The exome is composed of all of the exons within the genome, the sequences which, when transcribed, remain within the mature RNA after introns are removed by RNA splicing. This includes untranslated regions of messenger RNA (mRNA), and coding regions. Exome sequencing has proven to be an efficient method of determining the genetic basis of more than two dozen Mendelian or single gene disorders. [1]
The human exome consists of roughly 233,785 exons, about 80% of which are less than 200 base pairs in length, constituting a total of about 1.1% of the total genome, or about 30 megabases of DNA. [2] [3] [4] Though composing a very small fraction of the genome, mutations in the exome are thought to harbor 85% of mutations that have a large effect on disease. [5]
It is important to note that the exome is distinct from the transcriptome, which is all of the transcribed RNA within a cell type. While the exome is constant from cell-type to cell-type, the transcriptome changes based on the structure and function of the cells. As a result, the entirety of the exome is not translated into protein in every cell. Different cell types only transcribe portions of the exome, and only the coding regions of the exons are eventually translated into proteins.
Next-generation sequencing (NGS) allows for the rapid sequencing of large amounts of DNA, significantly advancing the study of genetics, and replacing older methods such as Sanger sequencing. This technology is starting to become more common in healthcare and research not only because it is a reliable method of determining genetic variations, but also because it is cost effective and allows researchers to sequence entire genomes in anywhere between days to weeks. This compares to former methods which may have taken months. Next-gen sequencing includes both whole-exome sequencing and whole-genome sequencing. [6]
Sequencing an individual's exome instead of their entire genome has been proposed to be a more cost-effective and efficient way to diagnose rare genetic disorders. [7] [8] It has also been found to be more effective than other methods such as karyotyping and microarrays. [9] This distinction is largely due to the fact that phenotypes of genetic disorders are a result of mutated exons. In addition, since the exome only comprises 1.5% of the total genome, this process is more cost efficient and fast as it involves sequencing around 40 million bases rather than the 3 billion base pairs that make up the genome. [10]
On the other hand, whole genome sequencing has been found to capture a more comprehensive view of variants in the DNA compared to whole-exome sequencing. Especially for single nucleotide variants, whole genome sequencing is more powerful and more sensitive than whole-exome sequencing in detecting potentially disease-causing mutations within the exome. [11] One must also keep in mind that non-coding regions can be involved in the regulation of the exons that make up the exome, and so whole-exome sequencing may not be complete in showing all the sequences at play in forming the exome.
With either form of sequencing, whole-exome sequencing or whole genome sequencing, some have argued that such practices should be done under the consideration of medical ethics. While physicians strive to preserve patient autonomy, sequencing deliberately asks laboratories to look at genetic variants that may be completely unrelated to the patient's condition at hand and have the potential of revealing findings that were not intentionally sought. In addition, such testing have been suggested to have imply forms of discrimination against particular groups for having certain genes, creating the potential for stigmas or negative attitudes towards that group as a result. [12]
Rare mutations that affect the function of essential proteins constitute the majority of Mendelian diseases. In addition, the overwhelming majority of disease-causing mutations in Mendelian loci can be found within the coding region. [5] With the goal of finding methods to best detect harmful mutations and successfully diagnose patients, researchers are looking to the exome for clues to aid in this process.
Whole-exome sequencing is a recent technology that has led to the discovery of various genetic disorders and increased the rate of diagnoses of patients with rare genetic disorders. Overall, whole-exome sequencing has allowed healthcare providers to diagnose 30–50% of patients who were thought to have rare Mendelian disorders.[ citation needed ] It has been suggested that whole-exome sequencing in clinical settings has many unexplored advantages. Not only can the exome increase our understanding of genetic patterns, but under clinical settings, it has the potential to the change in management of patients with rare and previously unknown disorders, allowing physicians to develop more targeted and personalized interventions. [13]
For example, Bartter Syndrome, also known as salt-wasting nephropathy, is a hereditary disease of the kidney characterized by hypotension (low blood pressure), hypokalemia (low potassium), and alkalosis (high blood pH) leading to muscle fatigue and varying levels of fatality. [14] It is an example of a rare disease, affecting fewer than one per million people, whose patients have been positively impacted by whole-exome sequencing. Thanks to this method, patients who formerly did not exhibit the classical mutations associated with Bartter Syndrome were formally diagnosed with it after the discovery that the disease has mutations outside of the loci of interest. [5] They were thus able to gain more targeted and productive treatment for the disease.
Much of the focus of exome sequencing in the context of disease diagnosis has been on protein coding "loss of function" alleles. Research has shown, however, that future advances that allow the study of non-coding regions, within and without the exome, may lead to additional abilities in the diagnoses of rare Mendelian disorders. [15] The exome is the part of the genome composed of exons, the sequences which, when transcribed, remain within the mature RNA after introns are removed by RNA splicing and contribute to the final protein product encoded by that gene. It consists of all DNA that is transcribed into mature RNA in cells of any type, as distinct from the transcriptome, which is the RNA that has been transcribed only in a specific cell population. The exome of the human genome consists of roughly 180,000 exons constituting about 1% of the total genome, or about 30 megabases of DNA. [16] Though composing a very small fraction of the genome, mutations in the exome are thought to harbor 85% of mutations that have a large effect on disease. [17] [18] Exome sequencing has proved to be an efficient strategy to determine the genetic basis of more than two dozen Mendelian or single gene disorders. [19]
The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the nuclear genome and the mitochondrial genome. Human genomes include both protein-coding DNA sequences and various types of DNA that does not encode proteins. The latter is a diverse category that includes DNA coding for non-translated RNA, such as that for ribosomal RNA, transfer RNA, ribozymes, small nuclear RNAs, and several types of regulatory RNAs. It also includes promoters and their associated gene-regulatory elements, DNA playing structural and replicatory roles, such as scaffolding regions, telomeres, centromeres, and origins of replication, plus large numbers of transposable elements, inserted viral DNA, non-functional pseudogenes and simple, highly repetitive sequences. Introns make up a large percentage of non-coding DNA. Some of this non-coding DNA is non-functional junk DNA, such as pseudogenes, but there is no firm consensus on the total amount of junk DNA.
The coding region of a gene, also known as the coding sequence (CDS), is the portion of a gene's DNA or RNA that codes for a protein. Studying the length, composition, regulation, splicing, structures, and functions of coding regions compared to non-coding regions over different species and time periods can provide a significant amount of important information regarding gene organization and evolution of prokaryotes and eukaryotes. This can further assist in mapping the human genome and developing gene therapy.
Molecular genetics is a branch of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the structure and/or function of genes in an organism's genome using genetic screens.
A frameshift mutation is a genetic mutation caused by indels of a number of nucleotides in a DNA sequence that is not divisible by three. Due to the triplet nature of gene expression by codons, the insertion or deletion can change the reading frame, resulting in a completely different translation from the original. The earlier in the sequence the deletion or insertion occurs, the more altered the protein. A frameshift mutation is not the same as a single-nucleotide polymorphism in which a nucleotide is replaced, rather than inserted or deleted. A frameshift mutation will in general cause the reading of the codons after the mutation to code for different amino acids. The frameshift mutation will also alter the first stop codon encountered in the sequence. The polypeptide being created could be abnormally short or abnormally long, and will most likely not be functional.
Genetics, a discipline of biology, is the science of heredity and variation in living organisms.
Trinucleotide repeat disorders, a subset of microsatellite expansion diseases, are a set of over 30 genetic disorders caused by trinucleotide repeat expansion, a kind of mutation in which repeats of three nucleotides increase in copy numbers until they cross a threshold above which they cause developmental, neurological or neuromuscular disorders. Depending on its location, the unstable trinucleotide repeat may cause defects in a protein encoded by a gene; change the regulation of gene expression; produce a toxic RNA, or lead to production of a toxic protein. In general, the larger the expansion the faster the onset of disease, and the more severe the disease becomes.
Genetic analysis is the overall process of studying and researching in fields of science that involve genetics and molecular biology. There are a number of applications that are developed from this research, and these are also considered parts of the process. The base system of analysis revolves around general genetics. Basic studies include identification of genes and inherited disorders. This research has been conducted for centuries on both a large-scale physical observation basis and on a more microscopic scale. Genetic analysis can be used generally to describe methods both used in and resulting from the sciences of genetics and molecular biology, or to applications resulting from this research.
In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA, that is transcribed to produce a functional RNA. There are two types of molecular genes: protein-coding genes and non-coding genes.
Hajdu–Cheney syndrome, also called acroosteolysis with osteoporosis and changes in skull and mandible, arthrodentoosteodysplasia and Cheney syndrome, is an extremely rare autosomal dominant congenital disorder of the connective tissue characterized by severe and excessive bone resorption leading to osteoporosis and a wide range of other possible symptoms. Mutations in the NOTCH2 gene, identified in 2011, cause HCS. HCS is so rare that only about 50 cases have been reported worldwide since the discovery of the syndrome in 1948
ASH1L is a histone-lysine N-methyltransferase enzyme encoded by the ASH1L gene located at chromosomal band 1q22. ASH1L is the human homolog of Drosophila Ash1.
GeneDx is a genetic testing company that was founded in 2000 by two scientists from the National Institutes of Health (NIH), Sherri Bale and John Compton. They started the company to provide clinical diagnostic services for patients and families with rare and ultra-rare disorders, for which no such commercial testing was available at the time. The company started in the Technology Development Center, a biotech incubator supported by the state of Maryland and Montgomery County, MD. In 2006, BioReference Laboratories acquired GeneDx. Since then, GeneDx has operated as a subsidiary of this parent company under the leadership of Bale and Compton. In October 2016, Benjamin D. Solomon was appointed as managing director.
RNA-Seq is a technique that uses next-generation sequencing to reveal the presence and quantity of RNA molecules in a biological sample, providing a snapshot of gene expression in the sample, also known as transcriptome.
Exome sequencing, also known as whole exome sequencing (WES), is a genomic technique for sequencing all of the protein-coding regions of genes in a genome. It consists of two steps: the first step is to select only the subset of DNA that encodes proteins. These regions are known as exons—humans have about 180,000 exons, constituting about 1% of the human genome, or approximately 30 million base pairs. The second step is to sequence the exonic DNA using any high-throughput DNA sequencing technology.
Genome instability refers to a high frequency of mutations within the genome of a cellular lineage. These mutations can include changes in nucleic acid sequences, chromosomal rearrangements or aneuploidy. Genome instability does occur in bacteria. In multicellular organisms genome instability is central to carcinogenesis, and in humans it is also a factor in some neurodegenerative diseases such as amyotrophic lateral sclerosis or the neuromuscular disease myotonic dystrophy.
ZTTK syndrome is a rare multisystem disease caused in humans by a genetic mutation of the SON gene. Common symptoms include developmental delay and often mild to severe intellectual disability.
ANNOVAR is a bioinformatics software tool for the interpretation and prioritization of single nucleotide variants (SNVs), insertions, deletions, and copy number variants (CNVs) of a given genome.
Personalized onco-genomics (POG) is the field of oncology and genomics that is focused on using whole genome analysis to make personalized clinical treatment decisions. The program was devised at British Columbia's BC Cancer Agency and is currently being led by Marco Marra and Janessa Laskin. Genome instability has been identified as one of the underlying hallmarks of cancer. The genetic diversity of cancer cells promotes multiple other cancer hallmark functions that help them survive in their microenvironment and eventually metastasise. The pronounced genomic heterogeneity of tumours has led researchers to develop an approach that assesses each individual's cancer to identify targeted therapies that can halt cancer growth. Identification of these "drivers" and corresponding medications used to possibly halt these pathways are important in cancer treatment.
Deborah Ann "Debbie" Nickerson was an American human genomics researcher. She was professor of genome sciences at the University of Washington. Nickerson founded and directed of one of the five clinical sites of the Gregor Consortium and was a major contributor to many genomics projects, including the Human Genome Project and the International HapMap Project.
Personalized genomics is the human genetics-derived study of analyzing and interpreting individualized genetic information by genome sequencing to identify genetic variations compared to the library of known sequences. International genetics communities have spared no effort from the past and have gradually cooperated to prosecute research projects to determine DNA sequences of the human genome using DNA sequencing techniques. The methods that are the most commonly used are whole exome sequencing and whole genome sequencing. Both approaches are used to identify genetic variations. Genome sequencing became more cost-effective over time, and made it applicable in the medical field, allowing scientists to understand which genes are attributed to specific diseases.
Jenny Carmeron Taylor is a British geneticist who is Professor of Genomic Medicine at the University of Oxford. Taylor is the Director of the Oxford Biomedical Research Centre Genetics Theme. Her research considers whole genome sequencing and ways to integrate genetic research into the National Health Service.