Genetic Testing Flashcards
Different types of genetic testing
- Population carrier screen
- Prenatal test
- Prenatal screen
- New born screen
- Diagnostic test
- Predisposition test
- Predictive test
What is purpose of population carrier screen.
Identifies heterozygotes – people with one copy of a mutant gene.
What is purpose of prenatal test.
Detects mutant allele in a foetus for a condition present in a family
What is purpose of prenatal screen .
Test embryos or foetuses from a population for increased risk of a condition based on family history.
What is purpose of new born screen .
Population wide testing for several treatable inborn errors of metabolism
What is purpose of diagnostic test .
Confirms diagnosis based on symptoms
What is purpose of predisposition test.
Detects allele associated with an illness but not absolutely diagnostic of it.
What is purpose of predictive test.
Detects highly penetrant mutation with adult onset in an individual at high risk based on family history.
How do evaluate the quality of genetic tests.
- Analytic validity: is the test result correct?
- Clinical validity: does the result correctly diagnose disease or its absence?
- Clinical utility: does the result guide medical management ?
- Ethical validity: how well does the test meet the expected ethical standards?
What are the two key indicators of analytic validity.
- Sensitivity: proportion of affected individuals who are correctly identified by testing positive in the test ( if highly sensitive = few false negative)
- Specificity: proportion of unaffected individuals who are correctly identified by testing negative in the test. (If highly specific =few false positives)
What is the result of a false diagnosis .
Normal individual sent for unnecessary extra test and costs, affected individuals not being treated and may die.
What is a false negative or positive.
False negative : affected individuals incorrectly give a negative test results.
False positive: affected individuals incorrectly give a negative test result.
What is a true positive of negative
True positive: affected individuals who tested positive
True negative: unaffected individuals who tested negative
Define Sensitivity
Fraction of person with disease who have the susceptibility genotype.
+ve present /+ve present +-ve present
Define specificity
Fraction of persons without disease who do not have the susceptibility genotype.
-ve absent/+ve absent + -ve absent
Explain the clinical validity of tests
- Primary concern of the clinician: how accurate is a positive and negative test results.
- Use predictive values: directly related to the prevalence of disease= number of cases in a defines population at a specific time.
Explain Positive predictive value (PPV)
Probability that following a positive test result, that person will truly have the disease.
+ve present/ +ve present + +ve absent
Explain negative predictive value (NVP)
Probability that following a negative test result, that person will truly not have the disease.
-ve absent / -ve present + -ve absent
What is the false positive rate
- Proportion of positive tests that are false.
- If a test has significant false positive rates then the predictive value is very low and cannot be used in screening .
- +ve absent/ + ve present + +ve absent
Explain what a False negative rates is.
- Proportion of negative tests that are false.
- If a test has significant false negative rates then the predictive value is very low and cannot be used in screening
- -ve present/ -ve present + -ve absent
What is a direct [DNA] diagnosis of genetic testing.
- Used to detect mutations in patients when the disease gene is known
- Disease -caused variants characterised.
What is a indirect [DNA] diagnosis of genetic testing.
- Method of choice when the disease gene has been localised to a specific chromosomal region, but not necessarily cloned/ characterised.
- Or for very large genes with many exons, difficult -to -find mutations, may be involved in the disease.
- Performed by linkage analysis.
List some indirect DNA diagnosis.
- Distinguish the two chromosomes in the relevant parents. (Linked marker)
- Determine the phase (which chromosome )
- Work out which chromosomes the person who is being tested inherited.
Applications of chromosome analysis.
- Prenatal diagnosis
- Pre-implantation diagnosis
- New-born screening
- Clinical diagnosis
- Cancer genetics
- Research
We test for the Presence or absence of chromosome abnormalities and CNV before birth.
Due to ….
- Maternal age(over 35 years)
- Previous abnormalities (de novo)
- One of the parents carrier of a structural chromosome abnormalities.
- Family history (greater family)
- Abnormal antenatal sonar
- Screening of maternal serum indication a high risk .
- Maternal anxiety
Explain what human karyotype is.
Karyotyping allow visualisation of numerical and large structural chromosome changes during arrested in mitosis.
Pros of G-banded karyotype
- Whole genome analysis
- Overall impression
- Can use blindly, no prior knowledge required
- Can detect balanced chromosome changes
- Can detect polyploidies and mosaics
Cons of G-banded karyotype
- Low resolution (1 – 10Mb)
- Culturing needed (needs metaphases)
- Labour intensive and specialised (very little automation possible)
- Time consuming, time delay to results (3 – 14 days)
Explain what fluorescence in situ hybridisation.
- Combine Karyotype & Hybridization techniques
- Chromosomes/Chromatin Immobilized on microscope slide
- DNA probes anneal to specific target DNA sequence
- Probes are labelled with fluorescent reporter molecules
- Presence or absence of a particular genetic aberration can be viewed under fluorescence microscope.
Pros of FISH
- Higher resolution (depend on application / probe design)
- Can use both inter- and metaphase cells
- Quick technique to detect abnormal chromosome numbers
- Many fluorochromes available, can detect multiple probes simultaneously
- Spectral karyotyping - can evaluate complete karyotype in single experiment
Cons of FISH
- Need prior information (area of interest)
- Information gained limited to design of probe
- Not general screening tool
Characterise quantitative fluorescence PCR
- Pre and Postnatal Screening test
- Short Tandem Repeats (STR) Markers for common aneuploidies
- Chromosome 13, 18, 21, X and Y
- Fluorescently labelled primers used for PCR
- Analysis by Capillary Electrophoresis
Material used in QF-PCR
Any DNA containing tissue, uncultured or cultured.
Amniotic cells , chorionic villi, foetal or peripheral blood, baccalaureate cells
Explain QF-PCR technique
- Extract DNA
- Perform Multiplex PCR using Fluorescently labelled primers.
- Separate PCR products through Capillary Electrophoresis according to size
- Analyse using suitable Software e.g. GeneScanTM or GeneMapperTM
Pros of QF-PCR
- Can use any nucleated cells, cultured and uncultured
- Rapid results, 24 to 48 hours
- Many SRTs available, can do multiplex PCR
- Can be use with Non-Invasive Foetal Aneuploidy Screening
- High accuracy in identifying specific chromosome
- Largely automated