Genetics Week 2

Question 1

What is imprinting?

Answer 1

A naturally occurring epigenetic difference between certain alleles expressed at the paternal and maternal loci of a chromosome set.

In this certain subset of genes, either the maternal set or the paternal set is expressed, but not both. Imbalance of this leads to imprinting disorders.

Both the paternal and maternal copies of the genes must be there for functional expression.

Question 2

what causes Prader-Willi Syndrome?

Answer 2

An interstitial microdeletion on chromosome 15 at site q11-q13.

PATERNAL CHROMOSOME

Essentially,the genes at position q-12 are gone from #15.

Question 3

Which has more mental dysfunction: a child with Prader-Willi or a child with Angelman?

Answer 3

Angelman - only moderate retardation with Prader-Willi

Question 4

Can a person inherit an epigenetic alteration of DNA?

Answer 4

Yes, but anything epigenetic is REVERSIBLE

Question 5

What are the 3 examples of epigenetics she gave us?

Answer 5

1. Methylation - regulation of gene expression, may be dynamic.
2. X inactivation - heritable from cell to cell but not passed to offspring
3. Genomic Imprinting: Parent of origin effects; transmitted through gametes.
   - what happens to the kid is based on which parent they inherit the imprint from.

Question 6

What protein is lost in Prader-Willi Syndrome?

Answer 6

Paternal chromosome 15,q12 codes a SNRP called

SNRPN –> only copies we have are from Dad.

(Small Nuclear Ribonucleaprotein N)

Question 7

What protein is lost in Angelman Syndrome? How is this related to the symptoms of the disease?

Answer 7

UBE3A -Ubiquitin Protein Ligase 3

Only the maternal UBE3A is expressed in the brain, but both copies are expressed everywhere else in the body.

This results in the SEVERE MENTAL RETARDATION associated with Angelman.

Question 8

What do we blame for the micro deletions in Prader-Willi and Angelman syndromes?

Answer 8

Low Copy Repeats causing unequal crossing over.

The common deletion points encompass the critical region for BOTH DISEASES

Question 9

Describe the Etiology of Prader-Willi Syndrome.

Answer 9

70% paternal micro deletion of chromosome 15

28% maternal uniparental disomy

2% mutation of imprinting center

Question 10

Describe the Etiology of Angelmann Syndrome?

Answer 10

70% maternal microdeletion

Less than 5% paternal uniparental disomy (RARE)

10% UBE3A point mutations (hereditary)

10% unknown

Women are always more complicated.

Question 11

What is uniparental disomy?

Answer 11

An individual inherits both chromosomes of one homologous pair from a single parent and NO copy of that chromosome from another parent.

Question 12

WTF is trisomy rescue?

Answer 12

Initially, a nondisjuction arises in meiosis that leads to trisomy at conception.

The rescue part occurs when a SECOND nondisjuction occurs (this time in MITOSIS) that results in loss of a chromosome in early embryogenesis.

This can result in UNIPARENTAL DISOMY if the lost chromosome was from the parent opposite that of the original meiotic nondisjuction.

Question 13

What is the chance the paternal chromosome gets lost in normal Trisomy Rescue?

Answer 13

1/3, because there are 3 chromosomes that can get lost.

Question 14

What has to happen to yield UNIPARENTAL DISOMY?

Answer 14

Tw independent errors. One in meiosis, then trisomy rescue nondisjuction error in MITOSIS during early embryogenesis.

Question 15

FACT: VERY RARE to see Parental disomy in Angelmann syndrome. More common to see MATERNAL UNIPARENTAL DISOMY in Prader-Willi syndrome.

Answer 15

Yes.

Question 16

What is the most common form of inherited mental retardation?

Answer 16

Fragile X Syndrome - 1 in 5000 males.

Question 17

What is the genetic basis of Fragile X?

Answer 17

Trinucleotide Repeat Expansion Disorder.

CGG expansion in the 5’ UNTRANSLATED region of the FMR1 gene. Expansion is accompanied by METHYLATION which shuts off gene expression.

Question 18

What percentage of males that inherit the fragile X expansion are affected? Females?

Answer 18

100% of males (only 1 X chromosome)

50% of females due to random X inactivation

Question 19

Is Fragile X syndrome considered a LOF or a GOF mutation?

Answer 19

LOSS OF FUNCTION - FMR1 gene is shit down by expansion and resulting methylation

Question 20

HOw many CGG repeats must accumulate for a person to express a Fragile X allele?

Answer 20

200-1300 repeats = FRAGILE X

6-54 is NORMAL

55-200 is PERMUTATED ALLELE (threatens dysfunctional expansion in future generations)

Question 21

Fact: FMR1 is a polymorphic gene. It naturally has variation in copy number. It’s when this copy number gets expanded by CGG repeats that it has disease tendency.

Remember: 6-54 = normal

Answer 21

Yes.

Question 22

Expansion of premutated (carrier) FMR1 genes to full-blown Fragile X Syndrome only occurs in __________ __________.

Answer 22

Female Meiosis

Question 23

Expansions of a trinucleotide (CAG) coding for GLUTAMINE is characteristic of what trinucleotide repeat expansion disorder?

Answer 23

Huntingdon’s

Question 24

What are the differenced between Fragile X and Huntingdon’s?

Answer 24

FRAGILE X:

• LOF
• Noncoding sequence
• CGG repeat
• 200+ repeats = disease
• FMR1 gene
• X-linked
• childhood onset

HUNTINGDONS:

• GOF
• Coding sequence
• CAG = polyglutamine repeats
• 36+ repeats = disease
• Autosomal Dominant
• Adult onset

Question 25

What causes the neuronal damage associated with Huntingdon’s?

Answer 25

Abnormal proteins aggregate near neurons.

Question 26

What TNR (Trinucleotide repeat) Expansion causes LOF and decreased gene expression?

Answer 26

Fragile X

Question 27

What TNR causes expansion of polyglutamine tract, and produced toxic protein aggregates?

Answer 27

Huntingdon’s

Question 28

What TNR has 3’ expansion of the untranslated region, DISRUPTING SPLICING?

Answer 28

Mytonic Dystrophy

slide 12 of handout

Question 29

You’re more likely to get Huntingdon’s disease from a paternal or maternal meiosis expansion of the gene?

What about Fragile X?

Answer 29

PATERNAL Meiosis expansion = Huntingdon’s

MATERNAL Meiosis expansion = Fragile X

Question 30

What does the term “anticipation” mean when applied to Trinucleotide Repeat Expansion Disorders?

Answer 30

A general feature of TNR disorders is progressive expansion of the gene with each successive generation. Copy number increases from generation to generation.

Progressive earlier onset and increased severity as families continue to reproduce.

Question 31

Affected Huntingdon’s individuals have greater than _____ copy #s. What about Fragile X?

Answer 31

Huntingdons: More than 36

Fragile X: More than 200

Question 32

“Direct testing” is also called…..

Answer 32

Genotyping for one or more specific mutations.

Question 33

What are the pros and cone of direct genetic testing? (AKA genotyping)

Answer 33

Pro: Very accurate and specific FOR THE GENES TESTED

Con: Only tests for the specific genes you determine, so it can only test for KNOWN MUTATIONS.

Question 34

What molecular method is used in direct testing?

What is its limitation?

Answer 34

DNA hybridization with ALLELE-SPECIFIC NUCLEOTIDES

Limitation: Only perfect hybrids will be detected.

Question 35

WTF is a homoduplex?

Answer 35

2 matching, complimentary sequences of DNA that can hybridize

Question 36

How do you use direct testing to screen for a Trinucleotide Repeat Disorder?

Answer 36

Direct testing and allele sizing.

1. Use gene-specific primers on flanking regions of the trinucleotide repeat region.
2. PCR amplification of flanked region
3. Elecrophoresis to separate by size.

SIZE OF PCR PRODUCT REFLECTS REPEAT #