162
Alternatively Spliced Genes
splicing transcripts. In addition, a trans-
splicing group I ribozyme was shown
to
convert
mutant
transcripts
to
nor-
mal mRNAs in the beta-globin and p53
genes.
4.2.3
SMaRT
Spliceosome-mediated RNA trans-splicing
(SMaRT) was developed utilizing the en-
dogenous trans-splicing activity in mam-
malian cells to correct aberrant splicing.
SMaRT-mediated repair was reported to
partially correct splicing defects in cys-
tic Fbrosis transmembrane conductance
(C±TR)
gene
in
cultured
cells
and
bronchial xenografts.
4.2.4
Chemical Compounds
A number of chemical compounds have
been shown to interact with RNA and/or
RNA-binding proteins. Using chemical
compounds to correct pre-mRNA splic-
ing defects is being actively explored as a
new therapeutic approach. Although the
underlying mechanisms remain unclear,
aclarubicin and sodium butyrate increase
the inclusion of exon 7 in SMN2 tran-
scripts in Fbroblasts derived from spinal
muscular atrophy patients or in transgenic
mice, suggesting the therapeutic potential
of chemical compounds in treating dis-
eases associated with defective splicing.
Other
chemical
compounds
are
be-
ing tested that are capable of modifying
splicing regulators, for example, kinases
or phosphatases, that influence SR pro-
tein functions.
5
Concluding Remarks
After more than two decades of studies, a
general picture of mammalian pre-mRNA
splicing and alternative splicing regulation
has begun to emerge. The basic com-
ponents of the mammalian splicing ma-
chinery have been identiFed. The highly
dynamic process of spliceosome assembly
involves multiple networks of RNA–RNA,
RNA–protein and protein–protein inter-
actions. Recognition of splice sites and
splicing regulatory sequences has been in-
vestigated in a number of genes, leading to
identiFcation and characterization of both
cis-acting regulatory elements and trans-
acting factors. We have begun to appreciate
the contribution of alternative pre-mRNA
splicing to creating genetic diversity, es-
pecially in mammals. A large number
of splicing mutations that cause human
diseases are being identiFed and charac-
terized. We now have a glimpse of the
complex picture of the involvement of
aberrant or defective splicing in the patho-
genesis of human diseases. ±urthermore,
efforts are being made to improve the diag-
nosis and treatment of diseases associated
with pre-mRNA splicing defects.
Despite the signiFcant progress, we are
only at the beginning stage of understand-
ing the molecular mechanisms controlling
pre-mRNA splicing and alternative splic-
ing regulation. A number of important
questions remain to be addressed.
±or the majority of genes, we have little
knowledge about their complete expres-
sion proFles of different splicing isoforms
in different cell types. We do not know how
the splicing events of different genes are
coordinated during development or in re-
sponse to environmental changes. How a
cell senses the environmental stimuli and
responds by producing different splicing
products remains largely unknown. Molec-
ular pathways that transduce extracellular
signals into the nuclear splicing machin-
ery have not yet been delineated. ±urther
development of new technologies such as
previous page 162 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 164 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off