7.6 Controlled Splicing in S. cerevisiae 145alternative splicing are more frequent. Examples are mutations in TDP-43 and
FUS, which are connected to amyotrophic lateral sclerosis (ALS) and other neu-
rodegenerative disorders [203, 204].
Consequently, modulation of disease-causing aberrant splicing is used as a
therapeutic approach [196, 205]. In spinal muscular atrophy, a motor neuron
disease, the smn2 gene is the only source for the essential survival motor neuron
(SMN) protein due to an inactivation of smn1. Inefficient inclusion of exon 7 in
the smn2 mRNA, due to a silent mutation disrupting an ESE, leads only to the
production of residual amounts of full-length protein [206]. Antisense oligonu-
cleotides (ASOs) have been developed that force inclusion of exon 7 by masking
a downstream ISS. ASOs are small oligonucleotides that base pair with exons,
splice sites, or splicing factor binding sites to subsequently modulate splicing
decisions [205]. This leads to the increased production of functional SMN pro-
tein, resulting in enhanced motor neuron function and survival (from 10 to
500 days) in a mouse model of severe disease [207]. One of these ASOs is now
the first antisense drug functioning via splicing correction and the first FDA-
approved treatment for SMA [208]. In general, ASOs show high efficacy, delivery
to several tissues, the ability to cross the blood–brain barrier and, until now, no
severe side effects, making them promising new therapeutics for the treatment
of splicing-related diseases.
7.6 Controlled Splicing in S. cerevisiae
7.6.1 Alternative Splicing
Alternative splicing events in S. cerevisiae are rare with only three examples
known so far. The most extensively alternatively spliced gene is the nuclear
export factor mtr2 [33]. Mtr2 contains an intron in its 5′ UTR, which includes
two 5′ splice sites and three 3′ splice sites. Five of the six possible combinations
and the unspliced transcript are detectable. The six different transcripts either
encode proteins with different N-termini or 5′ UTRs containing differing num-
bers (up to three) of upstream open reading frames (uORFs). The function of
these different encoded proteins/5′ UTRs or how splice site selection is regulated
is unknown.
A further example for alternative splice site usage in S. cerevisiae is src1. SRC1
acts in sub-telomeric gene expression and TREX-dependent mRNA export
[209]. Its intron contains two overlapping 5′ splice sites: GCAAGUGAGU (No. 1
underlined, No. 2 bold [210]). Usage of the downstream 5′ splice site results in
the expression of a long protein isoform that codes for two transmembrane
domains [209]. Usage of the upstream 5′ splice site results in a shorter protein
with only one transmembrane domain and reduced activity. Again, it is not
known how (and if ) splice site selection is regulated.
In S. cerevisiae, three SR-like homologs (NPL3, HRB1, and GBP2) and one
hnRNP-like protein (HRP1) have been identified. Mutagenesis studies indicate
that only NPL3 may be involved in splicing [211]. However, RNA-binding pro-
teins important for the splicing of individual transcripts have been reported.