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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

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medical biochemistry

Medical Biochemistry

Biochemical and Genetic Basis of Disease

Lecture 77

classes of biomolecules affected in disease
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
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
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
Organelle Involvement
  • Almost every cell organelle has been involved in the genesis of various diseases
different mechanisms similar effect
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
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)
genetic diseases8
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
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
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
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
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 metabolism13

Increased X,Y



S  P

Increased S  Decreased P



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
inborn error of metabolism14
Inborn Error of Metabolism

Increased phenylpyruvic acid


  • 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 tyrosine


genetic linkage studies
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 studies16
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
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 genes18
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

Structure of CFTR gene and deduced protein

Mutations in CFTR gene

Identifying defect in disease gene
  • Once disease gene identified, still can be arduous task identifying actual genetic defect
ethical issues
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
Molecular Medicine
  • Knowledge of human genome will aid in the development of molecular diagnostics, gene therapy, and drug therapy
gene expression in diagnosis
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 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
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 therapy25
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 beactively growing forintegration into genome
      • usually performed ex vivo
gene therapy26
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 therapy27
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
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