Disease relevance of SMN2

• A negative element in SMN2 exon 7 inhibits splicing in spinal muscular atrophy [1].
• Since VPA is a drug highly successfully used in long-term epilepsy therapy, our findings open the exciting perspective for a first causal therapy of an inherited disease by elevating the SMN2 transcription level and restoring its correct splicing [2].
• We have exploited the existence of a second copy of the human SMN gene (SMN2) to develop a high-throughput screening strategy to identify potential small molecule therapeutics for the genetic disease spinal muscular atrophy (SMA), which is caused by the loss of the SMN1 gene [3].
• This result suggests that SMN2 deletions could act as a susceptibility factor for sporadic lower motor neuron disease in adults [4].
• Significant increase in the number of the SMN2 gene copies in an adult-onset Type III spinal muscular atrophy patient with homozygous deletion of the NAIP gene [5].

High impact information on SMN2

• Here we show that the C/T transition functions not to disrupt an exonic splicing enhancer (ESE) in SMN1 (ref. 4), as previously suggested, but rather to create an exonic splicing silencer (ESS) in SMN2 [1].
• Spinal muscular atrophy results from the lack of functional survival of motor neuron 1 gene (SMN1), even though all affected individuals carry a nearly identical, normal SMN2 gene [6].
• Our findings explain the basis of defective SMN2 splicing, illustrate the fine balance between positive and negative determinants of exon identity and alternative splicing, and underscore the importance of antagonistic splicing factors and exonic elements in a disease context [7].
• Bifunctional antisense oligonucleotides provide a trans-acting splicing enhancer that stimulates SMN2 gene expression in patient fibroblasts [8].
• This discrepancy is because of a single nucleotide difference in SMN2 exon 7, which disrupts an exonic splicing enhancer containing an SF2ASF binding site [8].

Chemical compound and disease context of SMN2

• Valproic acid increases the SMN2 protein level: a well-known drug as a potential therapy for spinal muscular atrophy [2].
• Hydroxyurea enhances SMN2 gene expression in spinal muscular atrophy cells [9].
• The benzamide M344, a novel histone deacetylase inhibitor, significantly increases SMN2 RNA/protein levels in spinal muscular atrophy cells [10].

Biological context of SMN2

• We show that this ESS functions as a binding site for a known repressor protein, hnRNP A1, which binds to SMN2 but not SMN1 exon 7 RNA [1].
• Denervation was assessed in 89 spinal muscular atrophy (SMA) 1, 2, and 3 subjects via motor unit number estimation (MUNE) and maximum compound motor action potential amplitude (CMAP) studies, and results correlated with SMN2 copy, age, and function [11].
• Finally, SMN2 copy number is conclusively shown to ameliorate the phenotype and provide valuable prognostic information [12].
• Taken together with our previous data, we may reasonably hypothesize that the SMN2 gene copy number is more critical in determining the severity of the disease compared to the NAIP genotype [5].
• The two genes undergo alternative splicing with SMN1, producing an abundance of full-length mRNA transcripts, whereas SMN2 predominantly produces exon 7-deleted transcripts [8].

Anatomical context of SMN2

• Homozygous absence of the survival motor neuron gene (SMN1) is the primary cause of SMA, while SMA severity is mainly determined by the number of SMN2 copies [2].
• We show that in fibroblast cultures derived from SMA patients treated with therapeutic doses (0.5-500 microM) of valproic acid (VPA), the level of full-length SMN2 mRNA/protein increased 2- to 4-fold [2].
• Exon 8 of SMN2 was amplified in 96% of both normal and SMA-affected leukocytes [13].
• We report that sodium butyrate effectively increases the amount of exon 7-containing SMN protein in SMA lymphoid cell lines by changing the alternative splicing pattern of exon 7 in the SMN2 gene [14].
• Combined treatment with dibutyryl cAMP and forskolin significantly increased the level of both the full-length and exon 7-deleted SMN (exonDelta7SMN) transcript in primary hepatocytes from mice expressing two copies of human SMN2 gene [15].

Associations of SMN2 with chemical compounds

• Here, we report three compounds (sodium vanadate, trichostatin A and aclarubicin) that effectively enhance SMN2 expression by inducing Stat5 activation in SMA-like mouse embryonic fibroblasts and human SMN2-transfected NSC34 cells [16].
• We tested a therapeutic strategy by treating cultured lymphocytes from patients with SMA with hydroxyurea to modify SMN2 gene expression and to increase the production of SMN protein [9].
• Here we show that the novel benzamide M344, an HDAC inhibitor, up-regulates SMN2 protein expression in fibroblast cells derived from SMA patients up to 7-fold after 64 h of treatment [10].
• Trans-activating factors such as Htra2-beta1, as well as various drugs like sodium butyrate or aclarubicin, are able to restore the full-length SMN2 RNA to large amounts [17].

Physical interactions of SMN2

• The SR-like trans-acting splicing factor Htra2-beta1 was shown to interact with this ESE and to restore full-length SMN2 expression in vivo in a concentration-dependent manner [18].

Regulatory relationships of SMN2

• RT-PCR studies of SMN transcripts in control and patients with the same SMN2 copy number showed that the full-length/Delta7 ratio is influenced by the SMN1 genotype although it seems independent of the SMN2 copy number [19].

Other interactions of SMN2

• A single nucleotide difference that alters splicing patterns distinguishes the SMA gene SMN1 from the copy gene SMN2 [20].
• The SR-like splicing factor Htra2-beta1 facilitates correct splicing of SMN2 exon 7 through direct interaction with an exonic splicing enhancer within exon 7 [21].
• Analysis of polymerase chain reaction-restriction fragment length polymorphism as well as single-strand conformation polymorphism on exons 7 and 8 of the SMN genes revealed the SMN2-deletion [22].
• Htra2-beta 1 stimulates an exonic splicing enhancer and can restore full-length SMN expression to survival motor neuron 2 (SMN2) [23].
• Here we show that such synthetic effectors can mimic the functions of SR proteins and specifically restore wild type splicing when directed to defective BRCA1 or SMN2 pre-mRNA transcripts [24].

Analytical, diagnostic and therapeutic context of SMN2

• A cell-based bioassay was generated that detects SMN2 promoter activity, on which greater than 550,000 compounds was tested [3].
• Natural history of denervation in SMA: relation to age, SMN2 copy number, and function [11].
• We present a new, fast, and highly reliable quantitative test, based on real-time LightCycler PCR that amplifies either SMN1 or SMN2 [25].
• SMN2, a copy gene, influences the severity of SMA and may be used in somatic gene therapy of patients with SMA in the future [25].
• New insights on the evolution of the SMN1 and SMN2 region: simulation and meta-analysis for allele and haplotype frequency calculations [26].

References