Exome Sequencing

Exome sequencing, often referred to as whole exome sequencing (WES), is a high-throughput next-generation sequencing (NGS) technology used to evaluate the protein-coding regions of the human genome.
Although the exome constitutes only 1% to 2% of the 3 billion base pairs in the human genome (approximately 200,000 exons), it contains the vast majority of known disease-causing pathogenic variants.
WES is capable of identifying single-nucleotide variants (including pathogenic loss-of-function and missense mutations), small insertions, and small deletions within the coding regions.
Some advanced laboratory bioinformatics pipelines are now also able to call smaller copy number variants (CNVs) routinely as part of the WES analysis.

The technique involves the fragmentation of genomic DNA into multiple smaller segments, followed by a targeted "capture" of specific sequences corresponding to the exonic (protein-coding) regions.
The captured DNA is then processed for sequencing in parallel, allowing thousands to millions of sequences to be analyzed simultaneously at a high speed and comparatively lower cost than traditional Sanger sequencing.
The diagnostic yield and accuracy are significantly enhanced by employing a "trio approach," wherein the affected patient and both biological parents are sequenced simultaneously.
Trio sequencing simplifies the bioinformatics analysis by determining the inheritance pattern (segregation) of deleterious sequence variants and readily identifying de novo mutations (pathogenic variants not present in either parent).
Bioinformatics analysis utilizes population and disease-causing databases to filter thousands of identified variants down to a few candidate variants, classifying them as pathogenic, likely pathogenic, variants of unknown significance (VUS), likely benign, or benign.

WES is strongly indicated in clinical scenarios characterized by extreme genetic heterogeneity, atypical syndromic presentations, or when initial tiered testing (such as chromosomal microarray) is non-diagnostic.

Category	Specific Clinical Indications for Exome Sequencing
Neurodevelopmental	Unexplained global developmental delay (GDD), intellectual disability (ID), and autism spectrum disorder (ASD).
Complex Phenotypes	Multiple congenital anomalies or dysmorphic features that lack a clearly recognizable syndromic pattern.
Atypical Presentations	Atypical clinical features of known conditions (e.g., atypical Cornelia de Lange, Coffin-Siris, or Kabuki syndromes) where targeted panels are negative.
Extreme Heterogeneity	Disorders where a massive number of genes could be implicated, making single-gene or small panel testing impractical or not cost-effective.
Dual Diagnoses	Patients presenting with two or more unrelated phenotypes (e.g., oculocutaneous albinism and neutropenia), suggesting oligogenic phenotypes.

WES significantly reduces the time to diagnosis and is often more cost-effective than the traditional sequential diagnostic pathway of ordering multiple individual gene tests.
In cases of unexplained intellectual disability and severe global developmental delay, WES provides an additional diagnostic yield of approximately 30% to 40%.
WES offers the potential to identify novel disease-gene associations and uncover presentations caused by more than one altered gene.
A definitive molecular diagnosis via WES provides vital information on prognosis, guides medical management and surveillance, ends the diagnostic odyssey, and allows for accurate recurrence risk counseling and prenatal family planning.

Limitation Category	Description
Coverage Gaps	WES strictly evaluates the amplified DNA segments restricted to the coding regions, meaning it will miss pathogenic variants located in intronic, regulatory, or promoter regions.
Structural Variants	While improving, WES is generally less sensitive than chromosomal microarray or whole genome sequencing for detecting large structural variations, translocations, inversions, large deletions/duplications, and triplet repeat expansions.
Variants of Unknown Significance (VUS)	WES identifies an enormous amount of genetic variation, commonly revealing over 30,000 VUS in a single individual. Interpreting the pathogenicity of these variants requires extensive functional evidence and can cause diagnostic ambiguity.
Incidental Findings	WES may uncover pathogenic variants entirely unrelated to the primary clinical indication (e.g., uncovering a cancer predisposition gene or Alzheimer's susceptibility in a child tested for developmental delay).
Ethical Counseling	Pre-test genetic counseling is mandatory to inform families about the possibility of incidental findings and to document their preference regarding the disclosure of medically actionable versus non-actionable secondary findings.