18.03.06 AD - Gain of function

Question 1

What are the different types of gain of function mutations?

Answer 1

Hypermorph - allele that produces an increase in quaitntiy or activity of its product.

Neomorph - an allele with a novel activity or product.

Question 2

Why do gain of function mutations usually result in dominant phenotypes?

Answer 2

Because the presence of the normal allele foes not prevent the mutation allele from functioning abnormally.

Question 3

Give an example of a hypermorph.

Answer 3

Often involves a control or signalling system behaving aberrantly; signalling when it should not, or failing to switch a process off.

Example: Mutation in G-protein coupled hormone receptor genes lead to activation of the receptor even in the absence of a ligand such as with the GNAS gene in McCune-Albright Syndrome.

Question 4

Give an examples of a neomorph.

Answer 4

The product may acquire a novel function; a chromsome rearrangement may create a novel chimeric gene by fusing the exons of two different genes.

Example: BCR-ABL1 translocation in CML

Question 5

Give an example of a gene in which gain or function and loss of function mutations results to different diseases.

Answer 5

1. PMP22 (17p12) Gain CMT, loss = HNPP

Question 6

Give three examples of disease associated with GoF mutations.

Answer 6

1. PMP22 overexpression = CMT
2. GNAS receptor permanently on = GNAS
3. PI acquisition of new substrate = Alpha 1 antitrypsin
4. SCN4a ion channel permanently open = Paramyotonia congenita
5. HTT protein aggregation = Huntington disease
6. BCR-ABL1 chimeric gene = CML

Question 7

What is the mechanism underlying HD?

Answer 7

1. (CAG)n repeat expansion in exon 1 of HTT at 4p16.3.
2. Repeats translated into a polyglyutamine tract which is though to acquire a novel deleterious function leading to neuronal dysfunction and neurodegeneration.
3. PolyG expansions form neuronal intranuclear inclusions containing huntingtin, chaperone proteins and ubiquitin.
4. Mutant HTT forms abnormal protein structures e.g. B-sheets and is truncated by caspase-6 cleavage producing toxic n-terminal fragments.
5. This leads to processing of abnormal proteins
6. Toxicity of HTT affected by post-tranlsational modification and nuclear localisation.
7. Mutant HTT intereferes with gene transcription and may have direct or indirect effects on mitochondira, affecting metabolism and leading to oxidative stress.
8. Abnormal vesicle transport with decreased transport and release of BDNF and increased excitotoxicity

Question 8

What are the repeat ranges for HD

Answer 8

Normal <27
Intermediate 38-35
Reduced penetrance 36-39
Penetrance >40
Juvenile >60

Question 9

What is the evidence that the CAG repeat expansion is a gain of function mutation in HD?

Answer 9

1. Patients with chromsome deletions at 4p16.3 do not have HD
2. Patients without HD have been shown to have translocations affecting 4p16.3
3. Homozygotes are clinically identical to heterozygotes.
4. Dominant phenotype suggests mRNA or ptoein product of the disease allele has acquired a new property or is expressed inappropriately
5. HD levels of polypeptides encoded by normal and mutant alleles are identical.

Question 10

What is the mechanism of molecular pathogenesis in DM1/DM2 carriers

Answer 10

Toxic RNA gain-of-function (CUG)n and (CCUG)n repeats

Expansions form stable structures which sequester RNA-binding proteins e.g. MNBL

Altered expression and activity of these RNA regulatory proteins affects splicing

In DM patients embryonic splicing patterns of several genes were observed instead of adult splicing.

Question 11

Which SCAs are associated with a protein gain of function mechanism?

Answer 11

SCA1
SCA2
SCA3
SCA6
SCA7
SCA17

Question 12

What is the molecular pathology underlying SCA8?

Answer 12

SCA8 = late onset neurodegenerative disease with CTG/CAG expansions expressed in the brain and cerebellum.

Proposed that it may result from a combination of both RNA and protein gain of function mechanism.

The expansion associated with the SCA8 phenotype is located in both the 3’ untranslated region of ATXN8OS and a short polyglutamine ORF in the more recently identified overlapping gene ATXN8 [

The CTG·CAG repeat is adjacent to a CTA·TAG repeat that is highly polymorphic but stable when transmitted from one generation to the next

Bi-directional transcription through the repeat region

AXTN8 antisense CAG-containint transcript is translated into ataxin 8 protein found in intranuclear inclusions in Purkinje cells and in brainstem neurons of SCA8 patients.

Concomitant expression of untranslated sense CTG mutation in the 3’UTR of the ATXN8OS gene generates CUG repeat transcript the becomes toxic via a RNA GoF mechanism (similar to DM1)

Question 13

Which gene is associated with achondroplasia? What is its function?

Answer 13

Achondroplasia - caused by a mutation in/close to transmembrane domain of FGFR3 (4p16.3)

FGFR3 negatively regulates bone growth by inhibiting chondocytes.

Mutations result in constitutive activation of the FGF receptor - severely limiting bone growht.

Question 14

Which is the common FGFR3 mutation associated with achondroplasia? What is the effect of this mutation?

Answer 14

FGFR3 p.(Gly380Arg)
Enhances dimerization of the protein that catalyses downstream signalling.
Mutation rate of this codon is disproportionately high for unknown reasons.

Question 15

Which other disorders are caused by mutations in FGFR3?

Answer 15

Thanatophoric dysplasia types 1 and 2
Hypochondroplasia
Crouzon syndrome with ancanthosis nigricans
Wolf-Hirschhorn syndrome (microdeletions(
Muenke’s coronal craniosynostosis

Question 16

Give and example of a disease in which a novel functional gene has been created through a chromosomal rearrangement.

Answer 16

CML and ALL with chimeric gene BCR-ABL1 which leads to uncontrolled cell proliferation.

Question 17

What % of cases of adult leukaemia cases does CML account for?

Answer 17

15-20%

Question 18

What % of cases does ALL account for?

Answer 18

80%

Question 19

What is the function of the BCR-ABL1 fusion gene?

Answer 19

oncogene promotic genomic instability.

Production of an abnormal tyrosine kinase molecule with increased activity. Four fusion transcripts can result from the rearrangement.

Question 20

Give an example of a disease caused by overexpression of a gene, including its clinical features.

Answer 20

CMT Type 1a (70-80% of CMT) causeed by 1.5MB dup at 17p12, including PMP22

PMP22 - 22kDa glycoprotein peripheral myelin protein-22 presente in myelin membranes of peripheral nerves where it arrests Schwann cell division.

Duplication caused by misalignment and subsequent recombination between homologous sequences which flank PMP22

Recombination event usually occurs in male gametogeneis.