A loss-of-function (LOF) mutation is an alteration in the DNA sequence that results in the gene’s protein product being either completely absent or significantly impaired in its function. This change reduces or eliminates the gene’s biological activity, which can disrupt cellular processes and lead to a disease state.
Defining Loss of Function
The biological outcome of an LOF mutation is a measurable decrease in the activity of the resulting protein. The most severe form is the null allele, or amorph, which causes a complete absence of function, often because no stable protein is produced. A less severe outcome is a hypomorphic allele, which involves a partial reduction in function. In this scenario, the protein is still made but works inefficiently or at a much lower level than normal. The distinction between a null and a hypomorph is important because a partial reduction in function may be tolerated by the cell, while a complete loss often leads to a more severe disease.
Molecular Mechanisms That Cause Inactivation
The specific change in the DNA sequence dictates how a gene’s function is lost. Frameshift mutations are highly disruptive, occurring when an insertion or deletion of one or two nucleotides shifts the entire reading frame. Since the ribosome reads the genetic code in triplets, this shift causes every subsequent amino acid to be incorrect, almost always resulting in a non-functional, truncated protein.
Nonsense mutations are caused by a single-base substitution that converts an amino acid codon into a premature stop codon. This premature signal halts protein synthesis, yielding a shortened protein product that typically lacks the necessary structure to function. In many cases, the cell recognizes these faulty messenger RNA molecules and degrades them through nonsense-mediated decay, preventing the production of any protein.
Large deletions can remove entire exons or the entire gene sequence, leading to a complete absence of the protein. Regulatory mutations occur outside the protein-coding sequence, often in promoter or enhancer regions. These mutations do not change the protein’s structure but prevent the cell’s machinery from efficiently reading the gene, dramatically reducing the amount of functional protein produced.
How Loss of Function Affects Inheritance
The way an LOF mutation causes disease depends on how much protein is required for normal cellular function. In recessive disorders, the body is typically haplo-sufficient, meaning one functional copy of the gene produces enough protein to prevent disease. The disease only manifests if a person inherits two non-functional copies, one from each parent, as seen in conditions like cystic fibrosis.
A more complex inheritance pattern arises when the cell is haplo-insufficient, meaning a 50% reduction in protein level is enough to cause disease. In these cases, an LOF mutation in just one of the two gene copies results in a dominant disorder, such as many developmental syndromes. The single remaining working copy cannot produce enough protein to maintain health, causing the condition to appear.
The dominant negative effect occurs when the mutant protein actively interferes with the function of the normal protein produced by the healthy gene copy. This is often observed when proteins must assemble into multi-unit complexes, such as channels or structural components. The faulty protein incorporates itself into the complex, rendering the entire assembly non-functional and exerting a dominant effect.
Therapeutic Strategies for Restoring Function
Addressing diseases caused by LOF mutations often focuses on replacing the missing or non-functional protein. Gene therapy aims to deliver a functional copy of the affected gene into the patient’s cells using a viral vector. The new gene acts as a template, allowing the cells to produce the correct protein and restore the missing function, which is particularly effective for null mutations.
Protein replacement therapy involves the regular infusion of the correct, laboratory-produced protein for diseases where the protein is missing but can function outside the cell. This approach is common for lysosomal storage disorders or certain enzyme deficiencies where the missing component circulates in the bloodstream. While not a permanent cure, it effectively manages the deficiency by temporarily restoring the necessary protein levels.
Small molecule drugs offer another strategy, particularly for hypomorphic mutations where residual function remains. These drugs can act as pharmacological chaperones, helping a misfolded protein fold correctly and traffic to its proper cellular location. Other small molecules may be designed to enhance the activity of the partially functional protein, boosting its output enough to reach a therapeutic level.