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Dosage > SMA > Flashcards

SMA Flashcards

1
Q

What is the mode of inheritance for SMA?

A

Autosomal recessive

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2
Q

What is the classification of different types of SMA based upon?

A

Age of onset and motor development milestones.

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3
Q

Which form of SMA is the most severe?

A

Type 1, onset is typically within 6 months of age.

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4
Q

Age of onset: 6 months of age. Patient is unable to sit up and has severe respiratory dysfunction.

A

Type 1

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5
Q

Which type of SMA is considered ‘intermediate’ with a variable clinical course?

A

Type 2

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6
Q

Age of onset: <2 years of age. Patient is able to sit up but unable to walk.

A

Type 2

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7
Q

Age of onset >2 years. Patient is able to stand and walk unaided at least in infancy

A

Type 3

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8
Q

Which is the mildest form of SMA which is also adult on-set?

A

Type 4

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9
Q

Describe the 4 types of SMA.

A

Acute, Intermediate, Mild and Adult.

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10
Q

The complexity and instability of which chromosomal region accounts for 98% of SMA cases?

A

5q13

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11
Q

Why is the SMA critical region considered to be unstable?

A

Highly repetitive

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12
Q

In proximal SMA, which muscles are earliest and most severely affected?

A

Extremities closest to trunk

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13
Q

What is the estimated carrier frequency for SMAs?

A

1/40 - 1/60.

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14
Q

What is the estimated incidence of SMAs?

A

1/ 10,000

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15
Q

What complex structure is present at the 5q13 SMA region?

A

500kb duplication ad inversion.

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16
Q

SMN1 is the telomeric homolog of which highly similar gene?

A

SMN2

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17
Q

Of the two SMN genes, which is considered to be the pseudogene?

A

SMN2

18
Q

Is SMN2 telomeric or centromeric?

A

Centromeric

19
Q

SMN1 and SMN2 are known to be homologues. They differ by how many base-pairs?

A

5

20
Q

In which exons do SMN1 and SMN2 differ?

A

Exon 7 and exon 8

21
Q

Name the mutational mechanisms which account for 95% of SMA cases.

A
  1. Homozygous Deletion

2. Sequence-converted SMN1

22
Q

Most cases of SMA type 1 arise from…

A

Homozygous SMN 1 deletion

23
Q

Milder cases (type 2 and 3) are more likely to be caused by…

A

Sequence-conversion from SMN1 to SMN2.

24
Q

Are patients suffering from milder forms of SMA expected to have more or less copies of SMN2?

A

More

25
Q

Why would patients with milder forms of SMA have more copies of SMN2?

A

Patients with milder forms are more likely to have arisen from SMN1 to SMN2 sequence conversion, which results in more copies of SMN2.

26
Q

Which cells does SMA affect?

A

Anterior horn cells (lower motor neurons)

27
Q

Is brain function normal in patients with SMA?

A

Yes

28
Q

Is sensation affected in patients with SMA?

A

No

29
Q

Which muscles are affected in patients with SMA?

A

Extremities (usually those closest to the trunk) and Respiratory muscles.

30
Q

What is bulbar dysfunction?

A

Poor suck ability, reduced swallowing and respiratory failure.

31
Q

Are facial muscles usually affected in type 1?

A

No

32
Q

Is cognitive ability impaired?

A

No

33
Q

What prenatal observation is present in ~30% of cases?

A

Reduced fetal movement

34
Q

When would prenatal testing for SMA be appropriate?

A

Previously affected child, parent known to be a carrier

35
Q

Name 2 mechanisms which cause deletions of SMN1

A
  1. Homozygous deletion of SMN 1

2. Gene conversion of SMN1 into SMN2

36
Q

What is the clinical significance of a homozygous deletion of both SMN1 and SMN2?

A

Lethal

37
Q

In general, what type of SMA results from homozygous gene deletion events?

A

Type 1

38
Q

In general, what type of SMA results from a gene conversion event and a deletion in the other allele?

A

Type 2

39
Q

In general, what type of SMA result from a gene conversion in both alleles?

A

Type 3/ type 4.

40
Q

Name 2 potential sources of false negative carrier test results

A
  1. 2/0 carriers

2. Point mutations not currently detectable via dosage analysis.

41
Q

Can increased SMN2 copy number compensate for SMN1 deletion?

A

No, although some protein is still produced not as much, hence milder forms of SMA are associated with SMN2 copy number increase.

42
Q

Why does the SMN2 gene produce a less adequate protein than SMN1?

A

Exon skipping via alternative splicing (80% skips exon 7 and terminates at exon 8) = unstable protein.

Dosage (2 decks)

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