Lecture 6 - Congenital hyperinsulinism of infancy Flashcards
Patch-clamp: what does it do?
Measures ion passage through a channel - we can know which ion channel our glass pipette is on
Hyperinsulinism in infancy: what is it also known as, what are the statistics, when does it present, what does it result in, and what is the treatment?
Congenital hyperinsulinism in infancy (CHI)
1 in 28,389 live births in the UK (up to 1 in 2500 in consanguineous communities)
Presents in the neonatal period
- Poor feeding
- Lethargy
- Seizures
- Coma
- Brain damage
- Death
- Persistent hypoglycaemia
- Relative hyperinsulinemia
- Low serum fatty acids
- Low ketone bodies
Often unresponsive to drug therapy - pancreatectomy is often required
CHI: how is diagnosed and how is it managed?
Establish diagnosis:
* Blood glucose < 3 mmol/l
* Inappropriate insulin
* Glucose infusion > 8 mg/kg/min
* Hypoketotic
* Hypofattyacidaemic
* Serum ammonia?
Management:
* Commence diazoxide therapy
* Commence octreotide (somatostatin) therapy
* Commence nifedipine therapy
* Refer for 18F-L-dopa PET/CT scan
* Resect pancreas
kATP channels: what is its structure and what are the roles of the components?
Composed of four pore-forming Kir6.2 subunits and four regulatory sulfonylurea receptor 1 (SUR1) subunits
Kir6.2 - ATP inhibition site, open/closes pore
SUR1 - regulatory subunits
Both parts have nucleotide binding sites - allowing ATP/ADP to bind and convey the metabolic messages to the channel
CHI: what differences are there in the kATP channels compared to normal beta cells?
kATP channels don’t make it to the cell membrane - K⁺ ions not moved out of the cell, inappropriate depolarisation despite glucose presence
Electrical activity of beta cells
Depolarisation:
* Caused by kATP deactivation
* Less K⁺ ions moved out of the cell, the membrane potential is raised
* VGCC cause an influx of Ca²⁺, promoting insulin secretion
Repolarisation:
* VGCC inactivation - less Ca²⁺ in
* VGKC activation - More K⁺ out
Genetic causes of CHI
- kATP channel mutations - ABCC8/KCNJ11
- GLUD1 GoF mutations
- Glucokinase (GCK) GoF mutations
- HADHSC GoF mutations (SCHAD)
5) UCP2 mutations (uncoupling protein 2) (ER)
6) SLC16A1 mutations (monocarboxylate transporter 1) (ER)
7) HNF4A mutations (HNF4α transcription factor) (ER)
kATP channel mutations: what are some examples, what type of mutations are they, and what are the types?
ABCC8/KCNJ11
LoF mutations - mostly autosomal recessive (& dominant)
- Persistent CHI ABCC8 > ~200 mutations
- Persistent (& Transient) CHI KCNJ11 ~ 20 mutations
- Focal
- Diffuse
Genetics of focal CHI: what is it caused by and how does it work?
Loss of heterozygosity event during fetal development from the maternal side on Chromosome.11p15
This acts along with the mutated paternal genes to cause CHI
Genetics of focal CHI: SUR1, Kir6.2, IGF2, H19, and p57ᵏᶦᵖ²: what are they, what are they coded by, and how do they act in CHI?
SUR1 - Subunit of kATP channel, paternal side, mutated in CHI
Kir6.2 - Subunit of kATP channel, paternal side, mutated in CHI
IGF2 - promote beta cell growth, paternal side, affected by mutations in H19 and p57ᵏᶦᵖ²
H19 - mRNA that regulates IGF2 maternal side, loss of heterozygosity causes excessive IGF2, promoting beta cell hyperplasia
p57ᵏᶦᵖ² - Controls cell cycle arrest, maternal side, loss of heterozygosity results in beta cell hyperplasia
Diagnosis of focal CHI: why is it important to ascertain if it’s focal and what is the diagnostic method?
No need to remove the whole pancreas if the issue is focal
Use an ¹⁸F-DOPA PET-CT scan:
* Use in combination with rapid genetic screening
* Put ¹⁸F-DOPA in the body and watch its activity
* If a clear, bright spot is seen, then that is the location of the focal CHI
* If there is no clear bright spot, diffuse CHI can be assumed
¹⁸F-DOPA: what is it used for, what are its key features, what is it used to make, and what enzyme catalyses this process?
Focal CHI diagnosis - ¹⁸F-DOPA PET-CT scan
- Short half-life
- Produces bright spots in overactive beta cells
18F-DOPA - Dopamine
DOPA decarboxylase
GLUD1: what is it, how is it mutated in CHI, what other effects may be seen, and what cells are affected?
Glutamate dehydrogenase 1
Autosomal dominant GoF mutation - causes inappropriate insulin release due to increased metabolism (increased α-ketoglutarate production causing increased ATP production)
hyperammonemia syndrome - higher baseline plasma ammonia and plasma ammonia increment after oral protein
ß-Cells and Liver (and CNS - epilepsy more common in HI/HA than other forms of HI)
GLUD1: what is it, what is its normal function, what is allosteric promoted/inhibited by, and how may it gain gain of function mutations?
Glutamate dehydrogenase 1
Catalyses the oxidative deamination of
glutamate to α-ketoglutarate & ammonia
- ADH
- Leucine
- GTP
- SCHAD
Mutations in the regulatory domain may result in GoF mutations
Glucokinase mutations: what type of mutations are there, how may they cause CHI, and how may these mutations manifest?
- Autosomal dominant GoF mutations (CHI)
- Autosomal dominant LoF mutations (MODY/PND)
Cause inappropriate metabolism of glucose by glucokinase, increasing insulin release
Mutated allosteric effector site -
GCK: what is it, what does it do, and how may it cause CHI?
Glucokinase
Converts glucose into glucose-6-phosphate
GoF mutation - higher activation, higher metabolism, higher ATP, higher insulin secretion
HADHSC: what is it, how may it cause CHI, when does it present, and what is its molecular mechanism?
The gene coding for SCHAD
Loss of Function autosomal recessive mutations (may reach rates of up to 8/10 from consanguineous families)
Presents in early childhood
Lack of correct SCHAD function, reduced inhibition of GDH
SCHAD: what is it, what does it do, and how may it be mutated to cause CHI?
Short-chain 3-OH-acyl-CoA dehydrogenase
Part of the mitochondrial fatty acid oxidation chain - binds to GDH, inhibiting it and therefore the conversion of glutamate into α-KG
Lack of correct SCHAD function, reduced inhibition of GDH
Drug treatment of CHI 1: what are the different types and what do they do?
- Chlorothiazide - activate VGKC
- Octreotide - activate VGKC, inhibit insulin secretion
- Nifedipine - inhibit VGCC
- Polyunsaturated fatty acids - stabilise membrane potential via VGCC
- Diazoxide - activate kATP channels
- Chaperone therapy - Increase kATP channel trafficking
- Sirolimus (rapamycin) - mechanism unknown
- GLP-1R antagonists - block GLP-1 signalling
- RX358 - INS-R antagonist, undergoing trials still
If drugs fail, then a pancreatectomy is required
VGCC and VGKC: what are they?
Voltage gated calcium channel
Voltage gated potassium channel
CHI pathology
Channels porduced, trapped in cell, never make it to membrane
Transient
Temporary
Focal
One small section causing pathology
Diffuse
Large areas causing pathology - most has to be removed
