Genetics of Childhood Disorders: myasthenia gravis and Rasmussen’s encephalitis

[Guest guest]

J Am Acad Child Adolesc Psychiatry,40:9,1115-1117 September 2001

Genetics of Childhood Disorders:

Autoimmune Disorders

Myasthenia Gravis and Rasmussen’s encephalitis

J. Lombroso, M.D., and Marcos T. Mercadante, M.D., Ph.D.

How can autoimmunity cause illness? Over the years, immunologists have

debated this issue regarding many different diseases. The consensus today is

that several criteria must be fulfilled to demonstrate an autoimmune

etiology. For those disorders in which a humoral etiology is proposed,

autoantibodies should be found in the sera of affected individuals. In

addition, removal of these autoantibodies should lead to an improvement of

symptoms. Finally, transfer of the autoantibodies to an experimental animal

should reproduce some of the clinical symptoms of the disorder. In this

column, we review the evidence that myasthenia gravis and Rasmussen’s

encephalitis are autoimmune disorders. Similar strategies will need to be

used by investigators attempting to demonstrate that illnesses such as

Tourette’s syndrome and obsessive-

compulsive disorder are caused by an autoimmune mechanism.

Myasthenia gravis is a relatively rare neurological illness that affects

approximately 1 of every 200,000 individuals. A key clinical finding is

muscle weakness, especially after repeated activity. Although the course of

the illness is highly variable, the disease typically affects eye and eyelid

muscles leading to double vision and ptosis. In more severe cases, muscles

controlling swallowing, speaking, and breathing become affected.

The typical treatment is to administer inhibitors of acetylcholinesterase.

Acetylcholine is a neurotransmitter normally found at the neuromuscular

junction. To initiate a muscle contraction, the nerve terminal releases the

neurotransmitter acetylcholine into the synaptic cleft. The neurotransmitter

rapidly diffuses across and binds to acetylcholine receptors on the surface

of the muscle cell. The binding of neurotransmitter to its receptor leads to

a rapid change in ion flow across the membrane, which in turn propagates a

muscle contraction.

Acetylcholine must be rapidly removed from the synaptic cleft to allow for

multiple signals to arrive at the muscle and to allow for repeated

contractions in sustained muscle activity. The rapid removal is accomplished

in part by the action of acetylcholinesterase that degrades the

neurotransmitter. The finding that drugs that inhibit this enzyme improved

the muscle weakness in patients with myasthenia gravis suggested that the

illness was caused by a disturbance in some component of the acetylcholine

signaling pathway.

Little progress was made, however, until the chance observations of two

investigators at the Salk Institute in 1973. Jim and Jon Lindstrom

were interested in determining the location of the nicotinic acetylcholine

receptor within the CNS. In order to do this, they wanted to generate

antibodies against the receptor that could be used in immunocytochemical

localization studies. Generating antibodies is a time-honored procedure by

which investigators produce a probe that will bind to and help visualize a

specific protein under investigation.

Typically, rabbits are used to produce such antibodies. When a protein or a

portion of the protein is injected into a rabbit, the animal will mount a

humoral response against the foreign antigen. Large amounts of antibodies

are thereby produced that are capable of recognizing different portions of

the injected protein. Repeated boosts with the antigen lead to the

production of large amounts of antibodies in the sera of these animals. The

antibodies can then be separated from other components of the sera and used

in immunocytochemical studies within the CNS.

Unexpectedly, the rabbits that were immunized with the acetylcholine

receptor developed severe muscle weakness. and Lindstrom saw a

similarity between the animals’ behavior and the weakness seen in patients

with myasthenia gravis. When they treated the animals with an

acetylcholinesterase inhibitor, the rabbits got better. For the first time,

the specific hypothesis that the acetylcholine receptor was the target of

autoantibodies could be tested.

Relatively quickly, researchers established that the sera of patients with

myasthenia gravis contained antibodies and that these antibodies recognized

a subunit of the acetylcholine receptor complex of proteins (Fig. 1). This

was an important first step. It is currently believed that the immunogenic

portion of the acetylcholine receptor lies on one of the five subunits that

assemble to form the receptor. The a subunit is not only the site for

binding of acetylcholine, but it also contains the amino acid sequence that

elicits the antibody reaction. When the autoantibody binds to the receptor,

it is believed that the receptor is internalized by the muscle cell and

degraded. In some rare cases, the binding of autoantibody to the a subunit

blocks access of acetylcholine to its binding site. The binding of

autoantibodies, however, initiates a series of events that promotes

complement binding and focal lysis of the postsynaptic membrane. The net

result is that there are fewer functional acetylcholine receptors at the

neuromuscular junction.Muscle cells are innervated by axons with terminal

arbors that form synapses at structures called neuromuscular junctions. The

neuromuscular junction is composed of a nerve terminal that releases

acetylcholine neurotransmitter and the postsynaptic component on the muscle

cell where the acetylcholine receptors are found. Autoantibodies produced in

myasthenia gravis recognize amino acid sequences on the a subunit of the

acetylcholine receptor. Binding of the antibody to the receptor is thought

to lead to its internalization, leading to a loss of functional receptors at

the neuromuscular junction, as well as the eventual attack of the muscle

cell by lymphocytes.}]

If true, this decline in receptors would explain why treatment with

inhibitors of acetylcholinesterase is effective. The increase in the

relative amount of neurotransmitter at the neuromuscular junction

compensates for the loss of functional receptors. Removal of the antibodies

from sera of affected individuals leads to improvement of clinical symptoms.

In fact, plasmapheresis has now become a standard treatment for myasthenia

gravis.

It is interesting how history repeats. Twenty years later, in the same

neurobiology department at the Salk Institute, investigators discovered the

target of a second autoimmune disorder. Once again they did so by immunizing

rabbits with a receptor. This time they were interested in localizing one of

the glutamate receptors within the CNS. Surprisingly, some of the immunized

rabbits developed intractable seizures and neuropathological brain lesions

indistinguishable from those seen in humans with Rasmussen’s encephalitis.

Rasmussen’s encephalitis is a rare form of epilepsy that is associated with

progressive neurological dysfunction and destruction of a single cerebral

hemisphere. The disorder usually begins during the first decade of life with

the appearance of seizures, hemiparesis, and severe cognitive and language

impairments. The seizures are often unresponsive to standard antiseizure

medications, and surgical removal of the affected hemisphere is the standard

treatment. It was recently discovered that the etiology of this devastating

disease is an autoimmune response to one of the glutamate receptors, GluR3.

The glutamate neurotransmitter system has captured the attention of so many

investigators for several reasons. Glutamate is the most abundant

transmitter within the CNS. It has been implicated in a wide range of

complex neuronal processes including development, apoptosis, learning, and

memory. Its capacity to participate in these processes is due to its ability

to stimulate a wide variety of intracellular signals, and this is due in

part to the large number of distinct receptors through which glutamate acts.

There are at least 20 different genes that encode for glutamate receptors.

These receptors can be classified into two broad groups: the metabotropic

receptors and the ionotropic receptors. The metabotropic receptors are

members of the G-coupled family of receptors that are membrane-associated

proteins capable of stimulating a cascade of intracellular pathways when

activated. The ionotropic receptors also bind glutamate directly, but these

receptors act as ion channels. Binding to glutamate rapidly changes ion

currents across the cell membrane. The ionotropic receptors can be further

divided through their distinctive response to pharmacological reagents that

activate them: NMDA (N-methyl-D-aspartate), AMPA

(a-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic acid), and kainic acid.

The ionotropic glutamate receptors are formed by several subunits that

associate with each other to form a pore through the cell membrane. Each

receptor subtype is formed by combinations of five protein subunits. In this

way, a large number of receptor subtypes can be constructed. It is in fact

the varying subunit compositions that determine exactly which ions are

allowed through the channels. The central pore formed in AMPA/kainate

receptors primarily allows passage of Na+ and K+ ions and in some cases

small amounts of Ca++, while activation of NMDA receptors results in the

influx of large calcium currents.

Once it was discovered that many different subtypes of glutamate receptors

existed, investigators became interested in determining their localization

patterns. Why were there so many different glutamate receptors, and could

localizing them within the CNS clarify their varying functions? The group at

the Salk was particularly interested in studying the GluR3 receptor, a

subtype of the AMPA family. Rabbits were immunized with the GluR3 protein in

order to make specific antibodies against the receptor. As mentioned above,

several of the immunized rabbits developed intractable seizures and, on

histopathological examination, the brains showed a similar pattern of

perivascular lymphocytic infiltrate and microglial nodules that is commonly

seen in Rasmussen’s encephalitis.

This initial study suggested that perhaps humans with Rasmussen’s

encephalitis developed their illness as a consequence of an autoimmune

response directed against the GluR3 receptor. Investigators quickly

determined that anti-GluR3 antibodies were in fact present in the sera of

affected individuals. In addition, several patients responded dramatically

to the removal of the autoantibodies through plasmapheresis; this finding

suggests that the autoantibodies contributed at least in part to the

progressive neuronal loss and hemispheric atrophy so characteristic of the

disease. Attempts to create an animal model have not been successful to

date.

One question that has puzzled investigators is the fact that tissue

destruction occurs in only one cortical hemisphere. Circulating autoantibody

that crosses the blood-brain barrier should not show unilateral specificity,

especially as the antigen, the GluR3 receptor, is expressed at many sites in

both cortices. A proposed model is that the illness is initiated through a

local traumatic event (such as a blow) to the head that disrupts the

blood-brain barrier in a very limited region. Autoantibody is then able to

leak into a limited area of a single hemisphere.

Exactly how the autoantibody causes cellular damage is also a matter of

considerable debate. One hypothesis is that the GluR3 autoantibody is

excitotoxic. This would occur if the binding of the antibody to the

glutamate receptor activates the receptor and leads to a massive influx of

ions. Activation of glutamate receptors is a well-known mechanism that

precedes neuronal cell death. Lymphocytic infiltration then occurs and

causes local inflammation and a further disruption of the blood-brain

barrier which permits entrance of additional damaging autoantibodies.

A competing hypothesis is that the autoantibody that binds to GluR3

receptors attracts specific components of the complement system. Complement

cascades are activated and lead to neuronal death and lymphocytic

infiltrations. Both hypotheses are being tested and would explain the

progressive neuronal death that occurs in this degenerative seizure

disorder.

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