Title: Mechanisms of Teratogenesis
1Mechanisms of Teratogenesis
Lynda B. Fawcett, Ph.D. Assistant Professor of
Pediatrics Thomas Jefferson University Alfred I
duPont Hospital For Children Wilmington, DE
2Structural Anomalies/Dysmorphology
- Malformation the normal developmental process is
altered such that a given structure cannot form
or forms improperly, the error is intrinsic to
the morphogenetic process itself. - Deformation alterations to already existing
structures. Alterations in body shape, form, or
position. Often due to uterine constraint
(twinning, oligohydramnios etc). ex dislocation
of the hip, clubfoot, and some facial anomalies - Disruption alterations to already existing
structures, often involving extensive destruction
of tissue, and/or prevention of later tissue or
organ formation. Can arise from mechanical or
physiological factors. ex amniotic band
syndrome, fetal vascular occlusion, placental
emboli, localized or general hypoxia, vascular
insults.
3Disruption Syndactyly/adactyly
Hypoxia, hemorrhage, necrosis ROS Usually
asymmetrical
EX Cocaine CVS
4Mechanism of action
Mode of Action
Outcomes
cell death altered cell-cell interactions cell
signaling reduced biosynthesis impaired cell
migration reduced/impaired proliferation alter
ed differentiation
ROS/Lipid peroxidation DNA damage Altered gene
expression Metabolic disturbance Enzyme
inhibition Growth factor imbalance Receptor
agonist/antagonist
Malformation Deformation Death Functional
deficit Growth retardation
Pathogenesis
5Cytotoxic Teratogens
- Irradiation
- 2Deoxyadenosine
- 4-CP (metabolites)
- Hyperthermia
- Ethanol
Whether a toxicant induces cell death depends on
dose and timing of exposure as well as the nature
of the toxicant
6Apoptosis
Necrosis
The blebs fuse and become larger, no organelles
in blebs
The nucleus begins to break apart, DNA breaks
into small pieces organelles in to blebs
Blebs
The cell membrane ruptures and releases the
cells contents, the organelles are not
functional
The cell breaks into several apoptotoc
bodies the organelles are still functional
7Apoptosis
SIGMA-ALDRICH
Receptor mediated - Extrinsic (TNF Signaling)
Mitochondrial Intrinsic CytC, caspase 9
8Characteristics of Cell Death Mediated
Teratogenesis Replication associated cytotoxicity
- Tissue susceptibility changes over time, however
usually rapidly dividing undifferentiated tissues
are most affected with agents that induce direct
cytotoxicity - Tissue sensitivity greatest during organogenesis
in the neuroepithelium, limb buds and least in
heart and yolk sac - Characteristic malformations include NTDs,
anopthalmia, limb defects but, generally, not
heart defects (time dependent). - Occurs because
- DNA more accessible in S phase
- Defects manifest sooner, critical mass, windows
of oportunity - Molecular mechanisms
9Tissue sensitivity Irradiation/mouse
studies/organogenesis
- Molecular basis for tissue sensitivity may be due
to differential expression of P53 in tissues - Temporal/spacial expression of P53 mirrors that
of tissues most effected by cytotoxic teratogens - Irradiation caused activation of P53 in rapidly
proliferating, undifferentiated tissues (used a
lac Z reporter model). Neuroepithelium, limb
buds, arches (Gottli et al, 1997)
10(No Transcript)
11P53 KO studies
Consistent findings / apoptosis, cell cycle
check G1/S /- apoptosis and necrosis -/-
necrosis, no cell cycle check (M,S)
12Replication associated cytotoxicity
Open NT, reduced tissues Anopthalmia Growth
retardation
Control 11.5 d
AA deficiency (Met)
13Demethylating agents. Whole Mount Immunostaining
165 kD NF
Control
-AA (Met)
CL (-SAM)
NTD only ?165 Kd NF Mechanism probably more
specific, not general cytotoxicity
NTD GR, Reduced Size Protein Somite Pairs
limb buds Anopthalmia No heart defects
14Effects of Homocysteine on Rat Embryos Cultured
in Vitro from 9.5-11.5 p.c Cardiac Defects
Control
Hcy
Hcy
Control
15Effects of Hcy on Rat Embryos Cultured in Vitro
from 9.5-11.5 p.c Somite/Vertebral/Arch
Abnormalities (450 ?g/ml)
control
16- Although defects such as NTDs may result from
death, other mechanisms can also result in NTD - Specific mechanisms of action
- Receptor mediated - directly or indirectly
- Display some type of ligand specificity (enzyme)
- downstream alterations in cell signaling (growth
factors differentiation) - altered expression of key genes involved in
morphogenesis. - Closure defects have discrete window of
susceptibility, and less chance for recuperation.
for NTD reduced proliferation in neuroepithelium,
failure of neural fold elevation, failure of
neural fold fusion due to specific mechanisms may
resemble cytotoxicity at term
17Tissue specificity and TeratogenesisSpecific
Mechanism Usually receptor mediated
- Only affect tissues bearing appropriate
receptors - ex sex steroids - genitourinary
18Receptor-Mediated Teratogenesis
19Retinoic Acid
- Accutane 13-cis retinoic acid, used for
dematological and oncology therapies
RAE Spectrum of defects and potential for all
organ systems to be affected Most common defects
are craniofacial, external ear (BA 1-2)
20Specificity Teratogenesis RA
- Vitamin A derivatives involved in signaling
pathways during normal development - Morphogen
directly alter/control developmental programs. - 1) Induces differentiation or apoptosis of
various cell types breast carcinoma, prostate
cancer, AL - 2) Produces homeotic mutations in experimental
animals (limb duplications) - Proposed Mode of action for defects
- Promotes excessive cell death in regions
undergoing programmed cell death (Alles Sulik,
89, 90 etc) - Alters specification of tissues (NC) HOX genes
- Alters expression of other RARE genes
21RA receptors
- RAR ??? (all trans and 9-cis) RXR???(9 cis)
- Multiple isoforms exist for all RAR
- RAR/RXR form heterodimers RXR homodimers,
heterodimers with thyroid hormone R, peroxisome
proliferators and others - RAR, RXR differentially expressed in embryo
temporal and spatial pattern - RA interacts with other receptors (CRABP)
22CRABP1
RA
RA
RA
RA
RA
RAR/RXR
CRABP-II
(???)
RA
Other involvment
CRABP1, II
Biosynthesis RA ADH - ETOH hypervitamitosis vitami
n deficiency
Transcription RARE
AP-1 transcription factor block
RAR/RXR
Nucleus
23- RAR? -/- Vitamin A deficiency, Resistant to RA
induced limb defects. Agonists
RAR?gtRAR?gtRAR? for inducing limb defects
in wild type -
- RAR? -/- Normal, Resistant to BA 1-2 fusion
by RA - / RAR? agonist caused BA fusions
- RAR? upregulated in limbs following RA
- RAR? -/- Vitamin A deficiency defects, normal
craniofacial and posterior axial skeleton
resistant to RA induced posterior axial
skeleton truncation but not cranial, partial
resistance to other except limb - RXR? -/- Lethal, cardiac. Resistant to RA induced
limb defects
Limb
Craniofacial
Posterior
Cardiomorphogenesis
(Matt et al., 2003 Collins and Mao, 1999)
24Chromatin condensation suppresses RAREs (RARß2)
VPA
Potential for increased RARß2 destabilization of
chromatin Inappropriate expression of other
genes (p21 p53 cell death)
RARE
25Summary
- Teratogenic mechanisms can be broadly separated
into specific (receptor mediated) and
non-specific (cell death) - Teratogens can have both specific and
non-specific mechanisms (alcohol) - Even general cytotoxic/genotoxic teratogens may
have a molecular component that alters tissue
susceptibility - Initial molecular mechanisms have the potential
to result in myriad responses cell death,
differentiation, altered signaling. Pathogenesis
must be considered when evaluating the action of
teratogens - Understanding the molecular mechanisms of
teratogens furthers our understanding of
development, and allows for the development of
therapeutics with reduced chance of
teratogenicity