44 - DNA Testing Flashcards
Unstable repeat expansions
Characterised by an expansion of a segment of DNA within the specific gene.
Consists of repeating units of three or more nucleotides in tandem
Often trinucleotide repeats
Molecular basis of repeat expansion disorders
Gene has a normal repeat region.
When a number of repeat units increases to above the number in the normal range -> condition
Below threshold repeats are stable in germline and somatic cells
Become unstable above threshold
Dynamic mutations
Size of expansion changes
Anticipation
Expansion size of repeats increases in following generations.
Relationship between number of repeats and age of onset
Increased number of repeats lowers the age of onset
Mechanism of repeat expansion
Slipped misrepairing.
During replication, replicating strand detaches inappropriately from template.
Replicating strand slips from its proper alignment with the template strand by one repeat length.
Newly synthesised strand has one extra repeat.
Four examples of triplet repeat expansion disorders
Huntington’s
Fragile X
Dyotonic dystrophy
Friedrich ataxia
Class one unstable repeat expansion
Non-coding repeats causing a loss of protein function (EG fragile X syndrome)
Class two unstable repeat expansion
Non-coding repeats that confer novel properties on mRNA.
Leads to a toxic gain of function (EG myotonic dystrophy, fragile X syndrome associated tremor ataxia)
Class three unstable repeat expansion
Repeats in a codon that confer novel properties on the affected protein.
Leads to novel gain of function which is toxic, or overrides normal function of protein (EG Huntington’s)
Characteristics of late-onset neurodegenerative disorders caused by repeat expansions 1 2 3 4 5
• Characterised by loss of movement control
• Variable and overlapping clinical presentation
• Late onset disorders (mostly)
• Symptoms worsen over time
• DNA testing can aid diagnosis, especially in differential
diagnosis
– Also for predictive testing (pre-symptomatic at-risk
relatives and in prenatal testing)
Inheritance of Huntington’s
Autosomal dominant
Prevalence of HD
1 in 10,000 to 20,000
Age of onset of HD
LAte onset (mean age is 40-50 years)
Three main classes of features of HD
Movement disorder
Cognitive disorder
Psychiatric/emotional disorder
Early features of HD
Clumsiness, agitation, irritability, apathy, anxiety, disinhibition, delusions/hallucinations, abnormal eye movements, depression
Very generic
Middle features of HD
Involuntary movements, chorea, trouble with balance and walking, trouble with activities that require manual dexterity, slow voluntary movements, general weakness, weight loss, speech difficulties (dysarthria), stubbornness
Late features of HD
Rigidity, bradykinesia (difficulty initiating and continuing movements), serious weight loss, inability to walk, inability to speak, danger of choking on food, inability to care for oneself
Genetic basis of HD
CAG repeat in HTT gene found on chromosome 4
Repeat is in exon 1, CAG codes for glutamine
Role of huntingtin
Roles in transcription (EG of brain-derived neurotrophic factor), intracellular transport of other molecules, intracellular signalling and metabolism and in reducing apoptosis
Pathology of HD
Progressive degeneration and loss of medium spiny
neurons in striatum of the basal ganglia
Molecular pathology of HD
1
2
3
PolyQ-huntingtin cleaved by caspases and other enzymes generating N-terminal fragments with altered conformation - these are toxic.
These form aggregates and nuclear inclusions (this might not be toxic. Might even be protective cellular response)
Loss of functional HTT and possible mRNA toxicity might also contribute to pathology
Normal number of CAG repeats in HTT gene
Under 26
Number of CAG repeats in HTT gene that has 100% penetrance for Huntington’s disease
Over 40 repeats
Number of CAG repeats in HTT gene that will not affect individual, but could expand in germline cells
27-35 repeats
Why is there more paternal transmission of HT than maternal?
Intermediate number of repeats more likely to expand in sperm than eggs.
Something that can mitigate effect of CAG repeats
CCG interruptions of CAG repeats
HD testing
PCR fragment analysis on capillary electrophoresis with fluorescent tag.
Look at number of repeats (on two alleles)
Inheritance pattern of spinocerebellar ataxias
Autosomal dominant
Age of onset of spinocerebellar ataxias
Late-onset
Main features of spinocerebellar ataxias
Progressive degeneration of cerebellum, brain stem and
spinocerebellar tracts (gait, hand coordination, speech, eye
movements)
Unusual spinocerebellar ataxia
SCA6.
Mutation is in a subunit of a Ca2+ channel.
No anticipation.
Inheritance pattern of Friedrich ataxia
Autosomal recessive
Prevalence of Friedrich ataxia
2 - 4 in 100,000
Age of onset of Friedrich ataxia
Usually around puberty (mean age is 10 - 15 years, usually under 25)
Main features of Friedrich ataxia
1
2
3
- Progressive limb and gait ataxia
- Cardiomyopathy (~65%) (often the cause of death)
- Diabetes mellitus (~30%)
Repeat in Friendrich ataxia
GAA repeat in FXN gene found on chromosome 9
Location of GAA repeat in Friedrich ataxia
Located in intron 1 of FXN gene
Effect of Friedrich ataxia GAA repeat
Causes abnormal DNA secondary structure or induces heterochromatin, resulting in reduced protein production
Number of Friedrich ataxia cases attributable to repeat expansion
94-96%
Effect of GAA repeat in Friendrich ataxia
Defect causes mitochondrial iron accumulation, leading to
oxidative damage
Normal range of GAA repeats in Friendrich ataxia
5 to 33
Affected range of repeats in Friendrich ataxia
66 to 1700 repeats
Premutation range of repeats in Friendrich ataxia
34 to 65 repeats
