Transcript Medical Biochemistry Biochemical and Genetic Basis of Disease Lecture 77 Classes of Biomolecules Affected in Disease • All classes of biomolecules found in cells are.

Medical Biochemistry
Biochemical and Genetic Basis of
Disease
Lecture 77
Classes of Biomolecules Affected in
Disease
• All classes of biomolecules found in cells are affected
in structure, function, or amount in one or another
disease
– Can be affected in a primary manner (e.g., defect in
DNA) or secondary manner (e.g., structures, functions,
or amounts of other biomolecules)
Rate of Biochemical Alterations
• Biochemical alterations that cause disease may
occur rapidly or slowly
– Cyanide (inhibits cytochrome oxidase) kills within a
few minutes
– Massive loss of water and electrolytes (e.g., cholera)
can threaten life within hours
– May take years for buildup of biomolecule to affect
organ function (e.g., mild cases of Niemann-Pick
disease may slowly accumulate sphingomyelin in liver
and spleen)
Deficiency or Excess of Biomolecules
• Diseases can be caused by deficiency or excess of
certain biomolecules
– deficiency of vitamin D results in rickets, excess
results in potentially serious hypercalcemia
– Nutritional deficiencies
• primary cause - poor diet
• secondary causes - inadequate absorption, increased
requirement, inadequate utilization, increased
excretion
Organelle Involvement
• Almost every cell organelle has been involved in
the genesis of various diseases
Different Mechanisms, Similar Effect
• Different biochemical mechanisms can produce
similar pathologic, clinical, and laboratory findings
– The major pathological processes can be produced by a
number of different stimuli
– e.g., fibrosis of the liver (cirrhosis) can result from
chronic intake of EtOH, excess of copper (Wilson’s
disease), excess of iron (primary hemochromatosis),
deficiency of a1-antitrypsin, etc.
– different biochemical lesions producing similar end
point when local concentration of a compound exceeds
its solubility point (excessive formation or decreased
removal)  precipitation to form a calculus
• e.g., calcium oxalate, magnesium ammonium phosphate, uric
acid, and cystine may all form renal stone, but accumulate for
different biochemical reasons
Genetic Diseases
• Many disease are determined genetically
– Three major classes: (1) chromosomal disorders, (2)
monogenic disorders (classic Mendelian), and (3)
multifactorial disorders (product of multiple genetic and
environmental factors)
Genetic Diseases
• Polygenic denotes disorder caused by multiple
genetic factors independently of environmental
influences
• Somatic disorders - mutations occur in somatic
cells (as in many types of cancer)
• Mitochondrial disorders - due to mutations in
mitochondrial genome
Chromosomal Disorders
• Excess or loss of chromosomes, deletion of part of
a chromosome, or translocation
– e.g., Trisomy 21 (Down syndrome)
• Recognized by analysis of karyotype
(chromosomal pattern) of individual (if alterations
are large enough to be visualized)
• Translocations important in activating oncogenes
– e.g., Philadelphia chromosome - bcr/abl)
Monogenic Disorders
• Involve single mutant genes
• Classification:
(1) autosomal dominant - clinically evident if one
chromosome affected (heterozygote)
• e.g., Familial hypercholesterolemia
(2) autosomal recessive - both chromosomes must
be affected (homozygous)
• e.g., Sickle cell anemia
(3) X-linked - mutation present on X chromosome
• females may be either heterozygous or homozygous
for affected gene
• males affected if they inherit mutant gene
• e.g., Duchenne muscular dystrophy
Multifactorial Disorders
• Interplay of number of genes and environmental
factors
– pattern of inheritance does not conform to classic
Mendelian genetic principles
– due to complex genetics, harder to identify affected
genes; thus, less is known about this category of
disease
– e.g., Essential hypertension
Inborn Error of Metabolism
• A mutation in a structural gene may affect the
structure of the encoded protein
• If an enzyme is affected, an inborn error of
metabolism may result
– A genetic disorder in which a specific enzyme is
affected, producing a metabolic block, that may
have pathological consequences
Inborn Error of Metabolism
E
SP
Normal
Increased X,Y

*E
Increased S  Decreased P
Block
• A block can have three results:
(1) decreased formation of the product (P)
(2) accumulation of the substrate S behind the block
(3) increased formation of metabolites (X, Y) of the
substrate S, resulting from its accumulation
• Any one of these three results may have
pathological effects
Inborn Error of Metabolism
Increased phenylpyruvic acid

*E
Increased phenylalanine  Decreased tyrosine
Block
• Phenylketonuria - mutant enzyme is usually
phenylalanine hydroxylase
– synthesize less tyrosine (often fair skinned), have 
plasma levels of Phe, excrete phenylpyruvate and
metabolites
• If structural gene for noncatalytic protein affected by
mutation can have serious pathologic consequences
(e.g., hemoglobin S)
Genetic Linkage Studies
• The more distant two genes are from each other on the
same chromosome, the greater the chance of
recombination occurring between them
• To identify disease-causing genes, perform linkage
analysis using RFLP or other marker to study inheritance
of the disease (marker)
Genetic Linkage Studies
• Simple sequence repeats (SSRs), or
microsatellites, small tandem repeat
units of 2-6 bp are more informative
polymorphisms than RFLPs; thus
currently used more
Methods to clone disease genes
• Functional approach
– gene identified on basis of biochemical defect
– e.g., found that phenotypic defect in HbS was
GluVal, evident that mutation in gene encoding
b-globin
• Candidate gene approach
– genes whose function, if lost by mutation, could
explain the nature of the disease
– e.g., mutations in rhodopsin considered one of the
causes of blindness due to retinitis pigmentosa
Methods to clone disease genes
• Positional cloning
– no functional information about gene product, isolated
solely by it chromosomal position (information from
linkage analysis
– e.g., cloning CF gene based on two markers that
segregated with affected individuals
• Positional candidate approach
– chromosomal subregion identified by linkage studies,
subregion surveyed to see what candidate genes reside
there
– with human genome sequenced, becoming method of
choice
Identifying defect in disease gene
• Once disease gene identified, still can be arduous
task identifying actual genetic defect
Structure of CFTR gene and
deduced protein
Mutations in CFTR gene
Ethical Issues
• Once genetic defect identified, no treatment options may
be available
– Will patients want to know?
– Is prenatal screening appropriate?
– Will identification of disease gene
affect insurability?
• e.g., Hungtington’s disease - mutation due to trinucleotide
(CAG) repeat expansion (microsatellite instability)
– normal individual (10 to 30 repeats)
– affected individual (38 to 120) - increasing length of
polyglutamine extension appears to correlate with  toxicity
Molecular Medicine
• Knowledge of human genome will aid in the
development of molecular diagnostics, gene
therapy, and drug therapy
Gene expression in diagnosis
• Diffuse large B-cell lymphoma
(DLBCL), a disease that includes a
clinically and morphologically varied
group of tumors that affect the lymph
system and blood. Most common subtype
of non-Hodgkin’s lymphoma.
• Performed gene-expression profiling with
microarray containing 18,000 cDNA
clones to monitor genes involved in
normal and abnormal lymphocyte
development
• Able to separate DLBCL into two
categories with marked differences in
overall patient survival.
• May provide differential therapeutic
approaches to patients
Treatment for Genetic Diseases
• Treatment strategies
(1) correct metabolic consequences of disease by
administration of missing product or limiting
availability of substrate
• e.g., dietary treatment of PKU
(2) replace absent enzyme or protein or to increase its
activity
• e.g., replacement therapy for
hemophilia
(3) remove excess of stored
compound
• e.g., removal of iron by periodic
bleeding in hemochromatosis
(4) correct basic genetic abnormality
• e.g., gene therapy
Gene Therapy
• Only somatic gene therapy is permissible in
humans at present
• Three theoretical types of gene therapy
– replacement - mutant gene removed and replace
with a normal gene
– correction - mutated area of affected gene would be
corrected and remainder left unchanged
– augmentation - introduction of foreign genetic
material into cell to compensate for defective
product of mutant gene (only gene therapy
currently available)
Gene Therapy
• Three major routes of delivery of genes into humans
(1) retroviruses
• foreign gene integrates at random sites on
chromosomes, may interrupt (insertional mutagenesis)
the expression of host cell genes
• replication-deficient
• recipient cells must be
actively growing for
integration into genome
• usually performed ex vivo
Gene Therapy
(2) adenoviruses
• replication-deficient
• does not integrate into host cell genome
– disadvantage: expression of transgene gradually
declines requiring additional treatments (may
develop immune response to vector)
• treatment in vivo, vector can be introduced into
upper respiratory tract in aerosolized form
(3) plasmid-liposome complexes
Gene Therapy
• Conclusions based on recent gene therapy trials
– gene therapy is feasible (i.e., evidence for expression of
transgene, and transient improvements in clinical
condition in some cases
– so far it has proved safe (only inflammatory or immune
reactions directed toward vector or some aspect of
administration method rather than toward transgene
– no genetic disease cured by this method
– major problem is efficacy, levels of transgene product
expression often low or transient
Genetic Medicines
• Antisense oligonucleotides
– complementary to specific mRNA
sequence
– block translation or promote
nuclease degradation of mRNA,
thereby inhibit synthesis of protein
products of specific genes
– e.g., block HIV-1 replication by
targeting gag gene
• Double-stranded DNA to form
triplex molecule