M.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar

M.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar

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 1-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

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

S P Increased S Decreased PE

Normal Block

Increased X,Y

*E

Inborn Error of Metabolism

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)

Increased phenylalanine Decreased tyrosineBlock

Increased phenylpyruvic acid

*E

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

GluVal, evident that mutation in gene encoding -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

Mutations in CFTR geneStructure of CFTR gene and

deduced protein

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 forintegration 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