Lecture 23: Metabolic Disorders Flashcards
How are most metabolic disorders inherited?
Autosomal recessive.
Exception: Fabry’s is X-linked.
What characterizes metabolic disorders?
- Absence or abnormality of an enzyme or its cofactor.
- Accumulation or deficiency in a specific metabolite.
What are the three primary products of carbohydrate metabolism?
- Glucose
- Galactose
- Fructose
What is sorbitol an alcohol of?
Fructose
Where is glycogen stored in the body?
- Liver
- Muscle
What do glycogen storage disorders result in?
Accumulation of glycogen in the tissues due to a defect in metabolism.
What is the typical clinical presentation of a GSD in the liver?
- Fasting hypoglycemia and ketosis that improves with eating.
- Possible hepatomegaly
What is the typical clinical presentation of a GSD in the muscles?
- Delayed growth in children
- Exercise intolerance
- Progressive weakness
What labs would we expect for a GSD pt?
- Fasting hypoglycemia
- Elevated LFTs and CPK
How do we generally manage a GSD?
- Avoid hypoglycemia
- Enzyme replacement for specific ones.
- Symptomatic therapy
What are the 3 subtypes of fructosemia?
- Deficiency of fructose 1,6-diphosphatase (FDPase)
- Deficiency of fructose 1,6-biphosphatase aldolase (Aldolase B/Hereditary fructose intolerance)
- Deficiency of fructokinase (Essential fructosuria)
What is the mechanism of FDPase deficiency and the result?
Mechanism: Conversion of Fructose 1,6-biphosphate to Fructose 6-phosphate.
Results in inadequate gluconeogenesis during periods of fasting.
How does a FDPase deficiency present clinically?
- Hypoglycemia symptoms
- Acidosis symptoms (buildup of lactate)
How do we manage FDPase deficiency?
- Avoid fructose
- Avoid fasting for too long
What is the effect of an aldolase B or fructose 1,6-biphosphatase aldolase deficiency?
Toxic accumulation of Fructose-1-phosphate in the Liver, Kidney and SI, causing cell death.
How does an Aldolase B deficiency present clinically?
Ingestion of fruit or sweetened cereal leads to…
- Severe abdominal pain
- Failure to thrive
- Jaundice and hepatomegaly
- Hypoglycemia
How do we diagnose and manage an aldolase B deficiency?
Genetic testing to diagnose.
Management includes complete avoidance of fructose and sucrose.
What is the most benign form of fructosemia?
Lack of fructokinase, aka essential fructosuria.
What is the pathophysiology of essential fructosuria?
Cannot convert fructose to anything, so you pee it out instead.
How do you manage essential fructosuria?
No management needed, as you will pee it out.
What causes galactosemia?
Autosomal recessive defect of chromosome 9p13.
What is galactosemia?
Toxic accumulation of galactose-related molecules due to an inability to metabolize it.
Where does galactose typically come from?
- Milk
- Baby formula
- Celery, kiwi, plum, avocado, and figs.
What 3 enzyme deficiencies result in galactosemia?
- Galactose-1-phosphate uridyl transferase (GALT), AKA Type 1 galactosemia, as it is the MC and severe.
- Galactokinase (GALK), Type 2.
- Galactose epimerase (GALE), Type 3.
How does Type 1, or classic galactosemia present clinically?
- Normal at birth.
- Within a few days of getting galactose, failure to thrive, vomiting, hepatomegaly, and jaundice can occur.
- Within 2 weeks, buildup of galactitol can cause cataracts.
What is galactitol?
Unmetabolized galactose.
What is the primary clinical finding in Type 2 or GALK galactosemia?
Cataract formation.
How does Type 3 or GALE galactosemia typically present clinically?
- Mild to classic galactosemia.
What lab value is elevated in a GALT deficiency? Decreased?
- RBC galactose-1-phosphate elevated.
- GALT enzyme activity depressed.
How do we manage galactosemia as a whole?
- Minimizing dietary milk.
- DC breastfeeding
- Use soy-based formulas
- Avoid dairy as they get older.
How do we manage galactosemia long-term?
- Annual ophthalmologic exam.
- CBC, LFTs, RBC galactose-1-phosphate
- Females: LH, FSH, and estradiol starting at age 10. (Females tend to rely on dairy)
What causes PKU?
Autosomal recessive mutation of chromosome 12.
What enzyme is someone with PKU deficient in? What is the function of that enzyme?
Phenylalanine hydroxylase (PAH), which converts phenylalanine to tyrosine
What does a buildup of phenylalanine in the brain result in?
Destruction of myelin covering individual nerve fibers.
Who is PKU MC in?
Caucasians and Native-Americans.
What is classic PKU?
Complete absence of PAH.
What kind of soda might contain phenylalanine?
Diet sodas
How does untreated PKU present clinically?
- Light hair and skin
- Neurologic dysfunction due to loss of myelin.
How do we diagnose PKU?
- Newborn screening
- Elevated serum phenylalanine
How do we manage PKU?
- Avoid foods with phenylalanine.
- Sapropterin to activate PAH to breakdown phenylalanine. (Kids/Adults)
- Pegvaliase to degrade phenylalanine. (ADULT ONLY)
What is maple syrup urine disease?
Autosomal recessive disorder with a deficiency in the enzyme that breaks down BCAAs.
What 3 BCAAs are affected by maple syrup urine disease? (MSUD)
- Leucine
- Isoleucine
- Valine
What specific enzyme is MSUD deficient in?
Branched-chain ketoacid dehyrogenase complex (BCKDC)
What does excess leucine and isoleucine do to the body?
- Excess leucine = neurological symptoms.
- Excess isoleucine = maple syrup odor in urine, like burnt caramel.
What are the two types of maple syrup urine disease? (MSUD)
- Classic MSUD: < 3% residual enzyme activity. (MC)
- Non-classic MSUD: 3-30% residual enzyme activity.
How does classic MSUD tend to present?
Symptoms within 48hrs to 2 weeks (delayed by breastfeeding.)
* Irritability
* Poor feeding
* Seizures
* Cerebral edema
* Death
How does non-classic MSUD tend to present?
Delayed onset until childhood, exacerbated by physical stress.
Variable symptoms (Like in classic MSUD)
How do we manage MSUD?
- Strict protein restriction using medical-grade formula/food.
- Trial of thiamine supplement for 4 weeks. (some subtypes respond)
A patient with MSUD demonstrates rapid buildup of leucine. What is the immediate intervention to rapidly lower their leucine?
Hemodialysis
A patient with MSUD has just underwent hemodialysis for elevated leucine. What are the next steps in their management?
- D/C protein intake for 24-48h
- IV glucose (inhibits protein catabolism)
- IV insulin if BG > 130 (enhances protein synthesis)
When is a liver transplant indicated for MSUD?
Last resort for classic MSUD.
10% of BCKDC is found in the liver.
What is homocystinuria?
Dysfunction of methionine formation , which results in a buildup of homocysteine.
What is the main enzyme deficiency that results in homocystinuria?
Cystathionine beta-synthase
What are the classic symptoms associated with homocystinuria?
- Failure to thrive
- Dislocated optic lenses and mental retardation (MC)
- Marfanoid habitus (very tall and thin)
- MSK deformity with osteoporosis
- Thromboembolic related symptoms.
What are the MSK deformities associated with homocystinuria?
- Pes excavatum (sternum sunken in)
- Pes carinatum (sternum protrudes out)
- Genu valgum (Knees bent inward, knock knees)
How do we assess for homocystinuria?
- Newborn screening (WV)
- Elevated homocysteine and methionine in plasma or urine.