Spinal Muscular Atrophy Flashcards

1
Q

what is SMA

A

a multisystem disorder

  • primarily/prefers to attack MNs since main signs are muscle wasting and weakness following degeneration and death of MNs
  • includes a wide spectrum of NMDs with a variation in onset and severity
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2
Q

SMA genealogy

A
  • autosomal recessive - both parents need to be carriers for children to have it
  • it’s the 2nd more common autosomal recessive genetic disorder after CF
  • most common cause of infant death resulting from a genetic defect
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3
Q

SMA classification

A
  • diagnosis based usually on genetic testing of WBCs
  • SMA patients present with muscle wasting and many other symptoms
  • there are 5 types of SMA numbered 0-4 with zero being the most severe
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4
Q

type 0 SMA

A

is the most severe
affects only a small percentage of people
- we don’t usually talk about it and consider type 1 SMA to be most severe

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5
Q

what happens as SMA type number increases?

elaborate

A

the severity of the disorder decreases

  • type 1 has earlier onset and is more severe than the higher number types
  • as SMA type number increases, live expectancy increases too and onset is often later
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6
Q

SMN2 copy number and disease severity relationship?

also go write out the table of SMN copy numbers for each SMN type

A

as SMN2 copy number increases, disease severity decreases

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7
Q

do healthy individuals need SMN2?

A

no
we have SMN1
- in fact, we often have lower copy numbers of SMN2 but it’s irrelevant bc we have SMN1

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8
Q

what’s the number 1 genetic modifier for SMA?

A

SMN2 copy number for SMA patients

- it determines the severity of the disease

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9
Q

can someone be missing both SMN1 and SMN2?

A

nope, they would not survive

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10
Q

SMN1 and SMN2 copy number healthy individuals?

A

2 of each, one on each 5q

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11
Q

what are the % SMA pts with SMN2 copy numbers for:
SMA 1
SMA 2
SMA 3

A

SMA 1: 80% have 1 or 2 SMN2 copies
SMA 2: 80% have 3 SMN2 copies
SMA 3: 95% have 3 or 4 SMN2 copies

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12
Q

what’s the difference b/w SMN1 and SMN2?

A
  • there’s a C to T transition on exon 7 from SMN1 to SMN2 - it’s a sense mutation so the same AA is produced, but the transition affects pre-mRNA splicing so that exon 7 is not included in mature mRNA from SMN2
  • 80-90% of the time the premRNA code for a mature mRNA that produces dysfunctional protein
  • 10-20% of the time the premRNA includes exon 7 which codes for full length functional SMN protein
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13
Q

pre-mRNA splicing regulation of SMN1

A
  • exonic splicing enhancer (ESE) on 5’ end of exon 7 facilitates splicing of exons 6 and 7 together (this is good)
  • ESE attracts snRNPs and ASFs which aid in splcing
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14
Q

pre-mRNA splicing regulation of SMN2

A

C to T transition in exon 7 changes splicing enhancer (ESE) to splicing silencer (ESS) which inactivates ASF and promotes exclusion of exon 7

  • hnRNP1
  • ISS N1
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15
Q

hnRNP1

A

the main factor that bind to the ESS to suppress the splicing of exons which eliminates exon 7

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16
Q

ISS N1

A

intronic splicing silences N1

- contains 2 hnRNP A1 binding sites that have a much stronger effect on exon 7 exclusion than the C to T transition

17
Q

roles of SMN protein

A
  1. SMN complex - is important for snRNP assembly which are critical for splicing and processing mRNA
  2. transport of mRNAs from cell body down the axon where it needs to go
18
Q

spinraza/nusinersen facts

A
  • 1st drug approved for SMA in 2016
  • improves and prolongs quality and quantity of life
  • an ASO that increases the amount of SMN that can be produced by acting on SMN2, not SMN1
19
Q

what’s an ASO?

A

antisense oligonucleotide

  • a sequence of synthetic nucleotides generated in labs by biotech companies
  • drug is usually injected intrathecally (spinal canal into CSF)
  • antisense bc is complementary to the RNA
  • designed to be specific and stable
20
Q

nusinersen mechanism

A

binds to a site on intron 7 of SMN2 pre-mRNA:
- keeps splicing suppressor proteins hnRNP A1 and A2 from binding (ASO displaces them)
- allows U1 snRNPs to bind to 5 splicing sites
all in all, supports inclusion of exon 7 into SMN mRNA

21
Q

how well does nusinersen work to produce full length SMN protein? (%)

A

normally full length of an SMN protein produced by SMN2 is 20% but nusinersen increases this to 50%

22
Q

gel electrophoresis to measure SMN expression & length of SMN proteins technique

A
  • tissue sample put in reaction buffer to give it a negative charge
  • load sample into well with a thick gel matrix and then apply an electric field to the gel
  • molecules move from + to - to stretch out the molecule (mRNA or protein)
  • levels of mRNA or proteins in the sample were then identified using probes (oligonucleotide for mRNA and antibodies for proteins)
  • gel electrophoresis separates molecules according to size - smaller molecules move further
23
Q

does ASO nusinersen increase full length SMN protein?

what was measured to confirm this?

A

yes - in many tissues

  • measured % inclusion (the amount of full length SMN mRNA produced)
  • as drug dosage increased, % inclusion increased
24
Q

is there a dose-dependant relationship for ASO nusinersen efficacy?
is re-dosing necessary?

A

dose-dependent? yes - mice who received greater doses lived significantly longer
re-dosing? yes - mice had ASO effects decrease after 30 and 180 days, suggesting re-injections would be good

25
Q

how does ASO nusinersen improve quality of life (mechanism)?

A
  • the ASO increases the number of αMNs AND CSA of muscle fibres which makes them comparable to healthy individuals (mouse study)
  • ie. the increase in SMN caused by the ASO corresponds to an increase in the survival of MNs
26
Q

ASO nusinersen administration methods

which is better?

A

2 options:

  • ICV - intracerebroventricular
  • SC - subcutaneous
  • SC is better
27
Q

ICV vs SC administration of nusinersen

A

ICV

  • injection directly into CSF where ASO has direct effect on the brain
  • researchers found SMN increased in the CNS but not anywhere else (not muscle or liver)
  • doubled animals lifespan

SC

  • injection into peripheral NS, much less invasive
  • mice saw increased SMN in muscle, heat and other tissues while also increasing lifespan 25 times!
28
Q

can ASOs cross the blood brain barrier?

A

no. so subjects must be dosed while they are young when the BBB hasn’t completely formed yet - in mice anyway (can’t rely on SC injections after P5)

29
Q

what happened in phase 2 clinical trial when nusinersen was given to SMA infants?

A
  • drug found to be safe
  • hammersmith test found functional improvements (stat. sig) for subjects (better score = better performance)
  • nusinersen was able to move through CNS, along with other tissues of the body
  • kids who received drug had greater amount of full length SMN in SC after multiple intrathecal injections
30
Q

what might be a next step for drug companies and SMA research

A

look for a drug or ASO that can block the ESS found just upstream of exon 7 so that kids with SMA see even better functional improvements

31
Q

zolgensma - what is it?

A

a viral mediated gene therapy for SMA approved May 2019

- a virus delivers a functional copy of the SMN1 gene which has nothing to do with SMN2

32
Q

zolgensma - does it work? in comparison to nusinersen?

A

yes
- in studies, nusinersen increased survival rates in subjects compared to placebo group
BUT - everyone who received zolgensma survived
- subjects also see a 22% increase in function after 1 injection (only one needed ever) so it seems to be more effective than nusinersen

33
Q

McCartney 1988 study: details, significance, results?

A
  • @ Mcmaster
  • 1st study done where we specifically identified exercise effects on SMA pts
  • resistance training in type 3/4 SMA pts
  • found subject’s % improvement increased for everyone
  • patients got stronger and more fatigue resistant
  • found resistance training to be safe for SMA patients
34
Q

Madsen 2015 study: details, results?

A
  • aerobic training in SMA patients
  • fitness measured by VO2 max and outcomes measured by ADL questionnaire pre and post training
  • after training, SMA pts saw 25-30% increase in VO2max (2x that of healthy controls)
  • ADL questionnaire found training wasn’t quite as positive, resulted in a lot of fatigue, need to sleep in SMA patients
  • overall found exercise to be safe
35
Q

in mice with SMA, what are the effects of exercise? is running or swimming better? how was this identified?

A
  • both types of exercise increase lifespan, mice were stronger and more fatigue resistant
  • swimming was better than running for protecting medium and large diameter MNs (>600um) - ID’d by ChAT immunofluorescence
  • running was better at protecting smaller diameter MNs (<600um) - ID’d by estrogen related receptor Beta (ERRB)
  • all neuroprotective effects were independent from SMA expression - in the ventral horn of SC - there could be changes in SMA expression in other tissues