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A transgene carrying an A2G missense mutation in the SMN gene modulates phenotypic severity in mice with severe (type I) spinal muscular atrophy.

Monani UR, Pastore MT, Gavrilina TO, Jablonka S, Le TT, Andreassi C, DiCocco JM, Lorson C, Androphy EJ, Sendtner M, Podell M, Burghes AH - J. Cell Biol. (2003)

Bottom Line: We suggest that only in the presence of low levels of full-length SMN is the A2G transgene able to form partially functional higher order SMN complexes essential for its functions.Animals homozygous for the mutant transgene are less severely affected than heterozygotes.This demonstrates the importance of SMN levels in SMA even if the protein is expressed from a mutant allele.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, Ohio State University, Columbus, OH 43210, USA. monani.2@osu.edu

ABSTRACT
5q spinal muscular atrophy (SMA) is a common autosomal recessive disorder in humans and the leading genetic cause of infantile death. Patients lack a functional survival of motor neurons (SMN1) gene, but carry one or more copies of the highly homologous SMN2 gene. A homozygous knockout of the single murine Smn gene is embryonic lethal. Here we report that in the absence of the SMN2 gene, a mutant SMN A2G transgene is unable to rescue the embryonic lethality. In its presence, the A2G transgene delays the onset of motor neuron loss, resulting in mice with mild SMA. We suggest that only in the presence of low levels of full-length SMN is the A2G transgene able to form partially functional higher order SMN complexes essential for its functions. Mild SMA mice exhibit motor neuron degeneration, muscle atrophy, and abnormal EMGs. Animals homozygous for the mutant transgene are less severely affected than heterozygotes. This demonstrates the importance of SMN levels in SMA even if the protein is expressed from a mutant allele. Our mild SMA mice will be useful in (a) determining the effect of missense mutations in vivo and in motor neurons and (b) testing potential therapies in SMA.

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Related in: MedlinePlus

A molecular model of SMA that may explain the motor neuron specificity of the disease. A protein with a low affinity for SMN expressed selectively in the motor neurons would be unable to bind and form a functional complex in these cells in an SMA patient. This would result in degeneration and loss of motor neurons but not other tissues.
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fig7: A molecular model of SMA that may explain the motor neuron specificity of the disease. A protein with a low affinity for SMN expressed selectively in the motor neurons would be unable to bind and form a functional complex in these cells in an SMA patient. This would result in degeneration and loss of motor neurons but not other tissues.

Mentions: It is intriguing to consider how the A2G mutation on an SMN2;Smn−/− background results in a mild SMA phenotype. The mutation is the most 5′ one reported so far. It is neither in the oligomerization domain in exon 6 nor in exon 2b. It is not in the Sm protein binding domain nor does it affect SIP-1 binding (unpublished data). Yet, we have shown that the A2G mutation does disrupt self-association and affects SmN binding presumably by disrupting the formation of SMN oligomers. A to G mutations have been shown to profoundly affect α-helices (Lyu et al., 1990; Chakrabartty et al., 1991). It is not unlikely that the mutation disrupts the three-dimensional structure of the protein enough to affect the oligomerization domains in exon 2b and/or exon 6 as well as the Sm binding domain. Although the A2G protein retains some level of self-association activity, it is possible this deficiency results in that mutant's inability to form higher order complexes necessary for SMN function and interactions with protein partners. This could explain why SMN A2G does not rescue Smn−/− embryonic lethality. If, however, full-length SMN serves as a scaffold, then low levels of the protein might promote the formation of higher order FL-SMN:SMN A2G oligomers with an enhanced ability to bind other interacting proteins such as SmN. Our data show that SMN A2G binds with a higher affinity to FL-SMN than does Δ7 SMN. FL-SMN:SMN A2G complexes would therefore be more stable than FL-SMN:Δ7 SMN ones. Furthermore, it has been shown that protein levels in patients with missense mutations correlate with the disease phenotype (Lefebvre et al., 1997). These data and our observations lead us to a plausible molecular model of SMA (Fig. 7) in which SMN forms a number of complexes with different partners. These partners compete for the same sites on SMN, binding being determined by their relative affinities for SMN. In the presence of low levels of full-length SMN, low affinity binding partners are out-competed by high affinity proteins. It is possible that the low affinity binding partners are selectively found in motor neurons and are unable to form functional complexes in SMA patients due to the low SMN levels. Loss or disruption of the low affinity complex in motor neurons would result in the degeneration of these, but not other, tissues. Our model is consistent with the observation of increased SMN levels and a mild phenotype in mice carrying the A2G missense mutation on a severe SMA genetic background.


A transgene carrying an A2G missense mutation in the SMN gene modulates phenotypic severity in mice with severe (type I) spinal muscular atrophy.

Monani UR, Pastore MT, Gavrilina TO, Jablonka S, Le TT, Andreassi C, DiCocco JM, Lorson C, Androphy EJ, Sendtner M, Podell M, Burghes AH - J. Cell Biol. (2003)

A molecular model of SMA that may explain the motor neuron specificity of the disease. A protein with a low affinity for SMN expressed selectively in the motor neurons would be unable to bind and form a functional complex in these cells in an SMA patient. This would result in degeneration and loss of motor neurons but not other tissues.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172739&req=5

fig7: A molecular model of SMA that may explain the motor neuron specificity of the disease. A protein with a low affinity for SMN expressed selectively in the motor neurons would be unable to bind and form a functional complex in these cells in an SMA patient. This would result in degeneration and loss of motor neurons but not other tissues.
Mentions: It is intriguing to consider how the A2G mutation on an SMN2;Smn−/− background results in a mild SMA phenotype. The mutation is the most 5′ one reported so far. It is neither in the oligomerization domain in exon 6 nor in exon 2b. It is not in the Sm protein binding domain nor does it affect SIP-1 binding (unpublished data). Yet, we have shown that the A2G mutation does disrupt self-association and affects SmN binding presumably by disrupting the formation of SMN oligomers. A to G mutations have been shown to profoundly affect α-helices (Lyu et al., 1990; Chakrabartty et al., 1991). It is not unlikely that the mutation disrupts the three-dimensional structure of the protein enough to affect the oligomerization domains in exon 2b and/or exon 6 as well as the Sm binding domain. Although the A2G protein retains some level of self-association activity, it is possible this deficiency results in that mutant's inability to form higher order complexes necessary for SMN function and interactions with protein partners. This could explain why SMN A2G does not rescue Smn−/− embryonic lethality. If, however, full-length SMN serves as a scaffold, then low levels of the protein might promote the formation of higher order FL-SMN:SMN A2G oligomers with an enhanced ability to bind other interacting proteins such as SmN. Our data show that SMN A2G binds with a higher affinity to FL-SMN than does Δ7 SMN. FL-SMN:SMN A2G complexes would therefore be more stable than FL-SMN:Δ7 SMN ones. Furthermore, it has been shown that protein levels in patients with missense mutations correlate with the disease phenotype (Lefebvre et al., 1997). These data and our observations lead us to a plausible molecular model of SMA (Fig. 7) in which SMN forms a number of complexes with different partners. These partners compete for the same sites on SMN, binding being determined by their relative affinities for SMN. In the presence of low levels of full-length SMN, low affinity binding partners are out-competed by high affinity proteins. It is possible that the low affinity binding partners are selectively found in motor neurons and are unable to form functional complexes in SMA patients due to the low SMN levels. Loss or disruption of the low affinity complex in motor neurons would result in the degeneration of these, but not other, tissues. Our model is consistent with the observation of increased SMN levels and a mild phenotype in mice carrying the A2G missense mutation on a severe SMA genetic background.

Bottom Line: We suggest that only in the presence of low levels of full-length SMN is the A2G transgene able to form partially functional higher order SMN complexes essential for its functions.Animals homozygous for the mutant transgene are less severely affected than heterozygotes.This demonstrates the importance of SMN levels in SMA even if the protein is expressed from a mutant allele.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, Ohio State University, Columbus, OH 43210, USA. monani.2@osu.edu

ABSTRACT
5q spinal muscular atrophy (SMA) is a common autosomal recessive disorder in humans and the leading genetic cause of infantile death. Patients lack a functional survival of motor neurons (SMN1) gene, but carry one or more copies of the highly homologous SMN2 gene. A homozygous knockout of the single murine Smn gene is embryonic lethal. Here we report that in the absence of the SMN2 gene, a mutant SMN A2G transgene is unable to rescue the embryonic lethality. In its presence, the A2G transgene delays the onset of motor neuron loss, resulting in mice with mild SMA. We suggest that only in the presence of low levels of full-length SMN is the A2G transgene able to form partially functional higher order SMN complexes essential for its functions. Mild SMA mice exhibit motor neuron degeneration, muscle atrophy, and abnormal EMGs. Animals homozygous for the mutant transgene are less severely affected than heterozygotes. This demonstrates the importance of SMN levels in SMA even if the protein is expressed from a mutant allele. Our mild SMA mice will be useful in (a) determining the effect of missense mutations in vivo and in motor neurons and (b) testing potential therapies in SMA.

Show MeSH
Related in: MedlinePlus