Clinical Exome

General Description

The genome, which is the DNA sequence of the human organism, is composed of small portions called exons, which are separated by introns that lack genetic information. The set of all exons in a genome, responsible for producing proteins, is called the exome. Clinical exome refers to the sequencing or genetic study of a person's exome. This analysis is highly relevant from a medical perspective because it enables the detection of genetic mutations associated with numerous diseases.

There are different types of exome studies depending on the number of genes analyzed and the characteristics of the patient:

• Targeted clinical exome: Specific genes are selected for analysis based on the patient’s symptoms, as it is performed to confirm or rule out a specific disease.
• Comprehensive clinical exome: The exomes of all genes associated with a pathology are analyzed. The primary objective is to provide guidance on the prognosis and treatment of a patient with limited clinical information.
• Whole exome: All known exons are sequenced, usually for research purposes.

Thanks to advances in genetic medicine, particularly next-generation sequencing (NGS) technology, exome analysis has become simpler and faster. With this technique, approximately 85% of known mutations can be analyzed, leading to the diagnosis of diseases in patients presenting with relevant symptoms.

When Is It Indicated?

Exome sequencing is one of the best tools for early diagnosis of genetically-based diseases, allowing for the identification of the most appropriate treatment as soon as possible to reduce symptoms and improve patient quality of life. It is particularly indicated for:

• Complex diseases that may be caused by alterations in multiple genes.
• Pathologies with non-specific symptoms.
• Genetic mutations that have not yet been extensively studied.
• Cases where differential diagnosis is challenging.

The genetic study of the exome is useful for determining the origin of certain types of cardiopathies and ataxias (loss of muscle control), as well as for diagnosing conditions associated with infertility, such as premature ovarian failure.

How Is It Performed?

To perform exome sequencing, DNA must first be extracted. This can be done in various ways:

• Blood sample: Similar to routine blood tests, a small amount of peripheral blood (usually from the arm) is drawn using a needle and syringe. In these cases, sterile tubes with a purple cap containing ethylenediaminetetraacetic acid (EDTA) are used to prevent blood coagulation before reaching the laboratory for processing and analysis.
• To extract the DNA, the sample is mixed until homogeneous and then processed using:
• Precipitation technique: Sodium sulfate is added to break the plasma membrane and access the cells. Then, alcohol (ethanol or isopropanol) is used to remove the nuclear membrane and access the DNA after centrifuging the sample for five minutes.
• Separation technique: DNA is captured using magnetic beads coated with silica, which helps retain nucleic acids.
• Currently, technological devices allow for the sample to be processed in an automated system to obtain DNA.
• Saliva sample: The inner cheek is swabbed with a cotton-tipped stick. The sample is then stored in a sterile container without preservatives. These samples can be kept at room temperature for up to a year or frozen for long-term storage, as their properties remain unchanged indefinitely.
• DNA is obtained through the following steps:
• A test tube is filled with distilled water, salt, and a drop of soap.
• Saliva is added and stirred until a foamy mixture is obtained.
• 96% alcohol or isopropanol is added.
• After a short time, whitish filaments appear suspended in the liquid. These filaments are precipitated DNA.

Using electronic devices, the patient’s DNA is compared to standard genomic reference data. The sequences to be analyzed are determined based on each case. The equipment provides information based on guidelines set by the American College of Medical Genetics (ACMG), which specialists then interpret.

Risks

Undergoing exome sequencing poses no health risks.

In some cases, the needle puncture for blood sample collection may cause mild bruising, which typically disappears quickly.

What to Expect from Clinical Exome Sequencing

Sample collection takes only a few minutes, and patients can resume their daily routine immediately.

• Blood sample: It is advisable to wear short sleeves or clothing that can be easily rolled up to expose the arm. During extraction, the patient remains seated with the arm extended. The fist should be closed and lightly squeezed to promote blood flow. A slight prick may be felt when the needle is inserted, but this sensation is brief. To reduce the risk of bruising, applying pressure to the area for a few minutes is recommended.
• Saliva sample: The mouth should be kept open, and the tongue should be retracted to facilitate swabbing.

Results for targeted clinical exome sequencing are typically available within a few days, while comprehensive clinical exome results may take several weeks.

Medical Specialties That Request Clinical Exome Analysis

Exome analysis is conducted by medical geneticists.