Alternative splicing is a regulated process in which a single gene can give rise to multiple messenger RNA (mRNA) variants and therefore multiple protein products. During gene expression the initial transcript contains both exons (coding segments) and introns (noncoding segments); by selecting different combinations of exons, cells produce distinct mature mRNAs that are later translated into different proteins. This capacity expands the functional repertoire of genomes without increasing gene number.
Mechanism
Alternative splicing occurs during the processing of the primary RNA transcript after transcription and as part of the broader process of RNA splicing. The spliceosome, a large ribonucleoprotein complex, recognizes short sequence cues at exon–intron boundaries and catalyzes removal of introns and ligation of exons. Decisions about which splice sites are used depend on sequence motifs in the RNA (cis-elements) and on proteins or RNAs that bind them (trans-acting factors).
Common patterns
There are several recurring ways splice patterns are varied; these produce predictable types of mRNA isoforms. Typical categories include:
- Exon skipping (cassette exons) — an exon may be included or omitted.
- Mutually exclusive exons — two alternative exons are never included together.
- Alternative 5' or 3' splice sites — change the exon boundary and thus coding frame or UTR length.
- Intron retention — an intron remains in the mature RNA and may affect translation or stability.
- Alternative promoters or polyadenylation — produce different first or last exons and modify regulatory regions.
Regulation
Splicing is controlled by activator and repressor proteins that bind enhancers and silencers within exons or introns. Families of factors such as SR proteins and heterogeneous nuclear ribonucleoproteins (hnRNPs) often promote or inhibit particular splice sites. Splicing choices are also influenced by the speed of transcription, chromatin structure and RNA secondary structure, allowing integration of developmental, tissue-specific and environmental signals.
Biological importance and examples
Alternative splicing is widespread in eukaryotes, notably in animals: most human multi-exon genes produce multiple splice isoforms, contributing to cell-type specialization, developmental transitions and adaptability of signaling pathways. Different isoforms can alter enzymatic activity, subcellular localization, interaction partners or regulation by other molecules, so splicing provides a versatile means of modulating protein function.
Clinical relevance and quality control
Errors in splicing or mutations that affect splice sites underlie many human genetic disorders and can contribute to cancer by creating aberrant protein variants. Cells use surveillance pathways such as nonsense-mediated decay to eliminate transcripts with premature stop codons produced by faulty splicing. Therapeutic approaches that modify splicing — for example antisense oligonucleotides that mask or restore splice sites — are an active area of medical research.
Origins and ongoing research
The recognition that single genes can yield multiple proteins emerged with the discovery of split genes and introns in the late 20th century; since then high-throughput RNA sequencing has revealed far greater complexity and context-dependence than previously appreciated. Current research maps tissue-specific splice programs, deciphers the regulatory code, and seeks to exploit splicing modulation for diagnostics and treatments.