Biomedical Engineering Reference
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life. Recently, the gene therapy approach was taken to an advanced
level by using recombinant scAAV serotype 9 expressing a human
SMN cDNA. A single injection of 5 × 10
11
genomic copies of
scAAV9-SMN into temporal vein of SMN
7 mice on postnatal
day 1 (P1) resulted in a dramatic restoration of motor function and
neuromuscular physiology as well as a signifi cant increase in life
span up to 250 days [
3
]. In both studies, the transgene (human
SMN cDNA), time of application, and the mouse model (SMN
ʔ
7)
were identical. Therefore, the underlying mechanism for this dra-
matic difference in survival was the robust expression of SMN by
scAAV, wide transduction of the CNS by serotype 9, CBA pro-
moter expressing the transgene, and delivery route. We will discuss
all of the related gene therapy studies to emphasize the signifi cance
of each contributing factor in rescuing the SMA phenotype.
ʔ
In contrast to traditional single-stranded AAV (ssAAV), scAAV
vector packages a single viral genome possessing both sense and
antisense strands of the transgene. After uncoating in the trans-
duced cells, the single-stranded DNA folds into a double-stranded
DNA due to its self-complementary nature and forms an appropri-
ate Pol II template for transcription [
56
]. As a result, scAAV
achieves maximal expression levels within 48-72 h post-
transduction. Furthermore, in some instances, transduction effi -
ciency of scAAV is 10- to 100-fold higher than ssAAV [
57
,
58
]. An
alternative study demonstrated the magnitude of benefi ts using
scAAV versus ssAAV. ssAAV8 expressing human full-length SMN
was ICV injected into SMN
Advantages of scAAV
7 mouse model and extended life to
~50 days [
59
]. Interestingly, the expression of SMN using scAAV8
through ICV injection extended the survival up to 157 days [
59
].
Several studies have determined the temporal requirement for
SMN induction using transgenic SMA mice and AAV vectors.
These studies have concluded that restoration of SMN before the
symptomatic stage is necessary for a signifi cant increase in life span
[
3
,
60
,
61
]. Based on these results, it is not surprising that an effi -
cient rescue of SMA mice has never been achieved with other viral
vectors or traditional ssAAV, and thus far, scAAV is the most com-
petent viral vector for SMA gene therapy.
ʔ
AAV9 was shown to reach the CNS following intravenous adminis-
tration in neonatal mice and adult cats [
62
,
63
], suggesting that this
serotype might be well suited for the delivery of the SMN transgene
into SMA patients. The study by Passini et al. [
59
] which demon-
strated the superiority of scAAV to ssAAV also established the advan-
tages of serotype 9 to serotype 8. Mice treated with scAAV8-SMN
demonstrated highly improved motor function and neuromuscular
physiology compared to their control littermates but died prema-
turely compared to the mice injected systemically with scAAV9-
SMN (157 days vs. 250 days) as reported by Foust et al. [
3
].
Serotype 9 of AAV
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