SAQ 2017 Flashcards
A homozygous nonsense mutation (R577X) in the α-actinin-3 (ACTN3) gene was identified in two brothers with an early-onset form of muscular dystrophy. Upon testing, no expression of the ACTN3 gene was detectable in RNA samples from the brothers, since the mutation-bearing transcripts were degraded by nonsense-mediated decay.
Explain how three other classes of mutation typically result in no protein expression. (6 marks)
- Indel/frameshift – change in reading frame with likely introduction of premature termination codon upstream of last exon-exon junction. Induction NMD
- Splicing mutation - change in reading frame with likely introduction of PTC upstream of last exon-exon junction. Induction of NMD
- Deletion - causing removal of gene or cis elements required for transcription and/or translation
- Point mutation affecting start/regulatory codon
Types of mutation (5)
- nonsense mutation
causes a PTC (codon swapped for stop codon) - frameshift mutation
addition/deletion of a base pair causes change to reading frame - splicing mutation
inserts/deletes/changes nucleotides at splice site - deletion mutation
- point mutation
single base insertion/deletion/change
What are the main phases of the cell cycle, and what is happening within the cell in each case?
G0 – cells are in the quiescent phase. They are metabolically active but do not undergo cell division
G1 – cells are metabolically active and are growing
S – DNA synthesis and replication, the total amount of DNA is double the amount found in a cell in G0 or G1
G2 – Protein synthesis, growth and preparation for cell division occur
M – Mitotic phase – cells divide
Provide an example of a monogenic disorder and a polygenic disorder and compare and contrast the genetic basis of these diseases
Monogenic: Huntingtons
autosomal dominant CAG trinucleotide repeat disorder
late onset present in every generation in both males and females
rare
Polygenic: type 2 diabetes
multiple gene variants predispose to T2 diabetes, caused by both genetics and environment, can be family history, single nucleotide polymorphisms predispose to rather than cause disease
Draw a diagram illustrating the key sequence elements and protein complexes required for constitutive splicing and alternative splicing
constitutive splicing
- U1 binds to 5’ splice site on exon 1
- U2AF and BBP bind to branch point
- U2 kicks off U2AF and BBP
- U4/U6/U5 triple snrp create active site and loop pre-mRNA
- U6 kicks off U1 at 5’ splice site
- 3’ OH of exon 1 attacks 5’ site of exon 2
alternative splicing
regulatory regions are present within both introns and exons
Exon splicing enhancers (ESEs) and exon splicing silencers (ESSs) are found in exonic regions
Intron splicing enhancers (ISEs) and intron splicing silencers (ISSs) are found in intronic regions
ESEs and ISEs bind Serine Arginine (SR) proteins
Exon splicing silencers (ESSs) and intron splicing silencers (ISSs) usually bind hnRNPs
Sarah’s father has recently been diagnosed with Spinocerebellar ataxia type 6 (SCA6) which is a late onset, incurable, autosomal dominant disorder. In a genetic counselling session Sarah reveals that she wants both her and her 13 year daughter to undergo a presymptomatic test for SCA6
a) What is the difference between a presymptomatic and a diagnostic test
b) At what age would Sarah’s daughter usually be tested for SCA6; give two reasons why this is?
a) Diagnostic – when a patient is showing signs of particular disorder, presymptomatic is before a patient is showing signs, predicting what will happen in future
b) Presymptomatic tests are usually performed after the age of 18
Testing prior to 18 can
- Infringe on the child’s future autonomy
- Cause psychological damage growing up knowing that were going to affected by a serious late onset disorder
- Stigmatisation
In what situation would a presymptomatic test ALWAYS be indicated in a person under the age of 18 and give an example?
If the pre-symptomatic test could directly benefit the child by allowing medical or surgical intervention
Familial hypercholesterolaemia
What are the main differences between apoptosis and necrosis?
- Apoptosis is programmed cell death, necrosis is unprogrammed
- Apoptosis is physiological whilst necrosis is pathological
- Apoptosis elicits no inflammatory response, whereas necrosis is associated with an inflammatory response
- Apoptosis is elicited by cellular physiological stimuli such as lack of growth factors or activation of p53, whereas necrosis can be caused by external factors such as viruses, hypoxia, ischemia
A baby with a severe limb defect is born to a couple who are first cousins. Why is a consanguineous relationship more likely to produce offspring with a genetic abnormality?
All individuals carry a number of rare, deleterious, recessive mutations
When parents are related to each other it increases the chance of both parents carrying the same rare mutation
thus there is an increased risk of any offspring inheriting a mutation on both chromosomes
Define the following and state what percentage (0-10%, 11-20%, 21-35%, 36-50%) of the human genome they make up. Note that each percentage range can be used more than once
a) Intron
b) Exon
c) Promoter
d) 3’UTR
e) Coding Region
intron:
- Region of transcript, or corresponding DNA, that is not included in fully processed mRNA, having been excised during splicing
- 35%
exon:
- Region of transcript, or corresponding DNA, that is included in fully processed mRNA, an exon may be coding and generate transcripts which are translated into protein or non-coding and not be translated.
0-10%
promotor:
- A cis-acting regulatory DNA element upstream of a transcription unit which binds RNA polymerase.
0-10%
3’ UTR:
- The untranslated region between the stop codon and poly(A) tail.
0-10%
Coding region:
- The coding sequence is the entire exonic sequence that is translated (i.e. does not include 5’ and 3’UTRs)
0-10%
Genome-wide association studies have identified 128 common genetic variants associated with schizophrenia, but these variants only explain 5% of the genetic component of the disease. Briefly describe two potential sources of the remaining genetic component of the disease and how the influence of these factors might be assessed in future studies
- rare variants (not captured on SNP chips)
- structural variants
- non-additive effects/interaction
- epigenetics
- overestimated heritability
- epistasis
- linkage disequilibrium
When counselling a patient about their risk of developing an autosomal dominant disorder what other factors may you need to consider in addition to whether the patient has inherited the disease causing mutation? Explain with examples why these factors affect a patient’s risk of becoming affected by a disorder
- Penetrance
Not all disorders are fully penetrant therefore even if a patient has inherited mutation they may not develop the disease (i.e. Inherited Breast cancer has a penetrance of 50-80%) - Anticipation
When the disease develops at an earlier age in each generation, this important information to impart to the patient (i.e. trinucleotide repeat disorders)
