Single gene sequencing is a precise method used to read the specific order of DNA building blocks within a single, targeted gene. This process aims to identify genetic variations, such as changes in the DNA sequence, small deletions, or duplications, that might be linked to particular health conditions.
Understanding Single Gene Sequencing
Clinicians choose to focus on a single gene when there is a strong indication that a specific genetic condition is caused by variations within that gene. This targeted approach is efficient when a patient’s symptoms or family history point to a particular inherited disorder. The principle behind this method involves identifying changes in the DNA sequence of the chosen gene. These changes, called variants, can include single nucleotide variations, where one DNA building block is swapped for another, or larger alterations like deletions or duplications. Single gene sequencing specifically examines the exons, which are the protein-coding regions of the gene, because most single gene conditions result from variants in these sequences.
The Process of Single Gene Sequencing
The process of single gene sequencing begins with the collection of a biological sample, such as blood or saliva. DNA is extracted from the cells. This extracted DNA then undergoes amplification using Polymerase Chain Reaction (PCR) to create many copies of the targeted gene. After amplification, the sequencing reaction takes place using methods like Sanger sequencing or next-generation sequencing. In Sanger sequencing, modified nucleotides are introduced, which stop the DNA synthesis at specific points, creating DNA fragments of varying lengths that are then separated by size, with their fluorescent labels read by a machine to determine the sequence of the DNA building blocks.
Key Applications of Single Gene Sequencing
Diagnosing Genetic Conditions
Single gene sequencing diagnoses genetic conditions when a specific gene is suspected. For example, it identifies variants in the CFTR gene for cystic fibrosis, the HTT gene for Huntington’s disease, or genes associated with muscular dystrophy. This confirms a diagnosis, allowing for medical management and family counseling.
Carrier Screening
The technique is also valuable for carrier screening, which identifies individuals who carry one copy of a gene variant for a recessive disorder without showing symptoms themselves. This is particularly relevant for family planning, as it helps prospective parents understand the risk of passing on a genetic condition to their children. For instance, identifying specific variants in the PAH gene for phenylketonuria (PKU) allows for testing of other family members.
Pharmacogenomics and Cancer Syndromes
Pharmacogenomics utilizes single gene sequencing to predict an individual’s response to certain medications. Variations in specific genes can influence how a person metabolizes or reacts to a drug, guiding clinicians in selecting the most effective and safest treatment. Furthermore, single gene sequencing is applied in identifying familial cancer syndromes, such as inherited predispositions to breast and ovarian cancer linked to variants in the BRCA1 and BRCA2 genes.
Single Gene vs. Broader Genetic Tests
Single gene sequencing offers a highly focused approach, examining only one specific gene when there is a strong clinical suspicion of a condition linked to that gene. This method is generally cost-effective and provides deep coverage of the targeted gene, meaning it can detect even subtle variations. It is often chosen when a patient’s symptoms are highly indicative of a particular genetic disorder.
In contrast, broader genetic tests like whole exome sequencing (WES) and whole genome sequencing (WGS) analyze a much larger portion of an individual’s genetic material. WES focuses on the exome, which includes all the protein-coding regions of approximately 20,000 genes, representing about 1-2% of the entire genome. WGS, on the other hand, sequences nearly the entire genome, encompassing both coding and non-coding regions. These broader tests are typically employed when the genetic cause of a condition is less clear or when a more comprehensive genetic overview is desired.