- Exon skipping is one of the most important forms of alternative RNA splicing, a process that allows cells to produce different RNA transcripts from the same gene. During exon skipping, one or more exons present in a newly transcribed pre-mRNA are not included in the final mature mRNA. As a result, the mature RNA contains a different combination of exons and can potentially produce a different protein isoform. Exon skipping is a normal and highly regulated part of gene expression, although abnormal exon skipping caused by genetic variants or changes in splicing regulation can contribute to disease.
- To understand exon skipping, it is useful to first understand the organization of a gene. Many eukaryotic genes are transcribed into a precursor messenger RNA, or pre-mRNA, containing both exons and introns. Introns are removed during RNA splicing, while exons are generally retained in the mature RNA. However, not every exon has to be included in every mature transcript. When the cellular machinery selectively excludes an exon while joining the surrounding exons together, the process is known as exon skipping.
- Exon skipping depends on the accurate recognition of splice sites and other regulatory signals within the pre-mRNA. The spliceosome identifies important sequence features around introns, including the 5′ splice site, branch point, polypyrimidine tract, and 3′ splice site. At the same time, regulatory sequences known as splicing enhancers and silencers can influence whether a nearby exon is recognized and included. These elements provide binding sites for regulatory proteins and help determine which portions of the pre-mRNA will be incorporated into the mature transcript.
- The spliceosome is the central molecular machinery responsible for carrying out RNA splicing. It is a dynamic complex containing small nuclear RNAs and proteins organized into small nuclear ribonucleoproteins, or snRNPs. The major spliceosome includes U1, U2, U4, U5, and U6 snRNPs. During splicing, these components assemble around the pre-mRNA, recognize relevant splice signals, and catalyze the removal of introns and joining of exons. In exon skipping, differences in how splice sites and exons are recognized can result in an exon being bypassed while the surrounding exons are joined.
- Splicing factors play an important role in deciding whether an exon is included or skipped. Proteins such as SR proteins and heterogeneous nuclear ribonucleoproteins, or hnRNPs, can bind regulatory sequences in or around exons and introns. Depending on the particular protein, its binding location, and the cellular context, these factors can promote or inhibit exon recognition. The combined activity of many regulatory proteins helps determine the final pattern of RNA splicing.
- An exon that is alternatively included or skipped is often called a cassette exon. A cassette exon can be included in one mature mRNA but excluded from another produced from the same pre-mRNA. This creates different transcript versions without requiring a different gene. Cassette exon regulation is especially important in complex organisms, where different tissues and cell types frequently require different sets of proteins or protein variants.
- Exon skipping is therefore closely connected to alternative splicing. Alternative splicing can involve several different mechanisms, including exon skipping, alternative 5′ splice-site selection, alternative 3′ splice-site selection, mutually exclusive exons, and intron retention. Exon skipping is particularly common because it provides a relatively straightforward way of changing the structure of a mature mRNA. The resulting transcripts may encode different protein isoforms or may differ in stability, localization, or regulatory properties.
- The effect of skipping an exon depends strongly on its sequence and position. If the skipped exon contains a number of nucleotides that is a multiple of three, its removal may preserve the downstream reading frame. This can produce a protein that lacks a specific region while retaining much of the original sequence. Such an in-frame exon skipping event can therefore produce a shorter protein isoform. In contrast, if exon removal changes the reading frame, a frameshift can occur. This may introduce an abnormal amino acid sequence followed by a premature stop codon and can substantially alter or prevent production of a functional protein.
- Not every exon is entirely protein-coding. Some exons contain portions of the untranslated regions, or UTRs, of an mRNA. Therefore, exon skipping does not necessarily change the amino acid sequence of a protein. It can also influence how an mRNA is translated, localized, stabilized, or degraded. Changes in UTR-containing exons can therefore affect gene expression at levels beyond the protein-coding sequence itself.
- The decision to include or skip an exon can vary between tissues. A particular exon may be included efficiently in one cell type but skipped more frequently in another. This tissue-specific regulation allows the same gene to contribute to different cellular functions. Exon skipping can also change during development as cells differentiate and acquire specialized characteristics. In this way, alternative splicing provides an additional layer of gene regulation beyond simply determining whether a gene is transcribed.
- The cellular environment can also influence exon inclusion and skipping. Changes in the concentration or activity of splicing factors, signaling pathways, transcriptional activity, and RNA-binding proteins can alter how a pre-mRNA is processed. Because transcription and RNA processing can occur in a coordinated manner, the speed of transcription and the chromatin environment can sometimes influence the opportunities available for spliceosome assembly and exon recognition.
- Exon skipping can occur as part of normal biological regulation, but abnormal exon skipping can also result from genetic mutations or other sequence changes. A mutation may directly disrupt a splice site or alter a nearby regulatory sequence that normally promotes exon inclusion. Other variants may create or strengthen regulatory elements that favor exon exclusion. The consequence can be an abnormal mRNA containing an incorrect combination of exons.
- Some abnormal transcripts produced through exon skipping may contain a premature termination codon. In such cases, the cell may recognize the transcript as defective and target it for degradation through a quality-control pathway known as nonsense-mediated decay. This can reduce the amount of abnormal mRNA available for translation. The biological effect of a splicing mutation therefore depends not only on the RNA sequence produced but also on how the cell processes and regulates that transcript.
- Exon skipping has particular importance in the study of genetic disorders because changes in splicing can alter the amount, structure, or function of important proteins. Researchers can investigate these changes using techniques such as RNA sequencing, which allows different transcript forms to be detected and compared. Bioinformatics methods can then be used to identify exon-inclusion patterns, alternative transcripts, and changes in splicing between tissues, developmental stages, or disease states.
- Exon skipping is also an important area of therapeutic research. One approach uses short nucleic-acid molecules called antisense oligonucleotides to bind specific RNA sequences and alter how the splicing machinery recognizes an exon. Depending on the target and design, an antisense oligonucleotide can encourage the cellular machinery to skip a particular exon. This strategy can sometimes be useful when removing a specific exon produces a transcript or protein with a more favorable functional outcome than the original abnormal transcript. The precise therapeutic application depends on the gene, mutation, exon, and resulting protein.
- A well-known biological example of exon skipping involves the SMN2 gene, where alternative splicing determines whether exon 7 is included in the mature transcript. Most SMN2 transcripts naturally exclude exon 7, although a smaller proportion include it. This illustrates how relatively subtle differences in RNA sequence and splicing regulation can have important biological consequences. Another important example is the study of dystrophin transcripts in Duchenne muscular dystrophy, where specific exon-skipping strategies have been investigated to alter the resulting dystrophin protein. These examples demonstrate the broader principle that manipulating RNA splicing can sometimes change the consequences of genetic variation.
- Exon skipping is not limited to disease biology. It contributes to the normal diversity of proteins and RNA molecules found in multicellular organisms. Different exon combinations can allow a single gene to contribute to multiple molecular functions. However, it is important to distinguish between different RNA transcripts and functional proteins: an alternatively spliced transcript does not necessarily produce a stable or biologically active protein. Some transcripts may be rapidly degraded or translated inefficiently, while others may generate important protein isoforms.
- The evolution of exon skipping may also contribute to biological complexity. Changes in splicing regulatory sequences can modify how exons are used without necessarily changing the underlying gene itself. Over evolutionary time, changes in exon-inclusion patterns can contribute to differences in gene regulation, protein structure, and organismal traits. Comparative genomics and transcriptomics can help researchers investigate how alternative splicing patterns have evolved across species.
- Modern transcriptomic technologies have made it possible to study exon skipping at much greater resolution. RNA sequencing can reveal which exons are present in individual transcripts, while computational analysis can estimate how frequently particular exons are included or skipped. Long-read sequencing can provide additional information by capturing longer RNA molecules and helping researchers determine which combinations of exons occur within the same transcript. Single-cell approaches can further reveal differences in exon usage between individual cell populations.
- The relationship between exon skipping and the rest of RNA biology can be understood through the broader pathway of gene expression. DNA provides the genetic information, transcription produces pre-mRNA, RNA processing modifies the transcript, and RNA splicing removes introns and joins selected exons. Alternative splicing then allows different exon combinations to be generated. The resulting mature mRNA can move toward translation, where its sequence is interpreted according to the genetic code to determine an amino acid sequence.
- Exon skipping therefore represents an important connection between gene structure, RNA processing, gene regulation, and protein diversity. It demonstrates that the information contained within a gene is not used in only one fixed way. Instead, cells can regulate how that information is assembled into mature RNA, allowing different transcripts to be produced under different biological conditions. When this regulation works normally, exon skipping contributes to cellular specialization and biological complexity; when it is disrupted, it can contribute to abnormal gene expression and disease.
- Understanding exon skipping also provides a foundation for exploring other forms of alternative RNA splicing. Related mechanisms include the use of alternative splice sites, mutually exclusive exons, and intron retention, in which an intron that would normally be removed remains in the mature transcript. Together, these mechanisms demonstrate how extensively cells can regulate RNA processing after transcription. Studying these processes helps researchers understand how genes produce diverse RNA and protein products and how changes in RNA processing can influence human biology and disease.
- As research into RNA biology continues, exon skipping remains an important subject in genetics, molecular biology, transcriptomics, and therapeutic development. Its study connects fundamental mechanisms such as splice-site recognition and spliceosome activity with broader concepts such as alternative splicing, protein isoforms, genetic variation, and gene regulation. Understanding how cells decide whether an exon is included or skipped is therefore an essential step toward understanding how genetic information is transformed into the diverse biological functions observed in living organisms.
