Every function in our bodies is guided by instructions encoded within our cells. At the heart of these instructions is DNA, the genetic blueprint. This genetic information flows from DNA to RNA, and then to proteins, a concept known as the Central Dogma of molecular biology. Proteins are the cell’s workhorses, performing functions like catalyzing metabolic reactions, responding to stimuli, and providing structural support.
From Gene to Protein: The Splicing Process
A gene, a segment of DNA, contains the instructions for building a specific protein or functional RNA molecule. In eukaryotic cells, the initial RNA molecule transcribed from a gene, called pre-messenger RNA (pre-mRNA), is not immediately ready for protein synthesis. This pre-mRNA contains both coding regions, known as exons, and non-coding regions, called introns. Introns are non-coding sequences that must be removed for the mRNA to be properly translated.
The process of removing introns and joining exons together to form a mature messenger RNA (mRNA) molecule is called RNA splicing. This precise cutting and pasting is carried out by the spliceosome, a molecular machine made up of proteins and small RNA molecules. The spliceosome recognizes specific marker sequences at the ends of introns, ensuring their accurate removal and the correct joining of the flanking exons. Once splicing is complete, the mature mRNA can then leave the nucleus and be used as a template for protein synthesis in the cytoplasm.
The Concept of Alternative Splicing
While standard splicing produces a single type of mRNA from a gene, alternative splicing introduces a powerful layer of complexity. Alternative splicing is a process where different combinations of exons from the same pre-mRNA molecule are included or excluded in the final mature mRNA. This means that a single gene can give rise to multiple distinct mRNA transcripts, which in turn can be translated into various protein forms, often referred to as isoforms.
Alternative splicing occurs in several ways:
Exon skipping: One or more exons are left out of the final mRNA molecule.
Intron retention: A specific intron is kept within the mature mRNA, rather than being removed.
Other variations include the use of alternative 5′ or 3′ splice sites, which can shorten or lengthen an exon, leading to different protein sequences.
Mutually exclusive exons: Only one of two exons is retained in the mRNA.
These different splicing patterns are regulated by specific DNA sequences near the gene, called cis-acting elements, and by other molecules that bind to the DNA, known as trans-acting factors.
How Alternative Splicing Expands Protein Diversity
Alternative splicing increases the diversity of proteins an organism can produce from a limited number of genes. For example, it is estimated that over 90% of human genes undergo alternative splicing, allowing the human genome to generate far more proteins than its approximately 20,000 protein-coding genes would suggest. This process allows cells to create different protein versions with varied functions or properties.
These protein variations enable different cell types within an organism to perform specialized functions, even though they share the same underlying genetic code. For instance, one protein isoform might be active in a particular cellular pathway, while another isoform from the same gene might inhibit that pathway. This adaptability also allows organisms to respond effectively to diverse environmental conditions and contributes to biological complexity.
Alternative Splicing and Human Health
The regulation of alternative splicing is important for normal physiological processes. However, errors or dysregulation in this process can have consequences for human health. Aberrant alternative splicing can lead to the production of non-functional or harmful protein isoforms, disrupting normal cellular activities.
Such disruptions are implicated in many human diseases, including various cancers, where changes in splicing patterns can promote tumor growth or resistance to treatment. Neurodegenerative disorders, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and spinal muscular atrophy, are also linked to dysregulated alternative splicing. Alternative splicing defects are associated with approximately 15% of human genetic diseases, including conditions like Hutchinson-Gilford progeria syndrome and beta-thalassemia. Understanding the mechanisms of alternative splicing and how it goes awry in disease states is an avenue for developing new diagnostic tools and therapeutic strategies.