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					    Genetic Diseases

         Dr. Joseph de Nanassy
  Associate Professor, PALM, uOttawa
 Chief of Anatomical Pathology, CHEO
Site Chief of Laboratory Medicine, CHEO

       jdenanassy@cheo.on.ca
         613-737-7600 x 2897
              Objectives
☺ Develop a basic understanding of the
 genetic apparatus
☺ Comprehend definitions of major genetic
 abnormalities
☺ Correlate molecular abnormalities and
 genetic defects
                   Outline
I. Definitions
      Genetic code
      Chromosomes, Genes, Cell Division
      Molecular mechanisms
II. Abnormal fetal development
      Malformations, deformations, dysplasias,
      disruptions
III. Perinatal pathology
      Birth defects
      Metabolic disorders
The Cell
                Nucleus
☺ DNA: arranged in chromosomes
 (network of granules = nuclear chromatin)
☺ RNA: spherical intranuclear structure(s)
 - nucleolus / nucleoli
            Genetic Code
☺ A series of messages contained in the
 chromosomes
☺ This code regulates cell functions by way
 of directing the synthesis of cell proteins
☺ The code corresponds to the structure of
 the DNA
☺ The code is transmitted to new cells
 during cell division
DNA structure
DNA replication
mRNA and tRNA
            Chromosomes
☺ Exist in pairs – homologous: 22a + 1s
☺ Composed of double coils of DNA
☺ Basic unit: nucleotide
                    phosphate group
                    deoxyribose sugar
                    base: purine (A, G)
                          pyrimidine (T, C)
                 Genes
☺ A locatable region of genomic sequence,
 corresponding to a unit of inheritance
☺ A union of genomic sequences encoding
 a coherent set of potentially overlapping
 functional products; i.e. genes are one
 long continuum (2007)
☺ Determine cell properties, both structure
 and functions unique to the cell
                Genome
☺ Sum total of all genes contained in a
 cell’s chromosomes
☺ Identical in all cells
☺ Not all genes are expressed in all cells
☺ Not all genes are active all the time
☺ May code for enzymes or other
 functional proteins, structural proteins,
 regulators of other genes
           Gene Product
☺ A protein or RNA specified by a gene
☺ Transcribed into mRNA in the nucleus
☺ Translated through tRNA and
 cytoplasmic ribosomes into protein
             Human Genome
☺ 3 billion+ pairs of DNA nucleotides
☺ ~ 50,000 – 100,000 genes
☺ Protein-coding Genes = <10% (2%) of human genome
☺ Exons: parts of the DNA chain that code for specific
 proteins
☺ Introns: the parts in-between the exons
☺ Both exons and introns are transcribed but only the
 exons are translated (introns are removed from mRNA
 before leaving nucleus)
☺”Junk DNA”: no obvious function but 80% expressed
         Sex chromosomes
☺ Genetic sex = composition of X and Y
☺ Large X: many genes, many activities
☺ Small Y: almost entirely male sexual diff.
☺ Female: XX, male XY
☺ One X randomly inactivated and
 nonfunctional after first week of embryonic
 development
☺ Same inactivated X in descendant cells
(Mary) Lyon Law
(Murray) Barr body
            Y chromosome
☺ Stains with some fluorescent dyes
 - bright fluorescent spot in the nucleus
☺ Normal female: sex chromatin body
 but no fluorescent spot
☺ Normal male: fluorescent spot
 but no sex chromatin body
             Cell Division
☺ Mitosis: somatic cells (PMAT)
 Daughter cells have the same number of
 chromosomes as the parent cell.

☺ Meiosis: gametogenesis (1st and 2nd div)
 Number of chromosomes reduced by half.
              Chromatids
☺ Before mitosis, the DNA chains
 duplicate to form new chromosome
 material.
 The duplicated chromosome material lies
 side by side = two sister chromatids.
 Mitosis = the process by which conjoined
 chromatids separate into sister chromatids
 and move into new daughter cells.
                   Mitosis
☺ Interphase: DNA duplication to form
 chromatids just before mitosis
☺ Prophase: centriole migration, mitotic spindle
☺ Metaphase: chromosomes line up in centre,
 chromatids still joined at centromere
☺ Anaphase: chromosomes separate into sister
 chromatids
☺ Telophase: sister chromatids form new
 chromosomes, new nuclear membranes form,
 cytoplasm divides
Mitosis
                   Meiosis
☺ First meiotic division interphase: duplication of
 chromosomes to form paired chromatids
☺ Prophase 1 of meiosis: homologous
 chromosomes lie side by side over entire length
 = synapse.
 Interchange of segments of homologous
 chromosomes = crossover.
 2 Xs side by side just like the autosomes.
 X and Y end-to-end: no crossover.
                 Meiosis
☺ Metaphase 1: paired homologous
 chromosomes align at the equatorial plate
☺ Anaphase 1: homologous chromosome pairs
 migrate to opposite poles of the cell;
 each chromosome is composed of two
 chromatids, the chromatids are not separated
☺ Telophase 1: two new daughter cells form;
 each contains half the chromosome number =
 reduction of chromosomes by half; interchange
 of genetic material occurred during synapse
                Meiosis
☺ Second meiotic division = mitotic division
Prophase 2: DNA does not replicate
Metaphase 2: chromosomes align at the
  equatorial plate
Anaphase 2: sister chromatids migrate
  separately
Telophase 2: four haploid cells (half the
  normal number of chromosomes)
Meiosis
           Gametogenesis
☺ Gonads: testes, ovaries; contain
☺ Precursor cells or germ cells; mature into
☺ Gametes: sperm, ova; in gametogenesis
☺ Spermatogenesis, oogenesis
Gametogenesis
Primary follicles
Oogenesis vs. spermatogenesis
☺ One ovum (+ 3 polar bodies) vs. four
 spermatozoa
☺ Oocytes formed before birth vs. continuous
 spermatogenesis (‘fresh’ sperm)
 Prolonged Prophase 1 until ovulation –
 more frequent congenital abnormalities in ova of
 older women (longer exposure to potentially
 harmful environmental influences until meiotic
 division resumes at ovulation)
Chromosome Analysis
Karyotype
       Genes and Inheritance
☺ Locus: specific site of a gene on the
 chromosome. Since the chromosomes exist in
 pairs, genes are also paired.
☺ Alleles: alternate forms of a gene can occupy
 the same locus (homozygous, heterozygous)
☺ Recessive gene: expressed only when
 homozygous
☺ Dominant gene: expressed whether
 homozygous or heterozygous,
 both expressed when co-dominant
☺ Sex-linked gene: only X-linked in males,
 most are recessive, hemizygous (no allele on Y)
           Gene Imprinting
☺ Genes occur in pairs on homologous
 chromosomes, one from each parent
☺ Different effects of gene whether ♀ or ♂
☺ Genes modified during gametogenesis
☺ Gene imprinting: additional methyl
 groups added to DNA molecules
☺ Basic structure unchanged;
 in some diseases different expression
 (behaviour) depending on parent of origin:
 hereditary disease as a result of imprinting
       Genetic Engineering
☺ Insertion of a gene encoding a desired
 product (e.g. insulin) into a bacterium
☺ Bacterial gene spliced enzymatically,
 recombinant DNA inserted into plasmid
 (circular DNA segment in bacterium),
 dividing bacterial population produces
 desired protein
           Gene Therapy
☺ Normal gene inserted into defective cell
☺ Compensates for the missing or
 dysfunctional gene, in somatic cells only
☺ Can be inserted into mature cell (ly)
☺ Can be inserted into stem cell (bone
 marrow)
☺ Used to treat e.g. ADA deficiency, CF, …
 Congenital / Hereditary Diseases
☺ Congenital: present at birth
☺ Hereditary (genetic): result of
 chromosome abnormality or
 defective gene
     Causes of malformations
1. Chromosomal abnormalities
2. Gene abnormalities
3. Intrauterine injury (e.g. drugs, radiation,
   infection, environmental, etc)
4. Environmental effect on genetically
   predisposed embryo
   Chromosomal abnormalities
☺ Nondisjunction: failure of homologous
 chromosomes in germ cells to separate
 from one another during 1st or 2nd meiotic
 division
☺ Sex chromosomes or autosomes
☺ Extra chromosome: trisomy (24 or 47)
   Absent chromosome: monosomy (22 or
 45)
Nondisjunction in meiosis
☺ Chromosome Deletion: Broken piece of
 chromosome is lost from cell
☺ Translocation: Not lost, just misplaced
 and attached to another chromosome
 - reciprocal: between two nonhomologous
 chromosomes (no loss or gain of genetic
 material - no loss of cell function)
 - in germ cells: deficient or excess
 chromosome material – abnormal zygote
Translocation in gametes
Sex chromosome abnormalities
Turner syndrome
Klinefelter syndrome
     Autosomal abnormalities
☺ Loss of genetic material: aborted embryo
☺ Deletion of gene: congenital anomalies
☺ Trisomy: syndromic, e.g. 21, 18, 13
Trisomy 21 (Down)
             T21 causes
1. Nondisjunction during gametogenesis
   (95%)
2. Translocation (few)
3. Nondisjunction in zygote (rare)
Translocation T21
Zygote nondisjunction T21- Mosaic
    Abnormal gene diseases
☺ Individual gene abnormalities
☺ Hereditary diseases transmitted mostly
 on autosomes, only a few on sex
 chromosomes.
☺ Gene mutation: spontaneous
                    environmental
☺ Minor structural change may result in
 major functional abnormality (e.g. SCD:
 HgbSA, co-dominant, Hgb beta gene)
        Modes of Inheritance
☺ Autosomal dominant (a dominant gene
 expressed in the heterozygous state)
☺ Autosomal recessive (expressed only in
 homozygous individual, disease only if both
 alleles are abnormal, carrier if only one abN)
☺ Codominant (full expression of both alleles in
 heterozygous state)
☺ X-linked (usually affects male offspring; the
 abnormal X-linked gene acts as dominant gene
 when paired with the Y chromosome)
          Intrauterine Injury
1. Drugs: thalidomide (phocomelia), DES
    (cervical cancer), street drugs (IUFD),
    smoking (IUGR), alcohol (FAS), etc
2. Radiation: x-rays
3. Maternal infections:
    - Rubella virus (CVS, CNS, chr. infection)
    - CMV (microcephaly, chronic infection)
    - Toxoplasma gondii (hydrocephalus,
                           systemic infection)
Thalidomide baby
Prenatal CMV infection
     Multifactorial Inheritance
☺ Combined effect of multiple genes
 interacting with environmental agents,
 e.g. cleft palate, cardiac malformations,
 club foot, hip dislocation, spina bifida, etc
☺ Cause: developmental sequence fails to
 reach a certain point at an appropriate
 time (threshold)
Genetically determined variation
    in rate of development
Effect of harmful environmental agents
           on susceptibility for
       congenital malformations
Interaction of genetic predisposition
     and environmental factors
            in cleft palate
         Prenatal Diagnosis
     of Congenital Abnormalities
1. Examination of fetal cells for
   chromosomal, genetic or biochemical
   abnormalities
2. Examination of amniotic fluid for products
   secreted by the fetus
3. Ultrasound of the fetus to detect
   malformations (NTD, hydrocephalus,
   PCKD, etc)
    Prenatal Diagnosis
of Congenital Abnormalities
Main indications for amniocentesis
1. Maternal age (>35)
2. Previous infant with T21 or other
   chromosomal abnormality
3. Known translocation T21 carrier
4. Other chromosomal abnormality in either
   parent, e.g. t(7;21)
5. Risk of genetic disease in the fetus that
   can be detected prenatally (thalassemia)
6. Previous infant born with neural tube
   defect (multifactorial inheritance, ~5%)
 Methods of fetal DNA analysis
1. Enzyme analysis of DNA: resultant
   DNA fragments different in health and
   disease, e.g. sickle cell anemia
2. DNA probes: same complementary
   nucleotide arrangement as in defective
   DNA gene – binds to mutant gene
       Molecular Genetics of
       Solid Pediatric Tumors

☺ Mechanisms for tumor development
 1. Creation of novel fusion proteins
 2. Loss of tumor suppressor genes
 3. Activation of proto-oncogenes
Translocations, Oncogenes,
 Tumor suppressor genes
NB: MYCN amplification and
    1p deletion by FISH
NB: Double-minute chromosomes
           by FISH
      RB: MYCN probe to detect
   homogeneously staining region in
metaphase spread and interphase nuclei
  Ewing sarcoma: t(11;22)
EWS green, FLI-1 pink, t yellow
E-RMS: Spectral karyotype
   t(1;3), t(1;15), t(1;21)
Abnormal Fetal Development
☺   Malformation
☺   Deformation
☺   Dysplasia
☺   Disruption
Prenatal development, pre-embryonic
Prenatal development, early embryonic
Prenatal development, late embryonic
Fetal development
Normal gametogenesis
Meiosis
Abnormal gametogenesis
♂ & ♀ gametes
Sperm penetrating oocyte
Fertilization
Causes of human congenital anomalies
            Malformations
☺ Intrinsic abnormalities of blastogenesis
 and organogenesis affecting the
 morphogenetically reactive fields of the
 embryo = developmental field defects
☺ Occur alone or in combination
 (syndromes or associations)
☺ Severe (spina bifida aperta) or
 mild (spina bifida occulta)
            Malformations
☺ Causally heterogeneous
☺ Intrinsic causes: mendelian mutations,
 chromosome abnormalities,
 environmental interactions (multifactorial),
 mitochondrial mutations
             Disruptions
☺ Environmental (exogenous) causes
 producing abnormalities of morphogenetic
 field dynamics
☺ E.g. rubella, thalidomide, isotretinoin,
 alcohol, etc
Rubella embryopathy
Diabetic embryopathy
              Dysplasias
☺ Disturbances of histogenesis, occurring
 later and somewhat independently of
 morphogenesis
☺ Morphogenesis is prenatal,
 histogenesis continues postnatally in all
 tissues that have not undergone
 end differentiation
☺ Dysplasias may predispose to cancer
Neurofibromatosis
Tuberous sclerosis
             Deformities
☺ Secondary changes in form or shape of
 previously normally formed organs or body
 parts
☺ Caused by extrinsic forces (e.g. Potter
 syndrome) or intrinsic defects (e.g.
 fetal akinesia syndrome with congenital
 arthrogryposis)
Oligohydramnios (Potter) sequence
Arthrogryposis
              Sequences
☺ Secondary consequences of
 malformations, disruptions, dysplasias, or
 deformities
☺ E.g. renal adysplasia leads to Potter
 oligohydramnios sequence
   DiGeorge anomaly leads to tetany,
 hypoparathyroidism, heart failure,
 conotruncal congenital heart defect
          Minor Anomalies
☺ Disturbance of phenogenesis in fetal life
☺ Phenogenesis: the process of attaining
 final quantitative anthropometric traits of
 the race and family (variant familial
 developmental pattern)
☺ Causes:
     intrinsic (chromosome imbalance)
     extrinsic (teratogens)
             Syndromes
☺ Patterns of anomalies proven or
 presumed causally related
☺ Causes:
 - chromosome mutations
 - imprinting defects
 - aneuploidy
 - multifactorial disorders
 - teratogenic sequences
  Treacher-Collins syndrome
(mandibulofacial dysostosis) AD
       Leprechaunism
(defective insulin binding) AR
                Associations
☺ Idiopathic multiple congenital anomalies of
 blastogenesis
     Vertebral anomalies                  V
     Anorectal anomalies                  A
     TracheoEsophageal defects            TE
     Radial and Renal defects             R
☺ Single hit during gastrulation affecting multiple,
 morphogenetically closely related structural
 primordia
       Metabolic Disorders
☺ Most are inherited as AR, some are
 X-linked, a few are AD.
☺ Great variability in presentation
☺ Some present with dysmorphic features
☺ Storage material in RES and other
 tissues
         Storage Diseases
☺ Lysosomal Lipid Storage Diseases
   Nieman-Pick: sphyngomyelin
   Gaucher disease: glucocerebrosidase
   Tay-Sachs disease: Gangliosidoses
   Metachromatic leukodystrophy
☺ Mucopolysaccharidoses (I, II, III, VII)
   glycosaminoglycans and glycolipids
Hurler syndrome (MPS 1A) AR
          COH Disorders
☺ Glycogen Storage Diseases
☺ Galactosemia
Glycogen storage disease type II
Amino Acid Disorders
                 Misc.
☺ Fatty Acid Beta-Oxidation Defects
   (LCAD, MCAD, SCAD)
☺ Organic Acidemias
☺ Defects in Purine Metabolism
☺ Carnitine Deficiency
☺ Peroxisomal Disorders
☺ Disorders in Metal Metabolism
☺ Defects in Copper Metabolism
             References
☺ Wigglesworth: Textbook of Fetal and
 Neonatal Pathology
☺ Moore, Persaud: The Developing Human
☺ Perspectives in Pediatric Pathology,
 Volume 21, Society for Pediatric Pathology
☺ Gilbert-Barness: Potter’s Atlas of Fetal
 and Infant Pathology
☺ Crowley: An Introduction to Human
 Disease, Pathology and Pathophysiology
Thank you

				
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