Biomedical Engineering Reference
In-Depth Information
processes, interacting with an ever-growing list of RNA-associated
proteins [
31
]. SMN expression is ubiquitous with an intimate
involvement in a well-defi ned cellular process that relates to global
gene expression: snRNP biogenesis [
32
,
33
]. Since snRNPs are the
building blocks of pre-mRNA splicing machinery, it has been dif-
fi cult to resolve the apparently “general” snRNP activity with the
perceived motor neuron-specifi c pathology of the disease. However,
recent reports have demonstrated that different snRNP complexes
are differently affected by reduced SMN levels [
34
]. The recent
identifi cation of specifi c gene targets which are dysregulated in
SMN-defi cient models including
Drosophila
and mice has sup-
ported the hypothesis that SMA develops due to splicing abnor-
malities [
34
,
35
]. Additionally, SMN plays a role in neuronal
development such as growth cones and neurites [
36
,
37
] and func-
tions to transport beta-actin mRNA along developing axonal
extensions [
38
]. Even though the exact role that SMN loss of
function plays in disease progression is under a considerable debate,
therapeutic interventions have demonstrated that the restoration
of SMN protein very early on in motor neurons along with periph-
eral organs is the key for effi cient rescue [
39
].
In contrast to humans, rodents have a single copy of the
SMN
gene.
Murine
Smn
encodes a “C” at the 6th position of exon 7 and is,
therefore, analogous to human
SMN1
. The severe model of SMA
lacks murine
Smn
and contains two copies of human
SMN2
(
mSmn
−/−
,
hSMN2
+/+
). The gravity of disease symptoms in this
model leads to death 4-6 days postbirth [
40
]. The genotype of
SMN
1.1.3 SMA
Mouse Models
7 mice is identical to the severe model except for the addi-
tion of two copies of SMN
ʔ
ʔ
7 cDNA (
mSmn
−/−
,
hSMN2
+/+
,
SMN
7
+/+
) that lessens the disease severity [
41
]. This model still
demonstrates a severe phenotype with disease onset at 5-6 days and
an extended life span (13-16 days) compared to the severe model.
ʔ
The most common gene therapy approaches toward treating SMA
involves using therapeutic RNAs, antisense oligonucleotides
(ASO), and
trans
-splicing RNAs, to correct the faulty splicing of
the
SMN2
pre-mRNA. Several regulatory regions adjacent to the
exon 7 of
SMN
genes determine the constitutive (
SMN1
) or alter-
native (
SMN2
) splicing of
SMN
pre-mRNA (Fig.
1a
). One most
important element is a splice silencer (ISS-N1), located within
intron 7 inhibiting the inclusion of exon 7 in
SMN2
pre-
mRNA. Many ASOs were designed to act as an inhibitor of ISS-N1
to facilitate inclusion of exon 7 leading to the generation of a full-
length mRNA from
SMN2
gene [
42
-
49
]. Inhibition of ISS-N1
has been greatly improved by using modifi ed ASOs such as
1.1.4 Therapeutic
Interventions in SMA
2′
-O-2-methoxyethyl-ASO [
46
] as well as phosphorodiamidate
morpholino (PMO) ASO [
48
,
49
], both leading to a great level of
rescue using ICV injection [
48
,
49
] or combination of ICV and
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