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Question 222

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Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease affecting 1 in 10,000 live births.  The pathological hallmark of SMA is the loss of α-motor neurons in the spinal cord, which causes progressive paralysis and, ultimately, premature death in infants.  SMA is caused by insufficient levels of survival motor neuron (SMN) , a regulatory protein required for the maintenance of the spinal cord nerve cells that control voluntary muscle movement.The SMN protein is generated from the SMN1 and SMN2 genes, which differ in sequence by only a single nucleotide in exon 7.  In healthy individuals, the gene product of SMN1 is the full-length (FL) SMN protein, and the gene products of SMN2 are both the FL-SMN protein as well as a truncated unstable protein (SMNΔ7) that is rapidly degraded.  Approximately 90% of the SMN2 gene product is SMNΔ7, and only 10% is FL-SMN (Figure 1) .  In most patients with SMA, SMN1 is mutated such that its protein product is nonfunctional or deleted.  The disease phenotype arises because the SMN2 gene cannot fully compensate for the deficiency in FL-SMN protein synthesis.
Passage Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease affecting 1 in 10,000 live births.  The pathological hallmark of SMA is the loss of α-motor neurons in the spinal cord, which causes progressive paralysis and, ultimately, premature death in infants.  SMA is caused by insufficient levels of survival motor neuron (SMN) , a regulatory protein required for the maintenance of the spinal cord nerve cells that control voluntary muscle movement.The SMN protein is generated from the SMN1 and SMN2 genes, which differ in sequence by only a single nucleotide in exon 7.  In healthy individuals, the gene product of SMN1 is the full-length (FL) SMN protein, and the gene products of SMN2 are both the FL-SMN protein as well as a truncated unstable protein (SMNΔ7) that is rapidly degraded.  Approximately 90% of the SMN2 gene product is SMNΔ7, and only 10% is FL-SMN (Figure 1) .  In most patients with SMA, SMN1 is mutated such that its protein product is nonfunctional or deleted.  The disease phenotype arises because the SMN2 gene cannot fully compensate for the deficiency in FL-SMN protein synthesis. <strong>Figure 1</strong>  Structure of SMN1 and SMN2 transcriptsAntisense oligonucleotides (ASOs) that bind pre-mRNA were injected into SMA mice (SMN1<sup>−/−</sup>, SMN2<sup>+/+</sup>) exhibiting progressive tail necrosis, a characteristic measure of SMA severity.  ASO therapy effectiveness was assessed by measuring the tail length of wild-type (WT) mice and SMA mice exposed to increasing concentrations of ASOs (Figure 2) . <strong>Figure 2</strong>  Tail length progression of WT mice compared to SMA mice treated with either saline or ASOs Adapted from Lorson CL, Hahnen E, Androphy EJ, Wirth B. A single nucleotide in the SMN gene regulates splicing and is responsible for spinal muscular atrophy. Proc Natl Acad Sci USA. 1999;96(11) :6307-11. -The sequence below is a portion of exon 7 of the SMN1 mRNA transcript.5′ - UCAAGUGAUUCUCCU - 3′Which of the following represents the corresponding DNA coding strand sequence for this particular transcript? A) 5′ - AGGAGAATCACTTGA - 3′ B) 5′ - TCCTCTTAGTGAACT - 3′ C) 5′ - AGTTCACTAAGAGGA - 3′ D) 5′ - TCAAGTGATTCTCCT - 3′ Figure 1  Structure of SMN1 and SMN2 transcriptsAntisense oligonucleotides (ASOs) that bind pre-mRNA were injected into SMA mice (SMN1−/−, SMN2+/+) exhibiting progressive tail necrosis, a characteristic measure of SMA severity.  ASO therapy effectiveness was assessed by measuring the tail length of wild-type (WT) mice and SMA mice exposed to increasing concentrations of ASOs (Figure 2) .
Passage Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease affecting 1 in 10,000 live births.  The pathological hallmark of SMA is the loss of α-motor neurons in the spinal cord, which causes progressive paralysis and, ultimately, premature death in infants.  SMA is caused by insufficient levels of survival motor neuron (SMN) , a regulatory protein required for the maintenance of the spinal cord nerve cells that control voluntary muscle movement.The SMN protein is generated from the SMN1 and SMN2 genes, which differ in sequence by only a single nucleotide in exon 7.  In healthy individuals, the gene product of SMN1 is the full-length (FL) SMN protein, and the gene products of SMN2 are both the FL-SMN protein as well as a truncated unstable protein (SMNΔ7) that is rapidly degraded.  Approximately 90% of the SMN2 gene product is SMNΔ7, and only 10% is FL-SMN (Figure 1) .  In most patients with SMA, SMN1 is mutated such that its protein product is nonfunctional or deleted.  The disease phenotype arises because the SMN2 gene cannot fully compensate for the deficiency in FL-SMN protein synthesis. <strong>Figure 1</strong>  Structure of SMN1 and SMN2 transcriptsAntisense oligonucleotides (ASOs) that bind pre-mRNA were injected into SMA mice (SMN1<sup>−/−</sup>, SMN2<sup>+/+</sup>) exhibiting progressive tail necrosis, a characteristic measure of SMA severity.  ASO therapy effectiveness was assessed by measuring the tail length of wild-type (WT) mice and SMA mice exposed to increasing concentrations of ASOs (Figure 2) . <strong>Figure 2</strong>  Tail length progression of WT mice compared to SMA mice treated with either saline or ASOs Adapted from Lorson CL, Hahnen E, Androphy EJ, Wirth B. A single nucleotide in the SMN gene regulates splicing and is responsible for spinal muscular atrophy. Proc Natl Acad Sci USA. 1999;96(11) :6307-11. -The sequence below is a portion of exon 7 of the SMN1 mRNA transcript.5′ - UCAAGUGAUUCUCCU - 3′Which of the following represents the corresponding DNA coding strand sequence for this particular transcript? A) 5′ - AGGAGAATCACTTGA - 3′ B) 5′ - TCCTCTTAGTGAACT - 3′ C) 5′ - AGTTCACTAAGAGGA - 3′ D) 5′ - TCAAGTGATTCTCCT - 3′ Figure 2  Tail length progression of WT mice compared to SMA mice treated with either saline or ASOs
Adapted from Lorson CL, Hahnen E, Androphy EJ, Wirth B. A single nucleotide in the SMN gene regulates splicing and is responsible for spinal muscular atrophy. Proc Natl Acad Sci USA. 1999;96(11) :6307-11.
-The sequence below is a portion of exon 7 of the SMN1 mRNA transcript.5′ - UCAAGUGAUUCUCCU - 3′Which of the following represents the corresponding DNA coding strand sequence for this particular transcript?


A) 5′ - AGGAGAATCACTTGA - 3′
B) 5′ - TCCTCTTAGTGAACT - 3′
C) 5′ - AGTTCACTAAGAGGA - 3′
D) 5′ - TCAAGTGATTCTCCT - 3′

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