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GENATLAS PHENOTYPE
last update : 11/07/2008
Symbol SMA2
Location 5q13.2
Name spinal muscular atrophy, type 2, intermediate type
Corresponding gene SMN1 , SMN2 , NAIP
Main clinical features
  • onset 6-18 months of age, sits independently when placed, life expectancy 10-40 y
  • intermediate in severity between the infantile form of Werdnig-Hoffmann and the juvenile form of Kugelberg and Welander, onset usually between 3 and 15 months and survival beyond 4 years and usually until adolescence or later with proximal muscle weakness as in other forms of spinal muscular atrophy
    • Genetic determination autosomal recessive
    Function/system disorder neuromuscular
    Type disease
    Mechanism(s)
    Gene mutationChromosome rearrangementEffectComments
      deletion   large genomic deletions encompassing the SMN gene often extend to involve the NAIP gene
    Remark(s) SMN1 is considered to be the primary disease causing gene ; SMN2 cannot fully compensate lack of SMN1 expression because most normal SMN2 transcripts lack exon 7; the SMN2 copy number influences disease severity
    Genotype/Phenotype correlations phenotypes in SMN1 mutations span a continuum without clear delineation of subtypes, which are classified by age of onset and maximum function achieved ;homozygous absence of SMN1 associated in 62p100 of SMA3 patients (age of onset less than 3 years) with 2 or 3 SMN2 copies, whereas 65p100 of SMA3 patients (age of onset greater than 3 years ) with 4 to 5 SMN2 copies