Transcript the current state of treatment of genetic disease

Lecture 6-a
The Treatment of Genetic
Disease

The understanding of genetic disease at a
molecular level is the foundation of rational
therapy.
 The objective in treating genetic disease is to
eliminate or ameliorate the effects of the
disorder, not only on the patient but also on his
or her family.
THE CURRENT STATE OF TREATMENT OF
GENETIC DISEASE
Genetically Complex Diseases

Environmental interventions, such as
medications and lifestyle or diet changes, may
have a greater impact on the management of
genetically complex diseases.
 For example, cigarette smoking is an
environmental factor that all patients with
age-related macular degeneration or
emphysema should strictly avoid.

A striking example of a complex disorder
for which standard medical therapy is
increasingly successful is type 1 diabetes
mellitus, in which intensive insulin
replacement therapy greatly improves the
outcome.
 Surgical treatment of multifactorial
disorders can also be highly successful. For
example, three structural abnormalities
(congenital heart defects, cleft lip and
palate, and pyloric stenosis).
TREATMENT STRATEGIES
Therapy Directed at the Clinical Phenotype
 Treatment at the level of the clinical phenotype
includes all the medical or surgical interventions
that are not unique to the management of genetic
disease.
 Include:
– Avoidance
– Dietary restriction
– Replacement
– Diversion
– Inhibition
– Depletion
Dietary Restriction

Dietary restriction is one of the oldest and
most effective methods of managing genetic
disease.
 Diseases involving more than several dozen
loci are currently managed in this way.
 It usually requires lifelong compliance with
a restricted and often artificial diet.
 Many of the diseases treatable in this
manner involve amino acid catabolic
pathways, and therefore severe restriction of
normal dietary protein is usually necessary.
Replacement

The provision of essential metabolites,
cofactors, or hormones whose deficiency is
due to a genetic disease is simple in concept
and often simple in application.
 E.g., congenital hypothyroidism, 10% to
15% of which is monogenic in origin. This
disorder results from a variety of defects in
the formation of the thyroid gland or of its
major product, thyroxine.
Diversion



Diversion therapy is the enhanced use of
alternative metabolic pathways to reduce the
concentration of a harmful metabolite.
A successful application of diversion therapy is
in the treatment of the urea cycle disorders.
If the cycle is disrupted by an enzyme defect
such as ornithine transcarbamylase
deficiency, the consequent hyperammonemia
can be only partially controlled by dietary
protein restriction.

The ammonia can be reduced to normal levels by
diversion to metabolic pathways that are normally
of minor significance, leading to synthesis of
harmless compounds.
 Thus, the administration of large quantities of
sodium benzoate forces its ligation with glycine to
form hippurate, which is excreted in the urine.
 Glycine synthesis is thereby increased, and for
each mole of glycine formed, one mole of
ammonia is consumed.

Figure 13-4 The strategy of metabolite diversion. In this
example, ammonia cannot be removed by the urea cycle because
of a genetic defect of a urea cycle enzyme. The administration of
sodium benzoate diverts ammonia to glycine synthesis, and the
nitrogen moiety is subsequently excreted as hippurate.

A similar approach has been successful in
reducing cholesterol level in heterozygotes for
familial hypercholesterolemia.
 By the diversion of an increased fraction of
cholesterol to bile acid synthesis, the single
normal low-density lipoprotein (LDL) receptor
gene of these patients can be stimulated to produce
more hepatic receptors for LDL-bound cholesterol
 This treatment achieves significant reductions in
plasma cholesterol because 70% of all LDL
receptor-mediated uptake of cholesterol is by the
liver.

The increase in bile acid synthesis is achieved by
the oral administration of non-absorbable resins
such as cholestyramine, which bind bile acids in
the intestine and increase their fecal excretion.
 This example illustrates clearly an important
principle: autosomal dominant diseases may
sometimes be treated by increasing the expression
of the normal allele.
Inhibition

The pharmacological inhibition of enzymes
is sometimes used to modify the metabolic
abnormalities of inborn errors.
 Familial hypercholesterolemia also
illustrates this principle. When the
cholesterol load is decreased by diverting it
to other compounds or by removing it with
physical methods, the liver tries to
compensate for the decreased cholesterol
intake by up-regulating cholesterol
synthesis.

Consequently, the treatment of familial
hypercholesterolemia heterozygotes is more
effective if hepatic cholesterol synthesis is
simultaneously inhibited by a statin, a class of drugs
that are powerful inhibitors of 3-hydroxy-3methylglutaryl coenzyme A (HMG-CoA) reductase,
the rate-limiting enzyme of cholesterol synthesis.
 High doses of a statin typically effect a 40% to 60%
decrease in plasma LDL cholesterol levels in
familial hypercholesterolemia heterozygotes; when a
statin is used together with cholestyramine, the
effect is synergistic, and even greater decreases can
be achieved.

Figure 13-5 Rationale for the combined use of a bile
acid-binding resin and an inhibitor of 3-hydroxy-3methylglutaryl coenzyme A reductase (HMG CoA
reductase) in the treatment of familial
hypercholesterolemia heterozygotes.

Depletion
 Genetic diseases characterized by the
accumulation of a harmful compound are
sometimes treated by direct removal of the
compound from the body. This principle is
exemplified by the use of phlebotomy to
alleviate the iron accumulation that occurs in
hemochromatosis.
Table 13-3. Treatment of Genetic Disease by Metabolic Manipulation
Type of Metabolic
Intervention
Substance or Technique
Disease
Avoidance
Antimalarial drugs
Glucose-6-phosphate
dehydrogenase deficiency
Isoniazid
Slow acetylators
Dietary restriction
Phenylalanine
Galactose
Phenylketonuria
Galactosemia
Replacement
Thyroxine
Congenital hypothyroidism
Biotin
Biotinidase deficiency
Sodium benzoate
Urea cycle disorders
Oral resins that bind bile acids
Drugs that block the intestinal
absorption of cholesterol
Familial hypercholesterolemia
heterozygotes
Inhibition
Statins
Familial hypercholesterolemia
heterozygotes
Depletion
LDL apheresis (direct removal
of LDL from plasma)
Familial hypercholesterolemia
homozygotes
Diversion
THE MOLECULAR TREATMENT OF
DISEASE
Treatment at the Level of the Protein
 Enhancement
of Mutant Protein
Function with Small Molecule Therapy

Small molecules are that class of compounds
with molecular weights in the few hundreds to
thousands. Synthetic or natural.
 Vitamins, non-peptide hormones, and most
drugs are classified as small molecules.
 Vitamin-Responsive
Inborn Errors of
Metabolism

The biochemical abnormalities of a number of
metabolic diseases may respond, to the
administration of large amounts of the vitamin
cofactor of the enzyme impaired by the
mutation.
 In homocystinuria due to cystathionine
synthase deficiency, e.g., about 50% of patients
respond to the administration of high doses of
pyridoxine (vitamin B6, the precursor of
pyridoxal phosphate); in most of these patients,
homocystine disappears from the plasma.

The increased pyridoxal phosphate concentrations
may overcome reduced affinity of the mutant
enzyme for the cofactor or stabilize the mutant
enzyme.
 Nonresponsive patients generally have no residual
cystathionine synthase activity to augment.

Figure 13-7 The mechanism of the response of a mutant apoenzyme to the
administration of its cofactor at high doses. Vitamin-responsive enzyme
defects are often due to mutations that reduce the normal affinity (top) of the
enzyme protein (apoenzyme) for the cofactor needed to activate it. In the
presence of the high concentrations of the cofactor that result from the
administration of up to 500 times the normal daily requirement, the mutant
enzyme acquires a small amount of activity sufficient to restore biochemical
normalcy.
Small Molecules to Increase the Folding of
Mutant Polypeptides
 Many mutations disrupt the ability of the
mutant polypeptide to fold normally. If the
folding defect could be overcome, the mutant
protein would often be able to resume its
normal activity.
 The administration of small molecules may be
used to overcome a folding defect.
 Folding mutants of membrane proteins, for
example, fail to pass normally through the
endoplasmic reticulum and get "stuck" there,
leading to their degradation.
E.g., the ΔF508 mutation of the cystic fibrosis
protein. The mutant ΔF508 polypeptide is
recognized by a calcium-dependent chaperone
protein in the endoplasmic reticulum, retained
there, and degraded.
 An extraordinary correction of this defect has been
obtained in mice carrying the ΔF508 mutation by
the administration of curcumin, a nontoxic
mixture of compounds derived from turmeric, a
spice found in curry.


Curcumin inhibits a calcium pump in the
endoplasmic reticulum, thereby impairing
the binding of the mutant ΔF508 protein by
the calcium-dependent chaperone.
 The treated mice had a normalization of
chloride transport in the gut and nasal
epithelium and dramatically increased rates
of survival.
Small Molecule Therapy to Allow Skipping over
Mutant Stop Codons
 Aminoglycoside antibiotics, encourage the
translational apparatus to "skip over" a
premature stop codon and instead to
misincorporate an amino acid that has a codon
comparable to that of the termination codon.
 In this way, for example, Arg553Stop in CF is
converted to 553Tyr, a substitution that
generates a CFTR peptide with nearly normal
properties.
Protein Augmentation
 The prevention or arrest of bleeding
episodes in patients with hemophilia by the
infusion of plasma fractions enriched for
factor VIII is the prime example.
Augmentation of an Extracellular Protein: α1Antitrypsin Deficiency
 Human α1AT can be infused intravenously in
doses sufficiently large to maintain the interstitial
fluid α1AT concentration at an effective inhibitory
level for 1 week or even longer.
 An alternative approach still being studied
involves the delivery of α1AT directly to the lungs
by aerosol inhalation. This route of administration
is more attractive, since it requires only 10% of
the intravenous dose of α1AT.
Enzyme Replacement Therapy: Extracellular
Augmentation of an Intracellular Enzyme
Adenosine Deaminase Deficiency
Figure 13-8 Adenosine deaminase (ADA) converts adenosine to inosine and
deoxyadenosine to deoxyinosine. In ADA deficiency, deoxyadenosine accumulation
in lymphocytes is lymphotoxic, killing the cells by impairing DNA replication and
cell division to cause severe combined immunodeficiency (SCID).

Adenosine deaminase (ADA) is a critical enzyme of
purine metabolism that catalyzes the deamination of
adenosine to inosine and of deoxyadenosine to
deoxyinosine.
 The pathology of ADA deficiency, an autosomal
recessive disease, results entirely from the
accumulation of toxic purines, particularly
deoxyadenosine, in lymphocytes severe
combined immunodeficiency.
 The current treatment of choice, however, is bone
marrow transplantation from a fully HLAcompatible donor.
 Administration of the bovine ADA enzyme has been
shown to be effective.
Modified Adenosine Deaminase
 The infusion of bovine ADA modified by
the covalent attachment of an inert polymer,
polyethylene glycol (PEG), has been found
to be superior to the use of the unmodified
ADA enzyme.

Enzyme Replacement Therapy: Targeted
Augmentation of an Intracellular Enzyme
 Enzyme replacement therapy (ERT) is now
established therapy for two lysosomal storage
diseases, Fabry disease and Gaucher disease.

Fabry disease: α-galactosidase deficiency
– accumulation of globosides (glycosphingolipids)
– Reddish purple skin rash
– kidney and heart failure
– burning pain in lower extremities
Gaucher Disease
 This autosomal recessive condition is due to a
deficiency of the enzyme glucocerebrosidase. Its
substrate, glucocerebroside, is a complex lipid normally
degraded in the lysosome.
 The disease results from glucocerebroside
accumulation, particularly in the lysosomes of
macrophages in the reticuloendothelial system, leading
to gross enlargement of the liver and spleen.
 In addition, bone marrow is slowly replaced by lipidladen macrophages ("Gaucher cells") that ultimately
compromise the production of erythrocytes and
platelets, producing anemia and thrombocytopenia.
Bone lesions cause episodic pain, osteonecrosis, and
substantial morbidity.