Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
TERMINOLOGY • TERATOGEN = AN AGENT THAT DAMAGES THE SOMATIC TISSUE OF AN EMBRYO • MUTAGEN = AN AGENT THAT CHANGES THE GENETIC MATERIAL OF GERM CELLS • Most mutagens are teratogens, but few teratogens appear to be mutagens • Malformation - morphologic defect resulting from an intrinsically abnormal developmental process (structure is abnormal from its inception) • Deformation - abnormal form, shape, or position of a structure, caused by mechanical factors • Disruption - morphologic defect resulting from extrinsic interference with a normal process • Dysplasia - abnormal organization of tissue • Syndrome: "a recognizable pattern of anomalies which are known or thought to be causally related" (Spranger et al, 1982; Khoury et al, 1994)
TERMINOLOGY • Sequence: “pattern of anomalies derived from a known (or presumed) malformation or mechanical factor” • Complex: "those groups of heterogeneous disorders with overlapping characteristics that are difficult to separate into specific conditions" e.g., facio-auriculo-vertebral spectrum, hypoglossia-hypodactylia (Martinez-Frias, 1995) • Association: "derivatives of causally nonspecific disruptive events acting on developmental fields" or "abnormal markers of normal embryologic relationships“ (Lubinsky, 1986). In a sense, this is a "wastebasket" category that should constantly change with increasing understanding. • Developmental Field: "basic biological units of individual development and of evolution, and association to represent the idiopathic occurrence of multiple congenital anomalies during blastogenesis" (Opitz, 1985)
CONGENITAL MALFORMATIONS ETIOLOGY • A. Malformations are present from birth • B. Causes • 1. Genetic factors: chromosomal (numerical or structural) or gene abnormalities account for about 15% of malformations • 2. Environmental: a drug, disease, causative agent, malnutrition, etc. account for about 10% of malformations • 3. About 25% of malformations are due to multifactorial inheritance: a combination of genetic predisposition and environmental agents
CONGENITAL MALFORMATIONS ETIOLOGY • C. The presence of malformations is high in spontaneous abortions, indicating that the genetic defect is so severe that it halts embryogenesis; according to Connor and Ferguson-Smith (1991), chromosomal findings (those related to VISIBLE chromosomal defects) in early spontaneous abortions in the United Kingdom are: • 40% of embryos have apparently normal karyotopes • 60% of embryos have abnormal karyotypes: • 30% exhibit trisomy • 10% exhibit Turner syndrome (45, XO) • 10% exhibit triploidy, usually due to double fertilization or to diploid sperm (60% =69, XXY; 40% = 69, XXX) • 5% exhibit tetraploidy • 5% exhibit various conditions • Connor and Ferguson-Smith (1991) state that at least 7.5% of all conceptions have a chromosomal disorder
Critical Periods in Development • Cells are changing and following specified developmental programs • Interference with one stage prevents the proper sequence of events • There is a limited time frame in which sets of cells can complete their tasks.
Critical Periods in Development • During this time cells can be very sensitive to disruptive agents • This is the "critical period" for organ or tissue • E.g. disruption of neural tube closure leads to spina bifida, incomplete palate formation leads to cleft palate, etc.
II. GENETIC FACTORS • A.Genetic Abnormalities may be signaled by a common phenotype • 1. Characteristic facial features seen, for example, in Trisomy 21 (Down's syndrome); Trisomy 18 & Trisomy 13; Trisomy XXY (Klinefelter syndrome) • 2. Light colored skin, hair, and eyes seen in phenylketonuria • Phenylketonuria is one of the commonest inherited disorders - occurring in approximately 1 in 10,000 babies born in the U. S. • It occurs in babies who inherit two mutant genes for the enzyme phenylalanine hydroxylase (PAH)
II. GENETIC FACTORS • B.Chromosomal abnormalities include: • 1. Numerical abnormalities usually caused by nondisjunction; • a. Monosomy = the loss of a chromosome due to nondisjunction during oogenesis or spermatogenesis • 1. Loss of an autosome is lethal • 2. Loss of a sex chromosome results in Turner syndrome (45, XO)
TURNER SYNDROME (X Monosomy, Gonadal Dysgenesis) • a. Turner's syndrome is present in 1/2500 female births, but 99% of • Turner embryos spontaneously abort; • Turner's syndrome is due to nondisjunction with the loss of an entire chromosome or it can be due to deletion of the short arm of the X • b. The manifestations include: short stature, webbed neck, primary • Amenorrhea (due to ovarian dysgenesis); congenital heart malformations • c. Turner's syndrome is usually due to the deletion of the paternal X chromosome
TRISOMY Maternal Age Incidence of trisomy 21 atbirth 20 1/1500 25 1/1350 30 1/900 35 1/380 39 1/150 43 1/50 45 1/28 • a. Trisomy 21, Down's syndrome • 1. The manifestations of Trisomy 21 include: Mental retardation, simian crease, epicanthic folds, cardiovascular and gastrointestinal malformations • 2. Individuals with trisomy 21 can be expected to present eventually with presenile dementia; the progressive deposition of beta amyloid protein in the cerebral hemispheres is noted at autopsy in trisomy 21 patients • 3. Virtually all trisomy 21 individuals who live into their 40's have • Alzheimer's disease; thus current, research on Alzheimer's disease in the general population seeks genetic dysfunction associated with chromosome 21 • 4. The incidence of trisomy 21 increases with increasing maternal age;
TRISOMY • b. Trisomy 18, Edward's syndrome: 1:3000 • The manifestations of trisomy 18 include: • mental retardation; • micrognathia; • rocker bottom feet; • deformed ears; • prominent occipital region; • only about 10% of these babies live beyond the first year
TRISOMY • c. Trisomy 13, Patau's syndrome: 1:5000 • The manifestations of trisomy 13 include: • mental retardation; • bilateral cleft lip/palate; • small eyes; • only about 10% of these babies live beyond the first year
TRISOMY • d. XXY: Kleinfelter's syndrome; 1:1080 • The individual is phenotypically male (because of Lyonization), • tall with long legs, • a short body, small testes (he is infertile).
TRISOMY • e. XXX: Triple X; 1:960 • The individually is phenotypically female and of clinically normal appearance; fertile; but 15-25% are mildly mentally retarded • f. XYY: "Supermale",1:1080 (20/1000 in jails) • The individual is phenotypically male with a normal appearance; tends to be tall; • personality disorders are fairly common, especially increased aggressiveness
STRUCTURAL ABNORMALITIES OF CHROMOSOME • a. Translocation: a fragment of a chromosome is moved ("trans-located") from one chromosome to another - joins a non-homologous chromosome. • The balance of genes is still normal (nothing has been gained or lost) but can alter phenotype as it places genes in a new environment. • Can also cause difficulties in egg or sperm development and normal development of a zygote. • Acute Myelogenous Leukemia is caused by this translocation
STRUCTURAL ABNORMALITIES OF CHROMOSOME • b. Deletion: the chromosome breaks and the small piece without the centromere is lost • Cri-du-Chat Syndrome 46 XX or XY, 5p- (segmental deletion) • wailing, cat-like cry in about 50% of those afflicted • microcephaly • severe mental retardation • heart and other organ deformities • essentially, this is a partial monosomy
STRUCTURAL ABNORMALITIES OF CHROMOSOME • c. Duplication: parts of the chromosome are duplicated and are found either within the same chromosome or on another chromosome or as an independent piece • Fragile X syndrome (Martin-Bell syndrome) • Most people have 29 "repeats" at this end of their X-chromosome, those with Fragile X have over 700 repeats due to duplications. Affects 1:1500 males, 1:2500 females.
STRUCTURAL ABNORMALITIES OF CHROMOSOME • d. Inversion: a portion of the chromosome is reversed • e. Isochromosome: centromere divides transversely instead of longitudinally
STRUCTURAL ABNORMALITIES OF CHROMOSOME • C. Mutant genes • 1. Homozygous: both alleles at the locus are identical (AA, or aa) • Heterozygous: alleles at the locus are different (Aa) • 2. Mendelian diseases are due to a single mutant gene and are inherited acording to Mendelian laws • a. Autosomal diseases are encoded on one of the 22 pairs of autosomes; X-linked diseases are encoded on the X chromosome • b. Dominant genes code for traits that are expressed in both the homozygote and the heterozygote (AA, Aa); recessive genes code for traits that are expressed only in the homozygote (aa)
The Mechanism of Action of Teratogens • a. Inhibition of mitosis and/or increase in the length of the cell cycle • b. Inhibition of protein synthesis • c. Inhibition of cellular migration; known to be involved in some forms of mental retardation and in facial malformations • d. Damage to DNA, such as chromosomal breaks, nondisjunction and anaphase lag
The Mechanism of Action of Teratogens • e. Inhibition of nucleic acid synthesis • f. Cell death beyond the compensatory capacity of remaining cells • g. Inhibition of cell differentiation, including a slow down of developmental events resulting in developmental delay. • h. Production of vascular dysfunction leading to a disruption of perfusion to embryonic cells
Agents Affecting Critical Period • Teratogen: any agent that disrupts a developing embryo such as the following: • Mutated Genes: interfere with developmental program • Physical Agents: X-Rays, Heat • Chemical Agents: Steroids, Alcohol, Drugs • Viruses: herpes, Rubella (German Measles) viruses • Other: lack of a specific, essential component can also lead to abnormal development (e.g., FA & Spina Bifida)
Fetal Alcohol Syndrome (FAS) • Babies with FAS typically have a small head size, a narrow upper lip, an indistinct philtrim (the pair of ridges that runs between the middle of the upper lip and the nose), and a low nose bridge. • Abnormalities in neuron migration and a smaller brain size reflect the reduced mental ability of FAS children. • Abel and Sokal (1987. Drug and Alcohol Dependence 19: 51-70) note that While fragile-X and Down syndrome lead to the most cases of mental retardation, • FAS is third, affecting as many as 1 in 500-750 children in the US.
Fetal Alcohol Syndrome (FAS) • FAS children have a mean IQ of ~68 making them unable to manage their own lives or to learn from past experience. • There are likely many reasons for these effects of ethanol. • Neural crest migration is hindered in the presence of ethanol leading to premature differentiation of facial cartilage (Hoffman and Kulyk, 1999. Int. J. Dev. Biol. 43: 167-174). • Ethanol also induces apoptosis (death) of neurons (Ikonomidou et al, 2000. Science 287: 1056-1060). • These are just some of the insights that are being gained as researcher pursue this serious problem that has high costs for society and for the affected individuals.
Cleft Lip & Cleft Palate • Definition: a facial birth defect (congenital abnormality) • Common; ~ 1/1000 • Cleft Lip: affects the upper lip, from a notch to a fissure into the nose • Cleft Palate: affects the roof of the mouth, with a groove that may extend through the dental arch (hard palate). • Abnormalities may occur separately or together • Reason: left & right sides do not contact & fuse
Spina Bifida (SB) • A neural tube defect • Caused by the failure of the fetus’s neural tube to close properly during the first month of pregnancy • SB Infants may have an open lesion on their spine with nerve/spinal cord damage • SB may also cause bowel and bladder complications • Many children with SB have hydrocephalus (excessive accumulation of cerebrospinal fluid in the brain). • Spinal opening can be surgically repaired nerve damage can’t be fixed, leads to varying degrees of paralysis of lower limbs • Three Most Common Forms of SB • Myelomeningocele: the severest form--spinal cord and its protective covering (the meninges) protrude from an opening in the spine • Meningocele: spinal cord develops normally but meninges protrude from a spinal opening • Occulta: the mildest form, one or more vertebrae are malformed and covered by a layer of skin.
Spina Bifida (SB) • Problems: physical & mobility difficulties, most individuals have some form of learning disability • Early detection is done using amniocentesis • Large amounts of AFG (alpha-fetoprotein) indicates that Spina Bifida is likely • Ultrasound reveals a typical morphology after 20 weeks: Cranial scalloping is evident as a "lemon sign" in the BF fetus • Fetus makes a -fetoprotein • Normal: small amount crosses placenta & enters blood • Abnormal: high or low levels • (MSAFP) maternal serum a -FP screening • Opening in spine (spina bifida) or head, or an abdominal wall defect: a -FP more leaks out • Low a -FP levels cause concern, too (Down Syndrome, Trisomy 18) reasons for low a -FP unknown • SURGERY: to close the newborn’s spinal opening done ~24 h after birth to preserve existing function in the spinal cord. • TREATMENT: There is no cure for SB; Many need assistance devices such as braces, crutches, or wheelchairs. • PROGNOSIS: Depends on severity; most live into adulthood • PREVENTION: Folic Acid reduces incidence of neural tube defects; (0.4mg/day) should be taken by all women of childbearing age
PRENATAL DIAGNOSIS • There are a variety of non-invasive and invasive techniques available for prenatal diagnosis. • Each of them can be applied only during specific time periods during the pregnancy for greatest utility. • The techniques employed for prenatal diagnosis include: • Ultrasonography • Amniocentesis • Chorionic villus sampling • Fetal blood cells in maternal blood • Maternal serum alpha-fetoprotein • Maternal serum beta-HCG • Maternal serum estriol
PRENATAL DIAGNOSIS • Ultrasonography • This is a non-invasive procedure that is harmless to both the fetus and the mother. • The developing embryo can first be visualized at about 6 weeks gestation. • Recognition of the major internal organs and extremities to determine if any are abnormal can best be accomplished between 16 to 20 weeks gestation.
PRENATAL DIAGNOSIS • An ultrasound examination can be quite useful to (Indication): • Multiple gestations are present • Determine the size and position of the fetus, • Fetal age and growth • The size and position of the placenta, and umbilical blood flow. • The amount of amniotic fluid, • The appearance of fetal anatomy, there are limitations to this procedure. • Status of the uterine environment, including the amount of amniotic fluid • Presence or absence of congenital anomalies: A good example of this is Down syndrome (trisomy 21), such as nuchal thickening.
Ultrasonography • Fetal age and growth are assessed by crown-rump length during the 5th to 10th weeks of gestation. A • fter that, a combination of measurements; including the biparietal diameter (BPD) of the skull, femur length, and abdominal circumference are used • Congenital malformations that can be determined by ultrasound include: • The neural tube defects anencephaly and spina bifida; • Abdominal wall defects, such as omphalocele and gastroschisis; • Heart and facial defects, including cleft lip and palate.
Maternal Serum Screening • A search for biochemical markers of fetal status led to development of maternal serum screening tests. • One of the first of these tests assessed serum a-fetoprotein (AFP) concentrations. • AFP is produced normally by the fetal liver, peaks at approximately 14 weeks, and “leaks” into the maternal circulation via the placenta. Thus, AFP concentrations increase in maternal serum during the second trimester and then begin a steady decline after 30 weeks of gestation. • In cases of neural tube defects and several other abnormalities, including omphalocele, gastroschisis, bladder exstrophy, amniotic band syndrome, sacrococcygeal teratoma, and intestinal atresia, AFP levels increase in amniotic fluid and maternal serum. • In other instances, AFP concentrations decrease, as, for example, in Down syndrome, trisomy 18, sex chromosome abnormalities, and triploidy. • These conditions are also associated with lower serum concentrations of human chorionic gonadotropin (hCG) and unconjugated estriol.
PRENATAL DIAGNOSIS • Amniocentesis • This is an invasive procedure in which a needle is passed through the mother's lower abdomen into the amniotic cavity inside the uterus. • Enough amniotic fluid is present for this to be accomplished starting about 14 weeks gestation. • For prenatal diagnosis, most amniocenteses are performed between 14 and 20 weeks gestation. • Within the amniotic fluid are fetal cells (mostly derived from fetal skin) which can be grown in culture for chromosome analysis, biochemical analysis, and molecular biologic analysis. • In the third trimester of pregnancy, the amniotic fluid can be analyzed for determination of fetal lung maturity. This is important when the fetus is below 35 to 36 weeks gestation, because the lungs may not be mature enough to sustain life.
Amniocentesis • During amniocentesis, a needle is inserted transabdominally into the amniotic cavity and approximately 20 to 30 mL of fluid is withdrawn. • Because of the amount of fluid required, the procedure is not usually performed before 14 weeks gestation, when sufficient quantities are available without endangering the fetus. • The risk of fetal loss as a result of the procedure is 1%, but it is less in centers skilled in the technique. • The fluid itself is analyzed for biochemical factors, such as AFP and acetylcholinesterase. • In addition, fetal cells, sloughed into the amniotic fluid, can be recovered and used for metaphase karyotyping and other genetic analyses. • With special stains (Giemsa) and high-resolution techniques, chromosome banding patterns can be determined. • Furthermore, now that the human genome has been sequenced, more sophisticated molecular analyses using polymerase chain reaction (PCR) and genotyping assays will increase the level of detection for genetic abnormalities.
Chorionic Villus Sampling • Chorionic villus sampling (CVS) involves inserting a needle transabdominally or transvaginally into the placental mass and aspirating approximately 5 to 30 mg of villus tissue. • The risk of fetal loss from CVS is approximately twofold greater than with amniocentesis, and there have been indications that the procedure carries an increased risk for limb reduction defects. • Generally, these prenatal diagnostic tests are not used on a routine basis • Indications for using the tests include the following: • (1) advanced maternal age (35 years and older); • (2) previous family history of a genetic problem, such as the parents having had a child with Down syndrome or a neural tube defect; • (3) the presence of maternal disease, such as diabetes; • (4) an abnormal ultrasound or serum screening test.
PRENATAL DIAGNOSIS • Chorionic Villus Sampling (CVS) • In this procedure, a catheter is passed via the vagina through the cervix and into the uterus to the developing placenta under ultrasound guidance. • The introduction of the catheter allows sampling of cells from the placental chorionic villi. • The most common test employed on cells obtained by CVS is chromosome analysis to determine the karyotype of the fetus. • The cells can also be grown in culture for biochemical or molecular biologic analysis. • CVS can be safely performed between 9.5 and 12.5 weeks gestation.
PRENATAL DIAGNOSIS • Maternal blood sampling for fetal blood cells • This is a new technique that makes use of the phenomenon of fetal blood cells gaining access to maternal circulation through the placental villi. • The fetal cells can be sorted out and analyzed by a variety of techniques to look for particular DNA sequences, but without the risks that these latter two invasive procedures inherently have.
PRENATAL DIAGNOSIS • Triple" or "Quadruple" screen • Combining the maternal serum assays may aid in increasing the sensitivity and specificity of detection for fetal abnormalities. • The classic test is the “triple screen” for • alpha-fetoprotein (MSAFP), • beta-HCG, • estriol (uE3). • The "quadruple screen" adds inhibin-A.