What is Genetic Screening?
Genetic screening is a process in which DNA is tested to check if a person has any genetic indicators of diseases. It can also be defined as an experimental technique used for identification and selection of individual who possess phenotype of our interest in a mutangenized population. Thus genetic screening can also be called as phenotypic screening. It helps us to provide a better insight of functioning of gene in an individual. It has got applications in various genome project. The functioning of gene can be well determined by Forward genetic screen or Reverse genetic screen. Forward genetic screening first approaches towards the phenotype and then moves towards identifying genes i.e, genes responsible for a particular phenotype in an individual is identified. Whereas in reverse genetic screen it starts with gene or set of genes and moves towards analyzing the resultant phenotype by assaying its effect of disruption. A defined genetic background of organism being used and constant experimental procedure for identification of mutants of interest are the two key component of successful forward genetic screening. Define genetic background helps to identify and locate affected genes in mutant individual with greater efficiency (Herman .R.K, et al.,2005).
Many a times genetic screening and genetic testing are considered to be a common terms, however there is difference between genetic screening and testing. Genetic screening is a technique in which set of diagnostics test are used on large number of individuals to test these individuals who are at high risk of genetic disorders and are the carriers of that specific genes, whereas genetic testing uses different laboratory test to identify the genetic status of individual. The individuals who are already suspected to be at a higher risks for a particular genetic disorder undergo the genetic testing. It is based on medical history of family or on screening tests. But the similarity between both the test is both of them undergo laboratory tests to identify the presence of specific genes which may cause genetic disorders.
Genetic screening is a term that is also encountered in prenatal screening with pregnant women. Many researcher do screening on a broad spectrum in large number of people to find potential for genetic diseases. Nowadays genetic screening is also used by health insurance companies to screen the people for genetic disorders and avoid paying extra money. Genetic screening also include newborn genetic screening to identify new born who would benefit from early treatment. Reproductive genetic screening is also done to help reproductive decision making, and in this family history of the patient is checked to identify individuals who would get added benefit from additional prevention measure.
Types of screening variation
There are different screening variations which are used to identify genes that cause a mutants in phenotype of interest. Some of them are as follows :
Enhancer screening is used to screen an mutant individual with known genetic mutation. It can further be used to screen an individual with additional gene mutations that has its major role in biological and physiological processes. It is the mutation in one genes that causes the intensification of phenotype resulted due to mutation in another gene. The genetic enhancer screening identifies a mutations which enhance aphenotype of interest in already mutant individual.
Source http://www.wormbook.org/chapters/www_geneticenhancers/geneticenhancers.html Synthetic interaction betweenmec-8andsym-3.(A) Themec-8; sym-3double mutant has a highly penetrant defect: the anterior tip (arrow) of the pharynx is not properly joined to the anterior tip of the body (arrowhead), and a functional mouth is not formedThe mutants also have a bulbous nose, an enlargement of the anterior-most part of the body, which is particularly evident in the example shown here. The junction of the pharynx and anterior end of the body is normal in thesym-3single mutant (B) and in themec-8single mutant (C).
It is used to identify the suppressor mutations. Suppressor mutations revert the original mutations. It can be defined as the one which suppresses the phenotype of original mutations and are the another mutations on the site of chromosome which are distinct from the mutation under study. It has an intragenic suppression and extragenic suppression. Intragenic suppression is the one in which mutation is in same gene as in original mutation.whereas if there is a mutation in different gene then it is called as extragenic suppression or intergenic suppression.
It is a type of screening technique which involves performing temperature shifts to enhance a mutant shift. The organism grown at a lower temperature will have normal phenotype whereas at higher temperature the mutation in that particular gene will make it unstable. For example Lee Hartwell and Paul nurse independently carried out temperature sensitive screening to identify mutants defective in cell cycle S.cerevisiae and S.pombe.
Types of Genetic Screening :
forensic screening and
Presymptomatic screening is used to screen the patients whose health is in danger.
Carrier screening is used to carry out the analysis of individuals with a gene or a chromosome abnormality that may cause problems either for offspring or the person screened.it is carried out in healthy individuals where there can be a risk of genes harmful to offsprings or future generation. This can be done by testing of blood or tissue samples and can show the presence of a particular genetic trait, changes in chromosomes, or changes in DNA that are associated with inherited diseases in asymptomatic individuals. For example carrier screening is done for sickle cell anemia, Tay-Sachs disease, duchenne muscular dystrophy, hemophilia, Huntington’s disease, and neurofibromatosis.
Prenatal screening is carried out in a fetus when it is at risk for various identifiable genetic diseases or traits. It began in 1966.
Newborn screening is related with the analysis of blood or tissue samples taken in early infancy in order to detect genetic diseases for which early intervention can avert serious health problems or death. Newborn screening was started in 1960 with the ability to test newborns for a rare metabolic disease, phenylketonuria (PKU). Two other examples of newborn screening are the testing of African – American infants for sickle cell anemia and Ashkenazic Jews for Tay-Sachs disease.
Forensic screening is used in criminal investigations it is used to to discover a genetic linkage between suspects and evidence discovered in criminal investigations. As DNA of each and every person is unique, many people are reluctant to see such information become part of any national database, which might include information not only about identity but also about proclivity toward disease or behavior.
Susceptibility screening is a technique which is used to screen a selected population for genetic susceptibility to environmental hazards. It helps in the identification of workers who may be susceptible to toxic substances that are found in their workplace and may cause future disabilities.
Types and Uses of Genetic Tests
Type of Test
Examples of Genetic Disorders Detected
Possible Reasons for Choosing Test
Cystic fibrosis, Tay-Sachs disease
This type of test is often used by individuals with afamily historyof a specific disorder who wish to determine their risk for having a child affected with that disorder.
This type of test is generally used after an ultrasound detects an abnormality in afetusand themotherdesires a specific genetic diagnosis.
A number of suchtestsare required in the United States, and some states test for dozens of disorders. These tests are generally performed on a bloodsamplecollected immediately after birth.
Testing for late-onset disorders
Cancer, Huntington’s disease
Genetic testing for some disorders, like certain types of cancer, may allude to a predisposition to disease under specific environmental conditions (e.g., smoking). Other disorders, like Huntington’s disease, have a clear genetic cause but only manifest later in life and can be tested for at any time.
Patients requiring specific medicines, like warfarin, might undergo a genetic test to determine the likely rate of drugmetabolism, thus providing the doctor with critical information for proper dosage.
Genetic profile or DNA fingerprint
Genetic testing may assist in identifying an individual, as inpaternitycases or law enforcement situations.
Source :- Norrgard.K., 2008
Purpose of genetic screening
Genetic screening acts as an important tool in modern preventive medicine.
It is used to confirm the diagnosis in patients with symptoms.
Genetic screening also helps to advise other family members of the diagnosed patient to detect whether they also have the disease or have genetic markers for the disease even if they don’t have symptoms ,and also to check whether they are carriers or are neither.
It also helps in detecting when one partner is a sufferer or carrier, to test the other partner in order to advise whether their child will have the disease, be a carrier or will not have the disease.
If diseases observed in both the parents, the child must inherit the disease or the genetic markers for the disease. It may happen that one parent has the disease and the other partner also has the genetic markers for it but there are no symptoms for it, therefore the child may develop the disease or alternatively may not show any symptoms during its lifetime. If one parent has the disease and the other is a carrier, in each pregnancy there is a 50:50 chance of the child inheriting the disease.
In the population, screening is carried out to discover undiagnosed sufferers or those with genetic markers for the disease, to discover carriers and to aid research into the prevalence and severity of the disease and the carrier ratio.
Maternal serum alpha-fetoprotein (MSAFP) screening, enhanced MSAFP, amniocentesis, chorionic villus sampling (CVS), percutaneous umbilical blood sampling (PUBS),fetal biopsy and fetal cell sorting are some of the genetic screening tests which are currently available for pregnant women. MSAFP is a blood-screening test. It is performed at the 16-18 week gestation date and it tests for spina bifida. Enhanced MSAFP is also a similar type of blood-screening test that measures levels of certain biochemical markers to test for the presence of Down’s syndrome. But the only thing is that this test only has an accuracy of 60-65%. Amniocentesis is performed at the 16-18 week of gestation. Amniocentesis uses amniotic fluid to test for chromosomal abnormalities. It is also used to find biochemical abnormalities at the genetic level, it helps in detecting up to 180 genetic disorders. CVS i.e, chorionic villus sampling screening is performed at 10-12 weeks into gestation and it uses chorion tissue for chromosomal analysis in biochemical and DNA studies. It is not widely used because this test has a drawback of correlation to produce newborns with limb abnormalities. ).Percutaneous umbilical blood sampling, PUBS is performed after week 18 . PUBS is used only as a confirmation test based on results from previous other tests. In fetal tissues are taken for DNA testing . Fetal cell sorting includes an experimental procedure in whcich blood is taken from mother and fetal cells are tested in mothers blood.
Oberle,I. D.& Camerino.G, 1985 carried out genetic screening for hemophilia A(classic hemophilia) with polymorphic DNA probe. They collected blood samples after informed consent from13 families and carried out coagulation and immunologic assay. In this experiment they develop a new technique to screen the patients for hemophiliaA in the families at risk for the disease. A DNA probe (St14) that detects a very polymorphic region on the human X chromosome has been shown to be closely linked to hemophilia A. They observed that there was no recombination between the St14 locus and hemophilia A in 12 families studied. The odds in favor of linkage are 4.4 x 109to 1 (lod score, 9.65). They found that there was 0 to 6.5% probability of the 95 per cent confidence interval of a recombination between St14 and hemophilia A. This informative DNA probe which causes families in risk of hemophilia A in more than 90 per cent, can be used in conjunction with classic biologic assays to identify carriers with an accuracy of 96 per cent or more. If a small risk of misclassification due to crossover between the test and the disease loci is accepted, this DNA marker should allow first-trimester prenatal diagnosis of hemophilia A. Segregation analysis with St14 may thus represent a major improvement in genetic counseling for hemophilia A.
Social dilemmas of genetic screening
Though genetic screening is beneficial to society it has a got lot of controversies because by using genetic marker to check for genetic disorders is definitely beneficial to society because due to early diagnosis a preventive symptom treatment can be given by early medical assisantnce but however many a times it is observed that if a genetic disorder is detected in fetus then fetus is readily aborted. So here a controversy comes because no one has got right to end anyones life. Also many a times it happens that individuals carrying a genetic disorder or diseases are being discriminated by society or by health insurers and employers.
Hence this genetic screening has brought up a new legal, social and ethical dilemmas.
Karthikeyan. M.1999, Human Genetic Screening, http://www.ndsu.edu/pubweb/~mcclean/plsc431/students99/karthikeyan.htm
Norrgard.K., 2008, Medical Careers: Genetic Screening and Diagnostics, Nature Education1(1): pg no- 92
Burke.W., et al., 2011, Genetic Screening , Epidemologic reviews Oxford journal 33(1): pg no- 148–164 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3166195