Treatment & Prevention of Genetic Disease Flashcards
What are challenges of treating a single-gene disorder?
- Gene not always known/may have multiple genes causing disorder
- Pathophysiology may not be known (do not know what the gene specifically does, e.g., Huntington’s Disease- do not know how neuron is affected)
- Irreversible damage may be done by the time of birth/diagnosis
- If severe mutation, may be difficult to increase expression/function of gene product
What are the potential levels of intervention?
- Mutant gene (transplantation, gene therapy, expression modulation)
- Mutant mRNA (ribozyme gene transfer to degrade mutant)
- Mutant protein (protein replacement, enhancement of function)
- Metabolic/biochemical dysfunction (dietary/pharmacological compensation)
- Clinical phenotype (medical/surgical intervention)
- The family (genetic counseling, carrier screening, presymptomatic diagnosis)
What are the types of metabolic manupulations to treat genetic diseases?
- Avoidance
- Dietary restriction
- Replacement
- Diversion
- Inhibition
- Depletion
What are examples of avoidance as a metabolic intervention?
- Avoid antimalarial drugs for G6PD deficiency
- Avoid barbiturates for acute intermittent porphyria
What are examples of dietary restriction as a metabolic intervention?
- Restrict phenylalanine for PKU
- Restrict galatose for galactosemia
What are examples of replacement as a metabolic intervention?
- Thyroxine supplements for congenital hypothyroidism
- Biotin supplements for biotinidase deficiency
What are examples of diversion as a metabolic intervention?
- Sodium benzoate for urea cycle disorders (NH3 buildup fatal; convert NH3 via glycine to hippurate, which is readily excreted)
- Oral resins for familial hypercholesterolemia heterozygotes (cholestyramine binds bile acids in the gut and causes their excretion instead of reabsorption; cholesterol shunted to make more bile acids)
What are examples of inhibition as a metabolic intervention?
- Statins (Lovastatin, Lipitor) to treat familial hypercholesterolemia heterozygotes (HMG-CoA inhibitors prevent cholesterol synthesis)
What are examples of depletion as a metabolic intervention?
- LDL apheresis (direct removal of LDL from plasma) to treat familial hypercholesterolemia homozygotes
- Phlebotomy to treat hereditary hemochromatosis (limit iron overload)
What are the types of treatments for mutant/misfolded/missing proteins?
- Cofactor administration to increase enzyme activity
- Protein replacement (extra/intracellular)
- Pharmacological chaperone therapy
What is an example of cofactor supplementation as a treatment of mutant proteins?
For PKU, patient has some mutant apoenzyme activity (phenylalanine hydroxylase; PAH), but use supplementary BH4 cofactor (sapropterin) to maximize PAH action and decrease phenylalanine levels.
What is an example of pharmacological chaperone therapy as a treatment of misfolded proteins?
Use of AT2101 (Isofagomine; IFG) helps increase the production of normal glucocerebrosidase in Gaucher’s disease (lysosomal storage disorder).
What is an example of protein replacement as a treatment of mutant proteins?
Of extracellular proteins (most applicable):
- Factor VIII in hemophilia A
- Alpha1-antitrypsin in alpha1-AT deficiency
- PEG-adenosine deaminase (PEG-ADA) (modified protein) in ADA deficiency
Of intracellular proteins (enzyme replacement therapy; use in treatment of lysosomal storage disorders; tag with mannose-6-phosphate):
- Modified glucocerebrosidase in Gaucher disease
- MPS (I, II, VI)
- Fabry’s Disease
- Pompe Disease
Limiting factors: host immune system, frequent infusions, cost, blood-brain barrier
How do geneticists modify gene expression?
- Pharmacological modulation
- Partial modification of somatic genotype
- Via transplantation
- Via gene transfer into somatic tissues
How is hereditary angiodema treated?
- Modify gene expression
- Autosomal dominant disease results from mutation in C1 esterase inhibitor
- Danazol increases expression
How is sickle cell/thalassemia treated?
- Modify gene expression
- Decrease methylation of HbF by using Butyrate (a cytidine analog) so that CpG islands won’t be methylated
- Results in an increase in fetal hemoglobin production
- Note that response is heterogenous
What is Spinal Muscular Atrophy (SMA), what is its genetic etiology, and how is it treated?
- A group of autosomal recessive inherited diseases (SMA I, II, III) that cause muscle damage and weakness, which get worse over time and eventually lead to death.
- Caused by Survival Motor Neuron (SMN) gene mutation on chromosome 5
- SMN1 = telomeric copy
- SMN2 = centromeric copy
- Deletion of exon 7 from SMN1 leads to severely reduced SMN protein levels; SMN2 alone produces insufficient normal FL-SMN transcript/protein (10% vs. 90%; thymine base in exon 7 creates exonic splicing silencer).
- However, SMN2 copy number modifies the SMA phenotype; more copies, milder disease (e.g., SMA III has 3-4 SMN2 copies) due to greater transcript production.
- Thus, treated by modifying gene expression
How can sodium phenylbutyrate treate spinal muscular atrophy?
- In vitro:
- Increases FL-SMN2 mRNA
- Increases FL-SMN protein
- In vivo:
- Increased FL-SMND
- Increased muscle strength
- Improved motor function
How can antisense oligonucleotides (ASOs) treate spinal muscular atrophy?
- ASOs would promote the inclusion of exon 7 in SMN2.
- Normally, thymine base in SMN2 exon 7 (cytosine in SMN1 exon 7) recruits an exonic splicing silencer (ESS), thus producing transcripts without exon 7.
- ASOs would either block the ESS or create an exonic splicing enhancer (ESE), thus promoting FL-SMN in SMN2.
What are examples of molecularly-based treatments for inherited conditions?
-
Aminoglycosides
- Utilized in nonsense mutations that result in premature stop codons
- Molecule binds decoding site of rRNA, induces conformational change that reduces rNA discrimination ability
- Results in read-through of premature stop codons, leading to increased production of normal protein
- Example: Gentamicin in cystic fibrosis
- Other diseases with premature stop codons: Duchenne muscular dystrophy
-
Curcumin
- Prevents CFTR delta508 mutant from getting stuck in RER
What is an example of splicing modifications?
- The reading frame rule dictates that non-functional proteins will arise from deletion of exons containing a total number of nucleotides not divisible by 3.
- In Duchenne’s Muscular Dystrophy (DMD), a deletion of exon 50 in the dystrophin gene disrupts the reading frame, causing translation to stop prematurely, resulting in a truncated/unstable non-functional protein.
- Use antisense oligonucleotides to mask exon 51, causing it to be spliced out with exon 50, thus restoring the reading froame and producing a functional (albeit truncated) dystrophin protein.
- Not a cure, but still has some effect; similar to Becker’s Muscular Dystrophy (BMD).
What are examples of transplantation as a modification of the somatic genome?
- Liver transplant for inborn errors of metabolism.
- Bone marrow transplant for hemoglobinopathies, immunodeficiencies and storage disorders.
- Cord blood (human umbilical cord) has stem cells and is considered more sterile.
What is congenital adrenal hyperplasia and how is it treated prenatally?
- 21-hydroxylase deficiency leads to decreased cortisol and aldosterone.
- Buildup of precursors shunts them to the testosterone/estradiol pathway.
- Leads to virilized female fetuses (ambiguous genitalia) and shock due to hyperkalemia and hyponatremia.
- Treatment: Give cortisol to pregnant mother: induces negative feedback and stops excess testosterone production.
