INHERITED HUMAN METABOLIC DISEASE

Question 1

List some examples of inborn errors of metabolism with some examples

Answer 1

1. Lysosomal Storage Disorders
   
   • Tay-Sachs disease, Gaucher disease.
2. Glycogen Storage Diseases
3. Amino Acid Disorders
   
   • PKU, maple syrup urine disease.
4. Urea Cycle Disorders
   
   • Citrullinemia.
5. Fatty Acid Oxidation Disorders
   
   • Fatty acyl-CoA dehydrogenase deficiencies.
6. Organic Acidaemias
   
   • Methylmalonic acidaemia.

Question 2

Describe PKU, the importance of neonatal screening, and clinical features

Answer 2

Phenylketonuria (PKU)

• Most common IEM of amino acid metabolism.
• Due to mutation of phenylalanine hydroxylase gene (Chromosome 12).
• Autosomal recessive disorder.
• Variable phenotype - very mild to severe levels of hyperphenylalaninemia.
• If untreated leads to profound irreversible mental disability and seizures.
• Neonatal screening programs identify individuals with PKU - neonatal blood spot.
  
  • neonatal testing is especially important as parents tend to be asymptomatic carriers, and consequences of untreated PKU are severe
• PKU is detected at birth with blood tested
• Must be treated early
• Asymptomatic at first, but patient develops:
  
  • epileptic seizures
  • microcephaly
  • eczema and reduced skin pigmentation
  • intellectual disability
  • tremors and jerky movements

Question 3

Describe PKU metabolism and alternate pathways of metabolism

Answer 3

PKU - Importance of Phenylalanine Metabolism

• Phenylalanine typically metabolised to tyrosine
• Tyrosine can go to many different pathways e.g. melanin - affects skin pigmentation
  
  • Funnel into TCA intermediates → anywhere
    
    • Produce KBs
    • NTs like dopamine → noradrenaline and adrenaline

PKU - Metabolism of Phenylalanine

• PheOH catalyses the reaction
• enzyme is a mono-oxygenase i.e. one of the O-atoms appears in the substrate
• Tetrahydrobiopterin (BH4) is an electron carriers (cofactor for the enzyme)
• The BH4 forms quinonoid dihydrobiopterin or QH2 during hydroxylation
• QH2 is reduced to BH4 by the enzyme dihydrobiopterin reductase dHBPR
• PheOH is expressed in liver and kidney
• PheOH has quaternary organisation (Tetrameric)

PKU - Alternate Pathways of Metabolism
- alternative pathway for phenylaalanine metabolism: a shunt
- phenyl-pyruvate formed via a transamination reaction
- metabolites are excreted in urine
- phenyl-acetate imparts a characteristic odour to urine
- odour is frequently used as the initial spark for follow-up analysis
- metabolites are toxic to the brain: impair brain development and cause mental retardation

Question 4

Describe treatment of PKU

Answer 4

• Relatively little ingested Phe is used in protein synthesis ∴ majority of ingested Phe enters reaction sequence to generate Tyr
• Excess Phe competes for transport into brain

Therapeutic strategy:
- Restriction of dietary phenylalanine (Phe).
- Phe-free milk formulae.
- Avoid protein-rich foods (e.g., meats, fish, eggs, standard bread, cheeses, nuts).
- Avoid aspartame.
- Dietary compliance challenge – adolescence.
- BH4 cofactor can be helpful in some cases (sapropterin).
- Gene therapy: more permanent
- potential therapy: e.g. amino acid transporter inhibitors

Question 5

Describe monogenic diabetes and its diagnosis

Answer 5

• <5% diabetes.
• Any baby diagnosed with diabetes before the age of 6 months of age should have immediate genetic testing for neonatal diabetes. - as compared to T1D which typically is diagnosed after 6 months (need environmental input/insult to pancreas for development, not solely genetic influence)
• Children and adults diagnosed with diabetes in early adulthood that is not typical of type 1 or 2 diabetes that occurs in successive generations (suggestive of an autosomal dominant pattern of inheritance) should have genetic testing for maturity onset diabetes of the young (MODY).

Question 6

Provide some examples of neonatal diabetes, causative mutations and consequences

Answer 6

• causative mutations dictate treatment
  
  • **KCNJ11 and ABCC8: **affect ATP-sensitive K+ channels and are thus responsive to sulphonylureas, susceptible to developmental delays
  • INS: lack of insulin, requiring supplementation
  • KCNJ11, ABC88, and INS: all cause intrauterine growth restriction
  • 6q24: affects PLAGL1 and HYMA1, due to methylation defect resulting in transcription issues
  • GATA: results in pancreatic hypoplasia or agenesis, and other malformations e.g. cardiac. May require pancreatic enzyme supplementation
  • Elf2AK3: enzyme defect leading to skeletal dysplasia, exocrine pancreas insufficiency and needs insulin supplementation
  • **FOXP3: autoimmune diabetes and predisposition to other autoimmune diseases

Question 7

Describe the uses of genetic sequencing in neonatal diabetes

Answer 7

• if diabetes develops under 6 months of age, rapid non-selective genetic testing of multiple genes simultaneously e.g. with WGS techniques, independent of clinical features
• This informs:
  
  • treatment e.g. sulphonylureas for ABCC8 and KCNJ11, 6q24 methylation defect
  • explanation of associated clinical features: heart defects due to GATA4/6 mutations
  • anticipates clinical features: autoimmune disease and FOXP3, KCNJ11 and developmental delays, GATA and exocrine pancreas deficiency
  • intervention for comorbidities e.g. KCNJ11/ABCC8 and developmental issues

Question 8

Describe familial hypercholesterolaemia

Answer 8

FH is a genetic disorder characterised by high plasma cholesterol.
In particular, LDL cholesterol is elevated.

1/500 Australians are affected.
It is largely asymptomatic initially.

Yellow patches appear above eyelids and lumps in tendons (knee, hands, elbows).
- xanthelasma
- punctate lipid collections on buttocks
- xanthomas

High risk of atherosclerosis due to artery narrowing and plaque build-up.

Question 9

Describe the biochemistry of LDL cholesterol

Answer 9

• cells synthesise cholesterol from acetyl-CoA via HMG-CoA
• requirements above synthetic rate use LDL-cholesterol
• ‘insert’ LDL receptor into plasma membrane
• LDL receptor binds to B100 and apoE proteins
• LDL is internalised to secure and release the cholesterol

Question 10

Describe the role of PCSK9

Answer 10

• PCSK9 will bind LDLR and LDL particle complex
• This becomes endocytosed and leads to lysosomal degradation: PCKS9 facilitates a removal pathway

Question 11

Describe the genetics of FH

Answer 11

• Mutations of the LDL-receptor cause most cases of familial hypercholesterolaemia
• FH patients display mutations to the LDL-receptor gene
• Deficiency of LDL-receptors is found in hepatic and peripheral tissues
• Heterozygotes have CVD at 30 to 40 years
• Homozygotes exhibit severe disease in childhood
• Homozygous FH is rare: 1/1000000

LDLR mutations may produce one of the following:
- partial and complete gene deletions
- misfolded protein: retained in ER
- reduced affinity for LDL apoproteins
- inability of bound receptor to trigger endocytosis

Question 12

Describe the consequences of FH

Answer 12

• FH is characterised by hyperlipidemia
• lipoproteins aggregate on surface of endothelial cells
• LDL and contents oxidised by free radicals in blood
• Oxidised LDLs engulfed by circulating macrophages
• Macrophages laden with lipids: foam cells
• Foam cells die: fatty deposit on artery wakk
• Possible fibro-proliferative response results in formation of atheromatous plaque

Question 13

List and briefly describe the treatment of FH

Answer 13

• Diet
• Resins/Sequestrants (e.g., cholestyramine)
• HMG-CoA Reductase Inhibitors (Statins)
  
  • Inhibit cholesterol synthesis.
• Ezetimibe
  
  • Inhibits cholesterol absorption from the gastrointestinal tract.
• PCSK9 Inhibitors
  
  • Multiple ways; monoclonal antibodies most advanced.
• Genetic Therapies (Adenovirus)
• LDL Apheresis: filtration of blood to remove lipids
• Liver Transplant