Teratogenesis
https://www.youtube.com/watch?v=iN6uQYQ-hRU
30 min video on several teratogens- basic information
Teratogenesis
Learning objectives
1. Apply Wilson’s principles of teratology to understand what makes a teratogen.
2. Understand the different mechanisms by which teratogens exert their effects
on the fetus.
3. Know why teratogens act in specific windows of susceptibility.
4. Appreciate that different teratogens have different mechanisms and often act
through several different pathways.
5. Understand why commonly used substances such as alcohol and medications
can also be teratogenic.
The problem
• Teratogens – agents that cause non-heritable birth defects.
Teratogenesis
• The environment not only gives signals for normal development
but it also gives signals that can disrupt normal development.
• 2-5% of infants are born with anatomical abnormalities which are
caused by either genetics, the environment or a combination of
the two.
• Testing for teratogenicity is expensive and may not be transferable
to humans.
Teratogenesis
Wilson’s principles of teratology
1. Susceptibility to the teratogenic effect of an agent depends on
the genotype of the embryo, the genotype of the mother, and the
ways in which their genotypes allow mother and fetus to interact
with the adverse environmental factors.
2. There are critical periods of development when specific organ
systems are most susceptible to being adversely affected by
teratogenic agents.
3. Teratogenic agents act in specific ways on genes, cells and tissues
in the developing organism to interfere with normal developmental
events.
Wilson’s principles of teratology
4. Several factors affect the ability of a teratogen to interfere with
normal development. These include the nature of the agent itself,
the route and degree of maternal exposure, the ability of the
mother to detoxify or block the agent, the rate of transfer through
the placenta, the rate of absorption by the embryo or fetus, and the
genotype of the mother and her conceptus.
5. There are four major manifestations of abnormal development:
death, malformation, growth retardation, and functional defects.
6. Manifestations of deviant development increase in frequency and
degree as the teratogen dosage increases.
Teratogenesis - definitions
• Congenital anomaly – birth defect- can be structural or functional
• Functional = intellectual (cognitive), emotional physiological
• Structural = malformations, disruptions, deformations or dysplasias
Teratogenesis-definitions
• Malformation – structural birth defect which results from failure
of tissue to initially form properly.
• Disruption – Breakdown of a tissue that has initially formed
properly.
• Deformation- extrinsic mechanical forces on otherwise normal
tissue.
• Dysplasia – A lack of normal organization of cells in a tissue.
Teratogenicity
• Most teratogens produce structural defects only during certain
critical periods of development.
• Embryonic period – Conception to 8 weeks
• Fetal period – remaining time in utero
Dr. Ron Saulnier
HSS 4102
Fall 2020
• Rare to have congenital anomalies before 3 weeks because
teratogens damage either too many cells and it dies or too few cells
and it recovers (pluripotent cells).
• Maximum susceptibility is between 3-8 weeks when organs start to
form. Except for CNS which forms continuously until adolescence.
Windows of susceptibility
Types of teratogens
Mechanical forces
Teratogens
1.
2.
3.
4.
5.
Thalidomide
Mercury
Alcohol
Retinoic acid
Other teratogenic substances
Thalidomide History
• Thalidomide is a drug that was developed in the 1950s by the West
German pharmaceutical company Chemie Grünenthal GmbH.
• It was originally intended as a sedative or tranquiliser, but was soon
used for treating a wide range of other conditions, including colds,
flu, nausea and morning sickness in pregnant women.
• In the 1960s, two medical professionals; Dr Widukind Lenze and Dr.
William McBride, observed an association between the use of
thalidomide in expecting mothers and congenital malformations.
Thalidomide
• Doctors were slow to make this connection due to the wide range
of changes to foetal development. Limbs, internal organs including
the brain, eyesight and hearing could all be affected.
• Another reason why it took so long to establish the link to
thalidomide was that some of the damage caused by the drug was
very similar to certain genetic conditions that affect the upper or
lower limbs.
• It did not show teratogenic effects in rodents.
https://www.sciencemuseum.org.uk/objects-andstories/medicine/thalidomide#:~:text=Thalidomide%20is%20a%20drug%20that,morning%20sickness%20in%20pregnant%20women.
Thalidomide – window of susceptibility
Figure 5.1 During the early 1960s, doctors (primarily in Europe) began prescribing the drug
thalidomide to pregnant women as an effective mild sedative and remedy for morning sickness.
Thalidomide – window of susceptibility
Thalidomide- Effects
• Phocoemelia - a congenital deformity whereby the hands and feet are
bound to the child’s trunk, absent or grossly underdeveloped;
• Disfigurements of the ear;
• Ocular abnormalities;
• Facial palsies;
• Internal organ damage;
• Increase in the number of miscarriages
• Congenital heart disease.
• Caused phocomelia in over 7000 infants – could occur with one tablet.
• Over 80% of children born to mothers who took thalidomide had limb
defects.
• Only effective as a teratogen between days 20-36 after conception.
Thalidomide
• Thalidomide is a very complex molecule, requiring metabolic
breakdown to achieve activity and forming potentially over 100 byproducts. The major functions of these by-products are antiinflammatory or antiangiogenic.
• Blood vessels permit paracrine factor pathways necessary for limb
bud formation.
• NO promotes angiogenesis and protects from the effects of
thalidomide.
Thalidomide
• Metabolized into several potentially teratogenic or nonteratogenic metabolites.
• Acts primarily through blocking angiogenesis.
• Once smooth muscles cover the newly formed vessels,
thalidomide has no effect.
• Thalidomide also displays immunosuppressive activity. It inhibits
release of tumor necrosis factor-alpha from monocytes, and
modulates other cytokine action.
Thalidomide
Thalidomide
Figure 5.3 Thalidomide causes limb malformations by
interfering with blood vessel formation in limb buds
Current uses for thalidomide
• Thalidomide is still a valuable drug for cancer treatment.
• Also used in the treatment of some AIDS related conditions,
leprosy, multiple myeloma, and cancers, as well as Crohn's disease,
HIV, and others.
Thalidomide R vs S
Thalidomide story
Mercury poisoning
https://www.medicalnewstoday.com/articles/320563.php
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Teratogenic agents - Methylmercury
Heavy metal teratogens: industrial mercury and Minamata disease
• Release of mercury compounds caused neurological abnormalities
in nearly 10% of the children born near the area of Minamata Bay in
Japan in the 1950’s.
• A industrial plant producing acetaldehyde was releasing mercuric
sulfate into the bay which was metabolized by microbes into
methylmercury.
• Methylmercury concentrated in the shellfish.
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Methylmercury
• Cats were dying, crows fell from the sky, fish were floating and some
people had trouble seeing, hearing and swallowing.
• For pregnant women, mercury was selectively absorbed in regions of
the developing cerebral cortex.
• Mice given Hg during pregnancy had pups with small brains or eyes.
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Methylmercury toxicity
• During the third week of gestation, the human nervous system
begins to form in the embryo.
• Methylmercury has toxic effects on the nervous system during
embryonic development.
• Methylmercury readily crosses the placenta to the fetus, where
deposition within the developing fetal brain can occur.
• In the brain, methylmercury causes focal necrosis of neurons and
destruction of glial cells and is toxic to the cerebral and cerebellar
cortex.
• In children, defects due to methylmercury can result in deficits in
attention, behavior, cognition, and motor skills.
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Minamata disease
This video is part of the class notes
Figure 5.4
Methylmercury and
Minamata syndrome
1.Protein inhibition
2.Disruption of mitochondria function
3.Direct affect on ion exchange in a neuron
4.Disruption of neurotransmitters
5.Destruction of the structural framework
of neurons
Mercury during development
•
•
•
•
Methylmercury is especially dangerous to developing babies.
Highly toxic and can cross the placenta and the blood-brain barrier.
Concentrated in the brain of the developing fetus.
Children exposed to mercury may be born with symptoms
resembling cerebral palsy, spasticity and other movement
abnormalities, convulsions, visual problems and abnormal reflexes.
• The brains of children who have died as a result of mercury
poisoning show neuron loss in the cerebellum and throughout the
cerebral cortex.
• Mercury also appears to affect brain development by preventing
neurons from finding their appropriate place in the brain.
Mercury – mechanism of action
• Mercury has a strong affinity for sulfur, and mercury's primary mode
of toxic action in living organisms is thought to be the interference of
enzyme function and protein synthesis by binding to sulfhydryl or
thiol groups.
Alcohol
Alcohol as a teratogen
• Probably the most devastating teratogen in humans
• 1- in 750 births are affected in the US, may be as high as 1-100 in
some regions.
• Produces a syndrome of birth defects – FAS
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Fetal alcohol syndrome
Fetal alcohol syndrome
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Fetal alcohol syndrome
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Alcohol affected
Normal
Figure 5.6 Effects of alcohol on developing brains
Alcohol
• Also have intellectual defects and behavioural abnormalities.
• Behavioural changes can exist in the absence of gross physical
changes.
• Effects depend on the dosage and the developmental stage at time
of exposure.
• No safe level of alcohol during pregnancy.
• Mice have been used as a model for FAS and show effects at many
developmental stages.
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Alcohol
Figure 5.7 Alcohol-induced craniofacial and brain anomalies in mice
Alcohol
• There may be several different mechanisms including migration of
neural crests cells – prematurely initiate their differentiation into
facial bones.
• Genes related to the cytoskeleton, which affect migration, are
affected by alcohol.
• Cells forming median portion of the forebrain, upper midface and
cranial nerves may be killed by alcohol.
• Cell death may be caused by formation of superoxide free radicals –
antioxidants help prevent the effects of alcohol.
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Alcohol
Control
Ethanol treated
Ethanol + superoxide
dismutase
Figure 5.8 Cell death caused by ethanol-induced superoxide radicals is a possible
mechanism producing fetal alcohol syndrome
Neural Crest Cells
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Alcohol
• Both ethanol and acetaldehyde modify the intermediary
metabolism of carbohydrates, proteins, and fats.
• Both also decrease the transfer of amino acids, glucose, folic acid,
zinc, and other nutrients across the placental barrier, indirectly
affecting fetal growth due to intrauterine nutrient deprivation.
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Alcohol
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Alcohol
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Alcohol
• Alcohol also down-regulates sonic hedgehog (required
for formation of brain facial skeleton and organogenesis)
• Alcohol can lock neuroblasts into an undifferentiated state by
downregulating Sox5 and Ngn1 genes.
• Alcohol can induce an increase in DNA methyltransferases activity
but not histone transacetylase activity.
• Alcohol may affect the cell adhesion protein L1.
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Figure 5.9 The inhibition of
L1-mediated cell adhesion by
ethanol is another possible
factor in fetal alcohol
syndrome
Alcohol and the developing brain
• The brain undergoes significant structural and functional changes between
childhood and adolescence, with maturation continuing in early adulthood.
• Two important neurotransmitter systems that undergo substantial changes
during adolescence and are affected by alcohol consumption are dopamine
and gamma-aminobutyric acid (GABA).
Functions of Dopamine
•movement
•memory
•pleasurable reward
•behavior and cognition
•attention
•inhibition of prolactin production
•sleep
•mood
•learning
Alcohol and the developing brain
• GABA is the primary inhibitory neurotransmitter in the brain— it
represses the activity of other brain cells. Alcohol generally enhances
the effects of GABA on its receptors, leading to feelings of relaxation
and sleepiness.
• Gamma-aminobutyric acid (GABA) is the primary inhibitory
neurotransmitter known to counterbalance the action of the
excitatory neurotransmitter glutamate.
• A system using the neurotransmitter glutamate also appears to
undergo changes during adolescence. Glutamate interacts with
several receptors, including one called the NMDA receptor.
Alcohol and the developing brain
Difficulties learning new information
• One part of the brain that is affected by alcohol is the hippocampus. The
hippocampus is a sea horse shaped area deep inside your brain that is
responsible for learning and memory.
• Alcohol can damage or even destroy the cells that make up the
hippocampus, which is why some people experience fuzzy memories or
‘blackouts’ after drinking. Studies have shown that adolescents who drink
heavily and often can actually have a smaller hippocampus than their peers.
• Damage done to your hippocampus during adolescence can affect your
brain’s potential to learn and remember new things for the rest of your life.
Teratogenic agents
Retinoic acid
• It is an important compound involved in development but can
disrupt it if it is in the wrong amount or at the wrong times.
• RA is important for formation of the anterior-posterior axis in
mammals and for heart and jaw formation.
• If present in high amounts, cells usually not exposed to it will
respond to it.
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Retinoic acid
• Isotretinoin (13-cis-retinoic acid) Accutane was used for treatment of
acne in the early 80’s.
• Of 59 pregnant women exposed to Accutane
• 26 had no noticeable anomalies
• 12 aborted spontaneously
• 21 were born with anomalies
• The main known teratogenic effects of retinoids are face, skull,
cardiovascular, nervous system and thymic abnormalities.
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RA treated
Control
Figure 5.10 Effects of retinoic acid (RA) on mouse embryos
Retinoic acid
• RA can disrupt development through different mechanisms.
• Alters expression of HOX genes (genes involved in anteriorposterior axis and neural crests cells).
• Binds to neural crests cells and inhibits migration.
• Activates RA degrading enzymes causing a deficiency – which
causes similar anomalies.
Teratogenic agents
Retinoic acid and public health
• RA is a significant public concern because women of childbearing age are the
ones using acne meds.
• Excess vit A supplementation > 1000 IU/day had a 2% increase of malformations.
• Women require a pregnancy test before taking Accutane.
• The herbicide Glyphosate may upregulate the activity of endogenous RA.
• Phenotypes produced by Glyphosate based herbicides are mainly a
consequence of the increase of endogenous retinoid activity. This is consistent
with the decrease of Sonic hedgehog (Shh) signaling from the embryonic dorsal
midline, with the inhibition of otx2 expression and with the disruption of
cephalic neural crest development.
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Other teratogenic agents
Tobacco smoking
• Retards the growth of human fetuses and increases risk of fetal and newborn
death.
• Nicotine may damage fetal brain and lungs during development.
• Nicotine induces abnormalities in synapse formation and cell survival.
• Carbon monoxide in tobacco smoke can keep the developing baby from getting
enough oxygen.
• Tobacco smoke also contains other chemicals that can harm unborn babies.
Am J Respir Crit Care Med. 2016 Mar 1; 193(5): 486–494.
The Role of Nicotine in the Effects of Maternal Smoking during Pregnancy on Lung Development and
Childhood Respiratory Disease. Implications for Dangers of E-Cigarettes.
Eliot R. Spindel1 and Cindy T. McEvoy2 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4824926/
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Other teratogenic agents
•
•
•
•
•
Nicotine may alter the metabolism of fetal lung cells.
Increases the risk of lung problems later in life.
Lowers sperm in smoking men (>4 cigarettes/day).
May affect methylation pattern of tumor suppressor genes.
Teenagers of moms who smoked during pregnancy had increased methylation
of BDNF genes.
• By binding to nicotinic acetylcholine receptors in
the brain, nicotine elicits its psychoactive effects
and increases the levels of several
neurotransmitters in various brain structures –
acting as a sort of "volume control".
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Other teratogenic agents
Marijuana
• Cannabinols may impair testes function in rats by lowering
gonadotropin-stimulating hormone synthesis.
• No real evidence of teratogenicity but may have effects on the brain
which are only observed later in life.
• Cannabinoids altered migration of neuroblasts in rat cerebral cortex
which may explain cognitive and memory decline in the adult rats.
• Marijuana may be a subtle teratogen that alters brain development.
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Endocannabinoid system
• The cannabinoid receptors are G protein-coupled receptors that
are activated by endocannabinoids or exogenous agonists such as
tetrahydrocannabinol.
• Cannabinoid receptors are found in the parts of the brains
involved in emotional and behavioral reactions, homeostasis,
learning, memory, and decision-making.
• THC may help reduce anxiety however, the use of cannabis in
treating anxiety disorders may have adverse effects, such as
addiction and cognitive impairment.
• It is also consistent with animal experiments showing THC’s ability
to "prime" the brain for enhanced responses to other drugs.
Alcohol and nicotine also prime the brain for a heightened
response to other drugs.
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Endocannabinoid system
• The EC system communicates its messages “backward.” When the postsynaptic neuron is
activated, cannabinoids are made “on demand” from lipid precursors (fat cells) already present
in the neuron. Then they are released from that cell and travel backward to the presynaptic
neuron, where they attach to cannabinoid receptors.
• They can control what happens next when these cells are activated. In general, cannabinoids
function like a “dimmer switch” for presynaptic neurons, limiting the amount of
neurotransmitter (e.g., dopamine) that gets released.
• When a person smokes marijuana, THC overwhelms the EC system, quickly attaching to
cannabinoid receptors throughout the brain and body. This interferes with the ability of natural
cannabinoids to do their job of fine-tuning communication between neurons, which can throw
the entire system off balance.
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Cannabinoid receptors, located throughout
the body, are part of the endocannabinoid
system, which is involved in a variety of
physiological processes including appetite,
pain-sensation, mood, and memory.
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Long-term effects
• Rats exposed to THC before birth, soon after birth, or during adolescence show
notable problems with specific learning and memory tasks later in life.
• Cognitive impairments in adult rats exposed to THC during adolescence are
associated with structural and functional changes in the hippocampus.
• Frequent use starting in adolescence was associated with a loss of an average of
6 or up to 8 IQ points measured in mid-adulthood. Those who used marijuana
showed a significant decline in general knowledge and in verbal ability.
• Over time THC can change how the EC system works, which can lead to
problems with memory, addiction, and mental health.
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Prescription drugs
Category A
Adequate and well controlled studies have failed to demonstrate a risk to the fetus
in the first trimester of pregnancy (and there is no evidence of risk in later
trimesters).
Category B
Animal reproduction studies have failed to demonstrate a risk to the fetus and
there are no adequate and well-controlled studies in pregnant women.
Category C
Animal reproduction studies have shown an adverse effect on the fetus and
there are no adequate and well-controlled studies in humans, but potential
benefits may warrant the use of the drug in pregnant women despite potential
risks.
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Prescription drugs
Category D
There is positive evidence of human fetal risk based on adverse reaction data
from investigational or marketing experience or studies in humans, but potential
benefits may warrant use of the drug in pregnant women despite potential risks.
Category X
Studies in animals or humans have demonstrated fetal abnormalities and/or there
is positive evidence of human fetal risk based on adverse reaction data from
investigational or marketing experience, and the risks involved in use of the drug
in pregnant women clearly outweigh potential benefits.
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Prescription Drugs
Mechanisms
• A review of the literature by van Gelder et al, 2010 for mechanisms
causing major structural defects caused by prescription drugs
commonly taken by women of reproductive age found the
following mechanisms.
Dr. Ron Saulnier
HSS 4102
Fall 2020
•
•
•
•
•
•
Folate antagonism
Neural crest disruption
Endocrine disruption
Oxidative stress
Vascular disruption
Specific receptor or enzyme-mediated
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Van Gelder, M., van Rooij,I., Miller R., Zielhuis,G., de Jong-van den Berg L., and Roeleveld,N.
Teratogenic mechanisms of medical drugs. Hum Reprod Update. 2010. 16(4):378-94.
Read this paper for the details and examples of teratogenic mechanisms
https://www.nvp-volumes.org/p2_4.htm
More information on teratogens
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Angiotensin-converting enzyme
(ACE) inhibitors
• Angiotensin-converting enzyme (ACE) inhibitors can cause fetal
growth restriction and fetal death.
• Use in late pregnancy is associated with fetal toxicity and
intrauterine renal insufficiency and hyperkalemia
• Limb contractures, lung hypoplasia, facial anomalies, prematurity
are also reported.
• Effects are related to the hemodynamic effects on the fetus.
• Teratogenicity during the first trimester is low.
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Warfarin
• Warfarin, a blood-thinning drug, inhibits the vitamin K-dependent
synthesis of biologically active forms of the calcium-dependent
clotting factors II, VII, IX and X
• It can cause central nervous system defects, including mental
retardation, as well as problems with the optic nerves and hearing.
• Nasal hypoplasia and calcific stippling of the epiphyses
• Critical period is between 6-9 weeks
• Associated with a high risk of miscarriages
• Mechanism is of CNS damage is hemorrhage
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Valproic acid
• VA inhibits activity of histone deacetylases – causes
hyperacetylation. Many of these genes are regulated by RA.
• VA may cause autism-like developmental problems as a result of the
effect on histone deacetylases.
• VA causes both structural and functional birth defects
• VA blocks folate from being absorbed by the embryo leading to
neural tube defects.
• VA decreases the level of PAX1 transcription in chick somites
causing defects in ribs and vertebrae.
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Other teratogens
Pathogens
• Viruses are also teratogenic.
• Rubella virus makes a protein which stops mitosis by blocking
kinases required needed for cell division.
• First 5 weeks are the most critical.
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Pathogens
C: Chickenpox and shingles
H: Hepatitis B, C, D, E
E: Enteroviruses, a group of viruses including poliovirus
A: AIDS
P: Parvovirus B19, also known as fifth disease
T: Toxoplasmosis
O: Other infections such as group B streptococcus, listeria, candida
R: Rubella
C: Cytomegalovirus
H: Herpes simplex virus
E: Everything else sexually transmitted such as gonorrhea and chlamydia
S: Syphilis
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Pathogens
• CMV and Herpes are almost always fatal in early embryos.
• In late embryos – cause blindness, deafness, cerebral palsy and
mental deficiencies.
Heat
• An extended maternal temperature of 102F (38.9 C) or higher
during the first 6 weeks may affect closure of the neural tube.
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Herpes Simplex
• HSV can be a serious and potentially fatal condition for the fetus. While
neonatal herpes is a serious condition, it is also very rare.
• Less than 0.1% of babies born in the US each year get neonatal herpes. By
contrast, some 25-30% of pregnant women have genital herpes.
• Babies are most at risk for neonatal herpes if the mother contracts genital
herpes late in pregnancy because a newly infected mother does not have
antibodies against the virus, and a new herpes infection is frequently active, so
there is an increased risk of the virus being present in the birth canal during
delivery.
• Women who acquire genital herpes before they become pregnant have a very
low risk of transmitting the virus to their babies.
• Herpes can also be spread to the baby in the first weeks of life if he or she is
kissed by someone with an active cold sore (oral herpes).
http://www.ashasexualhealth.org/stdsstis/herpes/herpes-and-pregnancy/
Toxoplasmosis
• Toxoplasmosis normally causes a mild illness in people with healthy immune
systems, it's risky during pregnancy because it may harm your baby.
• The parasite can be found in meat, cat faeces, the soil where cats defecate and
unpasteurised goats’ milk.
• Toxoplasmosis is only a risk to an unborn baby if caught for the first time during
pregnancy or within a few weeks before you get pregnant. The damage the
infection may cause will depend on when in pregnancy you got the infection.
• On average, only 4 in 10 of such infections will pass to the baby. Caught during
pregnancy, toxoplasmosis can cause miscarriage, stillbirth or damage to the
baby’s brain and the eyes.
• However, most babies born with toxoplasmosis have no obvious damage at
birth but develop symptoms, usually eye damage, during childhood or even
adulthood. A few will have more serious symptoms such as blindness or brain
damage.
Cytomegalovirus
• Cytomegalovirus (CMV) is a common virus in the herpes​ virus family. 50% of people
have been infected by young adulthood and up to 85% by age 40.
• It is rare for a person to get symptoms after the initial infection unless their immune
system is weakened by severe illness and treatments (e.g. for cancer).
• Reactivation can occur during pregnancy in women who have had infection previously,
with a very small risk of transmission of CMV to the unborn baby.​
• If a woman is newly infected with CMV while pregnant, there is a risk that her unborn
baby will also become infected (congenital CMV).
• Hearing loss is the most common sign of congenital CMV. However, some infants with
congenital CMV infection who appear healthy at birth develop hearing or vision loss
over time.
https://www.health.nsw.gov.au/Infectious/factsheets/Pages/cmv-and-pregnancy.aspx