Infectious Disease and Microbiology – Prions

Overview

Prions are unusual infectious agents composed primarily of abnormally folded protein rather than conventional microorganisms. Unlike bacteria, viruses, fungi, and parasites, prions contain no known nucleic acid genome yet are capable of transmitting disease by inducing normally folded host proteins to adopt an abnormal conformation.

Prion diseases are collectively known as transmissible spongiform encephalopathies (TSEs). They are characterized by very long incubation periods, progressive neurodegeneration, spongiform changes in the brain, minimal inflammatory response, and ultimately death.

Classification

Genus: Not applicable

Species: Not applicable

Agent: Proteinaceous infectious particle (prion)

Major target organ: Central nervous system

Prions are fundamentally different from conventional infectious organisms because they lack:

DNA

and

RNA

Microbiologic Characteristics

Prions consist of an abnormal form of a host protein known as:

Prion protein (PrP)

The normal cellular form is commonly designated:

PrPᶜ

The abnormal disease-associated form has traditionally been designated:

PrPˢᶜ

The abnormal protein can promote conformational conversion of normal prion protein into additional abnormal molecules.

Basic Pathogenesis

Normal PrPᶜ

↓

Contact with abnormal prion protein

↓

Protein misfolding

↓

Conversion to abnormal prion conformation

↓

Accumulation and propagation of abnormal protein

↓

Neuronal dysfunction and degeneration

↓

Spongiform encephalopathy

High-Yield Microbiology Pattern

Infectious protein

• ●

No DNA or RNA

• ●

Progressive neurodegeneration

• ●

Spongiform brain changes

→ Think PRION DISEASE

Incubation Period

Prion diseases characteristically have:

Very long incubation periods

The interval between acquisition and clinical disease may extend for:

Years or even decades

depending on the specific prion disease and route of acquisition.

Once neurologic manifestations become clinically apparent, however, many prion diseases progress relentlessly.

Pathology

The characteristic neuropathologic process includes:

• Spongiform degeneration

• Neuronal loss

• Accumulation of abnormal prion protein

• Gliosis

• Minimal or absent conventional inflammatory response

The microscopic vacuolation gives affected brain tissue a:

Sponge-like appearance

hence the term:

Spongiform encephalopathy

Important Pathology Pattern

Rapid or progressive neurologic deterioration

• ●

Spongiform degeneration

• ●

Scant inflammatory response

→ Strongly suggests prion disease

Major Human Prion Diseases

Important human prion diseases include:

• Creutzfeldt–Jakob disease (CJD)

• Variant Creutzfeldt–Jakob disease (vCJD)

• Kuru

• Gerstmann–Sträussler–Scheinker syndrome (GSS)

• Fatal familial insomnia (FFI)

Human prion diseases can arise through:

Sporadic

Inherited

or

Acquired

mechanisms.

Sporadic Creutzfeldt–Jakob Disease

Sporadic CJD is the most common form of human prion disease.

It occurs without an identifiable external exposure or inherited pathogenic variant.

Typical manifestations include:

• Rapidly progressive dementia

• Myoclonus

• Ataxia

• Behavioral or cognitive changes

• Visual disturbances

• Pyramidal or extrapyramidal abnormalities

• Progression to severe neurologic disability

Classic CJD Pattern

Rapidly progressive dementia

• ●

Myoclonus

• ●

Ataxia

• ●

Characteristic MRI/EEG or CSF findings

→ Think Creutzfeldt–Jakob disease

Familial Prion Disease

Some prion diseases result from pathogenic variants involving:

PRNP

the gene encoding prion protein.

Inherited forms include:

• Familial CJD

• Gerstmann–Sträussler–Scheinker syndrome

• Fatal familial insomnia

Variant Creutzfeldt–Jakob Disease

Variant CJD (vCJD) became a major public health concern because of its association with exposure to the agent responsible for:

Bovine spongiform encephalopathy (BSE)

also known as:

“Mad cow disease.”

Variant CJD is distinct from the much more common sporadic form of CJD.

High-Yield Association

Bovine spongiform encephalopathy

→ Human exposure

→ Variant Creutzfeldt–Jakob disease

Kuru

Kuru is an acquired human prion disease historically identified among the Fore people of Papua New Guinea.

Transmission was associated with ritual mortuary practices involving consumption of tissues from deceased individuals.

The disease became a landmark example demonstrating the:

Transmissibility of prion disease

Gerstmann–Sträussler–Scheinker Syndrome

Gerstmann–Sträussler–Scheinker syndrome (GSS) is a rare:

Inherited prion disease

It typically produces progressive neurologic deterioration, often with prominent:

Cerebellar ataxia

and later cognitive decline.

Fatal Familial Insomnia

Fatal familial insomnia (FFI) is an inherited prion disorder characterized by prominent involvement of the:

Thalamus

The syndrome includes progressively severe:

Insomnia

along with autonomic and neurologic abnormalities.

The source’s description of:

“Insomnia secondary to thalamic destruction”

corresponds particularly well to this disorder.

High-Yield Pattern

Progressive severe insomnia

• ●

Autonomic dysfunction

• ●

Thalamic degeneration

• ●

Family history

→ Think Fatal familial insomnia

Clinical Course

Despite differences between individual prion diseases, the overall pattern is:

Long incubation

↓

Clinical neurologic manifestations begin

↓

Progressive neuronal degeneration

↓

Worsening cognitive and motor dysfunction

↓

Severe neurologic disability

↓

Death

Prion diseases are currently considered progressive and fatal.

Inflammatory Response

Unlike many infectious diseases, prion disease generally produces:

Little conventional inflammatory response

This is an important pathologic characteristic.

Thus:

Profound neurodegeneration

can occur despite:

Scant inflammation

Diagnosis

Modern diagnosis of suspected prion disease uses a combination of:

• Clinical presentation

• Brain MRI

• Cerebrospinal fluid testing

• EEG in appropriate cases

• Definitive neuropathologic evaluation when available

CSF RT-QuIC

An especially important modern diagnostic test is:

RT-QuIC

(real-time quaking-induced conversion)

This assay detects the ability of abnormal prion protein in a patient’s specimen to induce conformational changes in substrate prion protein.

CSF RT-QuIC has become an important test for supporting the diagnosis of:

Sporadic CJD

High-Yield Diagnostic Update

Suspected CJD

→ MRI brain

• ●

CSF RT-QuIC

with other clinical and laboratory findings used to establish diagnostic probability.

CSF 14-3-3 Protein

The source lists:

CSF 14-3-3 protein

as a diagnostic finding.

Elevated CSF 14-3-3 can support the diagnosis in an appropriate clinical setting, but it reflects:

Rapid neuronal injury

and is therefore not specific for prion disease.

It can be elevated in other neurologic disorders associated with extensive neuronal damage.

MRI

Brain MRI is extremely useful in suspected CJD.

Characteristic abnormalities may involve:

Cerebral cortex

and

Basal ganglia

with diffusion abnormalities being particularly important.

A classic MRI description in sporadic CJD is:

Cortical ribboning

High-Yield MRI Pattern

Rapidly progressive dementia

• ●

Myoclonus

• ●

Cortical ribboning on diffusion-weighted MRI

→ Strongly consider CJD

EEG

EEG may demonstrate characteristic abnormalities in some patients with sporadic CJD, classically:

Periodic sharp-wave complexes

However, EEG findings are not present in every patient and are interpreted together with MRI, CSF, and clinical findings.

Direct Visualization

Older descriptions may emphasize direct visualization of prion-associated material by:

Electron microscopy

However, electron microscopy is not the routine modern diagnostic approach for suspected CJD.

Modern evaluation relies much more heavily on:

MRI + CSF RT-QuIC + clinical findings

with neuropathology providing definitive confirmation in appropriate circumstances.

Neuropathology

Brain tissue examination may demonstrate:

• Spongiform change

• Neuronal loss

• Gliosis

• Abnormal prion protein deposition

Detection of disease-associated prion protein can provide strong diagnostic evidence.

Treatment

There is currently:

No established curative treatment

for human prion diseases.

Management is primarily:

Supportive and palliative

Supportive Management

Care may include management of:

• Myoclonus

• Pain or discomfort

• Psychiatric or behavioral manifestations

• Sleep disturbances

• Nutritional needs

• Mobility impairment

• Progressive neurologic disability

Because disease is progressive, supportive care and planning become increasingly important.

Infection-Control Importance

Prions are unusually resistant to many conventional:

Disinfection and sterilization procedures

This is especially relevant for instruments that contact:

High-infectivity nervous system tissues

Standard sterilization procedures may not be sufficient under circumstances involving suspected prion contamination, so specialized infection-control protocols are required.

Prions vs. Viruses

Prions

→ Protein only

→ No known DNA or RNA genome

→ Propagate through protein conformational conversion

→ Cause spongiform neurodegeneration

→ Extremely long incubation possible

Viruses

→ Contain DNA or RNA

→ Encode viral genetic information

→ Replicate using host-cell machinery

→ Produce a much broader spectrum of disease

CJD vs. Alzheimer Disease

Creutzfeldt–Jakob disease

→ Usually rapidly progressive cognitive decline

→ Myoclonus common

→ Ataxia may occur

→ Characteristic MRI abnormalities

→ CSF RT-QuIC may be positive

→ Progression often occurs over months

Alzheimer disease

→ Usually slowly progressive over years

→ Memory impairment prominent early

→ Myoclonus is not a typical early feature

→ Different pathologic protein abnormalities

Thus, the speed of progression is an important clinical clue.

CJD vs. Autoimmune Encephalitis

Both can cause:

Rapidly progressive cognitive and neurologic deterioration

However, autoimmune encephalitis is particularly important in the differential because some forms are:

Potentially treatable

Therefore, suspected CJD requires careful evaluation for alternative causes of rapidly progressive dementia.

Major Prion Disease Associations

Sporadic CJD

→ Most common human prion disease

Variant CJD

→ Associated with BSE

Kuru

→ Historical ritual exposure in Papua New Guinea

Gerstmann–Sträussler–Scheinker syndrome

→ Inherited, often prominent ataxia

Fatal familial insomnia

→ Inherited, prominent insomnia and thalamic degeneration

High-Yield Clinical Pattern

Rapidly progressive dementia

• ●

Myoclonus

• ●

Ataxia

• ●

Cortical ribboning on MRI

• ●

Positive CSF RT-QuIC

→ Think CREUTZFELDT–JAKOB DISEASE

Exam Essentials

Agent: Prion

Composition: Abnormally folded proteinaceous infectious material

Nucleic acid: Absent

Mechanism: Induces misfolding of normal host prion protein

Disease group: Transmissible spongiform encephalopathies

Incubation: Usually very long—potentially years to decades

Target: Central nervous system

Pathology: Spongiform degeneration + neuronal loss + gliosis

Inflammation: Characteristically minimal

Course: Progressive and ultimately fatal

Most common human prion disease: Sporadic CJD

CJD clue: Rapidly progressive dementia + myoclonus

Important MRI clue: Cortical ribboning

Important modern CSF test: RT-QuIC

Older/supportive CSF marker: 14-3-3 protein

Classic EEG: Periodic sharp-wave complexes

BSE association: Variant CJD

Papua New Guinea association: Kuru

Inherited ataxic syndrome: Gerstmann–Sträussler–Scheinker syndrome

Thalamic degeneration + insomnia: Fatal familial insomnia

Treatment: No established curative therapy; supportive care

Infection control: Prions have unusual resistance to conventional decontamination procedures

Key clinical pearl: Prions are infectious misfolded proteins that contain no nucleic acid yet propagate by inducing abnormal folding of normal host prion protein. The classic clinical syndrome is a rapidly progressive, fatal neurodegenerative disorder with spongiform brain changes and minimal inflammation; for sporadic CJD, high-yield modern diagnostic clues include cortical ribboning on MRI and a positive CSF RT-QuIC assay.