SMN2 copy number predicts acute or chronic spinal muscular atrophy but does not account for intrafamilial variability in siblings

J Neurol. 2006 Jan;253(1):21-5. doi: 10.1007/s00415-005-0912-y. Epub 2005 Jun 28.


Spinal muscular atrophy (SMA) is an autosomal recessive disorder that affects motor neurons. It is caused by mutations in the survival motor neuron gene 1 (SMN1). The SMN2 gene, which is the highly homologous SMN1 copy that is present in all the patients, is unable to prevent the disease. An SMN2 dosage method was applied to 45 patients with the three SMA types (I-III) and to four pairs of siblings with chronic SMA (II-III) and different phenotypes. Our results confirm that the SMN2 copy number plays a key role in predicting acute or chronic SMA. However, siblings with different SMA phenotypes show an identical SMN2 copy number and identical markers, indicating that the genetic background around the SMA locus is insufficient to account for the intrafamilial variability. In our results, age of onset appears to be the most important predictor of disease severity in affected members of the same family. Given that SMN2 is regarded as a target for potential pharmacological therapies in SMA, the identification of genetic factors other than the SMN genes is necessary to better understand the pathogenesis of the disease in order to implement additional therapeutic approaches.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Cyclic AMP Response Element-Binding Protein / genetics*
  • Cyclic AMP Response Element-Binding Protein / metabolism
  • Family Health*
  • Female
  • Gene Dosage*
  • Humans
  • Male
  • Middle Aged
  • Muscular Atrophy, Spinal / classification
  • Muscular Atrophy, Spinal / diagnosis*
  • Muscular Atrophy, Spinal / genetics*
  • Nerve Tissue Proteins / genetics*
  • Nerve Tissue Proteins / metabolism
  • RNA, Messenger / metabolism
  • RNA-Binding Proteins / genetics*
  • RNA-Binding Proteins / metabolism
  • Reverse Transcriptase Polymerase Chain Reaction / methods
  • SMN Complex Proteins
  • Survival of Motor Neuron 1 Protein
  • Survival of Motor Neuron 2 Protein


  • Cyclic AMP Response Element-Binding Protein
  • Nerve Tissue Proteins
  • RNA, Messenger
  • RNA-Binding Proteins
  • SMN Complex Proteins
  • SMN1 protein, human
  • SMN2 protein, human
  • Survival of Motor Neuron 1 Protein
  • Survival of Motor Neuron 2 Protein