Flashcards in Genetics in Medicine Deck (66)
Give 5 reasons for a clinical genetics referral?
1) Affected patient for investigation or diagnosis
2) Family history of a genetic disorder or condition with a genetic component
3) Fetal loss/abnormality
4) Recurrent miscarriages
5) Strong family history of cancer
What is the purpose of genetic counselling?
1) To inform
2) To talk about choices ie. whether to have tests or more children
What are the 3 stages involved in making a genetic diagnosis?
1) Family tree
2) Physical examination
3) Genetic tests
What defines the boundaries of synthesis in PCR?
What is the rough process of PCR?
1) Heat denaturation - 94 degrees
2) Primer annealing - 55 degrees
3) Primer extension - 72 degrees
How can PCR be used to determine an allele with a deletion?
Simply separating the products by size using gel electrophoresis to identify those with a deletion ie. smaller
Can identify single deleted base and shows homozygotes, heterozygotes and normal genes
A deletion of nucleotide 35 (guanine) in the connexin 26 gene is the most common cause of what?
Most common inherited cause of deafness
Using what kind of assay is allele specific mutation detection carried out and what is its application?
Oligonucleotide ligation assay
Used to distinguish between a known disease causing point mutation and the normal allele
How does an oligonucleotide ligation assay work? not the process just a general overview
1) Allele specific oligos (primers) are synthesised so that their 3' end nucleotides base pair with the changed nucleotide to identify normal and mutant alleles that differ by one base pair
2) The reaction will not take place when the 3' end is not perfectly base paired thus identifying the presence or absence of a particular allele that that oligo is specific for
What is the process of an oligonucleotide ligation assay?
1) Have 1 assay for each specific oligo (one that identifies normal and those that identify mutants)
2) The specific oligo along with a shared primer that will anneal to both the mutant and normal allele
3) If the oligo anneals completely to the sample (ie specifically the 3'nucleotide) then ligation can take place between the specific oligo and the shared primer, if the allele doesnt match the primer this cannot happen
4) Ligated and unligated products can then be identified using gel electrophoresis thus identifying the absence or presence of a specific allele
Other than DNA testing what other test is involved in the Leeds Method of Management algorithm for CF testing?
IRT (immunoreactive trypsin)
IRT - DNA - IRT
What are the limitation of the CF Genotyping assay?
It will only identify the 32 mutations for which it has been designed, there may be other CF-causing mutations that are not identified by this assay
Give 3 examples of templates which may be unsuitable for use in PCR and what is the alternative to PCR in these cases?
1) Gene may be too big for PCR
2) Fragile X mutation in FMR1 gene is a repetitive tract of (CGG)n sequence
3) GC-rich regions are difficult to PCR
Southern blotting technique is used as an alternative, old technique but still important for diagnostic work
What 'discovery' method is used in unknown mutations and what are the 2 types?
1) Sanger/traditional sequencing (using chain termination chemistry)
2) Clonal/next generation sequencing
What is the rough principle behind clonal sequencing?
Sample of DNA is cut into fragments, grown on a slide in dense clusters (to make the fluorescent signal stronger) , each kind of nucleotide is fluorescently labelled and these are added along with primers, as each new nucleotide is added a laser is used to activate the fluorescence and a computer reads which nucleotide has been added in each cluster thus reading the sequence
What is the advantage of next generation clonal sequencing over traditional sequencing?
Faster and cheaper
Can just sequence exons due to knowing the introns/exons from the human genome project
What is Cadasil?
Caused by a single gene, is a genetic predisposition to stroke
What is the definition of a multifactorial/complex genetic disorder?
Caused by the interaction of multiple genes in combination with environmental factors eg. type 2 diabetes, IHD
What is the definition of single gene/mendelian genetic disorder?
A mutation in a single gene, eg. CF
What is the definition of a chromosomal genetic disorder?
An imbalance or rearrangement in chromosome structure eg. aneuploidy, deletion, translocation
What is the definition of a mitochondrial genetic disorder?
Caused by a mutation in mitochondrial DNA
What is a somatic mutation?
Mutation within a gene in a defined population of cells that results in disease eg. cancer
Autosomal dominant inheritance normally affects what in the body?
1) Structural proteins
3) Transcription factors
What is the definition of penetrance?
The frequency with which a specific genotype is expressed by those individuals that possess it, usually given as a percentage. Can alter with age, ie by 80 years of age Huntingtons disease has 100% penetrance
What is meant by incomplete penetrance? Give an example
Not all relatives who inherit the mutation develop the disorder eg. BRCA1 mutation
What is the definition of expressivity?
Variation in expression - the extent to which a heritable trait is manifested by an individual. Ie. in Marfan syndrome, some people will have aortic dilatation, some lens dislocation and some stretch marks
What is the definition of anticipation?
The symptoms of a genetic disorder become apparent at an earlier age as it is passed from one generation to the next. In most cases, there is an increase in the severity of symptoms too
What 2 genetic diseases does anticipation occur in?
1) Myotonic dystrophy
2) Huntington's disease
What is the definition of a new dominant or de novo mutation?
a new mutation that has occurred during gametogenesis or in early embryonic development - the child is the first to be affected in the family but can now pass the mutation on to their own children