1.5.6 Single Nucleotide Variant Definition

Single Nucleotide Variant Definition is the description of a genetic alteration in which a single position within a DNA sequence differs from the corresponding position in a reference or normal sequence, representing the smallest possible unit of sequence-level genetic change and one of the most common forms of alteration observed in cancer genomes. A single nucleotide variant involves the substitution of one DNA base for another at a specific location, without any accompanying insertion or deletion of surrounding genetic material.

The Basic Nature of a Single Nucleotide Variant

A Change at One Specific Position

A single nucleotide variant is defined by its precise location, referring to an alteration occurring at one particular position within the genome, where the base normally present has been replaced by a different base.

Minimal Scale of Change

Compared with larger forms of genetic alteration, such as insertions, deletions, or structural rearrangements, a single nucleotide variant represents the smallest possible scale of sequence change, affecting only one position rather than a stretch of surrounding sequence.

Types of Single Nucleotide Variants by Functional Consequence

Variants That Change the Encoded Amino Acid

Some single nucleotide variants alter the specific instruction carried by that portion of the gene in a way that changes which building block is incorporated into the resulting protein, potentially affecting the protein's structure or function.

Variants That Do Not Change the Encoded Amino Acid

Other single nucleotide variants occur in a way that does not change the resulting protein product, since the genetic code contains some redundancy that allows certain sequence changes to have no effect on the protein ultimately produced.

Variants That Disrupt Normal Gene Processing

Certain single nucleotide variants occur at positions critical to how a gene's information is processed and assembled into a final functional product, and changes at these specific positions can significantly disrupt normal gene function even though only one base is affected.

Single Nucleotide Variants as Drivers or Passengers

Potential to Function as Driver Alterations

A single nucleotide variant can function as a genetic driver alteration if it occurs within a functionally important position of a gene relevant to cancer development and produces a meaningful change in that gene's activity or regulation.

Frequent Occurrence as Passenger Alterations

Because single nucleotide variants are a common and relatively frequent form of spontaneous genetic change, the great majority of those found within a cancer genome typically qualify as passenger alterations, lacking any meaningful functional consequence.

Relevance to Cancer Cell Biology

Single nucleotide variants represent one of the most extensively studied and readily detectable categories of genetic alteration in cancer, providing a precise, position-specific unit of genetic change that can be systematically cataloged, compared across many tumors, and evaluated for its potential role as a functionally significant driver within the broader landscape of cancer cell genetic alteration.