Trinucleotide Repeat Expansion Disorders Flashcards
Why do Trinucleotide Repeat Expansion disorders progress in severity with each generation?
- The nucleotide expansion gets longer
What is a trinucleotide repeat?
Repeats containing tandem repeats of 3 basepairs
- e.g. CAGCAGCAG…
T or F: after exceeding a certain threshold trinucleotide repeats become unstable from one generation to the next.
True
T or F: while TNR disorders are clinically diverse they share a common mechanism of mutation
True
What type of phenotypes are always present with Trinucleotide Repeat Expansion Disorders (TNR)?
- Phenotype: ALWAYS neurologic in nature
e. g. neurodegenerative, neuromuscular
What are flanking signals or Trinucleotide Repeats?
- are these static or dynamic?
- Sequences on either side of the TNR
- THESE DO NOT CHANGE
Fragile X Syndrome
- Chromosome and location
- Clinical signs
- FMR1 gene on X chromosome
- Developemental delay and cognitive impairment
Huntington Disease
- Chromosome and location
- Clinical signs
- HTT gene on Chromosome 4
- Progressive Neuropsychiatric Disorder
What is the defining Characteristic of Trinucleotide Repeat Expansion (TNR) Disorders?
- technical term
Anticipation: They show increased severity with successive generations
Friedrich Ataxia
- Chromosome and Location
- Clincial Signs
- FXN gene on Chromosome 9
- Progessive ataxia
Myotonic Dystrophy
- Chromosome and Location
- Clinical Signs
- DMPK gene on Chromosome 19
- Progressive Muscle Wasting
At what part of the chromosome does Fragile-X syndrome occur?
- Repeat type
5’ - UTR (untranslated region)
- CGG repeats are added here
At what part of the chromosome does Huntington’s Disease syndrome occur?
- Repeat type?
Occurs in EXON
- CAG repeats are added
At what part of the chromosome does Friedreich ataxia occur?
- Repeat type?
Occurs in INTRON
- GAA repeats are added
At what part of the chromosome does Myotonic Dystrophy occur?
- Repeat type?
3’ - UTR (untranslated region)
- CTG repeats
What are the characteristics of a person who is:
- Affected
- Intermediate
- Normal
Affected:
Has the disease (severity is variabe)
Intermediate:
Does not have disease BUT offspring are at high risk
Normal:
Under threshold and alleles are STABLE
T or F: Trinucleotide Repeat Diseases only are found in coding region of DNA.
FALSE, Friedreich Ataxia occurs in an intron
T or F: a family with normal sized alleles may have variance in allele size from generation to generation but they always stay in the normal range
FALSE, there is NO variability from generation to generation of families whose alleles are in the normal range
T or F: within the population many different families may have different sized alleles that fall within the normal range
TRUE, yet WITHIN each family the same sized allele is passed down from generation to generation
What happens to UNstable alleles (exceeding normal threshold) during replication?
- These get longer and may cause disease
Note: this happens in people with an intermediate phenotype
What type of individual has unstable alleles?
- Those with UNstable alleles have more than the ‘normal’ amount of repeats
- These individuals have an intermediate phenotype
When does allele expansion occur for individuals who have unstable alleles (aka intermediate phenotype)?
- During Meiosis (IN GERM CELLS)
What are the two main categories of Trinucleotide Repeat Disorders?
- Coding Sequence
- POLYGLUTAMINE (CAG) - NON-coding
- variable repeat type
Are expansions in the coding sequence considered gain of function or loss of function mutations?
- Gain of function because expression of the protein from just one chromosome leads to a DOMINANT DELETERIOUS EFFECT
What sequence is repeated in coding sequence expansions?
- CAG (glutamine codon)
Why are polyglutamine (CAG) sequences deleterious?
- When glutamine gets too high it has a TOXIC EFFECT ON NEURONS
- Protein Fragment Aggregates become visible
T or F: Huntington’s is a coding sequence disease TNR disorder.
True, it is an error in the exon
T or F: fragile X syndrome is a coding sequence disorder.
False, TNR repeat expansions for fragile X occur in the 5’-UTR (untranslated region)
In what gene regions do non-coding TNR occurs?
- its VARIABLE
could be in:
- 5’-UTP region
- 3’-UTP region
- Intron
Are proteins produced in non-coding TNR defects?
- NO
Describe where each of 5’-UTR and 3’-UTR errors occur relative to the start and stop codons?
5’-UTR - BEFORE START codon
3’-UTR - AFTER STOP codon
As far as noncoding TNR errors go, what leads to the deleterious effects in:
- 5’-UTR expansion
- 3’-UTR expansion
(aka what happens to the DNA or mRNA)
5’-UTR:
Expansion leads to methylation and loss of transcription (gene silencing) - NO mRNA produced
3’-UTR:
Expansion Disrupts normal splicing of other genes (deleterious affects at RNA level)
As far as coding TNR errors go, what leads to the deleterious effects in:
- Huntington’s Disease
Expanded polyglutamine (CAG) causes production of TOXIC AGGREGATES
(PROTEINS ACTUALLY BEING PRODUCED HERE)
What leads to increases in severity of TNR disorders from one generation to the next?
- What is the root cause of this problem (when in cell cycle)?
Anticipation - caused by EXPANSION of the already irregular repeat region during GAMETOGENESIS
T or F: increased severity of disease refers to increased clinical problems associated with disease AND/OR earlier age of onset
TRUE, disease itself doesn’t always have to be worse, the age of onset may just be earlier
How do we test for TNR defects?
- PCR primers for the FLANKING regions are added
- We run the PCR experiment and allow transcription of gene region
- We then look at the size of the PCR fragments
- DISEASED PEOPLE WILL HAVE LONGER FRAGMENTS FROM the Repeated region
What is the limitation for TNR tests?
- PCR is used so large alleles may be difficult to amplify
T or F: for most disorders determination of allele size is used to establish a DIAGNOSIS.
TRUE
Can we predict how much the region will expand from one generation to the next?
NO
Huntington’s
- Mode of Inheritance
- Age of onset
- signs/symptoms
- Autosomal Dominant
- 40 y/o (~15 years survival)
- Cognitive involvement with involuntary jerky and writhing movements
**Note that the presence of NORMAL intermediates have DISEASED children is a Trend in TNR disorders
**Note that the presence of NORMAL intermediates have DISEASED children is a Trend in TNR disorders
For Huntington’s Disease, how do we measure disease severity?
AGE:
- More repeats in the expanded EXON region lead to a worse manifestation of the disease
What are the chances of passing Huntington’s on?
1 in 2, its on autosomal dominant disorder
Friedrich Ataxia (FRDA)
- Mode of Inheritance
- Location of Repeat in gene
- Repeat Sequence
Mode:
- AUTOSOMAL RECESSIVE
Location of Repeat Gene:
- Intron 1 (9q13)
Repeat Sequence:
- (GAA)n
Fragile-X site A (FRAXA)
- Mode of Inheritance
- Location of Repeat in gene
- Repeat Sequence
Mode:
- X-Linked
Location:
- 5’ Untranslated Region (Xq27.3)
Repeat Sequence:
- (CGG)n
Myotonic Dystrophy (DM1)
- Mode of Inheritance
- Location of Repeat in gene
- Repeat Sequence
Mode:
- Autosomal Dominant
Location:
- 3’-Untranslated Region (19q13)
- so causes RNA toxicity
Repeat Sequence:
- (CTG)n
What are the signs/symptoms of Myotonic Dystrophy type 1 (DM1)?
**Often have difficulty letting go of things
(aka MYOTONIA - prolonged muscle contractions)
others:
- Cataracts
- Cardiac Conduction Defects
- Variable Expressivity with severity correlating with allele size
For Myotonic Dystrophy (DM1) do we measure severity by age of onset or by level of dysfunction?
- Level of Dysfunction
What is RNA toxicity?
- when does it occur
RNA Toxicity:
Occurs with Errors in the 3’-UTR
Process:
- mRNA accumulates in the cell and sequesters RNA binding proteins
- Affects the ability of RNA to undergo normal processing
- Toxicity causes DOMINANT TOXIC EFFECT
What test would you run if you suspected someone had Myotonic Dystrophy (DM1) and wanted to make a diagnosis?
- You use the PCR method like with Huntington’s
What are some clinical Signs and Symptoms of Friedreich Ataxia?
- Progressive Ataxia with onset at puberty
- Cardiomyopathy may accompany this
What is the frequency of individuals affected by fragile X syndrome?
- 1/5000
T or F: females with full mutation for fragile X syndrome with have full expressivity?
FALSE, 1/2 to 2/3 of females are affected (so reduced penetrance) and often expression of the disorder is less severe
(because of x inactivation)
What are the signs and symptoms of fragile X syndrome?
- Delayed Developemental Milestones
- Long Face, large ears, prominent jaw
- Macro-orchidism(postpubertal enlarged testes)
- Abnormal Temperament with Tantrums
- Autism
- IQ 30-50
What does the protein do that is encoded for by the FMR1 gene?
- Binds mRNA and functions in shuttling it from the nucleus to the cytoplasm
What happens as a result of the expansion of the FMR1 repeat region?
Methylation increased in expanded region - this prevents expression
- Note: expansions here cause increased fragility in Karyotype preparations causing that part of the X chromosome to break off
Where is the expanded region located in Fragile X syndrome?
Between the promoter region and the start codon
How many females are prematuration carriers of fragile X syndrome?
- size of mutation
- associated risk
- 1/100 females carries
- Size of Expansion and Risk of having and affected child are variable
If a males is a prematuration carrier for fragile X will his daughter have any chance of being affected?
- NO
- REPEAT REGION DOES NOT EXPAND IN MALES, this assumes that the mother is not also a carrier of fragile X
Does the son of a father with fragile X syndrome have any risk of getting the disease?
NO, its an X linked disease
T or F: the larger the premutation, the more likely it is that further expansion will occur
True
For the following diseases when is offspring at the greatest risk of having the disease when the mom is the prematuration carrier? Dad?
- Fragile X syndrome
- Myotonic Dystrophy
- Huntington’s Disease
Fragile X syndrome:
- Mom as prematuration = greatest risk
Myotonic Dystrophy:
- Mom as prematuration = greatest risk
Huntington’s:
- DAD as prematuration = greatest risk
**Greatest risk aka more likely to get screwed up in that parent’s meiosis
How might fragile X syndrome manisfest itself in someone who is a prematuration carrier of the disease?
- male
- female
BOTH:
- FXTAS (FraX-associated tremor/ataxia syndrome)
- MORE common in MALES because they only have one X
FEMALES only:
- POI (primary ovarian insufficiency) –> aka menopause before 40 y/o
T or F: while in affected individuals the Fragile X gene is underexpressed, in prematuration carriers it is OVERexpressed
True
Which category does PCR testing of allele size fall into? why?
- Targeted or
- Sequencing
Targeted, because arrays only look at the repeat region to get the repeat number they aren’t trying to find other point mutations in the gene or anything else like that
T or F: When testing for fragile X syndrome in a male only a single chromosome is used in analysis, this the case for PCR tesing of all TNR disorders
False, most TNR disorders you must look at BOTH alleles to check for the mutation, this is especially the case in Fredrick’s ataxia where the disease is Autosomal Recessive
T or F: the mutation causing Huntington’s disease occurs around the C-terminus
False, it occurs near N-terminus
T or F: while TNRs are a common cause of Friedreich Ataxia, the same disease could be caused by a loss of function mutation
True, I assume this is true for all LOSS of function diseases such as fragile X where the protein’s absence causes the disease regardless of what caused the protein’s absence
T or F: a heterozygous female may be affected by fragile X.
TRUE
What does the term transmitting male in Fragile X mean?
- The male will transmit his X to all daughters but the size of the repeat region will no expand
