PBL Cases 1-8 Flashcards
What is the inheritance pattern of cystic fibrosis?
autosomal recessive
What chromosome is the CFTR gene located on?
What is the most common mutation and what does it lead to?
- Genetic Mutation: CFTR gene located at chromosome 7
- Most Common Mutation: delF508, which is deletion of phenylalanine at 508-position
- Leads to misfolding and degradation
What is the epidemiology of cystic fibrosis?
- Which two populations have the highest incidence?
- What disease is present in high incidence populations?
- Epidemiology: Ashkenazi Jewish has highest (1/24), followed by Whites.
- High presence of cholera in high incidence populations. With half functional CFTR you do not die from dehydration when infected with cholera because of the ability to retain more chloride ions & therefore water than those who are not carriers.
What are the limitations of genetic screening exams?
- Limitations of Genetic Screening Tests for CF: multiple mutations, all of which are not deleterious
What does the CFTR gene encode for?
What is the role of the CFTR protein in epithelial cells?
- CFTR is a chloride channel located on the apical membrane of epithelial cells of different organ systems
- Mucous lubricates lining of airways, digestive system, reproductive tract, etc.
How does a normal CFTR protein function biochemically?
- Normal CFTR: increases the secretion of chloride ion via active transport and reduces the passive reabsorption of sodium ions “via regulation of epithelial Na channel”
- CFTR has direct inhibition of the Na channel → with chloride efflux, Cl- ions negatively inhibits sodium influx
How does a mutated CFTR protein function biochemically at the membrane?
- Mutated CFTR: decreases the secretion of chloride ion and augments the reabsorption of sodium ion (with passive water reabsorption)
- Hyperconcentrated, “dehydrated” mucous
How do defects in CFTR proteins translate to dysfunction in sweat glands?
- CFTR actively transports chloride ions out of sweat gland lumen into surrounding tissue
- In sweat, sodium follows chloride ions out of the sweat gland lumen
- Improper CFTR leads to increased NaCl released out of sweat gland lumen onto skin
How do defects in CFTR proteins translate to dysfunction in intestines/pancreas/lungs?
- Mutated CFTR channels → chloride is not actively transported into the lumen → ENaC (sodium channel) is not negatively inhibited → increased sodium reabsorption → increased water reabsorption → dehydrated mucous layer of lumen (muconeum ileus – obstruction of small intestines in neonates)
How does the sweat chloride test determine the presence of cystic fibrosis?
- Cl- conductance is virtually abolished because CFTR is the only outlet for chloride.
- When Na+ attempts to flow out of a CF duct through remaining sodium-selective pathways, it is unaccompanied by Cl- and so it creates an excess of negative charge in the duct that attracts Na+ and prevents its further absorption.
- The net result is that very little NaCl is reabsorbed, resulting in a high salt content in CF sweat.
What defects and symptoms occur in the respiratory tract due to CFTR dysfunction?
- thick and sticky mucous (assoc. with CF) clogs the tubes that moves air in and out of your lungs
- Symptoms: wheezing, breathlessness, lung infections
What defects and symptoms occur in the pancreas due to CFTR dysfunction?
- Pancreatic Insufficiency: The thick mucus can also block tubes that carry digestive enzymes from your pancreas to your small intestine. Without these digestive enzymes, your intestines aren’t able to completely absorb the nutrients in the food you eat.
- Symptoms: failure to thrive, CFRD (CF related diabetes)
What are three multi-organ system based treatments for cystic fibrosis?
- Enzymes to allow for digestion of food
- Shaking Vest to breakup mucous in lungs for easier breathing
- Antibiotics to fight off infections caused by ineffective mucous
What defects and symptoms occur in the reproductive system due to CFTR dysfunction?
- Reproductive System: vas deferens and fallopian tubes filled with thick mucous
What are the six classes of CFTR mutations, their functional consequences, and their treatments?
What is the inheritance pattern of G6PD deficiency and what are some causes of genetic selectivity?
- Inheritance pattern: X-linked recessive
- Males are hemizygous – one mutant gene leads to full blown deficiency
- Females are mosaic – x-link deactivation leads to differing levels of deficiency
- Causes of Genetic Selectivity: Carriers offer resistance to a deadly form of malaria called plasmodium falciparum
- Falciparum targets mature red blood cells
What are some of the most common mutations associated with G6PD deficiency that may alter protein structure/enzyme activity?
- Missense point mutations (change in one nucleotide that could alter AA) at Xq48
- Most abnormal alleles result in a functionally normal enzyme but have a shortened life span within the red cell
What is the role of G6PD in the hexose monophosphate shunt pathway and how can oxidative stress tip the balance of NADP-NADPH in the erythrocyte?
- G6PD oxidizes G6P while reducing NADP+ to NADPH
- NADPH can then reduce oxidized glutathione, in order to reduce H2O2
- G6PD is involved in the only pathway in RBCs that can reduce oxidants
In the G6PD case, oxidative stress was caused by…
Oxidative stress is caused by…
- Fava beans
- Bactrim (has a FR, which oxidizes GSH back to GSSG)
- Severe infections
What are the consequences of oxidative stress (think the G6PD case)?
- Oxidizes Fe2+ to Fe3+, changing hemoglobin to methemoglobin, which cannot bind O2
- Denatures cytosolic and membrane proteins → hemolysis → heme breakdown → bilirubin (toxic) → juandice
- Denatured hemoglobin → Heinz’s bodies → Bite cells after macrophages eat Heinz’s bodies
Explain how the imbalance between NADP-NADPH causes hemolysis. How does jaudice come from this?
- Decreased [NADPH] → decreased [GSH] → increased [ROS] → oxidation of Fe2+ to Fe3+ → hemoglobin converted to methemoglobin → increased [ROS] denatures cytosolic and membrane proteins → hemolysis → hemoglobin breakdown → heme exposed → porphorin ring exposed → formation of bilirubin (toxic) → increased blood [bilirubin] → juandice
What is the structure of hemoglobin and some characteristics?
- Hemoglobin
- Tetramer – each monomer is called globin
- Each monomer has a heme center that holds Fe2+ that binds an O2 molecule to transport to tissues
- Cooperative binding – when one O2 binds to one globin, binding affinity increases for other globins
What is the interrelation of Fe2+ and Fe3+?
- Interrelation of Fe2+ and Fe3+
- Fe2+ is oxidized to Fe3+ → hemoglobin is converted to methemoglobin → can no longer bind oxygen
What are the two routes that convert methemoglobin to hemoglobin?
- Two Routes of Converting Methemoglobin to Hemoglobin
- Cytochrome b5 Reductase: uses NADH produced from glycolysis
- Methemoglobin Reductase: converts methylene blue to leukomethylene blue using NADPH → leukomethylene blue reduces Fe3+ back to Fe2+
Relate the concepts of oxygen saturation, partial pressure of oxygen and the hemoglobin-oxygen dissociation curve.
- What factors affect the oxygen binding cuve and how does it move?
Describe four factors that can acutely change in a patient that will alter a drug’s volume of distribution and clearance.
- Dehydration – affects filtration rate of blood → less drug is filtered out
- The metabolism rates
- Children have an increase in metabolism, resulting in higher drug metabolism
- The elderly are affected due to diminished cellular function, receptor function and drug elimination difficulty.
- Renal Function affects the drug clearance and reabsorption.
- Liver diseases affect the biotransformations and absorption.
What are the steps of a normal NMJ?
Steps of Normal NMJ
- Depolarization of pre-synaptic nerve
- Voltage-gated Ca++ channels open at synapse
- Ca++ dependent vesicle binding
- Neurotransmitter (ACh) release
- Two ACh molecules bind to post-synaptic receptor (MuSK clusters AChR to post-synaptic membrane)
- Receptor opens allowing Na+ to flow into cell
- Depolarization of post-synaptic muscle
- Acetyl-cholinesterase terminates response
What are two problems that can occur at the NMJ in myasthenia gravis?
- Antibody binds to AChR → diminished end plate potential
- Binding – antibodies may initiate an inflammatory reaction that destroys them
- Blocking – antibodies prevent acetylcholine from binding to active site
- Modulating – increased endocytosis of receptor
- Antibody binds to MuSKR → diminished end plate potential
What makes diagnosing myasthenia gravis so challenging?
- Complaints are non-specific (i.e. tiredness, weakness)
- Other treatable conditions closely resemble MG
- Difficult to catch early due to slow progression of disease
What are the mechanisms of the three main tests used to diagnose myasthenia gravis?
- Injection of Edrephorium– inhibits Acetyl cholinesterase → increase in ACh activity
- Tensilon Test bad because it was non-specific acetyl cholinesterase inhibitor, causing heart arrhythmias (atropine is required to reverse effects)
- Nerve Stimulation Test – consistent nerve stimulation leads to decreased and delayed nerve response
- Amplitude of response remains same in normal individuals
- Serum AChR-AB Assay Blood Test – checks for AChR ABs are present
Draw the Urea Cycle and define what its function is.
- Definition of Urea Cycle: Method of excreting urea, a metabolite of protein catabolism.
What are the possible enzyme defects that occur in the urea cycle?
List:
- Disorders are associated with them
- Where the enzyme is located
- What product accumulates
What is the general pathway of the BCKD complex and what metabolites are formed?
How is the BCKD regulated?
What is the general mechanism of the BCKD complex or any other dehydrogenase complex?
