Muscular Dystrophies

Question 1

Which test is needed to detect tandem repeat disorders?

Answer 1

Repeat primed PCR/Southern blot

Question 2

If you want to test mtDNA, what type of sample is needed?

Answer 2

sample of affected tissue

will not be in the blood

Question 3

What is a unique test result issue for tandem repeat disorders that is variable and should be kept in mind?

Answer 3

There is somatic instability. The number of repeats can be different on different days. This is very unstable.

Question 4

How does the number of copies of SMN2 affect SMA patients?

Answer 4

If SMN1 is deleted, more than 2 copies of SMN2 is protective and results in more mild presentation.

Question 5

What type of testing methodologies are needed to detect repeat disorders?

Answer 5

• Southern Blot
• Repeat Primed PCR
• PCR
• Optical Mapping (FSHD only)

Question 6

What testing methodology is needed to detect SMN2 variants?

Answer 6

qPCR/ddPCR for dose analysis

Question 7

What testing methodoloy is needed to detect variants in SORD?

variants in SORD cause a genetic neuropathy

Answer 7

Nested PCR

because there is a pseudogene for SORD
A pseudogene is a segment of DNA that structurally resembles a gene but is not capable of coding for a protein

Question 8

Muscle biopsies may be needed for diagnostic confirmation BUT can also be…

Answer 8

nonspecific

Question 9

Describe the protective effects of SMN2 regarding SMN1 loss in SMA.

Answer 9

• SMN1 and SMN2 genes are almost identical. They only have 5 nucleotides that are different.
• 2 copies of SMN1 lost = SMA
• SMN2 makes 10% of the protein amount that SMN1 would have made, so each copy increases the protective effects of SMN2 copy number
• SMN2: makes 90% nonfunctional SMN protein and 10% functional SMN protein

Question 10

How is SMA classified and what is the pattern of classification?

Answer 10

Type 0 –> Type IV
most severe –> least severe

Question 11

Nusinersen (Spinraza)

Answer 11

• SMA drug
• targets exon 7, helping SMN2 compensate for SMN1 loss
• lenghtens event free survival and overall survival

Question 12

Treatment modalities for SMA

Answer 12

1. antisense oligonucleotides
2. gene delivery
3. small molecule

all work well

Question 13

Gene therapy for SMA

Answer 13

Onasemnogene Aberparvovec

Question 14

Duchene Dystrophy gene therapy

Answer 14

Elevydis

approved June 2023

Question 15

Duchene Muscular Dystrophy

Answer 15

• gene = DMD
• X-linked
• largest human gene
• high rate of spontaneous variation

Question 16

Genetic causes of DMD

Answer 16

• 70% = exon level deletion
• 10% = exon-level duplication
• 20% = smaller del/dup + point mutations

Question 17

Frameshift mutation in DMD gene leads to

Answer 17

DMD

Question 18

In frame mutation in DMD gene leads to

Answer 18

BMD
Becker Muscular Dystrophy

Question 19

Symptoms of DMD

Answer 19

• gross motor delay
• abnormal gait “clumsy” child
• persistent toe-walking
• speech or cognitive delay
• loss of ambulation by age 13
• CK levels very high
• neurodivergence can be present

cod = heart failure

Question 20

Backer Muscular Dystrophy

Answer 20

• similar to DMD but much milder
• symptom onset early childhood and young adulthood
• loss of ambulation after 15 years

Question 21

Motor Signs & Symptoms of DMD

Answer 21

• delayed walking
• waddling gait, toe-walking
• difficulty walking, running, jumping, and negotiating stairs
• loridosis
• frequent falls
• Gowers sign
• enlarged calves

Question 22

Median survival for DMD

Answer 22

28 years old

Question 23

Duchene Muscular Dystrophy timeline of progression

Answer 23

Question 24

There are multiple gene based therapeutics to restore dystrophin protein. None are available to all paitients, so treatment choice is specific to patient.

DMD & BMD

Answer 24

• Exon Skipping
• Splicing Correction
• Mutatant RNA removal
• Stop Codon Readthrough
• Gene Therapy: AAV mediated gene replacement
• CRISPR - future