This chapter deals primarily with APGO Educational Topic Areas:
TOPIC 9 PRECONCEPTION CARE
TOPIC 32 OBSTETRIC PROCEDURES
Students should be able to identify patients with genetic risk and counsel them regarding that risk as well as genetic screening options. They should also be familiar with the use of amniocentesis, chorionic villous sampling, and ultrasound to evaluate patients with suspected genetic disorders.
Clinical Case
A new obstetric patient presents for her nurse history. She is an only child but reports that she has an aunt and a female cousin who had a great deal of difficulty in school. The aunt also has a son who is mentally retarded and seems to remember at least one other distant cousin with mental retardation. She wants to make sure her baby does not have the same problem. What additional consultations and evaluations will you offer this patient?
Recent discoveries in the field of genetics have led to the increased use of genetic principles and techniques in all areas of medicine, including obstetrics and gynecology. In obstetrics, prenatal screening is routinely performed to detect genetic disorders, such as Down syndrome and cystic fibrosis. In gynecology, clinicians can offer appropriate genetic testing for women deemed at high risk for genes that increase the risk of breast, bowel, and ovarian cancers. In the future, genetic evaluation may lead to earlier and more accurate diagnosis of conditions such as diabetes. Gene-based therapies may also be used to treat diseases with greater specificity and fewer side effects than conventional treatments.
BASIC CONCEPTS IN GENETICS
Knowledge of the basic principles of genetics and an understanding of their application are essential in current medical practice. These principles form the basis for screening, diagnosis, and management of genetic disorders.
Genes: Definition and Function
Genes, the basic units of heredity, are segments of deoxyribonucleic acid (DNA) that reside on chromosomes located in cell nuclei. DNA is a double-stranded helical molecule. Each strand is a polymer of nucleotides made up of three components: 1) a “base,” which is either a purine (adenine [A] or guanine [G]) or a pyrimidine (cytosine [C] or thymine [T]); 2) a five-carbon sugar; and 3) a phosphodiester bond. The strands of the DNA helix run in an antiparallel fashion, adenine binding to thymine and cytosine binding to guanine. These base pairs, in their nearly limitless combinations, constitute the genetic code.
The information in the DNA must be processed before it can be used by cells. Transcription is the process by which DNA is converted to a messenger molecule called ribonucleic acid (RNA). During transcription, the DNA molecule is “read” from one end (called the 5-prime [5 ′] end) to the other end (called the 3-prime [3 ′] end). A messenger RNA (mRNA) molecule is formed that is exported from the cell nucleus into the cytoplasm. This mRNA contains a translation of the genetic code into codons. Transcription is regulated by promoter and enhancer sequences. Promoter sequences guide the direction of translation from 5′ to 3′ and are located on the 5′ end. Enhancersequences have the same function but are found further down the 5′-end of the DNA molecule.
After transcription is complete, mRNA is used as a template to construct the amino acids that are the building blocks of proteins. In this process, called translation, each codon is matched to its corresponding amino acid. The amino acid strand grows until a “stop” codon is encountered. At this point, the now-completed protein undergoes further processing and is then either used inside the cell or is exported outside the cell for use in other cells, tissues, and organs. Errors in the DNA replication process can occur in a variety of ways and lead to a mutation, a change in the normal gene sequence. Most DNA replication errors are rapidly repaired by enzymes that proofread and repair mistakes.
Replication errors are of four basic kinds: 1) missense mutations, in which one amino acid is substituted for another; 2) nonsense mutations, in which premature stop codons are inserted in a sequence; 3) deletions; and 4) insertions. An example of a replication error causing a recognized disease is Huntington disease, in which an abnormal number of cytosine–adenine–guanine (CAG) repeats occurs in the Huntingtin gene. DNA can also be damaged by environmental factors, such as ultraviolet light, ionizing radiation, and chemicals.
Chromosomes
The genetic information in the human genome is packaged as chromatin, within which DNA binds with several chromosomal proteins to make chromosomes. A karyotype reveals the morphology and number of chromosomes. Somatic cells are all the cells in the human body that are not gametes (eggs or sperm). Germ cells, or gametes, contain a single set of chromosomes (n = 23) and are described as haploid in number. Somatic cells contain two sets of chromosomes, for a total of 46 chromosomes. These cells are diploid, signifying that they have a 2n chromosome complement (2n = 46). These chromosome pairs consist of 22 pairs of autosomes, which are similar in males and females. Each somatic cell also contains a pair of sex chromosomes. Females have two X sex chromosomes; males have an X and a Y chromosome.
Chromosome Replication and Cell Division
Chromosomes undergo two types of replication, meiosis and mitosis, which are significantly different and produce cell types with crucially different capabilities. Mitosis is the replication of chromosomes in somatic cells. It is followed by cytokinesis, or cell division, that results in two daughter cells containing the same genetic information as the parent cell. Meiosis only occurs in germ cells. It is also followed by cytokinesis; but, in this case, cytokinesis results in four daughter cells with a haploid count.
Somatic cells undergo cell division based on the cell cycle. There are four stages of the cell cycle: G1, S, G2, and M. G1, or gap 1, occurs immediately after mitosis and is a period of inactivity with no DNA replication. During G1, all the DNA of each chromosome is present in the 2n form. The next phase is S, or synthesis, in which the chromosomes double to become two identical sister chromatids with a 4nchromosome complement. During G2, or gap 2, the cells prepare for mitosis. G1, S, and G2 are also called interphase, which is the period between mitoses.
Mitosis
The goal of mitosis is to form two daughter cells that have a complete set of genetic information. Mitosis is divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. During prophase the chromatin swells, or condenses, and the two sister chromatids are in close approximation. The nucleolus disappears, and the mitotic spindle develops. Spindle fibers start to form centrosomes, microtubule-organizing centers that migrate to the poles of the cell. In prometaphase, the nuclear membrane vanishes and the chromosomes begin to disperse. They will eventually attach to the microtubules that form the mitotic spindle. Metaphase is the stage of maximal condensation. The chromosomes are in a linear formation in the center of the cell, between the two spindle poles. It is during metaphase where cells can most easily be analyzed to obtain a karyotype from an amniocentesis or chorionic villus sampling (CVS). Anaphase is initiated when the two chromatids separate. They form daughter chromosomes that are drawn to opposite poles of the cell by the spindle fibers. Finally, telophase is when the nuclear membrane starts to reform around the independent daughter cells, which then go into interphase (Fig. 7.1).
Meiosis
Meiosis differs from mitosis in that a haploid number of cells are initially produced in two successive divisions. The first division ( meiosis I) is termed a reduction division, because of the resulting decrease in chromosome number from diploid to haploid. Meiosis I is also divided into four stages: prophase I, metaphase I, anaphase I, and telophase I. Prophase I is further divided into five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis. In prophase I, the chromosomes condense and become shorter and thicker. It is during the pachytene substage that crossing over takes place, resulting in four distinct gametes. However, it is during anaphase where most of the genetic variation occurs. In anaphase I, the chromosomes go to opposite poles of the cell by independent assortment, signifying that there are 223, or >8 million, possible variations. Anaphase I is also the most error-prone step in meiosis. The process of disjunction, in which the chromosomes go to opposite poles of the cell, can result in nondisjunction, where both chromosomes go instead to the same pole. Nondisjunction is a common cause of chromosomally abnormal fetuses.
FIGURE 7.1. Stages of mitosis. (Modified from Sadler TW. Langman’s Medical Embryology. 10th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006:12.)
The second meiotic division (meiosis II) is similar to mitosis, except that the process occurs within a cell with a haploid number of chromosomes. Meiosis II is also divided into four stages: prophase II, metaphase II, anaphase II, and telophase II. The result of meiosis II is four haploid daughter cells. After anaphase II, the possibilities for genetic variation are further increased by 223 × 223, ensuring genetic variation (Fig. 7.2).
Abnormalities in Chromosome Number
Any alteration in the chromosome number is called heteroploidy. Heteroploidy can occur in two forms: euploidy and aneuploidy. In euploidy, the haploid number of 23 chromosomes is altered. An example of euploidy is triploidy, in which the haploid number has been multiplied by three. The karyotype is 69,XXX or 69,XXY. Triploidy results from double fertilization of a normal haploid egg or from fertilization by a diploid sperm. Such abnormalities usually result in conceptions of partial hydatidiform moles and end spontaneously in the first trimester.
In aneuploidy, the diploid number of 46 chromosomes is altered. The trisomies are aneuploidies in which there are three copies of an autosome instead of two. Examples include trisomy 21 (Down syndrome), trisomy 18(Edward syndrome), trisomy 13 (Patau syndrome), and trisomy 16. Most trisomies result from maternal meiotic nondisjunction, a phenomenon that occurs more frequently as a woman ages (Fig. 7.3 and Table 7.1).
Sex chromosome abnormalities occur in 1 of every 1,000 births. The most common are 45,X; 47,XXY; 47,XXX; 47,XYY; and mosaicism (the presence of two or more cell populations with different karyotypes).Numeric sex chromosome abnormalities can result from either maternal or paternal nondisjunction.
Abnormalities in Chromosome Structure
Structural alterations in chromosomes are less common than numerical alterations. Structural abnormalities that affect reproduction occur in 0.2% of the population.
Deletions
A deletion occurs when a portion of a chromosome segment is lost (Table 7.2). In a terminal deletion, the missing portion of the chromosome is appended to the end of the long or short arm. If the missing portion of the chromosome is appended to both the long and short arms of the same chromosome, a ring chromosome can result. An interstitial deletion occurs when the deleted portion lacks a centromere, or in cases involving chromosomal breakage.
Insertions
Insertions occur when the portion of an interstitially deleted segment is inserted into a nonhomologous chromosome.
Inversions
An inversion is the result of faulty repair of a chromosomal breakage. The broken portion is inserted into the chromosome in an inverted fashion. A paracentric inversion occurs when both breaks occur on one arm of a chromosome. These types of inversions do not include the centromere, the region where the chromosome pairs are joined. Paracentric inversions cannot be identified by a traditional karyotype because the arms appear to be of normal length. Fluorescence in situ hybridization ([FISH] see p. 91) with locus-specific probes is used to detect this type of abnormality. A pericentric inversion involves a break in each arm. The centromere is included and a notable gain or loss of genetic material can be identified on a karyotype. For a parent with an inversion, the risk of having an abnormal child depends on the method of detection, the chromosome involved, and the size of the inversion. The observed risk is approximately 5% to 10% if the inversion is identified after the birth of an abnormal child, and 1% to 3% if identified at some other time. An exception is pericentric inversion of chromosome 9, which is not associated with genetic defects in offspring.
FIGURE 7.2. Stages of meiosis. (Modified from Sadler TW. Langman’s Medical Embryology. 10th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006:13.)
Translocations
A translocation involves the transfer of two chromosome segments, usually between nonhomologous (nonpaired) chromosomes. They are the most common form of structural rearrangements in humans. A translocation is described as balanced when equal amounts of genetic material are exchanged between chromosomes, and unbalanced when the chromosomes receive unequal amounts of genetic material. Two types of translocations are possible. A Robertsonian translocation only occurs in acrocentric chromosomes—those in which the centromere is located very near one end (chromosomes 13, 14, 15, 21, and 22). Those with Robertsonian translocations are phenotypically normal, but the gametes they produce may be unbalanced. Whether the unbalanced gametes will result in abnormal offspring depends on the type of translocation, the chromosomes involved, and the sex of the carrier parent. The most clinically important Robertsonian translocations are those involving chromosome 21 and another acrocentric chromosome, most commonly chromosome 14. Carriers of these translocations are at increased risk for having a child with trisomy 21. The risk of trisomy 21 is 15% if the translocation is maternal and 2% or less if it is paternal.
Balanced reciprocal translocations may involve any chromosome and are the result of a reciprocal exchange of chromosome material between two or more chromosomes. Like those with Robertsonian translocations, individuals with a balanced reciprocal translocation are also phenotypically normal but may produce gametes with unbalanced chromosomes. The observed risk of a chromosomal abnormality in an offspring is less than the theoretical risk, because some of these gametes result in nonviable conceptions. In general, carriers of chromosome translocations identified after the birth of an abnormal child have a 5% to 30% risk of having unbalanced offspring. Children with an unbalanced chromosome translocation are at increased risk for mental retardation, neurodevelopmental delay, and other congenital abnormalities.
FIGURE 7.3. Comparison of normal and abnormal meiotic divisions. (A) Normal meiotic division. (B) Nondisjunction in the first meiotic division. (C) Nondisjunction in the second meiotic division. (Modified from Sadler TW. Langman’s Medical Embryology. 10th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006:15.)
Patterns of Inheritance
Single-gene (Mendelian) disorders display predictable patterns of inheritance related to the location of the gene (autosomal or X-linked) and the expression of the phenotype (dominant or recessive). Although Mendelian disorders were the first type of genetic disorders described, it is now known that there are many genetic and environmental factors that modify these genes, making true single-gene disorders relatively rare. Health care providers should be aware that many single-gene disorders are discovered each year and may be tracked using Internet databases, such as Online Mendelian Inheritance in Man (http://www.nslij-genetics.org/search_omim.html).
Autosomal Dominant
Each gene occupies a specific position, or locus, on a chromosome. At each locus, there are two possible variations of the genes, or two alleles. If the phenotype of a disease is based on one allele in a gene pair, the gene is dominant. If the gene is located on an autosomal cell, its pattern of inheritance is described as autosomal dominant. Individuals with one dominant allele for a disorder (described as being heterozygous for the gene) will express disease and transmit the gene to 50% of their offspring (Box 7.1). Examples of genetic disorders with autosomal dominant inheritance include Marfan syndrome, achondroplasia, and Huntington disease.
The phenotypic expression of autosomal dominant genes is not always straightforward and may vary depending on specific characteristics of the gene. Variable expressivity is the varying expression of a disease in an affected person. For example, some individuals with neurofibromatosis have only a few café au lait spots, whereas others have large tumors. Neurofibromatosis, however, demonstrates 100% penetrance. Penetrance describes the likelihood that a person carrying the gene will be affected. Retinoblastoma is an example of incomplete penetrance; not all affected individuals will express any obvious form of disease. Anticipation refers to an increase in severity and earlier expression of disease with each subsequent generation. An example of a genetic mutation that shows anticipation is Huntington disease, where an expansion of the trinucleotide CAG repeat leads to earlier expression of the disease in affected offspring.
BOX 7.1 Patterns of Inheritance Characteristics of Autosomal Dominant Disorders
• Gene expression rarely skips a generation.
• An affected individual will transmit the gene to progeny 50% of the time.
• There should be equal sex distribution among affected relatives; males should be able to transmit to males and females to females.
• An unaffected first-degree relative will not transmit the gene to his or her progeny.
Characteristics of Autosomal Recessive Disorders
• Gene expression may appear to skip generations.
• Both males and females are affected.
• Neither parent is usually affected; affected individuals usually do not have affected children.
• If one parent is a carrier, half of the offspring will be carriers of the gene. If both parents are carriers, the risk of transmission of the disorder is 25%.
• If the suspected disorder is noted to be rare, consanguinity should be suspected.
Autosomal Recessive
An autosomal recessive disease is only expressed when the affected individual carries two copies of the gene (described as being homozygous for the gene) (see Box 7.1). Individuals who are heterozygous for the gene express a normal phenotype. During pregnancy, unless a woman has been screened for a particular disease based on her risk factors (e.g., sickle cell disease and cystic fibrosis), carriers of a recessive gene will not know they are carriers until they have affected offspring. Other examples of autosomal recessive disorders include Tay-Sachs disease and phenylketonuria.
X-Linked Inheritance
In X-linked diseases, the affected gene is located on the X chromosome. Because males only have one X chromosome, they will manifest disease if their X chromosome carries the affected gene. The male carrier status is considered hemizygous, whereas the female is almost always heterozygous.
X-linked recessive diseases are much more common than X-linked dominant diseases (Box 7.2). Some examples of X-linked recessive diseases are hemophilia and color blindness. Hypophosphatemia is an example of an X-linked dominant disease.
Fragile X syndrome is an X-linked disorder that causes mental retardation. It is caused by a repeat in the cytosine– guanine–guanine sequence in a specific gene located on the X chromosome. Transmission of the disease-producing genetic mutation to a fetus depends on the sex of the parent and the number of repeats in the parental gene. If the number of repeats is between 61 and 200, the individual is said to have a “premutation.” These individuals are phenotypically normal, although women carrying the premutation are at increased risk for premature ovarian failure. A full mutation is characterized by more than 200 repeats. These individuals display the signs and symptoms of the disorder.
BOX 7.2 Differences Between X-Linked Recessive and Dominant Disease
X-Linked Recessive Disease
• More common in males than in females.
• An affected male will not pass the disease to his son, but all the daughters will be carriers.
• The disease is transmitted from carrier females to affected males.
X-Linked Dominant Disease
• The disease is usually twice as likely in females than in males.
• An affected male will transmit the disease to all of his daughters, but not to any of his sons.
• Heterozygous females will transmit the gene to 50% of their offspring, whereas homozygous females will transmit the gene to all of their offspring.
A male may transmit the unexpanded premutation gene to his offspring, but expansion to the full mutation is rare in a male with the premutation gene. A female with a premutation gene may also transmit the gene to her offspring; however, the premutation gene may expand during meiosis and result in a full mutation. Women with a family history of boys with developmental delay, extreme hyperactivity, and speech and language problems should be offered fragile X– carrier testing. Women with ovarian failure or an elevated follicle–stimulating hormone level before age 40 years without a known cause should be screened to determine whether they have the fragile X premutation.
Mitochondrial Inheritance
Mitochondrial inheritance is different from other patterns of inheritance. Mitochondria contain unique DNA (called mitochondrial DNA) that differs from the DNA carried in the cell nucleus. Any mutations in this DNA are only transmitted from the mother to all of her offspring, and, if a male fetus is affected, he will not pass it on to any of his offspring.
Multifactorial Inheritance
Multifactorial disorders are caused by a combination of factors, some genetic and some nongenetic (i.e., environmental). Multifactorial disorders recur in families but are not transmitted in any distinctive pattern. Many congenital, single-organ system structural abnormalities are multifactorial, having an incidence in the general population of approximately 1 per 1,000. Examples of multifactorial traits are cleft lip, with or without cleft palate; congenital cardiac defects; neural tube defects (NTDs); and hydrocephalus.
RISK FACTORS FOR GENETIC DISORDERS
Several factors have been identified that increase the risk of having a child with a chromosomal abnormality, including maternal or paternal age and exposure to certain drugs. Other factors, such as ethnicity or a family history of a disease, may indicate that an individual carries a gene for a Mendelian disorder. The first step in assessing risk is to document information about the patient’s family and personal history (see Appendix C). This record is an effective method for obtaining information concerning personal and family medical history, parental exposure to potentially harmful substances, or other issues that may have an impact on risk assessment and management. This information can be collected prior to conception during a preconception office visit or during the first prenatal visit in the first trimester.
Some infectious diseases, including cytomegalovirus, rubella, and sexually transmitted diseases (see Chapter 24), as well as certain drugs (see Chapter 6) have been linked to an increased risk of birth defects. Preexisting diabetes mellitus may also predispose a fetus to a congenital anomaly. Because these defects are not gene based, family history and genetic testing procedures, such as amniocentesis or CVS, cannot be used to detect these abnormalities. Ultrasonography is the mainstay of surveillance for infectious and teratogeninduced congenital abnormalities.
Advanced Maternal Age
Although the risk increases with age, the majority of cases of Down syndrome occur in women younger than age 35 years (Table 7.3). In addition to Down syndrome, other chromosomal abnormalities increase in frequency with advanced maternal age (see Table 7.1).
Previous Pregnancy Affected by Chromosomal Abnormality
Women who have had a previous pregnancy complicated by trisomy 21, 18, or 13 or any other trisomy in which the fetus survived at least to the second trimester are at risk for having another pregnancy complicated by the same or different trisomy. The risk of trisomy recurrence is 1.6 to 8.2 times the maternal age risk, depending on several factors: the type of trisomy, whether the index pregnancy was a spontaneous abortion, maternal age at initial occurrence, and maternal age at subsequent prenatal diagnosis.
Some, but not all, sex chromosome abnormalities carry an increased risk of recurrence. A pregnancy complicated by fetal XXX or XXY increases the recurrence risk by 1.6% to 2.5% the maternal age risk. Turner syndrome (monosomy X; XO) and XYY karyotypes impart a nominal risk of recurrence.
History of Early Pregnancy Loss
At least half of all first-trimester pregnancy losses result from fetal chromosomal abnormalities. The most common are monosomy X; polyploidy (triploidy or tetraploidy); and trisomies 13, 16, 18, 21, and 22.
Advanced Paternal Age
Increasing paternal age, particularly after age 50 years, predisposes the fetus to an increase in gene mutations that can affect X-linked recessive and autosomal dominant disorders, such as neurofibromatosis, achondroplasia, Apert syndrome, and Marfan syndrome.
Ethnicity
Many Mendelian disorders occur more frequently in certain groups. African Americans are at increased risk for sickle cell disease, the most common hemoglobinopathy in the United States. Approximately 8% of African Americans carry the sickle hemoglobin gene, which is also found with increased frequency in those of Mediterranean, Caribbean, Latin American, and Middle Eastern descent. Caucasians of Northern European descent are at increased risk for cystic fibrosis, with an estimated carrier percentage of 1 in 22. Tay-Sachs, Gaucher, and Niemann-Pick diseases occur with greater frequency in individuals of Ashkenazi Jewish descent. Other diseases associated with certain ethnic groups are β-thalassemia found at increased frequency in individuals of Mediterranean origin, and α-thalassemia in individuals of Asian origin.
PRENATAL SCREENING
Obstetricians are responsible for determining if a woman is at increased risk for fetal abnormalities and for describing and offering appropriate prenatal screening or diagnostic tests. The purpose of prenatal genetic screening is to define the risk of a genetic disease in a low-risk population. A screen-ing test differs from a diagnostic test in that screening tests only assess the risk that a child will have a genetic disease; they cannot confirm or rule out the presence of the disease. A diagnostic test is given if a screening test is positive, to assess whether the disease is present or absent in the developing fetus. Genetic screening tests are routinely offered to all women to detect NTDs, Down syndrome, and trisomy 18. In addition, individuals of certain ethnic groups can be tested to detect whether they carry a gene for a particular disorder.
First-Trimester Screening
First-trimester screening tests are used to assess the risk of Down syndrome, trisomy 18, and trisomy 13 in a developing fetus. First-semester serum screening for Down syndrome consists of tests for levels of two biochemical markers: free or total human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A (PAPP-A). An elevated level of hCG (1.98 of the median observed in euploid pregnancies [MoM]) and a decreased level of PAPP-A (0.43 MoM) have been associated with Down syndrome. An ultrasonographic marker for Down syndrome is the size of the nuchal transparency (NT), a fluid collection at the back of the fetal neck that can be seen between 10 and 14 weeks of gestation (Fig. 7.4). An increase in the size of the NT between 10 4/7 and 13 6/7 weeks of gestation is recognized to be an early presenting feature of a variety of chromosomal, genetic, and structural abnormalities. When used alone, NT measurement has a detection rate for Down syndrome of 64% to 70%. Combining NT measurement with the other first-trimester biochemical markers yields an 82% to 87% detection rate of Down syndrome, with a 5% false-positive rate, which is equal to or higher than second-trimester screening tests. Women found to have an increased risk of aneuploidy with first-trimester screening tests should be offered genetic counseling and diagnostic testing by CVS or amniocentesis in the second trimester.
FIGURE 7.4. Nuchal area. Measurement is taken of the lucent area in the posterior neck (calipers), with the posterior caliper placed just inside the echogenic skin (arrowhead). The amnion (arrow) should not be mistaken for the skin. (From Doubilet PM, Benson CB. Atlas of Ultrasound in Obstetrics and Gynecology. Philadelphia, PA: Lippincott Williams & Wilkins; 2003:10f.)
An advantage of first-trimester screening is that the tests are performed early enough so that decisions can be made regarding continuing the pregnancy, if necessary (Table 7.4). Disadvantages include the need for specialized training and appropriate ultrasound equipment to achieve optimal NT measurement and the availability of CVS. Detecting pregnancies at high risk for Down syndrome in the first trimester is of low utility if a diagnostic invasive test (i.e., CVS) cannot be performed to verify the findings.
Several other first-trimester ultrasonographic findings have been evaluated as potential markers for aneuploidy in the first and second trimesters. Discovery of a structural malformation of a major fetal organ or structure (Table 7.5) or the finding of two or more minor malformations (e.g., choroid plexus cyst, extra digit, and single umbilical artery) increases the risk of aneuploidy sufficiently to warrant genetic testing of the fetus, regardless of maternal age or parental karyotype.
Second-Trimester Screening
Second-trimester screening may be the only option if a woman is seen for the first time during the second trimester of her pregnancy. Women who have had first-trimester screening for aneuploidy should not undergo independent second-trimester serum screening in the same pregnancy. When these test results are interpreted independently, the false-positive rates are additive, leading to many more unnecessary invasive procedures (11%–17%). After first-trimester screening, subsequent second-trimester Down syndrome screening is not indicated, unless it is being performed as a component of an integrated test (explained below), stepwise sequential test, or contingent sequential test.
Triple and Quadruple Screening Tests
An association between low maternal serum α-fetoprotein (AFP) levels and Down syndrome was reported in 1984. In the 1990s, hCG and unconjugated estriol were used in combination with maternal serum AFP to improve the detection rates for Down syndrome and trisomy 18. The average maternal serum AFP level in Down syndrome pregnancies is reduced to 0.74 MoM. Intact hCG is increased in affected pregnancies, with an average level of 2.06 MoM, whereas unconjugated estriol is reduced to an average level of 0.75 MoM. When the levels of all three markers (triple screen) are used to modify the maternal age-related Down syndrome risk, the detection rate for Down syndrome is approximately 70%; approximately 5% of all pregnancies will have a positive screen result. Typically, the levels of all three markers are reduced when the fetus has trisomy 18. Adding inhibin A to the triple screen (quadruple screen) improves the detection rate for Down syndrome to approximately 80%. The median value of the maternal inhibin A level is increased at 1.77 MoM in Down syndrome pregnancies, but inhibin A is not used in the calculation of risk for trisomy 18. These biochemical screening tests are performed at 15 to 20 weeks of gestation.
Ultrasound Screening
In the second trimester, gross abnormalities, such as cardiac defects, as well as a group of subtle sonographic markers (soft markers), may be associated with an increased risk of Down syndrome in certain women (Box 7.3).Although findings of soft markers do not significantly increase the risk of Down syndrome, they should be considered in the context of first-trimester screening results, patient’s age, and history. The significance of ultrasonographic markers identified by a second-trimester ultrasound examination in a patient who has had a negative first-trimester screening test result is unknown. Studies indicate that the highest detection rate is achieved with systematic combination of ultrasonographic markers and gross anomalies, such as thick nuchal fold and cardiac defects. However, an abnormal second-trimester ultrasound finding identifying a major congenital anomaly significantly increases the risk of aneuploidy and warrants further counseling and the offer of a diagnostic procedure.
BOX 7.3 Some Ultrasonographic “Soft Markers” for Down Syndrome
Nuchal fold
Intracardiac echogenic focus
Mild ventriculomegaly
Echogenic bowel
Shortened femur or humerus
Absent nasal bone
Pyelectasis
Screening for Neural Tube Defects
Maternal serum AFP is also used to screen for NTDs, i.e., congenital structural abnormalities of the brain and vertebral column. NTDs occur in approximately 1.4 to 2 per 1,000 births in the United States and are the second most common major con-genital abnormality worldwide (cardiac malformations are the most common). Maternal serum AFP evaluation is an effective screening test for NTDs and should be offered to all pregnant women, unless they plan to have amniotic AFP measurement as part of prenatal diagnosis for chromosomal abnormalities or other genetic diseases. Most affected pregnancies can be identified by an elevated maternal serum AFP level, defined as 2.5 MoM for a singleton pregnancy. Women with a positive screening test should receive an ultrasound examination to detect identifiable causes of false-positive results (e.g., fetal death, multiple gestation, and underestimate of gestational age) and for targeted study of fetal anatomy for NTDs and other defects associated with elevated maternal serum AFP.
Approximately 90% of newborns with NTDs are born to women who have not been offered amniocentesis because they have no family or medication history that would indicate them to be at increased risk. Folic acid has been shown to prevent recurrence and occurrence of NTDs. Because most individuals at increased risk do not know it until they have an affected child, all women should be advised to take a vitamin that contains at least 0.4 mg of folic acid prior to conception. For women who previously have had a child with an NTD, the recommended dose is 4 mg daily.
Integrated Screening
The results of both first-trimester and second-trimester screening and ultrasound can be combined to increase their ability to detect Down syndrome. This “integrated” approach to screening uses both the first-trimester and second-trimester markers to adjust a woman’s age-related risk of having a child with Down syndrome. The results are reported only after both first-trimester and second-trimester screening tests are completed. Integrated screening provides the highest sensitivity with the lowest false-positive rate. The lower false-positive rate results in fewer invasive tests and, thus, fewer procedure-related losses of normal pregnancies. Although some patients value early screening, others are willing to wait several weeks if doing so would result in an improved detection rate and less chance that they will need an invasive diagnostic test. Concerns about integrated screening include possible patient anxiety generated by having to wait 3 to 4 weeks between initiation and completion of the screening and the loss of the opportunity to consider CVS if the first-trimester screening indicates a high risk of aneuploidy.
PRENATAL DIAGNOSIS OF GENETIC DISORDERS
Prenatal genetic diagnosis should be offered in circumstances in which there is a definable increased risk of a fetal genetic disorder that may be diagnosed by one or more methods. Prenatal screening or diagnosis should be voluntary and informed. In most circumstances, test results are normal and provide patients with a high degree of reassurance that a particular disorder does not affect a fetus, although there is no guarantee that the fetus is normal and with no abnormalities. Early prenatal genetic diagnosis also affords patients the option to terminate affected pregnancies. Alternatively, a diagnosis of a genetic disorder may allow a patient to prepare for the birth of an affected child and, in some circumstances, may be important in establishing a plan for care during pregnancy, labor, delivery, and the immediate neonatal period.
Carrier Testing
Individuals who have a family history of a specific genetic disorder but who show no signs of the disorder themselves may undergo carrier testing to determine the risk of passing the disorder on to their offspring. In addition, individuals with certain ethnic backgrounds predisposed to genetic disorders may undergo carrier testing. For example, the American College of Obstetricians and Gynecologists (College) recommends that individuals of Ashkenazi Jewish descent should be tested prior to pregnancy or early in pregnancy for Tay-Sachs disease, Canavan disease, cystic fibrosis, and familial dysautonomia. There are also recommendations for other ethnic groups.
Carrier testing involves testing of cells obtained from a saliva or blood sample. Genes responsible for many diseases have been located, and direct testing for the presence of a specific mutation can be performed. Examples of diseases for which direct tests exist are Tay-Sachs disease, hemophilia A, cystic fibrosis, sickle cell disease, Canavan disease, familial dysautonomia, and thalassemia. For disorders in which disease-causing mutations have not been delineated, indirect testing is required. Indirect testing refers to the process of determining DNA sequences of specific length that are linked to a mutation. These sequences, called restriction fragment length polymorphisms, can be tested for by the Southern blot technique. Indirect testing is not as accurate as direct testing.
One partner is usually tested first. If one partner is found to be a carrier of a particular disorder, the other partner is tested as well. If both partners are carriers, a genetic counselor can provide more information regarding the risk of transmitting the disorder.
Fetal Diagnostic Procedures
Prenatal analysis of DNA requires fetal nucleated cells, currently obtained by amniocentesis, CVS, or percutaneous umbilical blood sampling (PUBS).
Amniocentesis
Amniocentesis is the withdrawal of 20 to 40 mL of amniotic fluid transabdominally, under concurrent ultrasound guidance, with a 20- or 22-gauge needle. Traditional genetic amniocentesis is usually performed between 15 and 20 weeks of gestation. Direct analysis of the amniotic fluid supernatant is possible for AFP and acetylcholinesterase assays; such analyses permit the detection of fetal NTDs and other fetal structural defects (e.g., omphalocele and gastroschisis).
Studies have confirmed the safety of amniocentesis as well as its cytogenic diagnostic accuracy (>99%). The risk of pregnancy loss is less than 1%. Complications, which occur infrequently, include transient vaginal spotting or amniotic fluid leakage in approximately 1% to 2% of all cases and chorioamnionitis in less than 1 in 1,000 cases. The perinatal survival rate in cases of amniotic fluid leakage following midtrimester amniocentesis is greater than 90%.
Early amniocentesis performed from 11 to 13 weeks of gestation has significantly higher rates of pregnancy loss and complications than traditional amniocentesis as well as significantly more amniotic fluid culture failures. For these reasons, early amniocentesis before 14 weeks of gestation should not be performed.
Chorionic Villus Sampling
CVS was developed to provide prenatal diagnosis in the first trimester. CVS is performed after 10 weeks of gestation by transcervical or transabdominal aspiration of chorionic villi (immature placenta) under concurrent ultrasound guidance. Recent multicenter trials have demonstrated transabdominal CVS to have similar safety and accuracy rates to that of traditional (i.e., performed at or after 15 weeks’ gestation) amniocentesis; transcervical CVS carries a higher risk of pregnancy loss. Disorders that require analysis of amniotic fluid, such as NTDs, cannot be diagnosed with CVS. There is also a significant learning curve associated with the safe performance of CVS.
The rate of pregnancy loss associated with CVS appears to approach and may be the same as the loss associated with midtrimester amniocentesis. The most common complication of CVS is vaginal spotting or bleeding, which occurs in up to 32.2% of patients after transcervical CVS is performed. The incidence after transabdominal CVS is less. There have been reports that CVS performed before 10 weeks of gestation is associated with limb reduction and oromandibular defects. Although these associations are controversial, they should be discussed with the patient during counseling. Until further information is available, CVS should not be performed before 10 weeks of gestation.
Percutaneous Umbilical Blood Sampling
PUBS, also known as cordocentesis, is usually performed after 20 weeks of gestation and has traditionally been used to obtain fetal blood for blood component analyses (e.g., hematocrit, Rh status, and platelets), as well as cytogenetic and DNA analyses. The indications for PUBS are declining. One major benefit of PUBS is the ability to obtain rapid (18– 24 hours) fetal karyotypes. However, with the advent of FISH, the need for a procedure with more potential for complications (i.e., PUBS) has been obviated. The procedure-related pregnancy loss rate of PUBS has been reported to be less than 2%. Cordocentesis is rarely needed but may be useful to further evaluate chromosomal mosaicism discovered after CVS or amniocentesis is performed.
Other prenatal diagnostic procedures include fetal skin sampling, fetal tissue (muscle and liver) biopsy, and fetoscopy. These procedures are used only for the diagnosis of rare disorders not amenable to diagnosis by less invasive methods.
Other Tests
Once fetal cells are obtained, a variety of tests and analyses can be performed. A karyotype is a photomicrograph of the chromosomes taken during metaphase, when the chromosomes have condensed. A separate image is made of each individual chromosome from this micrograph. The chromosomes are then matched to their homologue, so that the karyotype shows the chromosome pairs. Because most fetal cells in amniotic fluid specimens obtained through amniocentesis are not in metaphase, these cells must first be cultured (grown) in order to perform a karyotype analysis. An advantage of CVS over amniocentesis is that CVS allows for rapid cytogenetic and DNA analyses, because cytotrophoblasts obtained from first-trimester placentas are more likely to be in metaphase than in amniotic fluid cells.
FISH is a technique that involves fluorescent labeling of genetic probes for specific chromosomes, most commonly 13, 18, 21, X, and Y. FISH can identify abnormalities in chromosome number, and results are usually available by 48 hours. Although FISH analysis has been shown to be accurate, false-positive and false-negative results have been reported. Therefore, clinical decision making should be based on information from FISH along with a traditional karyotype, ultrasound findings, or a positive screening test result. Spectral karyotyping (SKY) is similar to FISH but can be done for all chromosomes. SKY is useful in detecting translocations.
Comparative genomic hybridization (CGH) is an evolving method that identifies submicroscopic chromosomal deletions and duplications. This approach has been proved to be useful in identifying abnormalities in individuals with developmental delay and physical abnormalities, when results of traditional chromosomal analysis have been normal. At present, the use of CGH in prenatal diagnosis is limited because of the difficulty in interpreting which DNA alterations revealed through CGH may be normal population variants. Until more data are available, the use of CGH for routine prenatal diagnosis is not recommended.
Genetic Counseling
Many couples at increased risk for having children with genetic disorders can benefit from genetic counseling, in which the primary health care provider, a medical geneticist, or other trained professional provides information and options to individuals or families about genetic disorders and risks. By using a family and genetic history questionnaire such as those found in the College’s Antepartum Record, the provider may elicit genetic risk factors that prompt a referral to a genetic counselor. Geneticists will commonly complete at least a three-generation pedigree to visualize the mode of inheritance and specific individuals in the family who are at risk for a given genetic condition. Ideally, this counseling takes place before conception. The key elements in genetic counseling are accurate diagnosis, communication, and nondirective presentation of options. The counselor’s function is not to dictate a particular course of action but to provide information that will allow couples to make informed decisions. Counseling is directed at helping the patient or family in the following areas:
• Comprehending the medical facts, including the diagnosis, probable course of the disorder, and available management
• Appreciating the way in which heredity contributes to the disorder and the risk of occurrence or recurrence in specific relatives
• Understanding the options for dealing with the risk of recurrence, including prenatal genetic diagnosis
• Choosing the course of action that seems appropriate in view of the risk and the family’s goals and act in accordance with that decision
• Making the best possible adjustment to the disorder in an affected family member and to the risk of recurrence in another family member
Genetic counseling may also involve alternative reproductive options (e.g., pregnancy termination, permanent sterilization, selective pregnancy reduction, and donor insemination). Patients should also understand that outside parties, such as insurance companies, may be able to obtain the results of genetic testing.
GENETICS IN GYNECOLOGY: CANCER SCREENING
It is now known that certain breast and ovarian cancers have a genetic predisposition. Genetic tests have been developed for the detection of some of these genes. Gynecologists play a key role in identifying individuals with a genetic disposition for cancer and ensuring that they receive the appropriate screening tests. The most important initial step in identifying women at high risk for hereditary cancers is a thorough family history. Clues to possible hereditary cancers include a history of cancers in first-degree relatives, cancers occurring at young ages, cancers in multiple generations, or many different cancers in one individual. Based on these findings, further testing and genetic counseling may be indicated.
Breast and Ovarian Cancer
The BRCA1 and BRCA2 genes have been identified as responsible for the hereditary forms of both breast and ovarian cancers. Clinically important BRCA mutations have been found in about 3% of Ashkenazi Jewish women and are estimated to occur in about 1 in 300 to 800 women in the general non-Jewish U.S. population. Criteria developed by the College for BRCA testing referral are as follows:
• Women with breast cancer at age 40 years or younger
• Women with ovarian cancer, primary peritoneal cancer, or fallopian tube cancer of high grade, serous histology at any age
• Women with bilateral breast cancer (particularly if the first case of breast cancer was diagnosed at age 50 years or younger)
• Women with breast cancer at age 50 years or younger and a close relative with breast cancer at age 50 years or younger
• Women of Ashkenazi Jewish ancestry with breast cancer at age 50 years or younger
• Women with breast cancer at any age and two or more close relatives with breast cancer at any age (particularly if at least one case of breast cancer was diagnosed at age 50 years or younger)
• Unaffected women with a close relative that meets one of the previous criteria
Testing for BRCA in higher-risk populations is indicated if the following are present:
• Women with a personal history of both breast cancer and ovarian cancer
• Women with ovarian cancer and a close relative with ovarian cancer or premenopausal breast cancer or both Visit http://thePoint.lww.com/activate
• Women with ovarian cancer who are of Ashkenazi Jewish ancestry
• Women with breast cancer at age 50 years or younger and a close relative with ovarian cancer or male breast cancer at any age
• Women of Ashkenazi Jewish ancestry in whom breast cancer was diagnosed at age 40 years or younger
• Women with a close relative with a known BRCA1 or BRCA2 mutation
Other Cancers
In addition to breast cancer, other cancers have been found to have a hereditary component. A hereditary syndrome called hereditary nonpolyposis colorectal cancer type A (HNPCC type A), or Lynch I syndrome, increases the risk of developing colon cancer. A family history of colon, endometrial, ureteral, or renal cancers should alert the clinic to screen for the HNPCC-linked genes. HNPCC type B, or Lynch II syndrome, is an autosomal dominant inherited syndrome that increases the risk of all of the cancers in Lynch I syndrome as well as of ovarian, gastric, and pancreatic cancers. Individuals or families who meet certain criteria, which include the presence of HNPCC in two successive generations and the diagnosis of HNPCC in at least three relatives, can undergo genetic testing to determine whether they have the defective gene.
Clinical Follow-Up
Your patient’s genetic history is suspicious for fragile X syndrome. You explain how this might be inherited in her family and refer her for blood tests. If the result reveals that she might be a carrier, you explain what additional fetal testing is available, along with any potential procedure-related risks to the pregnancy.
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