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Medical Genetics I > Intro to Metabolic Genetics > Flashcards

Intro to Metabolic Genetics Flashcards

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USMLE® Step 1 flashcards Biology 101 flashcards
1
Q

Define metabolism

A

sum of all chemical reactions occurring w/in the cells of the body

  • breakdown of products
  • buildup of products
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2
Q

What is the goal of metabolism?

A

maintain constant source of energy for the body

Note: this is done by absorbing fuel from meals and storing for use during fasting periods

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3
Q

What are the 3 macronutrients?

A

carbs, proteins, fats

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4
Q

Polysaccharides are ___ ___ while Monosaccharides are ___ ___

A

complex carbs, simple sugars

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5
Q

What is excess sugar stored as and where?

A

glycogen in the liver and muscle

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6
Q

What happens to excess glucose once glycogen stores are full?

A

it’s converted to fatty acids and glycerol to be stored as triglycerides

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7
Q

There are ___ essential AAs and ___ non-essential AAs

A

9, 11

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8
Q

How are excess circulating AAs stored?

A

converted to glucose and fatty acids and stored as triglycerides

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9
Q

What are the 3 forms of fats?

A

Monoglycerides, Triglycerides, Free Fatty Acids

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10
Q

What happens to excess circulating fatty acids?

A

Incorporated into triglycerides and primarily stored in adipose tissue and sometimes muscle

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11
Q

What is the metabolic process that happens at 1-4 hours of fasting? 3-12 hours of fasting? 10+ hours of fasting?

A

1-4: Glycogenolysis (glycogen –> glucose)
3-12: Gluconeogenesis (AAs –> glucose)
10+: Ketogenesis/Fatty Acid Oxidation ( fatty acids –> ketones)

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12
Q

What is anabolism and what does it result in?

A

Anabolism is the feeding/fed state –> results in manufacture/synthesis of materials needed in the cell by building larger organic macromolecules from smaller subunits and storage of excess ingested nutrients that are not immediately needed for energy production or as cellular building blocks

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13
Q

What is catabolism and what does it result in?

A

Catabolism is the fasting state –> results in the breakdown/degradation of stored energy resources of large energy-rich organic molecules into smaller nutrients available for energy use

Note: think of cat at 2am as hungry and breaking stuff

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14
Q

What are the body’s primary energy users?

A

Brain, Muscle

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15
Q

What is the brain’s preferred energy source? What does it use in times of long-term starvation?

A

glucose, ketones

Note: the brain can’t store glycogen

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16
Q

What is the liver’s role in metabolism?

A

maintenance of normal blood glucose levels by storing glycogen, releasing glucose, and converting glycogen to glucose

Note: other liver functions important in metabolism are production of clotting enzymes and metabolism of bilirubin

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17
Q

What is a lysosome?

A

intracellular digestive system, digests substances that the cell no longer needs

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18
Q

What happens when a lysosome doesn’t work properly?

A

massive build-up of material that causes lysosome to swell and burst, which interferes w/ function of the cell

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19
Q

What 3 types of problems with lysosomes can result in metabolic disorders and which is the most common?

A
  • deficient activity of a single, lysosomal specific enzyme
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20
Q

What do mitochondria do?

A

Extract energy from nutrients via mitochondrial oxidative phosphorylation (aka respiratory chain) and carry out beta-oxidation of fatty acids

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21
Q

What is a peroxisome?

A

intracellular waste treatment center

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22
Q

What is the major product of the peroxisome?

A

H2O2 (hydrogen peroxide)

Note: H2O2 is potentially destructive to the cell outside of the peroxisome

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23
Q

What do the enzymes inside the peroxisome do?

A

process/beta-oxidation of very long chain fatty acids (VLCFA)

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24
Q

What are the 2 disease mechanisms that typically result in IEMs?

A

toxic accumulation of substances, reduction of normal compounds

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25
Q

What do the genes that typically cause IEMs code for?

A

enzymes that convert substrate into product

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26
Q

How many different IEMs have been identified?

A

~1000

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27
Q

How common are IEMs?

A

1/1500 births

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28
Q

At what age do IEMs present?

A

Any age –> about half are identified outside of the neonatal period

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29
Q

What is the inheritance pattern of most IEMs?

A

AR

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30
Q

What are the 3 different results of genetic mutations that cause IEMs?

A
  • reduce activity of enzyme
  • reduce effectiveness of cofactors/activators of an enzyme
  • produce defective transportation compounds in the body
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31
Q

Why is there variability in phenotypes for IEMs?

A

different types of mutations or combos of mutations

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32
Q

What are the 5 different approaches that can be taken for the treatment of IEMs?

A
  • precursor and substrate restriction (alter diet)
  • cofactor supplementation
  • supplement or provide deficient enzyme (transplant of enzyme replacement therapy)
  • increase use of alternative pathway
  • supplement products and downstream products
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33
Q

How does substrate reduction therapy work?

A

individuals are treated with substrate synthesis inhibitors which decreases the amount of substrate to match the amount of residual enzyme to prevent accumulation

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34
Q

How does molecular/pharmacological chaperone therapy work and what is the goal?

A

small molecules bind to and stabilize the functional form of a misfolded protein in the ER of the cell which can restore enzyme activity

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35
Q

Why might intrathecal therapy be used to treat an IEM?

A

IEM that affects CNS needs enzyme delivered directly to the brain/spinal cord so this system avoids the blood-brain barrier

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36
Q

What are clinical features associated with infants/children w/ IEMs?

A
  • vomiting, seizures, ataxia, lethargy, coma, hepato-encephalopathy
  • dysmorphic (coarse) features
  • skeletal abnormalities
  • poor feeding, FTT
  • DCM, HCM, hepatomegaly, jaundice, liver dysfunction
  • DD
  • hypo- or hypertonia
  • visual/auditory disturbances
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37
Q

What are clinical features associated with older children/adults w/ IEMs?

A
  • varying degrees of LD/DD/ID, ASD
  • exercise intolerance
  • muscle weakness (possible progressive)
  • behavioral disturbances (delirium, hallucination, agitation, aggressiveness)
  • ataxia
  • anxiety/panic attacks
  • seizures
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38
Q

Episodes of symptoms of IEMs might be…

A
  • acute
  • intermittent
  • precipitated by stress (mental and/or physical)
  • progressive
  • associated w/ feeding

Note: length of time since eating can provide estimate of what should be going on in the body to maintain glucose levels; specific disorders can also be symptomatic after ingesting the offending agent

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39
Q

What are some things that might be included in the hx of a pt w/ IEM?

A
  • onset of symptoms w/ change in diet and unusual dietary preferences/aversion
  • decompensation beyond what might be expected from infection
  • hx of deterioration after initial period of good health
  • (1 mo - 5 yr) might have hx of recurrent episodes of vomiting, ataxia, seizures, lethargy, coma, or hepato-encephalopathy
  • other findings: poor feeding, FTT, DD, failure to reach milestones
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40
Q

List 3 examples of symptoms of an underlying IEM that might sound like a dx

A

hypoglycemia, hyperammonemia, acidosis

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41
Q

If glucose is the main source of energy (ATP) for the body, why measure glucose instead of ATP if that’s the true issue?

A

much easier to measure blood glucose

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42
Q

What is the normal range for BG? What is the range for hypoglycemia in adults? children?

A
  • normal: 70-140 mg/dL
  • hypo for adults: <55 mg/dL
  • hypo for children: <50 mg/dL
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43
Q

What 3 alternate energy sources might be used when hypoglycemia occurs?

A

fatty acid oxidation, ketone production/oxidation (brain), metabolism of lactate

Note: none of these are as efficient as glucose metabolism but can be implemented when necessary

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44
Q

Name 2 types of IEMs w/ hypoglycemia as a prominent component
Name 2 types of IEMs w/ hypoglycemia as a significant secondary component

A
  • carb metabolism (GSD Type I), fat metabolism (MCAD deficiency)
  • disorders of protein metabolism, mito disease
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45
Q

Where is ammonia produced?

A

liver, intestinal mucosa, kidneys

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46
Q

How is free ammonia removed from the blood?

A

removed by liver and excreted in urine as urea after traveling through the urea cycle

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47
Q

What is normal range for ammonia? Abnormal range?

A
  • normal: <80 umol/L

- elevated” >80 umol/L

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48
Q

What occurs when excess ammonia accumulates?

A

ATP production is disrupted

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49
Q

What is the major differential of an elevated ammonia level?

A

IEMs or systemic diseases like liver failure

Note: can be difficult to tell whether it is primary or secondary hyperammonemia due to IEMs causing liver damage

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50
Q

What clears a lot of substances that accumulate in metabolic disorders in utero?

A

placenta and maternal circulation

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51
Q

In which types of IEMs can you see significant hyperammonemia?

A
  • urea cycle defects/protein metabolism (10-100x increases above normal)
  • mito disease and other protein metabolism issues (2-3x increases above normal)
52
Q

What is a major symptom resulting from hyperammonemia?

A

hepatic encephalopathy resulting in neurologic problems

53
Q

What is the normal pH? Acidosis pH? Alkalosis pH?

A
  • normal: 7.4
  • acidosis: <7.35
  • alkalosis: >7.45
54
Q

What body system plays an important role in maintenance of normal pH? How?

A

respiratory system through the ability to alter excretion rate of [H+] generating CO2

55
Q

Respiratory alkalosis is caused by excessive ___ of CO2 as a result of ___ (___ breathing).
Respiratory acidosis is caused by excessive ___ of CO2 as a result of ___ (___ breathing)

A
  • loss, hyperventilation, increased

- retention, hypoventilation, decreased

56
Q

What is metabolic alkalosis and what most commonly causes it?

A

reduction of plasma [H+] due to vomiting, ingestion of alkaline drugs (TUMS)

57
Q

What causes metabolic acidosis?

A

a net gain of [H+] or loss of HCO3- (bicarbonate)

58
Q

What are 4 physiological reasons someone might have metabolic acidosis?

A

sever diarrhea, diabetes mellitus, strenuous exercise, severe renal failure

59
Q

What are 5 secondary causes of metabolic acidosis?

A

severe infection/sepsis, advanced catabolic state, tissue hypoxia, dehydration, intoxication

60
Q

What is an anion gap and what is the normal value?

A

the difference between major cations (Na+ or K+) and major measured anions (Cl- and bicarbonate), normal is less than or equal to 16

61
Q

What is the job if the anion gap is elevated?

A

determine what is present in extra quantity that is increasing the gap such as lactate, ketones, or organic acids

Note: all 3 of these metabolites can be measured in blood or urine

62
Q

What are the 3 primary types of IEMs associated w/ metabolic acidosis?

A
  • disorders of protein metabolism (largest group typically associated w/ overwhelming metabolic acidosis)
  • disorders of fat metabolism
  • mito disease
63
Q

What is lactic acidosis?

A

too much lactic acid in the blood

64
Q

what are the 2 most valuable diagnostic markers of disturbed mitochondrial energy metabolism?

A

lactate and pyruvate

Note: lactate is produced from pyruvate but lactate is easier to measure in blood and CSF so it is usually the 1st of the metabolites to be noticed

65
Q

What 2 mechanisms can increase plasma lactate levels?

A
  • increased pyruvate production (glycolysis)

- decreased pyruvate consumption/oxidation (by pyruvate dehydrogenase complex or pyruvate carboxylase)

66
Q

What are 2 examples of primary hyperlacticacidemia?

What are 2 examples of secondary hyperlacticacidemia?

A
  • carb metabolism, mito disease

- protein or fat metabolism

67
Q

What is ketoacidosis?

A

Acidosis caused by too many ketones

68
Q

How are ketones formed?

A

from excess acetyl-CoA from carb and lipid metabolism, they are produced by the liver during catabolism

69
Q

Ketoacidosis results from either defects in ___ ___ or ___ ___ ___

A
  • ketone utilization
  • increased ketone production

Note: usually a secondary result of IEMs, primary IEMs for ketone metabolism are rare

70
Q

What can lead to elevated CK levels?

A

muscular dystrophy, trauma to muscle, mito disease, LSDs

71
Q

Measurement of the ___-___ ___ is specific for indicating damage to the myocardial cells

A

CK-MB isoenzyme

Note: this measurement is helpful in distinguishing whether there is heart muscle involvement

72
Q

When do CK elevations due to acute muscle trauma appear? when do they peak? when do they return to normal?

A
  • appear: 4-6 hours after
  • peak: 18-30 hours after
  • normal: by 72 hours after
73
Q

Define rhabdomyolysis

A

a sudden increase in serum concentrations of CK

74
Q

In which types of IEMs can rhabdomyolysis be seen?

A

carb, fat, and mito IEMs associated w/ muscle disease

75
Q

What is the range of CK levels for an episode of acute rhabdomyolysis?

A

50,000-200,000 U/L

76
Q

What can happen to the urine with severe rhabdomyolysis?

A

myoglobinuria: muscle tissue pigment turns the urine tea or cola colored

77
Q

What are fatty acid and protein backbone structures attached to when they are broken down?

A

Carnitine

78
Q

What does carnitine do in the cell?

A

serves as a shuttle across mitochondrial membrane and w/in the mitochondria as protein/fatty acids are metabolized into energy/ATP

79
Q

What is a major role for carnitine?

A

transports LCFAs to mitochondria for energy production

80
Q

What are the 2 forms of carnitine?

A
  • free

- bound (also called acylcarnitine)

81
Q

What determines how acylcarnitines are subdivided and what are the categories?

A

Length: long chain, very long chain, medium chain, short chain

82
Q

What are 3 defects in carb metabolism?

A

galactosemia, hereditary fructose intolerance, GSDs

83
Q

What are the hepatic forms of GSD?

A

GSD1 (von Gierke’s disease) and GSD III (Cori disease)

Note: these are treated w/ cornstarch; cause enlarged liver due to excess glycogen

84
Q

What are the muscular forms of GSD?

A

GSD II (Pompe disease - also LSD) and GSD V (McArdle’s disease)

85
Q

What causes classic galactosemia?

A

deficient activity of galactose-I-phosphate uridyltransferase which catalyzes production of UDP galactose from galactose-1-phosphate and UDP-glucose

86
Q

What is the genetic basis of classic galactosemia and the inheritance pattern?

A

mutations in the GALT gene, AR

87
Q

What percent of control values of GALT is consistent with classic galactosemia (G/G)?

A

5% or less (complete or near complete enzyme deficiency)

88
Q

What are 5 important things to know about partial tranferase deficiency?

A
  • more frequent than classic galactosemia
  • often discovered on NBS
  • enzyme activity is 10-50% of normal
  • generally asymptomatic
  • Duarte variant is best know variant
89
Q

How is galactosemia diagnosed on NBS?

A

abnormally high levels of galactose-1-phosphate, confirm dx by enzyme deficiency in heparinized whole blood or RBC

90
Q

On the mutation panel/sequence analysis of GALT for classic (G/G) galactosemia with 6 common mutations, what is the percentage of the following:

  • two common mutations
  • one common mutation and one private mutation
  • two private mutations
A
  • 2 common: 71%
  • 1 common, 1 private: 10%
  • 2 private: 19%
91
Q

What are the clinical features of untreated classic galactosemia?

A
  • feeding problems/vomiting/diarrhea
  • lethargy
  • FTT
  • hypoglycemia
  • liver failure/jaundice
  • bleeding
  • sepsis (E. coli)/shock
  • females-increased risk for primary or secondary POI (75-96% before age 30)
  • DD/ID
  • cataracts (can happen w/in first few days of life)
  • growth delay
  • speech apraxia
92
Q

How do you treat classic galactosemia?

A
  • dietary intervention for infants w/ GALT activity less than 10% of controls
  • lactose-free formula
  • calcium supplementation
93
Q

What are the branched-chain AAs (BCAA)?

A

Valine, Isoleucine, Leucine

94
Q

What are 5 IEMs caused by defects in protein metabolism?

A
  • PKU (maternal, biopterin defects)
  • Tyrosinemia
  • Alcaptonuria
  • Homocystinuria
  • Nonketotic hyperglycemia
95
Q

What causes classic PKU?

A

severe phenylalanine hydroxylase (PAH) deficiency

96
Q

What causes maternal PKU? How can this affect fetal development?

A
  • poor control of plasma Phe levels during pregnancy

- can cause IUGR, microcephaly, birth defects

97
Q

How is PKU typically diagnosed?

A

NBS

98
Q

What is the genetic etiology of PKU?

A

PAH gene, AR

99
Q

What symptoms will untreated infants with PKU develop?

A
  • seizures
  • severe-profound ID
  • microcephaly
  • behavioral problems
  • “mousy” odor from urine
  • very light skin and hair

Note: infants develop typically for the first few months of life before developing symptoms

100
Q

How is PKU treated? How long does treatment need to be continued?

A
  • PKU diet: small amounts of Phe for normal growth and development, avoid high Phe foods (meat, fish, eggs, high-protein foods), use Phe-free formula as supplement (Tyrosine)
  • Phe restriction is life-long
101
Q

What is biopterin cofactor deficiency tetrahydrobiopterin (BH4)? How is it diagnosed?

A
  • 2% of hyperphenylalaninemias
  • DHPR and biopterin in bloodspot
  • urine biopterins
102
Q

What are 2 biopterin recycling defects and what is the pattern of urine biopterins?
What are 2 biopterin synthesis defects and what is the pattern of urine biopterins?

A 
Recycling
- DHPR deficiency, PCD deficiency
-  high neopterin, low biopterin
Synthesis
- GTPCH deficiency, PTPS deficiency
- low neopterin, low biopterin
103
Q

What does a urea cycle defect cause?

A

elevated blood ammonia

104
Q

What is the purpose of the urea cycle?

A

rids body of waste nitrogen, mainly from AA metabolism

105
Q

How does the body get rid of waste nitrogen?

A

liver conjugates waste nitrogen as urea which is excreted by the kidneys

106
Q

Name 4 urea cycle defects from “inside the circle”

A
  • OTC deficiency
  • Citrullinemia
  • ASA lyase deficiency
  • Arginase deficiency
107
Q

What is one of the only IEMs that’s X-linked and what kind of IEM is it?

A

Ornithine Transcarbamylase (OTC) deficiency, urea cycle defect

108
Q

Name 2 urea cycle defects that are “outside the circle”

A
  • NAGS deficiency (secondary inhibition by organic acids)

- CPS deficiency

109
Q

What are the 4 treatments for urea cycle defects?

A
  • precursor and substrate restriction –> dietary protein restriction
  • provide/supplement deficient enzyme –> liver transplant
  • increase alternative pathway use –> oral meds to help remove nitrogen via alternative mechanisms
  • supplement products –> citrulline or arginine supplementation
110
Q

What are 7 organic acidurias?

A
  • Maple syrup urine disease (MSUD)
  • Isovaleric aciduria (IVA)
  • 3-methyl crotonyl-CoA carboxylase (3MCC) deficiency
  • Methylmalonic acidemia (MMA)
  • Propionic aciduria (PA)
  • Glutaric acidemia Type I (GA-I)
  • Biotinidase deficiency
111
Q

What are the features of classical organic acidurias (PA, MMA)?

A
  • well at birth and for first few days
  • toxic encephalopathy (vomiting, poor feeding, hypotonia, lethargy progressing to coma, macrocephaly, seizures, metabolic acidosis - organic acids accumulate, secondary hyperammonemia)
112
Q

What are the features of cerebral organic acidurias (GA-I, Canavan)?

A

neurological symptoms w/out typical metabolic/lactic acidosis and hypoglycemia

113
Q

How do you treat organic acidurias? Is it 100% effective in preventing metabolic crises? How long is treatment?

A
  • diet control (supplement AA, avoid AA, supplement carnitine)
  • emergency protocol (carnitine supplementation during times of stress, avoidance of fasting)
  • metabolic crises still possible w/ diet control
  • treatment is lifelong

Note: pts that survive the neonatal period w/out decompensation are still at risk to develop other neurological symptoms (LD/DD/ID)

114
Q

What urine odors are associated w/ the following organic acidurias?

  • MSUD
  • 3MCC deficiency
  • IVA
  • PKU
A
  • maple syrup
  • cat urine/ammonia
  • sweaty feet
  • mousy
115
Q

What happens during fat metabolism?

A

fatty acids are released by lipoprotein lipase and hepatic lipase cleaving triglycerides in endothelial cells and on the surface of hepatocytes, triglycerides are released from fat cells

116
Q

What does FAO stand for? What does FAOD stand for?

A
  • FAO = fatty acid beta-oxidation

- FAOD = fatty acid oxidation disorder

117
Q

Name 8 IEMs caused by defects of fat metabolism (not including carnitine disorders)

A
  • VLCAD deficiency
  • LCHAD deficiency
  • TFP deficiency
  • MCAD deficiency
  • SCAD deficiency
  • HADH deficiency
  • GAII and MADD
  • HMG deficiency
118
Q

What are 3 carnitine disorders?

A
  • carnitine transport defect (primary carnitine deficiency)
  • carnitine-acylcarnitine translocase deficiency (translocase)
  • carnitine palmitoyl transferase I & II (CPT I & II) deficiency
119
Q

What is the clinical relevance of FAO defects?

A
  • often cause infant/childhood deaths following minor illness w/ fasting
  • affect multiple siblings in a family
120
Q

What are symptoms associated with FAOD?

A
  • hypoglycemia
  • coma
  • seizures
  • heart failure
  • muscle breakdown
121
Q

True or False: there is treatment available for FAODs and it is highly effective in at least some

A

True

122
Q

What is MCAD deficiency?

A

deficiency of MCAD enzyme resulting in low plasma carnitine, high medium-chain acylcarnitines

123
Q

What is the genetic etiology of MCADD?

A
  • gene: ACADM
  • common mutation: 985A>G, K304E (represents 9-%)
  • 80% of pts are homozygous
  • 1 in 40 carrier frequency among Northern European
124
Q

What are the symptoms of MCADD?

A
  • vomiting
  • lethargy
  • seizures
  • breathing difficulties
  • brain damage
  • coma
  • sudden death (caused by hypoketotic hypoglycemia when ill/fasting)
125
Q

What is the treatment for MCADD?

A
  • typical infant feeding
  • typical diet should be ~30% of calories from fat
  • supplement carnitine as needed
  • avoid prolonged fasts
  • understand chronic health needs of an otherwise “well” child
  • provision of an emergency care protocol
126
Q

What are 7 IEM systems that aren’t related to carbs, proteins, or fats?

A
  • peroxisomal disorders (X-ALD, X-linked, associated w/ defect in 1 enzyme)
  • porphyrias (defects in synthesis of heme, can be acute - neurologic or cutaneous - affect skin primarily, symptoms: photosensitivity, mental disturbances, hepatic disease)
  • purines and pyrimidines (Lesch-Nyhan syndrome, XLR, self-injurous behavior)
  • metal metabolism (hemochromatosis - iron metabolism, Wilson disease - copper metabolism, Menkes disease - copper metabolism, X-linked, symptoms: sparse kinky hair, FTT, neurological symptoms)
  • bone metabolism/mineralization (familial X-linked hypophosphatemic rickets - XLD, hypophosphatasia - AR or AD)
  • cholesterol metabolism (SLOS)
  • congenital disorders of glycosylation (CDG) syndromes ( variable, umbrella term for disorders involving glycoproteins/glycolipids)

Medical Genetics I (18 decks)

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