160
Alternatively Spliced Genes
the
SMN2
gene is not usually mutated
in SMA patients, studies on stimulating
SMN2
exon 7 alternative splicing suggest
a potential therapeutic approach based
on the activation of alternative splicing
of genes homologous to mutated genes
(Sect. 4.2).
Myotonic dystrophy (DM), an autoso-
mal dominant disease, is an example of
trans-acting genetic mutations in which
remarkable progress has been made. DM
is a most common form of muscular
dystrophy affecting both skeletal muscle
and smooth muscles. Type I myotonic
dystrophy (DM1) is caused by a CTG
trinucleotide
expansion
in
the
3
0
un-
translated region (3
0
UTR) of the DMPK
(DM protein kinase) gene on chromo-
some 19q13.3. Type II myotonic dystrophy
(DM2) is associated with a large CCTG
repeat expansion in the intron 1 of the
ZNF9
gene. An ‘‘RNA gain-of-function’’
hypothesis has been proposed that these
CTG or CCTG repeat expansions cause
the
formation
of
aberrant
RNA
tran-
scripts containing large CUG or CCUG
repeats. Such RNA transcripts containing
long tracks of CUG/CCUG repeats may
disrupt the normal function of certain
RNA-binding proteins and induce sec-
ondary aberrant splicing of other genes.
The RNA-binding proteins involved are
likely to be the splicing regulators of CUG-
BP family. The disruption of CUG-BP
protein functions leads to aberrant splic-
ing of genes including cardiac troponin T,
insulin receptor, muscle-speciFc chloride
channel, and tau. The aberrant splicing of
these downstream genes can explain the
cardiac phenotype, insulin resistance and
myotonia found in DM patients. These
Fndings demonstrate the complex roles of
alternative splicing regulation in human
pathogenesis.
Several
trans-acting
splicing
defects
have been reported in autosomal domi-
nant retinitis pigmentosa (adRP). Recent
genetic studies demonstrate that muta-
tions in genes encoding general splic-
ing factors such as HPRP3, PRPC8, or
PRP±31
cause
adRP.
It
remains
un-
clear why these seemingly general defects
in
the
splicing
machinery
cause
such
neuron-speciFc diseases. The underlying
pathogenetic mechanisms await further
investigation.
4
Perspectives on Diagnosis and Treatment
of Diseases Caused by pre-mRNA Splicing
Defects
4.1
Diagnosis of Human Diseases Caused by
Splicing Defects
In
the
past
three
decades,
signiFcant
advances have been made in the diagno-
sis of human genetic diseases, including
diseases caused by splicing mutations.
However, currently available molecular di-
agnostic tools remain limited for detecting
splicing mutations.
DeFnitive diagnosis of splicing diseases
should be based on the following criteria.
±irst, the clinical manifestations should
correlate with defects in a given gene. Sec-
ond, corresponding splicing mutations are
detected in genomic DNA samples of the
patients. Third, aberrant or defective splic-
ing products should be detected in the
affected tissues or cells from patients. De-
tection of genomic mutations relies on
DNA sequence analysis. In most cases, ef-
forts in genomic DNA sequence analysis
have been focusing on either exonic re-
gions or near splice junctions. Detection
of aberrant or defective splicing products
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