Exome sequencing

Overview

Exome sequencing is a genomic technique for sequencing all of the protein-coding regions of genes in a genome, known as the exome. It is a cost-effective alternative to whole-genome sequencing, as it focuses on the 1-2% of the genome that encodes proteins, which are responsible for most known Mendelian disorders.

History

The development of exome sequencing was driven by the need to identify genetic variants associated with human diseases. The first successful application of exome sequencing was reported in 2009, when it was used to identify the genetic cause of Miller syndrome.

Methodology

Exome sequencing involves several key steps:

Sample Preparation

DNA is extracted from a biological sample, such as blood or saliva. The DNA is then fragmented into smaller pieces to facilitate sequencing.

Exome Capture

The fragmented DNA is hybridized to a set of probes that are complementary to the exonic regions of the genome. These probes "capture" the exonic DNA, allowing it to be separated from the non-coding regions.

Sequencing

The captured exonic DNA is then sequenced using high-throughput next-generation sequencing (NGS) technologies. This generates millions of short reads that are aligned to a reference genome.

Data Analysis

Bioinformatics tools are used to analyze the sequencing data, identify variants, and interpret their potential impact on protein function. This includes variant calling, annotation, and filtering to prioritize variants that may be pathogenic.

Applications

Exome sequencing has a wide range of applications in medical research and clinical diagnostics:

Disease Gene Discovery

Exome sequencing has been instrumental in identifying genes associated with rare Mendelian disorders. By comparing the exomes of affected and unaffected individuals, researchers can pinpoint mutations that are likely to cause disease.

Cancer Genomics

In cancer research, exome sequencing is used to identify somatic mutations that drive tumor development. This can inform targeted therapies and improve personalized medicine approaches.

Prenatal and Neonatal Diagnostics

Exome sequencing is increasingly used in prenatal and neonatal settings to diagnose genetic disorders early in life, allowing for timely interventions.

Limitations

While exome sequencing is a powerful tool, it has several limitations:

• It does not capture non-coding regions of the genome, which can also harbor important regulatory variants.
• Some exonic regions may be poorly captured or sequenced, leading to incomplete coverage.
• Interpretation of variants, especially those of unknown significance, remains challenging.

Future Directions

Advancements in sequencing technologies and bioinformatics are expected to improve the accuracy and utility of exome sequencing. Integration with other "omics" data, such as transcriptomics and proteomics, may provide a more comprehensive understanding of genetic diseases.

See Also

• Whole-genome sequencing
• Genomics
• Personalized medicine

External Links

• [NIH Exome Sequencing Project](https://esp.gs.washington.edu/drupal/)
• [Exome Aggregation Consortium (ExAC)](http://exac.broadinstitute.org/)

• 

  Exome sequencing workflow step 1a

• 

  Exome sequencing workflow step 1b

• 

  In-solution capture process