Genetic Diseases

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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_at_cheo.on.ca
• 613-737-7600 x 2897

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Objectives

• Develop a basic understanding of the genetic
  apparatus
• Comprehend definitions of major genetic
  abnormalities
• Correlate molecular abnormalities and genetic
  defects

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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

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The Cell
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Nucleus

• DNA arranged in chromosomes
• (network of granules nuclear chromatin)
• RNA spherical intranuclear structure(s)
• - nucleolus / nucleoli

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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

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DNA structure
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DNA replication
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mRNA and tRNA
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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)

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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

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Genome

• ? Sum total of all genes contained in a cells
  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

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Gene Product

• ? A protein or RNA specified by a gene
• ? Transcribed into mRNA in the nucleus
• ? Translated through tRNA and cytoplasmic
  ribosomes into protein

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Human Genome

• ? 3 billion pairs of DNA nucleotides
• ? 50,000 100,000 genes
• ? Protein-coding Genes lt10 (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

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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

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(Mary) Lyon Law
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(Murray) Barr body
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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

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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.

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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.

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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

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Mitosis
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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.

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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

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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)

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Meiosis
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Gametogenesis

• ? Gonads testes, ovaries contain
• ? Precursor cells or germ cells mature into
• ? Gametes sperm, ova in gametogenesis
• ? Spermatogenesis, oogenesis

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Gametogenesis
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Primary follicles
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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)

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Chromosome Analysis
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Karyotype
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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)

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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

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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

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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,

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Congenital / Hereditary Diseases

• ? Congenital present at birth
• ? Hereditary (genetic) result of chromosome
  abnormality or
• defective gene

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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

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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)

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Nondisjunction in meiosis
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• ? 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

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Translocation in gametes
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Sex chromosome abnormalities
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Turner syndrome
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Klinefelter syndrome
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Autosomal abnormalities

• ? Loss of genetic material aborted embryo
• ? Deletion of gene congenital anomalies
• ? Trisomy syndromic, e.g. 21, 18, 13

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Trisomy 21 (Down)
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T21 causes

1. Nondisjunction during gametogenesis (95)
2. Translocation (few)
3. Nondisjunction in zygote (rare)

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Translocation T21
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Zygote nondisjunction T21- Mosaic
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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)

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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)

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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)

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Thalidomide baby
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Prenatal CMV infection
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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)

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Genetically determined variationin rate of
development
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Effect of harmful environmental agents on
susceptibility for congenital malformations
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Interaction of genetic predisposition and
environmental factors in cleft palate
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Prenatal Diagnosis of Congenital Abnormalities

• Examination of fetal cells for chromosomal,
  genetic or biochemical abnormalities
• Examination of amniotic fluid for products
  secreted by the fetus
• Ultrasound of the fetus to detect malformations
  (NTD, hydrocephalus, PCKD, etc)

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Prenatal Diagnosis of Congenital Abnormalities
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Main indications for amniocentesis

1. Maternal age (gt35)
2. Previous infant with T21 or other chromosomal
   abnormality
3. Known translocation T21 carrier
4. Other chromosomal abnormality in either parent,
   e.g. t(721)
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)

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Methods of fetal DNA analysis

• 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

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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

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Translocations, Oncogenes, Tumor suppressor genes

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NB MYCN amplification and 1p deletion by FISH
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NB Double-minute chromosomes by FISH
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RB MYCN probe to detect homogeneously staining
region in metaphase spread and interphase nuclei
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Ewing sarcoma t(1122) EWS green, FLI-1 pink, t
yellow
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E-RMS Spectral karyotypet(13), t(115), t(121)
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Abnormal Fetal Development

• ? Malformation
• ? Deformation
• ? Dysplasia
• ? Disruption

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Prenatal development, pre-embryonic
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Prenatal development, early embryonic
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Prenatal development, late embryonic
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Fetal development
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Normal gametogenesis
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Meiosis
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Abnormal gametogenesis
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? ? gametes
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Sperm penetrating oocyte
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Fertilization
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Causes of human congenital anomalies
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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)

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Malformations

• ? Causally heterogeneous
• ? Intrinsic causes mendelian mutations,
  chromosome abnormalities,
• environmental interactions (multifactorial),
  mitochondrial mutations

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Disruptions

• ? Environmental (exogenous) causes producing
  abnormalities of morphogenetic field dynamics
• ? E.g. rubella, thalidomide, isotretinoin,
  alcohol, etc

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Rubella embryopathy
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Diabetic embryopathy
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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

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Neurofibromatosis
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Tuberous sclerosis
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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)

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Oligohydramnios (Potter) sequence
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Arthrogryposis
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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

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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)

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Syndromes

• ? Patterns of anomalies proven or presumed
  causally related
• ? Causes
• - chromosome mutations
• - imprinting defects
• - aneuploidy
• - multifactorial disorders
• - teratogenic sequences

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Treacher-Collins syndrome(mandibulofacial
dysostosis) AD
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Leprechaunism(defective insulin binding) AR
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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

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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

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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

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Hurler syndrome (MPS 1A) AR
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COH Disorders

• ? Glycogen Storage Diseases
• ? Galactosemia

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Glycogen storage disease type II
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Amino Acid Disorders
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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

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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 Potters Atlas of Fetal and
  Infant Pathology
• ? Crowley An Introduction to Human Disease,
  Pathology and Pathophysiology

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Thank you
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