19.03.06 Gain of function Flashcards
1
Q
Types of dominant mutations
A
1) AD haploinsufficiency
2) AD dominant negative effect
3) Gain of function mutations
2
Q
What are the two possible results of a GoF mutation?
A
1) Increase in gene expression (i.e. a hypermorph - an allele that produces an increased in quantity or activity of a product)
2) Develop a new function (i.e. a neomorph - an allele with a novel activity or product - this can happen when a translocation creates a chimeric gene fusion (BCR-ABL in CML))
3
Q
GoF muations
A
- Usually cause dominant phenotypes (as the one remaining normal allele can’t stop the mutated allele from acting abnormally)
- Often linked to an abnormal signally activity (activating when should be silent, or vice versa)
- Usually require a more specific change than LoF mutations.
4
Q
Examples of GoF genes
A
1) Overexpression of PMP22 in CMT
2) GNAS receptor being permantly activated in McCune-Albright syndrome
3) t(9;22) creating BCR-ABL chimeric gene product in CML
4) Protein aggregation of mutant HTT in HD
5
Q
Unstable repeats
Example - HD - what is the disease mechanism?
A
- Expansion of (CAG)n repeat in exon 1 of HTT gene (4p16.3)
Normal (<27), disease associated with >36 repeats - Repeats translated into polyglutamine tract - this acquires novel deleterious function when abnormally expanded
- Causing neuronal dysfunction and neurodegeneration
- Expanded tract results in formation of intranuclear inclusions containing huntingtin, chaperon proteins and ubiquitin (get greater levels in regions of the brain linked to phenotype, and in juvenile severe onset cases)
- However inclusions are not cause of disease
- Mutant HTT thought to form abnormal protein structures which are truncated to form toxic N-terminal fragments
-This leads to the processing of the abnormal HTT protein, via the ubiquitin pathway, that you see in HD - Mutant HTT interferes with gene transcription and metabolism - causing oxidative stress
- Also get abnormal vesical transport (reduced release of BDNF) and increased excitotoxicity.
6
Q
Unstable repeats
Example - HD - what evidence is there for GoF?
A
- Deletions of and translocations affecting HTT don’t cause HD
- HOMs clinically identical to HETs
- Dominant affect suggests mRNA has gained a new novel property or is expressed incorrectly
- Levels of polypeptides encoded by normal and mutant alleles are identical
7
Q
Unstable repeats
Example - DM1/DM2 - what is the molecular pathogenesis?
A
- toxic RNA GoF of expanded (CUG)n anf (CCUG)n repeats, respectively
- Repeat expansions form stable structures that take control of RNA-binding proteins
- Leads to altered expression and activity of the RNA binding proteins, which in turn affects alternative splicing
8
Q
Unstable repeats
Example - Spinocerebellar ataxia - what is the molecular pathogenesis?
A
- SCA1, SCA2, SCA3, SCA6, SCA7 and SCA17 proposed to arise from a protein gain of function mechanism
- CAG repeat mutations cause polyglutamine-rich proteins to be produced which form brain specific inclusions
- SCA8 is a later onset neurodegenerative disease with CTG/CAG expansions
- Thought to get both RNA and protein GoF mechamisms in SCA8
9
Q
Highly mutable codon
Example - Achondroplasia
A
- Most common form of inherited disproportionate short stature
- Normally caused by mutation in or close to the transmembrane domain of the FGFR3 gene at 4p16
- FGFR3 negatively regulates bone growth by inhibiting the proliferation of chondrocytes
- Mutations lead to the constant activation of the FGF receptor - severely limiting bone growth
- Two mutations, c.1138G>A/C p.(Gly380Arg), account for >99% of achondroplasia
- Both enhance dimerization of the protein that catalyses downstream signalling
- Mutation rate is disproportionately high - high de novo rate thought ot be due to variant conferring a proliferation advantage in spermatogonial stem cells
N.B. other variants in the same gene can cause other conditions:
1) Thanatophoric dysplasia types 1 and 2
2) hypochondroplasia (p.(Asn540Lys) in ~50% affected individuals)
3) Wolf-Hirschhorn syndrome (microdeletion at 4p16)
10
Q
Translocations producing novel fusion genes (in cancer)
Example - BCR-ABL fusion gene in CML
A
- 15-20% of all adult leukaemia is CML
-80% of all childhood laukaemia is ALL
~95% of all CML cases (and a minority of ALL cases) have t(9;22)(q34;q11) - called the philadelphia chromosome. - Get fusion of breakpoint cluster reigon (BCR) from 22 and c-ABL proto-oncogene on 9
- BCR-ABL acts as an oncogene and promotes genomic instability
- It causes formation of an abnormal tyrosine kinase molecule with increased activity, and this is what is thought to cause leukaemia
- Breakpoints can vary on 22, therefore there are four fusion transcripts which can be produced - b2a2, b3a2, e1a2, and e19a2
- b2a2 and b3a2 are detected mainly in CML and e1a2 is detected mainly in ALL
- e19a2 results from a rare rearrangement detected in CML
CML is treated with Imatinib, which requires accurate quantification of of the abnormal BCR-ABL clone (called measurement of ‘minimal residual disease’ (MRD)) - this monitors how effective treatment is
MRD normally carried out by RT-PCR by quantifying levels of BCR-ABL mRNA transcripts in blood and bone marrow
11
Q
Overexpression
Example - CMT and HMSN
A
- Common cause of CMT is 1.5Mb dup of 17p12 (PMP22)
- Accounts for 70-80% of CMT1 cases, and 30% of all CMT cases
- Seen in both familial and sporadic cases
- High mutation rate
- Most patients are HET (but a HOM case has been seen with very severe disease)
- PMP22 encodes 22kDa glycoprotein peripheral myeline protein-22
- Found in myelin membranes of peripheral nerves and is involved in schwann cell division
- Dup causes by misalignment and recombination between homologous sequences with flank the PMP22 gene (usually occurs in male gametogenesis)
- Also get reciprocal deletion due to the same unequal crossing over and this causes a relatively mild disorder - Hereditary Neuropathy with Liability to Pressure Palsy (HNPP).
