genetic disesase
DESCRIPTION
The Hardy-Weinberg Law states that the genotypes in the population from one generation to another generation will be in equilibriym.TRANSCRIPT
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