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Mutations in LAMB2 causing a severe form of synaptic congenital myasthenic syndrome.

Maselli RA, Ng JJ, Anderson JA, Cagney O, Arredondo J, Williams C, Wessel HB, Abdel-Hamid H, Wollmann RL - J. Med. Genet. (2009)

Bottom Line: The findings included: pronounced reduction of the axon terminal size with encasement of the nerve endings by Schwann cells, severe widening of the primary synaptic cleft and invasion of the synaptic space by the processes of Schwann cells, and moderate simplification of postsynaptic folds and intact expression of the endplate acetylcholinesterase.The endplate potential quantal content was notably reduced, while the frequencies and amplitudes of miniature endplate potentials were only moderately diminished and the decay phases of miniature endplate potentials were normal.This case, which represents a new type of synaptic CMS, exemplifies the wide variability of phenotypes associated with LAMB2 mutations and underscores the fundamental role that laminin beta2 plays in the development of the human neuromuscular junction.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, University of California Davis, Davis, CA, 95618, USA. ramaselli@ucdavis.edu

ABSTRACT

Background: We describe a severe form of congenital myasthenic syndrome (CMS) associated with congenital nephrosis and ocular malformations caused by two truncating mutations in the gene encoding the laminin beta2 subunit (LAMB2).

Methods and results: Mutational analysis in the affected patient, who has a history of a serious untoward reaction to treatment with acetylcholinesterase inhibition, revealed two frame-shifting heteroallelic mutations, a maternally inherited 1478delG and a paternally inherited 4804delC. An anconeus muscle biopsy demonstrated a profound distortion of the architecture and function of the neuromuscular junction, which was strikingly similar to that seen in mice lacking laminin beta2 subunit. The findings included: pronounced reduction of the axon terminal size with encasement of the nerve endings by Schwann cells, severe widening of the primary synaptic cleft and invasion of the synaptic space by the processes of Schwann cells, and moderate simplification of postsynaptic folds and intact expression of the endplate acetylcholinesterase. The endplate potential quantal content was notably reduced, while the frequencies and amplitudes of miniature endplate potentials were only moderately diminished and the decay phases of miniature endplate potentials were normal. Western blot analysis of muscle and kidney tissue and immunohistochemistry of kidney tissue showed no laminin beta2 expression.

Conclusion: This case, which represents a new type of synaptic CMS, exemplifies the wide variability of phenotypes associated with LAMB2 mutations and underscores the fundamental role that laminin beta2 plays in the development of the human neuromuscular junction.

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Related in: MedlinePlus

Ultrastructural findings at the neuromuscular junction (NMJ). Control (A) and the patient (B–F). (A) Normal NMJ from a control demonstrating normal nerve terminal size and highly complex postsynaptic membrane folding with well formed secondary synaptic clefts. The arrow heads point to Schwann cell processes, which cap the nerve terminal (N), without extending into the synaptic cleft. The arrows point to the primary synaptic cleft (top) and at a secondary synaptic cleft (bottom). The asterisk in A–C is placed over the Schwann cell. (B) Small nerve terminal partially encased by a Schwann cell process, which intrudes into the synaptic space. (C) Small nerve terminal retracted from the synaptic space and completely engulfed by the Schwann cell. Note also pronounced simplification of the postsynaptic membrane (arrow). (D) Bifurcated nerve terminal with one nerve ending completely engulfed (top arrowhead) and the other ending partially encased (bottom arrowhead) by the Schwann cell. Note also pronounced widening of the primary synaptic cleft and reduction of the density of synaptic vesicles in the nerve terminal. (E) Nerve terminal divided in three small endings, which are encased by the Schwann cell and retracted from the postsynaptic membrane. (F) Pronounced reduction of the area of apposition between the nerve terminal and the postsynaptic membrane, widening of primary synaptic cleft and invasion of the synaptic space by Schwann cell processes (horizontal arrowheads). The vertical arrowheads point to two active zones, which in contrast to the control, are not apposing postsynaptic secondary clefts. (G) and (H) Quantification of the area of apposition between the nerve and muscle. (G) Bar graph representing the average length of apposition between nerve and muscle in 11 controls and 11 patient endplates. (H) Percentages of direct nerve–muscle apposition relative to the total length of the synaptic cleft in 24 controls and 11 patient endplates (mean (SEM)). Calibration marks (A–F) represent 1 μm.
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jmg-46-03-0203-f01: Ultrastructural findings at the neuromuscular junction (NMJ). Control (A) and the patient (B–F). (A) Normal NMJ from a control demonstrating normal nerve terminal size and highly complex postsynaptic membrane folding with well formed secondary synaptic clefts. The arrow heads point to Schwann cell processes, which cap the nerve terminal (N), without extending into the synaptic cleft. The arrows point to the primary synaptic cleft (top) and at a secondary synaptic cleft (bottom). The asterisk in A–C is placed over the Schwann cell. (B) Small nerve terminal partially encased by a Schwann cell process, which intrudes into the synaptic space. (C) Small nerve terminal retracted from the synaptic space and completely engulfed by the Schwann cell. Note also pronounced simplification of the postsynaptic membrane (arrow). (D) Bifurcated nerve terminal with one nerve ending completely engulfed (top arrowhead) and the other ending partially encased (bottom arrowhead) by the Schwann cell. Note also pronounced widening of the primary synaptic cleft and reduction of the density of synaptic vesicles in the nerve terminal. (E) Nerve terminal divided in three small endings, which are encased by the Schwann cell and retracted from the postsynaptic membrane. (F) Pronounced reduction of the area of apposition between the nerve terminal and the postsynaptic membrane, widening of primary synaptic cleft and invasion of the synaptic space by Schwann cell processes (horizontal arrowheads). The vertical arrowheads point to two active zones, which in contrast to the control, are not apposing postsynaptic secondary clefts. (G) and (H) Quantification of the area of apposition between the nerve and muscle. (G) Bar graph representing the average length of apposition between nerve and muscle in 11 controls and 11 patient endplates. (H) Percentages of direct nerve–muscle apposition relative to the total length of the synaptic cleft in 24 controls and 11 patient endplates (mean (SEM)). Calibration marks (A–F) represent 1 μm.

Mentions: The most consistent structural abnormality of the NMJ shown by electron microscopy were: (1) small axon terminal size and encasement of nerve endings by the Schwann cell; (2) severe widening of the primary synaptic clefts with invasion of the synaptic space by processes of Schwann cells; and (3) moderate simplification of the postsynaptic membranes (fig 1). The small nerve terminals, which often appeared divided into multiple very small segments and retracted from the postsynaptic membrane, were in all cases partially or completely encased by processes of the Schwann cells. As a result of the encasement of the nerve terminal and invasion of the synaptic space by the Schwann cell, the areas of apposition between the nerve terminal and the postsynaptic membrane were extremely small, and even in these areas where presynaptic membranes were directly apposed to postsynaptic membranes the primary synaptic cleft was notably widened. Occasional active zones were seen in the nerve terminal, but they were not consistently apposed to the secondary clefts, as seen in the controls. Some nerve terminals showed a relatively normal number of synaptic vesicles, while others showed few synaptic vesicles. Overall, there was a mild reduction in the density of synaptic vesicles compared to the controls. In the postsynaptic region, there was simplification of the postsynaptic membranes, which was variable and minor compared to the extreme hypoplasia of the axon terminals. There was no other postsynaptic abnormality, with the exception of two sub-sarcolemmal nuclei containing degenerating membranous debris.


Mutations in LAMB2 causing a severe form of synaptic congenital myasthenic syndrome.

Maselli RA, Ng JJ, Anderson JA, Cagney O, Arredondo J, Williams C, Wessel HB, Abdel-Hamid H, Wollmann RL - J. Med. Genet. (2009)

Ultrastructural findings at the neuromuscular junction (NMJ). Control (A) and the patient (B–F). (A) Normal NMJ from a control demonstrating normal nerve terminal size and highly complex postsynaptic membrane folding with well formed secondary synaptic clefts. The arrow heads point to Schwann cell processes, which cap the nerve terminal (N), without extending into the synaptic cleft. The arrows point to the primary synaptic cleft (top) and at a secondary synaptic cleft (bottom). The asterisk in A–C is placed over the Schwann cell. (B) Small nerve terminal partially encased by a Schwann cell process, which intrudes into the synaptic space. (C) Small nerve terminal retracted from the synaptic space and completely engulfed by the Schwann cell. Note also pronounced simplification of the postsynaptic membrane (arrow). (D) Bifurcated nerve terminal with one nerve ending completely engulfed (top arrowhead) and the other ending partially encased (bottom arrowhead) by the Schwann cell. Note also pronounced widening of the primary synaptic cleft and reduction of the density of synaptic vesicles in the nerve terminal. (E) Nerve terminal divided in three small endings, which are encased by the Schwann cell and retracted from the postsynaptic membrane. (F) Pronounced reduction of the area of apposition between the nerve terminal and the postsynaptic membrane, widening of primary synaptic cleft and invasion of the synaptic space by Schwann cell processes (horizontal arrowheads). The vertical arrowheads point to two active zones, which in contrast to the control, are not apposing postsynaptic secondary clefts. (G) and (H) Quantification of the area of apposition between the nerve and muscle. (G) Bar graph representing the average length of apposition between nerve and muscle in 11 controls and 11 patient endplates. (H) Percentages of direct nerve–muscle apposition relative to the total length of the synaptic cleft in 24 controls and 11 patient endplates (mean (SEM)). Calibration marks (A–F) represent 1 μm.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2643050&req=5

jmg-46-03-0203-f01: Ultrastructural findings at the neuromuscular junction (NMJ). Control (A) and the patient (B–F). (A) Normal NMJ from a control demonstrating normal nerve terminal size and highly complex postsynaptic membrane folding with well formed secondary synaptic clefts. The arrow heads point to Schwann cell processes, which cap the nerve terminal (N), without extending into the synaptic cleft. The arrows point to the primary synaptic cleft (top) and at a secondary synaptic cleft (bottom). The asterisk in A–C is placed over the Schwann cell. (B) Small nerve terminal partially encased by a Schwann cell process, which intrudes into the synaptic space. (C) Small nerve terminal retracted from the synaptic space and completely engulfed by the Schwann cell. Note also pronounced simplification of the postsynaptic membrane (arrow). (D) Bifurcated nerve terminal with one nerve ending completely engulfed (top arrowhead) and the other ending partially encased (bottom arrowhead) by the Schwann cell. Note also pronounced widening of the primary synaptic cleft and reduction of the density of synaptic vesicles in the nerve terminal. (E) Nerve terminal divided in three small endings, which are encased by the Schwann cell and retracted from the postsynaptic membrane. (F) Pronounced reduction of the area of apposition between the nerve terminal and the postsynaptic membrane, widening of primary synaptic cleft and invasion of the synaptic space by Schwann cell processes (horizontal arrowheads). The vertical arrowheads point to two active zones, which in contrast to the control, are not apposing postsynaptic secondary clefts. (G) and (H) Quantification of the area of apposition between the nerve and muscle. (G) Bar graph representing the average length of apposition between nerve and muscle in 11 controls and 11 patient endplates. (H) Percentages of direct nerve–muscle apposition relative to the total length of the synaptic cleft in 24 controls and 11 patient endplates (mean (SEM)). Calibration marks (A–F) represent 1 μm.
Mentions: The most consistent structural abnormality of the NMJ shown by electron microscopy were: (1) small axon terminal size and encasement of nerve endings by the Schwann cell; (2) severe widening of the primary synaptic clefts with invasion of the synaptic space by processes of Schwann cells; and (3) moderate simplification of the postsynaptic membranes (fig 1). The small nerve terminals, which often appeared divided into multiple very small segments and retracted from the postsynaptic membrane, were in all cases partially or completely encased by processes of the Schwann cells. As a result of the encasement of the nerve terminal and invasion of the synaptic space by the Schwann cell, the areas of apposition between the nerve terminal and the postsynaptic membrane were extremely small, and even in these areas where presynaptic membranes were directly apposed to postsynaptic membranes the primary synaptic cleft was notably widened. Occasional active zones were seen in the nerve terminal, but they were not consistently apposed to the secondary clefts, as seen in the controls. Some nerve terminals showed a relatively normal number of synaptic vesicles, while others showed few synaptic vesicles. Overall, there was a mild reduction in the density of synaptic vesicles compared to the controls. In the postsynaptic region, there was simplification of the postsynaptic membranes, which was variable and minor compared to the extreme hypoplasia of the axon terminals. There was no other postsynaptic abnormality, with the exception of two sub-sarcolemmal nuclei containing degenerating membranous debris.

Bottom Line: The findings included: pronounced reduction of the axon terminal size with encasement of the nerve endings by Schwann cells, severe widening of the primary synaptic cleft and invasion of the synaptic space by the processes of Schwann cells, and moderate simplification of postsynaptic folds and intact expression of the endplate acetylcholinesterase.The endplate potential quantal content was notably reduced, while the frequencies and amplitudes of miniature endplate potentials were only moderately diminished and the decay phases of miniature endplate potentials were normal.This case, which represents a new type of synaptic CMS, exemplifies the wide variability of phenotypes associated with LAMB2 mutations and underscores the fundamental role that laminin beta2 plays in the development of the human neuromuscular junction.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, University of California Davis, Davis, CA, 95618, USA. ramaselli@ucdavis.edu

ABSTRACT

Background: We describe a severe form of congenital myasthenic syndrome (CMS) associated with congenital nephrosis and ocular malformations caused by two truncating mutations in the gene encoding the laminin beta2 subunit (LAMB2).

Methods and results: Mutational analysis in the affected patient, who has a history of a serious untoward reaction to treatment with acetylcholinesterase inhibition, revealed two frame-shifting heteroallelic mutations, a maternally inherited 1478delG and a paternally inherited 4804delC. An anconeus muscle biopsy demonstrated a profound distortion of the architecture and function of the neuromuscular junction, which was strikingly similar to that seen in mice lacking laminin beta2 subunit. The findings included: pronounced reduction of the axon terminal size with encasement of the nerve endings by Schwann cells, severe widening of the primary synaptic cleft and invasion of the synaptic space by the processes of Schwann cells, and moderate simplification of postsynaptic folds and intact expression of the endplate acetylcholinesterase. The endplate potential quantal content was notably reduced, while the frequencies and amplitudes of miniature endplate potentials were only moderately diminished and the decay phases of miniature endplate potentials were normal. Western blot analysis of muscle and kidney tissue and immunohistochemistry of kidney tissue showed no laminin beta2 expression.

Conclusion: This case, which represents a new type of synaptic CMS, exemplifies the wide variability of phenotypes associated with LAMB2 mutations and underscores the fundamental role that laminin beta2 plays in the development of the human neuromuscular junction.

Show MeSH
Related in: MedlinePlus