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.
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) .
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.
-Based on the data shown in Figure 2, which statement best explains the function of antisense oligonucleotides (ASOs) in SMA mice? ASOs affect:

Question

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.

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) .

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.
-Based on the data shown in Figure 2, which statement best explains the function of antisense oligonucleotides (ASOs) in SMA mice? ASOs affect:

Quizplus · Accepted Answer

11ecba2d_0f9a_5570_a3bf_4b6e06d16b80_MD0008_00 According to the passage, SMA mice (SMN1^−/−, SMN2^+/+) are an animal model of SMA that produces insufficient FL-SMN. SMA mice exhibit progressive tail necrosis, which is seen as a loss of tail length and serves as a functional measure of SMA severity. In contrast, WT mice produce normal levels of FL-SMN transcripts, and therefore show no tail necrosis. In this scenario, the effectiveness of ASO therapy was assessed by comparing the tail length of SMA mice following ASO or saline administration to that of untreated WT mice. Figure 2 shows that ASO-treated SMA mice have increased tail length compared to SMA mice treated with saline. The passage states the ASOs bind pre-mRNA, meaning the ASOs used in this study must function by affecting the processing of pre-mRNA into mature mRNA. In particular, the improvement in tail length can be attributed to ASO-induced inclusion of exon 7 during processing of SMN2pre-mRNA. Because SMN2 produces FL-SMN (contains exon 7) and SMNΔ7 (lacks exon 7), this exon 7 inclusion caused higher expression of FL-SMN compared to SMNΔ7. Consequently, ASO-treated mice had increased FL-SMN levels and decreased tail necrosis. (Choice A) The data presented in Figure 2 clearly show that the ASOs used in the study alleviate some of the symptoms associated with SMA. To have this effect on SMA disease pathology, ASO must alter the way the SMN2 pre-mRNA molecule is spliced. Splicing occurs during pre-mRNA processing, not during transcription. (Choices C and D) The mature SMN2 mRNA transcript (ie, 5′ cap added, polyadenylated, and spliced) is transported from the nucleus to the cytoplasm, where translation (protein synthesis) occurs. As stated in the passage, ASOs only bind pre-mRNA. As a result, ASOs do not affect transport of mature SMN2 mRNA to the cytoplasm or its translation. Educational objective: Processing (which includes splicing) of the pre-mRNA transcript into mature mRNA occurs in the nucleus.