Next-Generation Sequencing (NGS)

Next-generation sequencing (NGS) is a high-throughput molecular genetic technology capable of running thousands to millions of DNA sequences in parallel.
It provides significantly higher speed and is comparatively more cost-effective than traditional Sanger sequencing, which can only sequence a part of a gene at a time.
NGS identifies sequence-level alterations, including single-nucleotide variants (SNVs), small insertions, and deletions (indels) that disrupt normal gene expression and protein function.
The diagnostic yield of NGS is highly variable and depends on the specific disorder, the sequencing platform utilized, the capture kit, and the overall depth of gene coverage.

Targeted Gene Panels: This approach utilizes NGS to simultaneously test a smaller subset of genes known to be associated with specific, overlapping clinical phenotypes, such as neuromuscular disorders, epilepsy, or nonsyndromic deafness.
Whole Exome Sequencing (WES): WES selectively targets and sequences the protein-coding regions of the genome, known as the exome.
While the exome constitutes only 1% to 2% of the 3 billion base pairs in the human genome, it harbors the vast majority of identified disease-causing pathogenic variants.
Whole Genome Sequencing (WGS): WGS evaluates the entire genomic sequence, including both coding and noncoding (intronic or regulatory) regions.
WGS provides roughly 3,000 times more data than a standard chromosomal microarray and offers improved detection of structural variations and copy number variations (CNVs).

The NGS process begins with the extraction of nucleic acids (DNA or RNA) from the patient's sample, such as peripheral blood or other tissues.
Library preparation involves the fragmentation of the genomic DNA into multiple smaller segments.
The fragmented DNA undergoes a series of modifications, including end repair, A-tailing, and linker modification amplification.
For targeted panels and WES, specific DNA sequences corresponding to exonic regions or targeted genes are selectively captured or enriched; for WGS, the entire fragmented DNA library is processed without selective capture.
Following the sequencing run, raw data processing maps and aligns the sequenced base calls to a standard reference human genome sequence.

Bioinformatics analysis is a critical step that filters several thousands of identified variants down to a few candidate variants using established population and disease-causing databases.
Variants are then correlated with the patient's clinical phenotype, pedigree, and other relevant investigations to determine their clinical significance.
According to the American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) guidelines, variants are classified into five categories: pathogenic, likely pathogenic, variant of unknown significance (VUS), likely benign, and benign.
The detection of an enormous amount of genetic variation, especially in WES and WGS, frequently leads to the identification of VUS, which require extensive functional evidence or family segregation studies (trio sequencing) to determine pathogenicity.
Incidental or secondary findings (e.g., discovering an adult-onset cancer predisposition gene during a pediatric developmental workup) may be uncovered, mandating careful pre-test ethical genetic counseling.

Targeted gene panels are highly indicated for conditions exhibiting locus heterogeneity (where multiple genes cause the same condition) or disorders sharing a common biological pathway, such as RASopathies.
WES is the preferred modality for patients presenting with extreme genetic heterogeneity, atypical or indistinct phenotypes, dual diagnoses, or severe unexplained intellectual disability and autism spectrum disorders where de novo mutations are common.
In cases of unexplained global developmental delay and severe non-syndromic intellectual disability, WES provides an additional diagnostic yield of approximately 30% to 40%.
WGS is indicated when noncoding or structural variations are suspected, when critical illness (such as in the neonatal intensive care unit) demands rapid data generation, or when prior WES has been non-diagnostic.

Current standard NGS techniques possess inherent limitations and are generally not useful for detecting methylation disorders.
NGS also exhibits reduced sensitivity for accurately identifying triplet repeat expansions, certain large chromosomal rearrangements, and large structural deletions compared to targeted molecular cytogenetic assays.
Coverage gaps may exist, particularly in regions that are poorly captured or amplified, potentially leading to false-negative results if a pathogenic variant resides in an unsequenced area.