Genes and disease 2 Flashcards
Diagnosis of genetic disorders often involves what? (3)
1- A physical examination
- Examine dysmorphic features
- Distinctive facial features, head circumference, distance between the eyes, length of arms and legs, neurological etc
2- Detailed medical family history
Occurrence of the same condition in more than one family member can indicate a genetic disorder
3- Genetic Testing
Molecular, chromosomal, and biochemical testing are used to diagnose genetic disorders
What is cytogenetics?
-
Examination of whole chromosomes for abnormalities
> Chromosome abnormalities can be numerical (wrong number of complete sets of chromosomes)
> Or structural when one or more chromosomes contain the wrong DNA.
1- What is the significance of chromosomal abnormalities?
2- Give an example of a mechanism behind chromosomal abnormalities.
1- Large-scale changes in chromosomes mean several genes can be affected
2- Chromosomes break or fail to segregate during meiosis or mitosis
What is karyotyping? How is it done? What does it allow geneticists to do?
- Examination of whole chromosomes for numerical abnormalities or structural abnormalities.
- Needs mitotic cells eg WBCs
- Stain metaphase chromosomes with eg Giemsa
- Giemsa dye stains heterochromatic regions dark and euchromatic regions as light bands.
- Characteristic pattern of bands to ID each chromosome
What are structural abnormalities can be seen? (3)
What numerical abnormalities can be seen? (3)
- Aneuploid: Incorrect total number of chromosomes
Anything other than 22+XX or 22+XY - Monosomy: only one of a pair, one homolog is missing
- Trisomy: three copies of one chromosome
Why are numerical sex chromosome abnormalities less problematic?
- The Y chromosome carries few genes
- An X chromosome can be inactivated (reduces the effect if have an extra X chromosome)
Many genetic disorders are caused by mutations affecting a single base or small part of a gene.
These changes cannot be detected by karyotyping and requires DNA based molecular genetic tests.
- What is molecular genetic testing?
> Name + explain an example.
- Looking for a specific gene, a certain mutation within a specific gene or any mutation on a specific gene .
> Fluorescencein situhybridization (FISH)
Complementary DNA sequences hybridise
Synthesise a probe to region of interest - labelled with fluorescent
Allow probe to hybridise to DNA of patient
If patient DNA has mutation hybridisation is prevented
What is amplification based- gene testing?
- Amplify the gene of interest-PCR
- Sequence the gene to look for DNA changes
> Enables genomes to be sequenced very rapidly
DIFFERENT TYPES OF GENETIC TESTING:
- What is the difference between the following:
1- Diagnostic testing
2- Predictive + pre-symptomatic genetic tests
3- Carrier testing
1- To confirm a diagnosis when a disease is suspected
Patients may already experience symptoms
Involve chromosome analysis, biochemical analysis and molecular testing
Provide choices in management/treatment
2- A predictive test can provide information about whether or not a person will develop or is likely to develop a specific condition, usually at a later stage in life
Done if the condition can be prevented or symptoms can be treated
Also the information may be useful in decisions about lifestyle and important life decisions
3- To identify whether a healthy unaffected individual is a carrier of a genetic condition
A carrier status has implications for future offspring
Offered to individuals with a family history of certain
1-What does pre-natal testing include?
1- Screens and diagnostic tests
- Identify the risk of developing a genetic disease or condition e.g. Combined test for Down’s syndrome uses ultrasound scans to measure the nuchal translucency (LESS THAN 3.5MM IS NORMAL) and a blood test to measure the levels of PAPP-A (LOW LEVELS ASSOCIATED WITH DOWNS SYNDROME) and hCG ( HIGH LEVELS ASSOCIATED WITH DOWNS SYNDROME)
> If results indicate the risk to be more than 1 in 150 then diagnostic tests are offered
Pre-natal testing:
- Explain the following diagnostic tests:
1- Chorionic villus sampling
2- Amniocentesis
1- Done between 11 to 14 weeks of pregnancy
Cells are taken from the chorionic villi using a needle
Cells are analysed for chromosomal abnormalities
2- Done between 15 to 20 weeks of pregnancy
Cells are taken from the amniotic fluid and analysed for chromosomal abnormalities
What is Non-invasive prenatal testing?
- Blood test taken from the mother in pregnancy, that uses DNA sequencing technology to evaluate whether a baby has a high chance of a certain chromosomal condition.
Where can patient’s get specialist genetic advice?
- Regional Genetic Centres
What 4 groups of people require referral to a specialist genetic centre?
1- Those with a known / suspected genetic disorder
2- Children with dysmorphic features / learning difficulties
3- Couples with a history of miscarriage
4- Those with a history of cancer
Why is genetic counselling an important part of gene testing?
- Helps the individual understand the medical facts and future risks
- Helps them appreciate the mechanism of heredity
- Helps them make necessary adjustments
- Gives them practical and psychological support
What are the benefits of genetic testing?
- Gives certainty about genetic makeup
- Aids the accuracy of the diagnosis so appropriate treatment is offered
- Preventative measures and monitoring can be done if there is a known possible risk
- Helps if planning to have children
What are the limitations of genetic testing?
- May not provide all the answers (leading to decisions without being fully informed)
- Some conditions may not have specific treatments / interventions currently available some may
- It may lead to difficult, undesired decisions (e.g. termination of pregnancy after prenatal tests)
What are the ethical dilemmas surrounding genetic testing?
- Undesired options after genetic testing
- Consequences of false-positives and false negative results
- Uncertain implications for early interventions
- Should we undertake genetic testing even if no treatment or interventions exist
What is metabolic manipulation and what is it used for?
- A type of treatment for inborn errors of metabolism (genetic conditions caused by mutations disrupting specific enzymes)
Treatment sometimes includes dietary changes
What does protein augmentation involve?
- Purifying the missing protein and giving it to the patient
What does gene therapy involve? (4)
1- Replacing a mutated gene with a normal copy
2- Inactivating a mutated gene that is functioning improperly
3- Introducing a new gene to fight a disease
4- Homologous recombination or CRISPR/Cas9
> Transfers a normal copy of the gene to restore the function of the protein that is missing / dysfunctional
Viral vectors (retroviruses / adenoviruses) who’s genome has been modified to be non-pathogenic and includes the required gene transfer the DNA to cells
What is the mechanism of antisense therapy? (Gene slicing)
Dominant mutations can produce proteins which are toxic to the cells.
Antisense gene therapy can be used to block synthesis of toxic proteins.
1- Short single-stranded pieces of chemically modified nucleotides, known as oligonucleotides are engineered to be complementary to specific mRNA i.e gene in the cell.
2- Binding of antisense oligonulceotides (ASO) to target mRNA blocks its translation or can induce its degradation by RNase H.
What are morpholinos?
Similar to antisense mRNA
Backbone of morpholine rings rather than ribose
Can hybridise to mRNA and block translation
Can also be designed to hybridise with pre-mRNA to skip exons
Approved therapy for DMD
What is Crispr/Cas9?
- Clustered Regularly Interspaced Short Palindromic Repeats in bacterial genomes
- Cas9 is a bacterial protein that degrades foreign DNA (using CRISPR sequences to target)
In gene editing
Template RNA (sgRNA) and Cas9 introduced to a cell
sgRNA targets a gene
Cas9 cleaves the DNA which is then repaired
If a repair template is present, an edited gene can be inserted
If not NHEJ pathway is used
What is CAR-T?
Chimeric Antigen Receptor T-cells
Not used as a gene therapy
But does use gene editing to treat cancer
- Inserts a gene for a chimeric antigen receptor to a specific protein into a patients T-cells = CAR-T cells
- Amplify the cells in vitro
- Put back in patient
- CAR-T cells attack cells expressing the antigen and lyses them
