Spinal Muscular Atrophy

Question 1

What is the pathophysiological mechanism behind SMA?

Answer 1

• Brain would usually initiate movement by sending nerve signals down spinal cord (=anterior horn motor neurons)
• These motor neurons delay signals to the muscles which cause the muscles to contract
• Degeneration of these motor neurons in the spinal cord causes proximal, symmetrical limb and trunk muscle weakness

Question 2

What is the mode of inheritance in SMA?

Answer 2

Autosomal recessive

Question 3

What is the incidence of SMA and what is the carrier frequency?

Answer 3

• Incidence = approx. 1 in 10,000

- Carrier frequency = approx. 1 in 50

Question 4

What is the clinical presentation associated with SMA Type 1?

Answer 4

• Most severe and most common (~60% of all cases)
• Onset at less than 6 months
• Cannot sit unaided
• 50% die before 2nd birthday

Question 5

What is the clinical presentation associated with SMA Type 2?

Answer 5

• Onset 7-18 months
• May sit unaided but never walk unaided
• Have increased risk of respiratory problems
• Death at more than 2 years

Question 6

What is the clinical presentation associated with SMA Type 3?

Answer 6

• Onset later than 18 months

- Able to walk but eventually need to use a wheelchair

Question 7

What is the clinical presentation associated with SMA Type 4?

Answer 7

• Adult onset (more than 30 years)

Question 8

What gene is involved in SMA?

Answer 8

• Survival Motor Neuron gene (SMN) in two copies: SMN1 and SMN2

Question 9

What is the difference between SMN1 and SMN2?

Answer 9

• Only 5 base pairs differ at the 3’ end (2 are located in exon 7 and exon 8)
• These differences allow SMN1 to be distinguished from SMN2 and form the basis of the molecular test

Question 10

Provide some details on the base difference in exon 7 of the SMN1/2 genes

Answer 10

• C to T conversion in SMN2 resulting in a synonymous amino acid change
• This change disrupts an exon splice enhancer (ESE) site preventing efficient splicing of exon 7 in SMN2
• therefore lower levels of the protein are produced (~10%)

Question 11

The majority of the SMN protein is produced from what gene?

Answer 11

• SMN1 (90% of protein produced)
• SMA patients with a deletion of SMN1 always have at least one copy of SMN2 = 10% of protein
• Complete absence of SMN protein shown to be embryonically lethal in mice

Question 12

Does the number of SMN2 copies affect disease severity?

Answer 12

• Yes: the greater the number of SMN2 copies the less severe the disease
• number of copies acts as a dose-dependent disease modifier as the number of copies can vary between 0-5
• patients who have homozygous deletions of SMN1 with 5 copies of SMN2 have been reported in Asymptomatic individuals with family histories of SMA

Question 13

The number of copies of SMN2 copies is one factor in determining phenotype. How is this true in the context of the SMA subtypes?

Answer 13

Those with Type 3 have on average more copies of SMN2 than those with type 2 or type 1

Question 14

SMN1 copy number overlaps in SMA Type 1, 2 and 3. Provide details

Answer 14

• SMA type 1: deletion in both alleles
• SMA type 2: deletion in one plus gene conversion (to SMN2) in the other
• SMA type 3: gene conversion in both

Question 15

What are the three diagnostic result categories SMA patients can fall into?

Answer 15

1. Homozygous deletion of SMN1 (lack SMN1 due to either deletion or gene conversion) = 94%
2. Homozygous for pathogenic sequence variant = less than 1%
3. Compound heterozygotes: deletion/gene conversion on one allele and point mutation on another = 4%

Question 16

If a diagnosis of SMA is suspected then what genetic test can be performed?

Answer 16

• MLPA to determine the copy number of SMN1

- If only one copy is identified then point mutation analysis can be performed

Question 17

What are some of the problems involved with SMN1 carrier testing?

Answer 17

• approx. 4% of Normal chromosomes carry two copies of SMN1 gene on same allele which could mask a deletion
• Testing can’t distinguish between patients with one copy on each allele (1:1 = non-carriers) and those with two copies on one allele and none on the other (2:0 = carriers)
• De novo mutations occur in 2% of individuals with SMA
• Possible risk of germline mosaicism

Question 18

What further testing can you conduct in cases where parents may be (2:0)?

Answer 18

• Markers that flank SMN1 region can be used to aid in identification of at risk haplotype
• Genotyping grandparents of apparently non-carrier parent can help determine if they are a carrier as grandparents will more than likely have (1:0) and (2:1) genotype
• If mutation is de novo then the grandparents will have normal dosage levels (1:1)

Question 19

What is the procedure for prenatal testing in SMA?

Answer 19

• Markers located in and around the SMN1 gene can be used to confirm which haplotype has been inherited by the fetus
• Requires parental DNA samples and sample from proband

Question 20

What non-genetic tests may be used for SMA diagnosis?

Answer 20

• EMG: fibrillation and muscle denervation
• Motor and nerve conduction velocity: normal
• Creatine kinase: normal or mildly elevated
• Muscle histology: group atrophy of type 1 and 2 muscle fibres

Question 21

What are some differential diagnoses for SMA?

Answer 21

• Arthrogryposis multiplex congenita

- SMA and respiratory distress (SMARD) with diaphragmatic and intercostal muscle weakness (caused by IGHMBP2)