q., Huang, Z. Y., Zhang, B., Wang, T., Liu, C. F., Lu, H., Dong, X. P., Wu, Z. Y., Yu, S. Y., et al. (2018). Expert consensus on clinical diagnostic criteria for fatal familial insomnia. Chinese Medical Journal, 131(13), 1613-1617. https://doi.org/10.4103/0366-6999.235115

Cracco, L., Appleby, B. S., & Gambetti, P. (2018). Fatal familial insomnia and sporadic fatal insomnia. Handbook of Clinical Neurology, 153, 271-299. https://doi.org/10.1016/B978-0-444-63945-5.00015-5

Baldelli, L., & Provini, F. (2019). Fatal familial insomnia and agrypnia excitata: Autonomic dysfunctions and pathophysiological implications. Autonomic Neuroscience: Basic and Clinical, 218, 68-86. https://doi.org/10.1016/j.autneu.2019.02.007