Biochemical Basis Ofdisease

8/13/2019 Biochemical Basis Ofdisease

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Medical Biochemistry

Biochemical and Genetic Basis of

Disease

Lecture 77


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Classes of Biomolecules Affected in

Disease All classes of biomolecules found in cells are affected

in structure, function, or amount in one or anotherdisease

Can be affected in a primary manner (e.g.,defect inDNA) or secondary manner (e.g., structures, functions,or amounts of other biomolecules)


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Rate of Biochemical Alterations

Biochemical alterations that cause disease may

occur rapidly or slowly

Cyanide (inhibits cytochrome oxidase) kills within afew minutes

Massive loss of water and electrolytes (e.g., cholera)

can threaten life within hours

May take years for buildup of biomolecule to affectorgan function (e.g.,mild cases of Niemann-Pick

disease may slowly accumulate sphingomyelin in liver

and spleen)


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Deficiency or Excess of Biomolecules

Diseases can be caused by deficiency or excess of

certain biomolecules

deficiency of vitamin D results in rickets, excessresults in potentially serious hypercalcemia

Nutritional deficiencies

primary cause - poor diet

secondary causes - inadequate absorption, increased

requirement, inadequate utilization, increased

excretion


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Organelle Involvement

Almost every cell organelle has been involved in

the genesis of various diseases


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Different Mechanisms, Similar Effect

Different biochemical mechanisms can producesimilar pathologic, clinical, and laboratory findings

The major pathological processes can be produced by anumber of different stimuli

e.g.,fibrosis of the liver (cirrhosis) can result fromchronic intake of EtOH, excess of copper (Wilsonsdisease), excess of iron (primary hemochromatosis),deficiency of a1-antitrypsin, etc.

different biochemical lesions producing similar end

point when local concentration of a compound exceedsits solubility point (excessive formation or decreasedremoval) precipitation to form a calculus

e.g.,calcium oxalate, magnesium ammonium phosphate, uricacid, and cystine may all form renal stone, but accumulate for

different biochemical reasons


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Genetic Diseases

Many disease are determined genetically Three major classes: (1) chromosomal disorders, (2)

monogenicdisorders (classic Mendelian), and (3)

multifactorial disorders(product of multiple genetic and

environmental factors)


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Genetic Diseases

Polygenicdenotes disorder caused by multiplegenetic factors independently of environmental

influences

Somatic disorders- mutations occur in somaticcells (as in many types of cancer)

Mitochondrial disorders- due to mutations in

mitochondrial genome


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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)


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Monogenic Disorders

Involve single mutant genes Classification:

(1) autosomal dominant- clinically evident if onechromosome affected (heterozygote)

e.g., Familial hypercholesterolemia

(2) autosomal recessive- both chromosomes mustbe 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


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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 ofmetabolismmay result

A genetic disorder in which a specific enzyme is

affected, producing a metabolic block, that may

have pathological consequences


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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 thesubstrate S, resulting from its accumulation

Any one of these three results may have

pathological effects

S P Increased S Decreased PENormal Block

Increased X,Y*E


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Phenylketonuria- mutant enzyme is usuallyphenylalanine hydroxylase

synthesize less tyrosine (often fair skinned), have plasma levels of Phe, excrete phenylpyruvate andmetabolites

If structural gene for noncatalytic protein affected bymutation can have serious pathologic consequences(e.g.,hemoglobin S)

Increased phenylalanine Decreased tyrosineBlock

Increased phenylpyruvic acid*E


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Genetic Linkage Studies

The more distant two genes are from each other on thesame chromosome, the greater the chance ofrecombination occurring between them

To identify disease-causing genes, perform linkageanalysis using RFLP or other marker to study inheritance

of the disease (marker)


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Genetic Linkage Studies

Simple sequence repeats(SSRs), ormicrosatellites, small tandem repeatunits of 2-6 bp are more informative

polymorphismsthan RFLPs; thuscurrently used more


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Methods to clone disease genes

Functional approach

gene identified onbasis of biochemical defect

e.g.,found that phenotypic defect in HbS was

GluVal, evident that mutation in gene encodingb-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


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Methods to clone disease genes

Positional cloning

no functional information about gene product, isolatedsolely by it chromosomal position(information fromlinkage analysis

e.g., cloning CF gene based on two markers thatsegregated with affected individuals

Positional candidate approach

chromosomal subregion identified by linkage studies,

subregion surveyed to see what candidate genes residethere

with human genome sequenced, becoming method ofchoice


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Identifying defect in disease gene

Once disease gene identified, still can be arduoustask identifying actual genetic defect

Mutations in CFTR geneStructure of CFTR gene and

deduced protein


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Ethical Issues

Once genetic defect identified, no treatment options maybe available

Will patients want to know?

Is prenatal screening appropriate?

Will identification of disease gene

affect insurability?

e.g., Hungtingtons disease- mutation due to trinucleotide(CAG) repeat expansion(microsatellite instability)

normal individual (10 to 30 repeats)

affected individual (38 to 120) - increasing length ofpolyglutamine extension appears to correlate with toxicity


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Molecular Medicine

Knowledge of human genome will aid in the

development of molecular diagnostics, gene

therapy, and drug therapy


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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-Hodgkins 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


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Treatment for Genetic Diseases

Treatment strategies

(1) correct metabolic consequences of disease byadministration of missing product or limitingavailability of substrate

e.g.,dietary treatment of PKU

(2) replace absent enzyme or protein or to increase itsactivity

e.g.,replacement therapy forhemophilia

(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


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Gene Therapy

Only somatic gene therapy is permissible inhumans at present

Three theoretical types of gene therapy

replacement- mutant gene removed and replacewith a normal gene

correction- mutated area of affected gene would becorrected and remainder left unchanged

augmentation- introduction of foreign geneticmaterial into cell to compensate for defective

product of mutant gene (only gene therapycurrently available)


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Gene Therapy

Three major routes of delivery of genes into humans(1) retroviruses

foreign gene integrates at random sites onchromosomes, may interrupt (insertional mutagenesis)

the expression of host cell genes replication-deficient

recipient cells must beactively growing forintegration into genome

usually performed ex vivo


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Gene Therapy

(2) adenoviruses

replication-deficient

does not integrate into host cell genome

disadvantage: expression of transgene graduallydeclines requiring additional treatments (maydevelop immune response to vector)

treatment in vivo, vector can be introduced intoupper respiratory tract in aerosolized form

(3) plasmid-liposome complexes


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Gene Therapy

Conclusions based on recent gene therapy trials

gene therapy is feasible (i.e., evidence for expression oftransgene, and transient improvements in clinicalcondition in some cases

so far it has proved safe (only inflammatory or immunereactions directed toward vector or some aspect ofadministration method rather than toward transgene

no genetic disease cured by this method

major problem is efficacy, levels of transgene productexpression often low or transient


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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

targetinggaggene

Double-stranded DNA to form

triplex molecule

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Biochemical Basis Ofdisease