20.03.04 AR disorders - i.e. SMA

Question 1

Estimated incidence of SMA

Answer 1

1 in 6- 10,000

Question 2

Carrier frequency of SMA

Answer 2

1 in 50 in UK

Question 3

Is it a common cause of infant death

Answer 3

Yes, second most common AR disorder after CF.

Question 4

What is the pathology of SMA

Answer 4

• spinal cord and anterior horn motor neuron degeneration.

- The result of synaptic defects in motor neuromuscular junctions (NMJs)

Question 5

Clinical phenotype

Answer 5

• Progressive proximal limb and trunk muscle weakness
• Breathing difficulties (intercostal muscle weakness)
• paralysis of voluntary muscles
• Tremor in fingers
• Tongue fasiculations, poor suck and swallow.

Question 6

What is prenatal SMA

Answer 6

• Severe weakness of prenatal onset
• Arthrogryposis multiplex congenita. Congenital joint contractures, absence of movement (except face and extraocular), death from respiratory arrest before 1 month of age.

Question 7

What is type I SMA

Answer 7

• 60% of all SMA.
• Diagnosed before 6 months age.
• Floppy baby= profound hypotonia, tongue fasiculations, lack of motor development
• Progressive weakness in infancy and death at an early age (often due to aspiration pneumonia)

Question 8

What is type II SMA

Answer 8

• Intermediate subtype. 27% of SMA cases
• Onset between 6-12 months
• Low muscle tone, absent tendon reflexes. Can sit unaided but never able to walk
• Reduced life expectancy (70% reach adulthood)

Question 9

What is type III SMA

Answer 9

• 12% of SMA patients
• Onset of muscle weakness ~ after 10 months (IIIa) or 3 years (IIIb)
• Can stand and are ambulatory (probability decreases with age)

Question 10

What is type IV SMA

Answer 10

• 1% of SMA patients
• Onset of muscle weakness in 2nd or 3rd decade of life
• Can stand and walk alone

Question 11

What is atypical SMA

Answer 11

• SMA with congenital bone fractures.

- Floppiness, multiple fractures of long bones, CNS involvement and elevated CK levels

Question 12

Differential diagnosis for floppy babies

Answer 12

• SMA
• DM1
• PWS

Question 13

Which genes are associated with SMA

Answer 13

• SMN1 and 2

- 5q12.2-13.3

Question 14

SMA is caused by what

Answer 14

-Hom or het deletions/ mutations in SMN1

Question 15

How are SMN1 and 2 arranged

Answer 15

• SMN1 is telomeric

- SMN2 is centromeric

Question 16

How many base pairs are different between SMN1 and 2

Answer 16

5bp

Question 17

Is SMN1 and 2 amino acid sequence identical

Answer 17

Yes

Question 18

Which synonymous variant alters the splicing efficiency of exon 7 of SMN2

Answer 18

c.840C>T

Question 19

Why does c.840C>T reducing splicing of exon 7 in SMN2

Answer 19

• c.840C > T is translationally silent but disrupts an exonic splice enhancer (ESE) and creates a new exonic splice silencer (ESS)
• Causes exon skipping. Non-functional protein which is rapidly degraded

Question 20

What proportion of transcripts are full length for SMN1 and 2

Answer 20

• SMN1= 90% of transcripts are full length

- SMN2= 90% of transcripts lack exon 7

Question 21

What does SMN protein do in motor neurons

Answer 21

• Complexes with Gemin and acts as a chaperone to assist the assembly of small ribonuclear proteins (snRNPs) in motor neurons.
• Component of the spliceosome

Question 22

Is SMN protein essential for life

Answer 22

• Yes.
• Also SMA patients with hom SMN1 deletions have at least 1 copy of SMN2
• Mouse knock outs are embryonic lethal with no SMN protein

Question 23

What two hypotheses are there for SMA disease mechanism

Answer 23

• Loss of SMN impairs mRNA production and neurons are deficient in necessary proteins for growth and function
• SMN has a motor neuron specific function.

Question 24

What percentage of SMA patients have a hom deletion of at least exon 7

Answer 24

95-98%

Question 25

How can exon 7 of SMN1 be lost

Answer 25

- Deletion | - Conversion to exon 7 of SMN2 (unidirectional and non-reciprocal)

Question 26

What proportion of SMA patients are hom for inactivating point mutations

Answer 26

<1%

Question 27

Why is the incidence of SMA high even though it is lethal

Answer 27

-High mutation rate

Question 28

Has mosaicism been reported in SMA

Answer 28

Yes

Question 29

Is there a genotype phenotype correlation

Answer 29

Yes - SMA I= hom deletion of SMN1 - SMA II= SMN1 to SMN2 gene conversion and a deletion - SMAIII= gene conversion on both alleles

Question 30

Can SMN2 compensate for SMN1 loss

Answer 30

- Not fully but when SMN2 copy number increases, the small amounts of full length transcript can result in milder SMA II or III. - Correlation is not absolute or strong enough to have prognostic value, so not reported

Question 31

What genetic tests are offered in the diagnostic setting

Answer 31

- Exon 7 and 8 dosage of SMN1 - Often by MLPA, Real time PCR or qPCR - Probes are specific to the differences between SMN1 and 2

Question 32

What other testing is available to patients with clinical symptoms but only a het deletion

Answer 32

-Sequence analysis to look for 2nd undetected variant

Question 33

Why is carrier testing complex

Answer 33

- 6% of parents will have normal SMN1 dosage 1) 4% will have both copies on a single chromosome 2) De novo deletion in 2% cases (i.e. only 1 parent is a carrier) - Linkage analysis would clarify

Question 34

Strategies for SMN therapies

Answer 34

increase the expression of SMN protein

Question 35

Examples of SMN therapies in development

Answer 35

1) Antisense oligonucleotides (ASOs) to correct SMN2 splicing. e.g. Spinraza. Benefits: no toxicity and highly stable 2) gene conversion from SMN2 to SMN2 3) Stem cell or gene threrapy to compensate for the lack of SMN protein. AAV9 virus engineered to carry WT SMN. e.g. Zolgensma. However, has some side effects.

Question 36

When is treatment most effective

Answer 36

- Early - Motor neurons are non-dividing so once lost they cannot be replaced. Patients with advanced states of degeneration won't benefit from treatment