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Molecular genetics in clinical practice

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Also listed as: Genomics in clinical practice
Related terms
Background
Methods
Research
Implications
Limitations
Safety
Future research
Author information
Bibliography

Related Terms
  • Cancer, chromosome, DNA, deoxyribonucleic acid, FISH, fluorescence in situ hybridization, gene amplification, gene hybridization, genetics, genome, genomics in clinical practice, Human Genome Project, microarray, molecular diagnostics, molecular pathology, PCR, polymerase chain reaction.

Background
  • Genes are found inside the nucleus of the cells of all organisms. An individual's genes are contained in a large molecule called DNA (deoxyribonucleic acid), which looks like a twisted ladder. This unique shape is called a double helix. The sides of the double helix are made of alternating sugar and phosphate molecules. The "rungs" of the "ladder" are made of small molecules called bases. These molecules include adenine, thymine, cytosine, and guanine. Because they contain nitrogen, they are sometimes called "nitrogen bases."
  • All genes are composed of different combinations of these four molecules, which are arranged in single file. The sequence of these molecules provides the "code," or instructions, for each of the genes involved in the development, growth, and function of all the cells in the body.
  • Human have about 30,000 genes, each of which provides instructions for making a specific protein. Proteins are responsible for most of the chemical functions of the body. The genes, and a large amount of DNA that has other functions, are organized into 23 pair of chromosomes (containing 46 chromosomes). The total genetic composition of an individual is called the complete human genome.
  • RNA, short for ribonucleic acid, are short molecules similar to and derived from DNA. Messenger RNA and transfer RNA complete the process of protein formation from the DNA code. RNA contains adenine, cytosine, and guanine like DNA, but instead of thymine, it contains uracil.
  • A genetic disease or disorder is a condition that is caused by abnormal expression of one or more genes. This means that the gene produces too much or too little protein, or produces an abnormal protein. Abnormal gene expression occurs when the bases that make up an individual's genes are incorrectly deleted, added, or substituted. It may also occur when molecules that interact with and regulate DNA are abnormal. If a genetic mutation causes a protein in the body to stop functioning properly, the person may develop a disease or disorder.
  • The Human Genome Project (HGP), which began in 1990, was an international research program designed to map out all of the DNA that makes up a human being. All people have about 99.9% of the same genetic information. The differences in the remaining 0.1% may be responsible for different features such as hair and eye color and a variety of other different features or may cause disease.
  • Many illnesses have been linked to mutations in specific genes. Information obtained by mapping the human genome will help scientists better understand the causes and epidemiology of many diseases and may help them to prevent, diagnose, treat, and possibly cure genetic disorders.
  • Molecular genetics is the study of genes in the laboratory. Molecular diagnostics is the use of this genetic information to diagnose disease, provide information about the prognosis of the disease, and guide treatment. Molecular genetics can be used to detect variations in genes that are associated with most types of cancer, to diagnosis some types of inherited disease, to characterize diseases of bodily function, to perform parental and forensic testing, and for many other applications.
  • Molecular genetic testing can also be used to predict how individuals will respond to certain chemicals, such as medications. If patients have one type of gene variant, then they may be more likely to benefit from a specific drug. If they do not have that specific gene variant, the drug may have no effect or may be dangerous.
  • Molecular genomics refers to the study of the entire genome of a patient, as opposed to the study of a single gene. Molecular genomics testing looks at the interactions among many different genes and other parts of DNA and combines this information to determine the genetic profile of a patient. The genetic profile is like a map of an individual's DNA. It contains all of the individual variations that make an individual more susceptible or resistant to certain diseases and/or chemicals. It may be used to determine if a certain type of cancer will respond to a certain drug, or if a patient is at increased risk of side effects from a drug.

Methods
  • Fluorescence in situ hybridization (FISH): Fluorescence in situ hybridization (FISH) is a method that is used to determine whether a specific part of a chromosome is present in a cell. To perform FISH, researchers first identify a specific region of a chromosome that they are interested in studying (for example, a region that contains a specific gene). They then generate a probe, or a sequence of DNA, that can recognize and bind to the chromosomal region of interest. In FISH, the probes that researchers use fluoresce, that is, they glow under ultraviolet light if the labeled chromosomal region is present in a cell.
  • In some diseases (such as cancer), genetic mutations occur that can cause part of a chromosome to be deleted, repeated, or reversed in orientation. By looking for differences in how chromosomes from different cells stain with a probe, researchers may be able to observe these changes under the microscope and learn more about the genetic mutations that cause a particular disease.
  • Polymerase chain reaction (PCR): Polymerase chain reaction (PCR) is a method of multiplying a small piece of DNA millions of times in order to study it. To perform PCR, cells are collected from a patient's saliva, blood, or tissue sample. A probe, or sequence of DNA that recognizes the gene of interest, is added to the individual's DNA. After the probe attaches to the gene, proteins are added to make millions of copies of the gene. This allows laboratories to determine if a gene is present even in a very small sample of tissue. It may also identify how much of the gene is present.
  • PCR can identify genes that are abnormal, such as those present in some types of cancer and in inherited diseases. Using PCR, small segments of altered DNA that would otherwise be insufficient for detection can be identified and used for diagnosis or other purposes. Real time PCR (RT-PCR) is similar to PCR, but is performed much more rapidly. This process can amplify DNA in a matter of minutes or hours.
  • DNA microarray: DNA microarray is a method of studying multiple genes at once. Microarrays are used to detect changes in dozens, hundreds, or even thousands of genes. This provides more information about a person's specific genetic makeup. Because microarrays are a relatively new technology, they are primarily used in research.
  • DNA sequencing: DNA sequencing is a method of determining the exact pattern of adenine, guanine, cytosine, and thymine in a DNA fragment. To sequence DNA, the segment is mixed with an enzyme that copies the DNA, a primer, or segment of DNA that tells the enzyme where to begin copying, and the four nitrogen bases required to synthesize DNA.
  • Southern blots and northern blots: Southern and northern blots are methods of detecting certain DNA and RNA molecules based on their size and electrical charge. Southern blots are used to detect mutations that produce DNA fragments of abnormal size or charge, such as deletions and additions of genetic material. First, a patient's DNA is taken from a sample of blood or tissue. Then, the DNA is broken down into fragments by proteins that cut the DNA at specific points. Next, these fragments are placed in a gel. In a process called electrophoresis, a charge is applied to the gel, causing the DNA to move through the gel at different speeds based on electrical charge and size. The DNA particles are thus separated within the gel. If an abnormal DNA particle is present, it will be visible when the gel is dyed and viewed under fluorescent light. The presence of an abnormal DNA particle may signify the presence of a certain disease. Northern blots are performed in the same way as southern blots, except they use RNA instead of DNA.

Research
  • Molecular genetics may be used to diagnose some diseases that are related to gene mutations, such as cystic fibrosis and some types of cancer.
  • Early cancer diagnosis: Because molecular genetics can be used to detect gene mutations in very small samples of blood or tissue, it may provide the first indication of cancer. For example, mutations in a gene called p53 are found in many different cancers, and detection of these mutations may one day allow for early diagnosis of many different forms of cancer. This may be possible because some tumor cells and tumor DNA are released into the blood. In theory, it could be possible to detect tumor DNA in the blood with molecular genetic techniques.
  • Inherited disease: Inherited diseases are caused by underlying genetic mutations. Because genes are passed down from parents to their children, it is possible for them to pass on a genetic disease. Once researchers have determined exactly what mutation(s) is the cause of a specific inherited disorder, molecular genetic testing may be used to detect this mutation and diagnose the disease. For example, the genetic mutation that causes Huntington's disease (a disorder that causes abnormal movements and changes in mental function) can be detected by a molecular genetic test. It is also possible to diagnose many inherited disorders in a developing fetus.
  • Pharmacogenomics: Pharmacogenomics is the study of how someone's genes affect the way an individual responds to drugs. Genetic variations may influence whether an individual has a stronger or weaker response to certain drugs or how fast an individual metabolizes a drug. For example, an enzyme in the liver called CYP2D6 is responsible for the metabolism of the blood thinner warfarin (Coumadin®). Some people have a CYP2D6 gene variation that causes them to metabolize warfarin more slowly than normal. This causes higher levels of the drug in the blood and increases risk of spontaneous bleeding. Patients can now be tested for this variation, and if it is present, they can be treated with a lower dose of warfarin or with a different drug. This type of information will allow doctors to more accurately choose the right drug and dose for patients.
  • Molecular therapies: Another area of pharmacogenomics that is currently being researched is related to molecular therapies. These are drugs designed to target a specific molecule, usually a gene mutation, a protein produced by a mutated gene, or a protein produced in abnormal amounts due to a genetic mutation. Some molecular therapies are already in use for certain types of cancer. For example, the breast cancer drug trastuzumab (Herceptin®) requires a molecular genetic test for the abnormal gene before it can be prescribed.
  • For molecular therapies to be effective, the patient must have the target genetic mutation. Therefore, molecular genetics may be used to determine whether a patient has the mutation and would benefit from therapy. Molecular genetic researchers continue to develop tests for gene mutations so that molecular therapies can be used appropriately.

Implications
  • Molecular genetics allows some patients to be diagnosed and treated earlier, more accurately, and with fewer invasive procedures. Invasive procedures include those that require entering the patient's body to visualize organs or to take a piece of tissue. This includes surgery, biopsy, and endoscopic evaluations like a colonoscopy. Because molecular genetic tests can sometimes be performed on blood samples or on cells from the inside of the cheek, genetic testing may, in some cases, replace more invasive methods of diagnosis. This will decrease the risk to the patient of complications associated with invasive procedures.
  • Detection of genetic mutations may be performed before patients begin having symptoms from a disease. This may be done for patients who have family histories of genetic diseases, or for individuals who are at risk because of their race or ethnicity. If a disease is detected, patients may be treated earlier and followed more closely by their doctor to treat symptoms as they develop.

Limitations
  • Molecular genetics can detect the presence of certain mutations, but it cannot tell what those mutations mean. Even patients with identical genetic mutations may have different disease symptoms or responses to drugs. Determining the effect of a mutation on a specific patient will require regular visits with a doctor for evaluation and management.
  • In order to perform a molecular genetic test, the specific gene that is involved with a disease must be known. There are many diseases that are thought to have genetic mutations associated with them, but for which the specific gene involved is currently unknown.

Safety




Future research
  • Molecular genetics is a rapidly growing area of medicine. Researchers are currently working to expand the use of molecular genetics to diagnose an increasing number of diseases. Researchers are working to identify the genes that cause a number of different diseases so that molecular genetic tests can be developed for disease detection. Cancer, in particular, is an area of intense molecular genetic research. Other areas of research include inherited disease, chronic diseases such as arthritis, and pharmacogenomics (how an individual responds to drugs).

Author information
  • This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

Bibliography
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Copyright © 2011 Natural Standard (www.naturalstandard.com)


The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.

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