Introduction to teratology Department of Histology and Embryology LF MU Petr Vaňhara Teratology is the study of birth defects, and a teratogen is something that either induces or amplifies abnormal embryonic or fetal development and causes birth defects. INTRODUCTION TO TERATOLOGY Embryologyis the study of normal intrauterine embryonic and fetal development. INTRODUCTION TO TERATOLOGY Historical context τέρας (Greek) teras = monster Everything looking abnormal INTRODUCTION TO TERATOLOGY Historical context Since the 17th century related to abnormal births (development) INTRODUCTION TO TERATOLOGY 1905 1921 1929 1935 19371941 1944 1948 1952 19591961 What is a teratogen? INTRODUCTION TO TERATOLOGY • X-Rays • Lipid diet • Hypovitaminosis • Sex hormones • Virus infection • Chemical substances • Medications • Environmental pollutant (methyl mercury) Physical • Ionizing irraditation (UV, RTG, , , ), • Temperature • Mechanical factors in utero (pes equinus, amnion bands) Chemical • Nucleotide analogues (chemotherapy) • Organometall compounds • Hormones and endocrine disruptors • Medications (aminopterins, thalidomide, antiepileptic drugs, anticoagulants, vitamin A ) • Environmental pollutant (methylmercury, lead, heavy metals) • Alcohol, drugs, solvents • Many others Biological • Infection agents (Toxoplasmosis, Rubella, Cytomegalovirus, Herpes, Syphilis) • Disease of mother (diabetes, thyroid conditions, autoimmune disorders, PKU) INTRODUCTION TO TERATOLOGY Teratogens around us What does a teratogen do? INTRODUCTION TO TERATOLOGY • Intra uterine growth retardation • Failure of histogenesis, organogenesis • Embryonic/fetal death • Disrupts fine molecular or metabolic pathways • Growth retardation • Failure of histogenesis, organogenesis • Embryonic/fetal death INTRODUCTION TO TERATOLOGY https://www.govinfo.gov/content/pkg/FR-2008-05-29/pdf/E8-11806.pdf INTRODUCTION TO TERATOLOGY How to classify a teratogen? INTRODUCTION TO TERATOLOGY How to classify a teratogen? Pregnancy and Lactation Labeling Rule (PLLR) since 2015 in USA Instead of using an arbitrary lettering system, the PLLR provides detailed risk summaries and more comprehensive information derived from clinical experience (if available), animal data, and concerns related to the pharmacologic activity of the drug. In addition, the label includes information on the risks associated with untreated illness. This information helps to put the potential effects of the drug into perspective with the goal of providing a more individualized risk-benefit analysis. https://eu-rd- platform.jrc.ec.europa.eu/eurocat/preventio n-and-risk-factors/medication-during- pregnancy_en How to avoid teratogen? INTRODUCTION TO TERATOLOGY • Talk with your healthcare provider about any medications you’re taking. • Avoid cigarettes, alcohol and recreational drugs. • Don’t take any supplements, medications or prescription drugs without checking with your healthcare provider. • Avoid cleaning litter boxes. • Avoid hot tubs, saunas and anything that raises your internal body temperature. • Remove tuna, swordfish and other fish high in mercury from your diet. • Talk with your supervisor or human resources about harmful chemicals in your workplace. 65-75% Multifactorial or unknown 20-25% Genetical <5% • Environmental • Intrauterine infections • Metabolic disorder of mother • Drugs and medications • Radiation INTRODUCTION TO TERATOLOGY How to identify a teratogen? How to identify a teratogen? INTRODUCTION TO TERATOLOGY "Koch's Postulates„ adapted for teratology: • The agent must be present during the critical stage of development. • The agent produces a particular pattern of birth defects in animal studies. • The agent crosses the placenta and there is a dose-response relationship. • There is an abrupt increase in the frequency of a particular defect or group of defects (syndrome). • The increase of defects is associated with the use of a new drug or the widespread exposure to a chemical or environmental change. • There is an absence of other factors to explain the observations. • The mechanism of teratogenesis makes biological sense. Koch's Postulates in microbiology: • The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms. • The microorganism must be isolated from a diseased organism and grown in pure culture. • The cultured microorganism should cause disease when introduced into a healthy organism. • The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent. • Animal studies • Observations from human exposure INTRODUCTION TO TERATOLOGY How to identify a teratogen? INTRODUCTION TO TERATOLOGY • Animal studies • Observations from human exposure •Understand the biological context Wilson’s Six Principles of Teratology (1977) 1. Susceptibility to teratogenesis depends on the genotype of the conceptus and the way in which this interacts with environmental factors. 2. Susceptibility to teratogenic agents varies with the developmental stage at the time of exposure. 3. Teratogenic agents act in specific ways (mechanisms) on developing cells and tissues to initiate abnormal embryogenesis (pathogenesis). 4. The final manifestations of abnormal development are death, malformation, growth retardation, and functional disorder. 5. The access of adverse environmental influences to developing tissues depends on the nature of the influences (agent). 6. Manifestations of deviant development increase in degree as dosage increases from the no-effect to the totally lethal level. INTRODUCTION TO TERATOLOGY INTRODUCTION TO TERATOLOGY Mechanisms of action? • Epigenetic control of gene expression • The effects of small regulatory RNAs • The imbalance of gene products resulting from submicroscopic alterations of genomic structure such as copy number changes • Alterations of the cytoskeleton • Perturbations of the extracellular matrix • Effects of mechanical forces on embryogenesis • Disturbances of intracellular or intercellular signalling • Dysfunction of molecular chaperones • Effects on the distribution of molecules into subcellular compartments • Alterations of the integrity of intracellular organelles • Mutation • Chromosomal nondisjunction and breaks Mitotic interference • Altered nucleic acid integrity or function • Lack of precursors and substrates needed for biosynthesis • Altered energy sources • Enzyme inhibitions • Osmolar imbalance • Altered membrane characteristics 1977 2010 INTRODUCTION TO TERATOLOGY CASE STUDY Evidence for ZIKV induced microcephaly? CASE STUDY How to identify a new teratogen? INTRODUCTION TO TERATOLOGY • Observations from human exposure • Understand the biological context • Validate on animals Strength (effect size): A small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal. Consistency(reproducibility): Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect. Specificity: Causation is likely if there is a very specific population at a specific site and disease with no other likely explanation. The more specific an association between a factor and an effect is, the bigger the probability of a causal relationship. Temporality: The effect has to occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay). Biological gradient: Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect. In other cases, an inverse proportion is observed: greater exposure leads to lower incidence. Plausibility: A plausible mechanism between cause and effect is helpful (but Hill noted that knowledge of the mechanism is limited by current knowledge). Coherence: Coherence between epidemiological and laboratory findings increases the likelihood of an effect. Experiment: "Occasionally it is possible to appeal to experimental evidence". Analogy: The use of analogies or similarities between the observed association and any other associations. Reversibility: If the cause is deleted then the effect should disappear as well. Bradford Hill criteria CASE STUDY ZKV transmission CASE STUDY ZKV mechanism of action CASE STUDY • Teratology, teratogens → From Koch’s postulates to Wilson’s principles • Mechanisms of action → any embryology and/or cell biology textbook • Classification & clinical examples → any embryology textbook, FDA (EU) categories • Identification, validation → ZIIKA forest virus story & Bradford Hill criteria Take home message and further reading INTRODUCTION TO TERATOLOGY Suggested reading: • Friedman JM. The Principles of Teratology: Are They Still True? Birth Defects Research (Part A): Clinical and Molecular Teratology 88:766–768 (2010) • Varga I. Embryology Teaching: An Often-neglected Part of the Medical Curriculum. Rev Arg de Anat Clin. 9(2):47-51 (2017) • Ujhazy et al. Teratology – past, present and future. Interdiscip Toxicol. 5(4): 163–168 (2012) x INTRODUCTION TO TERATOLOGY Not every genetic anomaly results in a malfunction Thank you for attention pvanhara@med.muni.cz