Survival of Motor Neuron 2, Centromeric

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The Survival of Motor Neuron 2 (SMN2) gene is homologous to the SMN1 gene; both of which are located on a large region of inverted repeat on chromosome 5q. The SMN2 gene lies towards the centromeric side of this repeat, as opposed to the SMN1 gene, which lies on the telomeric side. The SMN1 protein is part of a complex which plays an important role in the spliceosomal snRNP assembly and thereby is integral for the process of pre-mRNA splicing that takes place in the nucleus. This function of the protein is especially evident in the motor neuronal cells, where a loss of function of the protein affects the development of axons and dendrites from these neurons. The SMN protein, therefore, plays an important role in the transmission of neuronal impulses. Both the SMN proteins have the same function. However, the SMN2 protein is produced in much smaller amounts than the SMN1 protein, thereby decreasing the relative significance of the SMN2 gene. The effect of the SMN2 gene, therefore, is dependent on its copy number.

In patients with Spinal Muscular Atrophy (SMA), the SMN1 gene may be affected. In such cases, higher copy numbers of the SMN2 gene may help in ameliorating the severity of the disease condition. Patients with defective copies of the SMA1 gene, who also have up to three copies of the functional SMA2 gene have much milder symptoms of SMA which begin later in life than those who have only the regular one or two copies of the SMA2 gene. Other studies also point out to an increased risk for the development of Amyotrophic Lateral Sclerosis (ALS) in individuals with reduced copies of the SMN2 gene.

Molecular Genetics

The SMN2 gene is located on chromosome 5 as part of a 500 kb inverted repeat. SMN2 gene spans a total length of 28 kb, while the protein weighs 31 KDa with almost 300 amino acids. Four different isoforms of this protein are produced as transcription variants, of which only the d-isoform is functional. SMN2 differs from SMN1 two single nucleotide changes; one each in exons 7 and 8. In exon 7, the silent c.840C>T transition disrupts an exonic splicing enhancer and results in the skipping of exon 7 and the production of an unstable protein with decreased activity. It has also been suggested that the c.840C>T transition instead creates an hnRNP A1 binding exonic splicing silencer, which causes exon 7 skipping in SMN2

The varying copy number of the genes is thought to be due to gene conversion events involving both SMN1 and SMN2. Recent research has meanwhile indicated that the gene duplication event probably occurred more than five million years ago before the delineation of chimpanzees and humans. However, the SMN2 gene itself appeared for the first time in humans.

Epidemiology in the Arab World

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Other Reports


A study conducted by Haider et al. (2001) on the pattern of exon deletion in SMN in Spinal Muscular Dystrophy included three SMA patients from Bahrain [See Kuwait > Haider et al., 2001].


Haider et al. (2001) investigated the pattern of exon deletion in survival motor neuron (SMN) genes in 46 patients with different phenotypes of spinal muscular atrophy from different ethnic backgrounds, which included 26 Kuwaiti patients. DNA from affected patients and 62 unaffected control subjects was subjected to PCR amplification of both exons 7 and 8 of SMN gene which were then digested with the restriction enzymes DraI and DdeI, respectively. Absence of the DdeI enzyme restriction site in exon 8 of SMN1 gene made it possible to distinguish between SMN1 and SMN2 genes. In all patients with type I SMA, both exons 7 and 8 of SMN gene showed deletions. All patients with type II SMA showed deletion in exon 7, while exon 8 showed deletion in almost all patients. Both patients with Type III SMA were Kuwaitis, and were found to have deletion in both exons 7 and 8. None of the controls were found to have deletions in either SMN gene.


A study conducted by Haider et al. (2001) on the pattern of exon deletion in SMN in Spinal Muscular Dystrophy included five Omani SMA patients [See Kuwait > Haider et al., 2001].

Saudi Arabia

A study conducted by Haider et al. (2001) on the pattern of exon deletion in SMN in Spinal Muscular Dystrophy included one Saudi SMA patient [See Kuwait > Haider et al., 2001].

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