Inherited metabolic diseases: 2022 overview of personalized therapies

Question 1

How can inherited metabolic disorders be defined?

Answer 1

A large group of monogenic disorders, caused by defects in the DNA leading to disrupted biochemical reactions or pathways.

Question 2

What are characteristics of inherited metabolic disorders?

Answer 2

• Energy deficiency and shortage of building blocks
• Accumulation of (toxic) substances
• Any organ(s) can be affected
• Static but more often progressive symptoms.

Question 3

Why should we care for rare neurometabolic disorders?

Answer 3

• Symptoms are often severe
• Epilepsy, developmental delay, regression, intellectual disability, movement disorders, neuropathies, psychiatric/behavioral.
• Window of opportunity

Question 4

There are different ways metabolic disorders can be treated, specifically by interveining in the metabolic pathway.
Name ways you can interveine here.

Answer 4

• Limit intake of the substrate
• Supplement the product
• Stimulate an alternate pathway
• Provide co-factor
• Supply enzyme
• Transplant organ
• Gene therapy

Question 5

See picture. This girl was diagnosed with a LARS1 deficiency. This gene encodes a cytosolic leucine-tRNA synthetase and is part of a family of related genes known as Aminoacyl-tRNA synthetases (ARSs). What is the main function of ARSs genes?

Answer 5

Loading tRNAs with cognate amino-acids to enhance translation.

Question 6

What is a possible hypothesis for the pathophysiology of ARS1 deficiency (think stepwise)?

Answer 6

So there is decreased LARS expression and acitivity, which results in a decreased tRNA aminoacylation. This results in stress, namely a high translational demand, with a low amino acid supply.
A high translational demand with a low amino acid supply can result in:
- decreased translation speed
- translation termination at LEE-codons
- misacylation → mistranslation → protein degradation

Question 7

A new treatment for ARS1 deficiency was developed, a targeted amino acid supplementation treatment. The activity and leucine rescue was researched in fibroblasts.
What was found before and after administration of the treatment (i.e. leucine supplementation)?

Answer 7

• A reduced residual acitivy in aminoacylation, which decreases with higher temperature, was found in LARS deficient fibroblasts
• Leucine supplementation resulted in rescue of function and survival.

Question 8

What are challenges that are ahead in regard to (rare metabolic) diseases such as a LARS1 deficiency?

Answer 8

• ARS1 deficiences are rare
• Hetereogeneous phenotypes
• Natural history limited → is the disease progressive?
• Low evidence levels
• Minimal access quality amino-acids (for supplementation)
• Optimal dosage unknown
• Lack of approval and reimbursement

Question 9

N-of-1 trial designs can be very important in researching rare diseases and in developing tailored treatment for these diseases. However, an n-of-1 trial on its own is not neccesarily high of statistical power. What can be done to yield an evidence level of 1 with a n-of-1 trial?

Answer 9

Using multiple n-of-1 trials and performing a meta-analysis on these multiple n-of-1 trials.

Question 10

UMD (United for Metabolic Diseases) is an national n-of-1-for all platform in the Netherlands. Their mission is to accelerate evidence, approval and access for inherited metabolic diseases. What tools do they have to achieve this?

Answer 10

• Expert board
• Toolbox and templates
• Alternative trial designs
• Personalized outcome measures
• Methodology and statistics
• Policymakers involvement

Question 11

Drug discovery and development:
What are challenges?

Answer 11

• Duration of drug discovery and development usually takes longer than 10 years.
• Expensive
• High risk and low success rate

Question 12

What are the advantages of drug repurposing compared to drug discovery and development?

Answer 12

• Duration of drug repurposing usually takes less than 5 years (instead of longer than 10 years).
• More affordable
• Known drug safety
• Reduced pharmacokinetic uncertainty
  (However, still not easy)

Question 13

Why is the use of CRISPR-Cas9 seen as a breakthrough?

Answer 13

• Gene editing without viral insertions, only patient’s own cells.
• More personalized → safer?

Question 14

What is RNA therapy?

Answer 14

RNA therapy is designed to correct mistakes or mutations in the RNA of someone with a genetic disease. By correcting the mistake, the RNA can then be used to create the protein that the cell needs, taking away the underlying cause of the disease. E.g. anti-sense oligonucleotides for splice site mutations.

Question 15

How can RNA therapy with oligonucleotides for a certain disease be personalized?

Answer 15

By identifying the mutation and the splice defect of the disease → constructing antisense oligonucleotides (ASOs) that target the splice defect → change in pre-mRNA splicing and protein expression which usually causes cleavage of the mRNA by enzymes (you can also construct ASOs that stabilize mRNA that is normally degraded)

Question 16

What is pyridoxine-dependent epilepsy (PDE)? And what is the problem with the treatment for PDE?

Answer 16

Intractable epilepsy responsive to vitamin B6. Here, a deficient enzyme called antiquitin (due to mutation), results in the build up of a molecule that interferes with vitamin B6 (i.e. pyridoxine). Antiquitin is normally involved in the breakdown of the amino acid lysine in the brain.
People with PDE get pyridoxine (i.e. B6) as a treatment, however >80% still has intellectual disability despite the treatment.

Question 17

Intra-cerebral lysine reduction is a therapy for pyridoxine-dependent epilepsy. What does the treatment consist of and what are the (expected) results from this treatment?

Answer 17

• A protein restricted diet (to limit the substrate (lysine) that leads to neurotoxic metabolite accumulation)
• Arginine supplementation (to compete for brain lysine influx)

Question 18

What is another treatment that can be used to treat pyridoxine-dependent epilepsy?

Answer 18

Targeting the AASS enzyme to reduce the accumulation of neurotoxins via drug repurposing and RNA therapy. AASS is an enzyme which lies upstream of the defective gene in the human lysine metabolic pathway.

Question 19

How can the targeting of the AASS enzyme for the treatment of pyridoxine-dependent epilepsy be researched for its effectiveness?

Answer 19

• In human model systems (e.g. iPSCs)
• In animal model systems (e.g. PDE zebrafish or mice)

Question 20

Children with antiquitin-deficient pyridoxine-dependent epilepsy have neurotoxic build-up of metabolites. One of these metabolites can be found in the blood and can be used as a new biomarker for e.g. newborn screening for PDE. What biomarker was found?

Answer 20

2-OPP