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Newborn Screening Ontario
Spinal Muscular Atrophy

Genetics of SMA

People have two copies of the SMN1 gene.  When one copy is working (carrier) or both copies are working then people are healthy.  When a baby is born with no working copies of the SMN1 gene then they will develop SMA.  SMA can be caused by deletions in the SMN1 gene (~97% of cases) or other genetics changes (~3%).  Newborn screening will only detect cases of SMA caused by deletions in the SMN1 gene. 

The type of SMA depends upon the number of copies of another gene, SMN2.  People can have 1 to 6 copies of the SMN2 gene.  The higher number of SMN2 copy numbers, the less severe the symptoms.

The table below gives the chance of having each type of SMA depending upon the SMN2 copy number.  For example, a baby found to have 1 copy of SMN2 has a 96% chance of having SMA type 1 and a 4% chance of having SMA type 2. 

SMA type

SMN2 copy number

1

2

3

4

SMA type 1

96%

79%

15%

1%

SMA type 2

4%

16%

54%

11%

SMA type 3

0%

5%

31%

88%*

*includes both SMA type 3 and type 4

Babies with a deletion of both their copies of SMN1, and with SMN2 copy numbers of 1, 2, 3 or 4 will be reported as having a screen positive result and referred for more testing.  This means that >99% of SMA type 1 and 2 cases and >97% of SMA type 3 cases caused by a deletion in the SMN1 gene will be picked up by screening.  SMA type 4 will also be detected if caused by SMN2 copy numbers of 4 or less.

 

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