Module 3 - Molecular cell biology of disease Flashcards
Mitochondrial targetting
A high content of positively charged residues with a strong tendency to form an amphipathic α-helix
Defective mitochondrial targetting: what is it caused by and what is the effect?
A point break mutation from an arginine amino acid into a proline which acts as a helix breaker
In a normal MTS, essentially 100% reaches the matrix, but in a mutated MTS, ~25% reach the matrix
This causes less pyruvate hydrogenase to reach the matric which greatly increases blood lactic acid concentration - congenital lactic acidosis
Defective ER targetting
Defective ER targetting: what is it caused by and what is the effect?
Point mutation of arginine into a cysteine
ERTS does not correctly interact with Sec61 and not be efficiently translocated into the ER and instead mostly be released into the cytosol. This causes toxic insulin build-up in the cytosol, killing beta cells
What happens to incorrectly targetted proteins?
Build-up in the cytosol and may be broken down by lysosomes and proteasomes
Cystolic proteasomes: what are they, what do they do, and what is the process behind them?
Large protease complex which degrades proteins by proteolysis
They degrade Short-lived proteins and misfolded proteins
- Proteins marked for proteolysis by attachment of a protein called ubiquitin
- Polyubiquitin chain recognised by the proteasome
- Ubiquitin recycled
- Protein degraded into amino acids
What quality control is there in the ER?
Misfolded proteins (as they are harmful if left around) are recognised by chaperone proteins and prevented from leaving the ER
Cystic fibrosis: what is it, how fatal is it, and what is it caused by?
A recessive genetic disorder that affects 1 in 3000 live births
Often fatal by the age of 40
Caused by mutations in a chloride channel called CFTR (cystic fibrosis transmembrane conductance regulator)
Cystic fibrosis mutation: what happens when there is no mutation and what happens when there is a mutation?
CFTR is expressed in epithelial cells in many organs (lung, liver, pancreas) and it pumps Cl⁻ ions out of cells, causing water to follow by osmosis which keeps mucus on the surface of epithelial cells hydrated
Cilia can beat to remove bacteria and debris
In cystic fibrosis, mucus becomes dehydrated and cilia cannot function
Common mutations of cystic fibrosis: the most popular form
The most common mutation is the deletion of phenylalanine at position 508 (dF508)
~90% of patients have at least one copy of the dF508 mutant gene
dF508 CFTR cannot fold correctly, so is retained by ER quality control system and does not reach the plasma membrane
What happens to misfolded CFTR proteins
Misfolded proteins that are recognised and retained by ER quality control must be removed
They are moved back into the cytosol and degraded by the proteasome
ER-associated degradation: ERAD
Potential treatments for CFTR mutations: trafficking/folding
The df508 mutation still functions as a Cl⁻ but just can’t make it to the plasma membrane.
Pharmacological chaperones can be used as ‘correctors’ (lumacaftor, tezacaftor) to allow CFTR to fold correctly and reach the membrane
Potential treatments for CFTR mutations: synthesis
Genetic approaches (gene editing etc)
Potential treatments for CFTR mutations: function
Potentiators (increasing channel opening frequency)
Accumulation of misfolded proteins: what does it do?
Triggerds the unfolding protein response
UPR - a response that attempts to restore homeostasis by increasing chaperone concentration and inhibiting protein synthesis
What happens if the UPR fails and what are the potential treatments for accumulated proteins?
Apoptosis may occur
- Reduce synthesis of the mutant protein (Gene editing, RNA interference)
- Stimulate degradation of the mutant protein (Proteasome activators (IU1), Autophagy enhancers (carbamazepine))
- Drugs to alter UPR signalling (Prevent activation of programmed cell death, Increase protein ER folding and degradation capacity )
Chylomicron biosynthesis: what is the process behind it?
Lipids and fatty acids enter the intestines where a Prechylomicron is formed in the ER and it is then sent to the Golgi in a transport vesicle to mature before being secreted as a mature Chylomicron
Chylomicron retention disease (CRD): what is it and what causes it to occur?
When there is a buildup of Prechylomicrons in the ER
Everything is folded correctly (so not a folding disease) but the ER export is defective - COPII fails to assemble correctly
What controls COPII assembly?
Sar1 GTPase controls the formation of COPII vesicles
Sar1p in the cytosol is in the ‘OFF’ form when bound with GDP but Sar1-GEF causes GDP to dissociate and nearby GTP to bind, activating the Sar1p
Sar1p then binds with Sec23 which binds with Sec24 to form the coat for the vesicle
What are the two forms of the Sar1 protein and which causes CRD
Sar1a and Sar1b - there are 90% identical but are just encoded by different genes
Sar1b can have 20 known gene mutations which may cause the GTP binding site to be ineffective or the protein to not be made
Why can the ER have normal cargo transport despite having Sar1b mutated?
Sar1a functions properly and so, since Sar1b focuses on only specific types of cargo, only a few cargos, including chylomicrons, are affected by the mutation
What are the symptoms and treatment of CRD?
Symptoms - Impaired fat, cholesterol and soluble vitamin absorptions which lead to slow growth, weight gain, and gastrointestinal/nervous system effects
Treatment - low-fat diet to minimise Prechylomicron concentration in the ER
Familial hypercholesterolemia: what is it and what can it cause?
An autosomal-dominant disease
The leading cause of coronary heart disease
What causes familial hypercholesterolemia?
Accumulation of cholesterol in the blood due to defects in cholesterol uptake (caused by LDL receptor defects)
The 6 classes of LDL receptor mutations
Class 1 - Receptor not synthesised
Class 2 - Absent from membrane (due to misfolding or withholding in the ER)
Class 3 - Normal synthesis but abnormal binding
Class 4 - Clustering into pits (LDL binds but does not enter the cell)
Class 5 - Receptors not recycled after endocytic complex occurs and readily degraded
Class 6 - Receptors fail to target the membrane
How many FH sufferers are affected by the class 2 mutation?
~50%
What is LDLRAP1, what does it do, and
LDL-receptor assistor protein - an adaptor protein required for the endocytic uptake of the LDL receptor
Mutations in the gene for LDLRAP1 lead to its loss, autosomal recessive hypercholesterolemia (ARH) occurs with symptoms similar to FH
What is LDLRAP1, what does it do, and
LDL-receptor assistor protein - an adaptor protein required for the endocytic uptake of the LDL receptor
Mutations in the gene for LDLRAP1 lead to its loss, and autosomal recessive hypercholesterolemia (ARH) occurs with symptoms similar to FH
Treatments for FH
Inhibit cholesterol synthesis
Inhibit dietary cholesterol absorption
Lysosomal storage diseases: how many are there, what do they do, and what are the main types?
Over 50 known types
Causes progressive accumulation of uncleaved/untransported substrates
- Niemann-pick type C (95%) - caused by NPC1 mutation
- Gaucher disease - caused by lysosomal acid β-glucosidase
Niemann-pick type C: what occurs with this, what type of disease is it, and what is the effect of the disease?
NPC1 should transport cholesterol (that came from LDLs) from lysosomes into cytosol
Misfolding disease
Inefficient cholesterol transport leads to the cell producing more of its own cholesterol which disrupts the production of other lipids
Gaucher disease: what occurs with this and what is the effect of the disease?
The absence of acid β-glucosidase in the lysosome causes the accumulation of glucosylceramide and other glycolipids
A range of symptoms
What are the treatments for Gaucher disease?
- Enzyme replacement therapy
- Substrate reduction therapy:
- Pharmacological chaperones
Enzyme replacement therapy: how does it work?
Inject synthetic enzymes which are added with M6P to bind to the M6P receptor and function in the lysosome
Substrate reduction therapy: how does it work?
Reduce the amount of glycosylceramide in lysosomes – don’t need so much lysosomal acid β-glucosidase (Miglustat inhibits glucosylceramide synthesis and is used to treat Gaucher disease)
Pharmacological chaperones: how does it work?
Drugs that promote correct folding can increase the amount of enzyme that escapes ER quality control and reaches the lysosome
