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Prevalence of SMN1 deletion and duplication in carrier and normal populations: implication for genetic counselling
  1. V Cusin1,
  2. O Clermont2,
  3. B Gérard1,
  4. D Chantereau1,
  5. J Elion1
  1. 1Laboratoire de Biochimie Génétique, Hôpital R Debré, 48 Boulevard Sérurier, 75019 Paris, France
  2. 2Laboratoire de Génétique Bacterienne EA3105, Hôpital R Debré, 48 Boulevard Sérurier, 75019 Paris, France
  1. Correspondence to:
 Dr V Cusin, Laboratoire de Biochimie Génétique, Hôpital R Debré, 48 Boulevard Sérurier, 75019 Paris, France; 
 veronica.cusin{at}wanadoo.fr

https://doi.org/10.1136/jmg.40.4.e39

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    Spinal muscular atrophy (SMA) is the second most frequent autosomal recessive disease, with a prevalence of 1 in 6000 live born infants.1 It is characterised by degeneration of motor neurones of the anterior horn of the spinal cord, leading to symmetrical muscular weakness and atrophy. The International SMA Consortium classification2 defines several degrees of severity in the SMA phenotype, depending on the age of onset and motor development milestones. Type I SMA, Werdnig-Hoffmann I disease, is the most severe form with onset within 6 months of birth. Patients are unable to sit up and have serious respiratory dysfunction. Type II SMA is the intermediate form with onset within the first 2 years; children can sit up but are unable to walk. The clinical course is variable. Type III (also called Kugelberg-Welander disease) begins after 2 years of age and usually has a chronic evolution. Children can stand and walk unaided at least in infancy. Adult form (type IV) is the mildest, with onset after 30 years of age; few cases have been reported and its prevalence is not accurately known.

    Spinal muscular atrophy is linked to locus 5q13 in more than 95% of patients.3–6 The critical region, containing several genes including the survival motor neurone (SMN) gene, is inverted and duplicated. Homozygous deletion of SMN1, located in the telomeric position, accounts for the disease in 98% of these cases and has been reported in infantile, intermediate, and adult onset disease.7–10 Linkage analysis in families with SMA shows large de novo deletions in 2% of patients.11–13SMN2 is a highly homologous gene located in the centromeric duplicated region.7,14 Most of the SMN1 transcripts are full length, whereas most of the SMN2 transcripts lack exon 7. In fact, a nucleotide substitution …

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