A splice mutation is a specific error in a gene’s DNA sequence. Genes contain the instructions for building proteins, which perform a vast number of tasks in our bodies. These instructions are not continuous but are like a rough draft that requires editing. A splice mutation acts like a typo in the editing marks of this draft.
This error alters how the cell processes genetic instructions, causing important parts to be left out or meaningless sections to be included. Unlike mutations that change a single component, a splice mutation corrupts the entire assembly process, leading to significant errors in the final protein structure.
The Normal Gene Splicing Process
A gene stored in DNA is not a continuous set of instructions. It is composed of alternating segments called exons and introns. Exons are the coding regions that contain the actual instructions for building a protein, while introns are non-coding regions that must be removed. Think of exons as the scenes of a movie and introns as the outtakes.
The process begins with transcription, where the cell creates a preliminary copy of the gene, including both exons and introns, called pre-messenger RNA (pre-mRNA). This copy then undergoes an editing step known as splicing. During splicing, a molecular machine called the spliceosome recognizes the boundaries between exons and introns, precisely cutting out the introns and joining the exons together.
This editing creates a mature messenger RNA (mRNA) molecule. The mRNA is a continuous blueprint containing only the coding sequences from the exons. This final blueprint then exits the nucleus and travels to the cell’s protein-building machinery to be read.
How Splice Mutations Disrupt Splicing
Splice mutations introduce errors into the DNA that disrupt the splicing process. These mutations occur at or near the specific sequences that mark the beginning and end of an intron. These locations, known as the splice donor and acceptor sites, act as signals for the spliceosome, telling it where to cut the pre-mRNA. A mutation can alter these signals, making them unrecognizable.
When the spliceosome cannot identify the correct boundaries, it may fail to cut or cut at the wrong spot. Another way these mutations disrupt splicing is by creating cryptic splice sites. These are new, incorrect splice signals that appear within an intron or an exon. This new cryptic site can trick the spliceosome into making a cut where it shouldn’t, such as removing part of an exon.
Consequences for Protein Production
The errors in splicing have direct consequences for the final mRNA blueprint and protein production. Two common outcomes are exon skipping and intron retention. In exon skipping, the spliceosome fails to recognize an exon, so it is omitted from the final mRNA. This is like a chef skipping a line in a recipe and leaving out an ingredient.
Conversely, intron retention occurs when the spliceosome fails to remove an intron. The non-coding intron sequence is kept in the final mRNA, inserting a block of disruptive instructions. Both errors alter the reading frame of the mRNA, leading to a severely altered protein. The resulting protein may be too short (truncated) or contain incorrect amino acids that cause it to misfold and lose its function.
Associated Genetic Disorders
Many genetic disorders are caused by splice mutations that lead to the production of faulty proteins. Examples include:
- Spinal Muscular Atrophy (SMA): A defect in the SMN2 gene prevents exon 7 from being included in the final mRNA. This results in an unstable Survival Motor Neuron (SMN) protein, causing the loss of motor neurons.
- Cystic Fibrosis: Certain mutations in the CFTR gene disrupt proper mRNA splicing. This limits the production of the functional CFTR protein needed for chloride ion transport across cell membranes.
- Duchenne Muscular Dystrophy: Splice mutations in the dystrophin gene disrupt the production of the dystrophin protein, which is required for muscle fiber strength and stability.
- Cancers: Some cancers, such as myelodysplastic syndromes, are associated with mutations in splicing factors. These mutations cause widespread splicing errors that can contribute to malignant cell growth.
Therapeutic Strategies Targeting Splicing Errors
Scientific advancements have led to therapies that can correct errors caused by splice mutations. A leading approach involves drugs known as antisense oligonucleotides (ASOs). ASOs are short, synthetic strands of nucleic acids designed to bind to a specific sequence on a pre-mRNA molecule. They act like molecular tape to mask a problematic sequence. This intervention corrects the splicing process, allowing for the creation of a functional protein.
An example of this strategy is the drug nusinersen (Spinraza), used to treat Spinal Muscular Atrophy. Nusinersen is an ASO that binds to a specific site on the SMN2 pre-mRNA. This binding blocks a splicing silencer that causes exon 7 to be skipped. By masking this silencer, the drug ensures that exon 7 is included in the mature mRNA, leading to the production of the functional SMN protein.