The Prenatal Diagnosis of Genetic Diseases

Background: Prenatal diagnosis is a subfield of clinical genetics and gynecology that exemplifies the effective integration of theoretical and clinical medicine. Milestones in its history include the development of cytogenetic, molecular genetic, and molecular cytogenetic methods as well as advances in ultrasonography. The latter technique not only improves the safety of invasive procedures, but also enables earlier and more reliable diagnosis of congenital malformations.

Methods: This article provides an overview of the subject in the light of selectively reviewed literature, guidelines, and recommendations.

Results and conclusion: Invasive prenatal diagnosis is most commonly performed to assess the embryonal/fetal chromosome set. An increasing number of monogenic diseases can be diagnosed prenatally by either genetic or biochemical testing, depending on the particular disease being sought. Polygenic and multifactorial diseases cannot be reliably diagnosed by genetic testing at present, although a number of malformations can be ascertained prenatally by ultrasonography. We discuss the applications and limitations of invasive and noninvasive techniques for prenatal diagnosis.

The term prenatal diagnosis strictly comprises all diagnostic modalities aimed at gaining information about the embryo or fetus. However in its narrower usage it refers to the prenatal identification of genetically determined diseases and their disposition. In recognition of progress in this area the German Medical Association (Bundesärztekammer, BÄK) published guidelines on prenatal diagnosis of illness and its disposition in 1998 (1).

A disease of wholly or partly genetic in origin is present in around 4% of all neonates. Genetically determined or co-determined diseases can be divided into three groups:

• Chromosomal aberrations
• Monogenetic diseases which are caused by single gene mutation
• Polygenetic/multifactorial diseases, which are caused by mutations in several genetic areas as well as exogenous factors.

The following article will focus on the applications and limitations of the prenatal diagnosis of chromosomal abnormalities and monogenetic diseases, but will not focus on ultrasonographic diagnosis of fetal anomalies, whether occurring in isolation or as part of syndromes including monogenetically inherited diseases.

The prenatal diagnosis of chromosomal abnormalities

Common reasons for karyotype analysis are:

• Maternal age: the probability of chromosomal anomalies increases with maternal age (Figure 1 gif ppt). Around half of chromosomal anomalies are accounted for by trisomy 21 (Down syndrome) (2)
• Following an abnormal result from non-invasive screening
• Following an ultrasound finding which raises the possibility of a chromosomal problem
• In the presence of a known translocation, inversion or insertion in one parent; in these cases, the probability of an unbalanced aberration is increased, over and above the maternal age-related risk.
• A chromosomal anomaly in an existing child of the couple’s. For example, after the birth of a child with free trisomy, the risk of a numerical chromosomal abnormality is increased by 1% for each subsequent child compared to other parents of the same age (3).

As chromosome analysis requires cellular material, an appropriate intervention is needed, of which several are available, depending on gestation, clinical question, and interventional risk (Table 1 gif ppt, eBox 1 gif ppt).

Amniocentesis

Amniocentesis is typically carried out under ultrasonographic control, between 15 and 17 weeks of pregnancy. The procedure specific risk is 0.5% to 1% (3). Around 15 mL of amniotic fluid would normally be aspirated. Chromosomal analysis requires prior cell culture, which takes on average two weeks. After this metaphase chromosomes are analyzed numerically and structurally (Figure 2 gif ppt). Uncultured amniotic fluid can be used to determine levels of alpha fetoprotein (AFP), which is present in increased concentrations in open neural tube defects and other disorders such as abdominal wall defects (e.g., gastroschisis). In the presence of a raised AFP level, acetylcholinesterase is measured as a marker of neural tube defects (eBox 2 gif ppt).

Amniocentesis can also be used in conjunction with a rapid screening test for numerical aberrations of specific chromosomes, which can be used on uncultured amniotic fluid, in conjunction with conventional cytogenetic analysis. Using FISH analysis (fluorescent in situ hybridization) with chromosome-specific probes at interphase nuclei or via molecular genetic analysis of highly polymorphic markers on a DNA probe isolated from uncultured amniotic fluid cells, information can be obtained regarding numerical abnormalities of chromosomes 13, 18 and 21, and the X and Y chromosomes (Figure 3 gif ppt). This test allows detection of the commonest chromosomal anomalies within one to three days. This test is of particular importance where morphological abnormalities potentially associated with the above conditions have been detected, and where a rapid diagnosis is required at a late stage of pregnancy. A prenatal rapid test can serve to reassure the pregnant woman if normal, but cannot replace formal karyotyping (http://www.gfhev.de/en/documents/index.htm).

Chorionic villus sampling

Chorionic villus sampling (CVS) is typically performed in the 11^th to 12^th weeks of pregnancy. It should not be carried out before the 11^th week in view of the increased risk of limb abnormalities associated with CVS prior to this date, which some commentators have linked to placental trauma and vascular infarction at a critical stage of development. Depending on placental site, CVS can be carried out either transcervically or transabdominally. Chromosome analysis is carried out either as a direct preparation or following brief culture (1 day) as well as after full culture (7 to 10 days). In experienced hands the procedure-specific risk of miscarriage is up to 1% (eBox 3 gif ppt).

Placental biopsy

Placental biopsy is in effect late transabdominal chorionic villus sampling. It can be used to obtain a rapid result at a late stage of pregnancy.

Cordocentesis

Cordocentesis is a technically challenging intervention in which the umbilical vein is entered where possible at the site of placental insertion. The commonest indications are suspected fetal anemia in association with rhesus disease, parvovirus B19 infection, or fetal hydrops. Cordocentesis can also be used for rapid karyotyping or molecular genetic diagnosis from 16 to 20 weeks, depending on indication. Cordocentesis is important where a rapid result is needed late in pregnancy, for example in the presence of ultrasonographically detected anomalies or severe growth restriction, which may point to a chromosomal abnormality. The result of chromosome analysis of lymphocytes from cord blood can be available in three to five days.

The limitations of cytogenetic diagnosis

Prenatal karyotyping is a reliable procedure, but like all tests, limited by factors which can be technical or biological. The probability of obtaining no fetal cellular material is less than 1% in experienced hands. Culture failure can occasionally occur.

One limitation of cytogenetic diagnosis arises via the optical resolution of the chromosomes. Structural chromosomal abnormalities smaller than the achievable optical resolution cannot be detected. A further limitation relates to the detection of a possible chromosomal mosaic, where two or more cell lines can be present. A mosaic can only be detected if chromosomally aberrant cells are present in the examined specimen.

The demonstration of certain structural abnormalities such as translocation or inversion often requires further investigation (eBox 4 gif ppt).

Noninvasive investigations

Invasive prenatal diagnosis on the grounds of maternal age is increasingly being replaced by combined assessment of risk factors, of which maternal age is just one. The miscarriage risk in particular drives a need for noninvasive alternatives to the above interventions. In addition to maternal age, certain biochemical parameters in maternal blood and first trimester ultrasonographic appearances allow individualized assessment of the risk of aneuploidy. Counselling should make it clear that these noninvasive methods modify the maternal age-related risk of certain chromosomal abnormalities, but cannot rule out chromosomal abnormality. They can however aid decision making for or against an invasive method.

Nuchal translucency screening

Increased nuchal translucency in the fetus is associated with increased risk of chromosomal abnormality and other diseases (4). Sonographic measurement of the thickness of the nuchal fold between 11+0 and 13+6 weeks of pregnancy, together with maternal age and biochemical markers allows an individualized risk of aneuploidies such as trisomy 21, 13, and 18 to be calculated. This allows, for a screen-positive rate of 5 %, 80 % (nuchal translucency only) to 90% (nuchal translucency plus biochemical parameters) of cases of trisomy 21 to be detected (Table 2 gif ppt). However, nuchal translucency measurement is not comparable with targeted anomaly diagnosis carried out in a specialist setting as part of a so-called extended first trimester screening. The aim of this type of early anomaly scan is to identify fetal anomalies, with nuchal thickness as an integrated part. The German Society for Ultrasound in Medicine (Deutsche Gesellschaft für Ultraschall in der Medizin, DEGUM) (5) advises gynecologists in possession of a nuchal translucency measurement certificate but who do not have a specialist qualification in anomaly scanning to refer women with abnormal nuchal thickness measurements (greater than the 95^th centile for gestational age) and multiple pregnancy to a specialist center (DEGUM level II or III). This patient group may conceal numerous additional diseases such as cardiac anomalies, in addition to chromosomal anomalies (4).

Prerequisites for an interpretable nuchal thickness measurement include operator qualification, choice of appropriate duration of investigation, and technical considerations. Inclusion of additional parameters such as measurement of the nasal bone, Doppler assessment of the tricuspid valve, and the ductus venosus, and the facial angle allows individualised detection rates for trisomy 21 to be increased to up to 95% (Table 2).

Biochemical parameters

In recent years, the measurement of human chorionic gonadotrophin (HCG) and pregnancy-associated plasma protein A (PAPP-A) in maternal serum between the 11^th and 14^th weeks of pregnancy have become increasingly established in combination with nuchal translucency measurement and maternal age (combined first trimester test). (5). Prior to this, the so-called triple test was offered (6), measuring alpha fetoprotein (AFP), HCG and free estriol between 15 and 20 weeks. A further biochemical parameter, Inhibin A, when added to the triple test, yields the so-called quadruple test (7). Accurate assessment of gestational age is essential for interpreting the biochemical parameters. As PAPP-A and HCG measurement in association with nuchal translucency measurement are always carried out in conjunction with fetal biometry, for example using the crown rump length, this allows concurrent assessment of gestational age. This is not the case with the triple test. Laboratories calculate individual risks for trisomy 21, 13 and 18 and for neural tube defects using the gestation as reported by the gynecologist. This often involves using the last menstrual period, which leads to a relatively high level of error. In our experience, couples are often worried unnecessarily by a wrongly calculated triple test. This, together with the possibility of earlier and more precise risk assessment in the first trimester, is an argument against the triple test.

Prenatal diagnosis of monogenetically determined diseases

At the present time around 5000 known disorders are inherited in a monogenetic mendelian fashion. Foremost among them are autosomal dominant, autosomal recessive and X-linked disorders, which carry a higher risk of illness than that conveyed by age-related risk. An autosomal dominant condition carries an a priori 50% inheritance risk where one parent is affected. An autosomal recessive disease carries a 25% inheritance risk for children of a healthy carrier couple. An X-linked recessive disorder carries a 50% risk for the son of a carrier mother.

Specific, albeit non-screening genetic tests are currently available for more than 1000 of these diseases. Unlike cytogenetic prenatal diagnosis based on maternal age, prenatal gene testing is not a screening test. Given the individuality of each case, prior planning is essential. Two differing strategies are possible: indirect and direct genetic testing.

Direct genetic testing involves the identification or exclusion of the relevant mutation(s), and assumes knowledge of existing mutations in the index patient.

Indirect genetic testing involves the demonstration or exclusion of the so-called high risk haplotype in the fetus. Indirect genetic testing uses the principle of genetic linkage, and requires examination of the family for polymorphic markers whose alleles are closely associated with disease in this family. In theory, all that is required in an informative family is the locus of the relevant gene. Diagnostic uncertainty arises where there is heterogeneity in the gene locus, i.e. when mutations in different genes lead to the same disease. An additional, albeit quantifiable uncertainty arises where a gene recombines with a linked marker. It is obvious that the reliable interpretation of indirect genetic testing presupposes that the stated relationships within the pedigree are correct.

In a positive prenatal diagnostic test, especially for an autosomal dominant condition, the possibility of variable expression and reduced penetrance must be borne in mind. Variable expressivity of a mutation occurs when the resulting phenotype is more or less strongly expressed in different members of the same family.

Reduced penetrance implies missing expression of the mutation. In this situation the phenotype can be normal despite the mutation. Variable expression and reduced penetrance can be explained by as yet largely unknown modifying factors. For this reason it is important to discuss these issues during genetic counselling.

For reasons dictated by time and technical considerations, a molecular genetic test is usually carried out in the context of CVS following polymerase chain reaction (PCR) for amplification of DNA isolated from chorionic villi, before eventual DNA sequencing can be carried out (eBox 5 gif ppt).

Inherited metabolic disorders can in some cases be diagnosed from chorionic villi or amniotic cells (10). The prerequisite for this is that the relevant gene is expressed in these cells and the metabolic defect has been detected in fibroblasts (via skin biopsy) in an index patient within the family. Some genetic disorders are sought directly in the amniotic fluid (eBox 6 gif ppt).

Genetic counselling in prenatal diagnosis

The German genetic testing act (Gendiagnostikgesetz) of February 1, 2010 stipulates that pregnant women must be offered genetic counselling before and after prenatal diagnostic testing (11). The following areas should be covered:

• Presentation of the background risk of congenital disease and anomaly, and individual increased risks (for example increased maternal age)
• The options and limitations for prenatal genetic diagnosis
• Possible diseases which can be detected
• Risks associated with the relevant tests
• Conflictual areas in relation to prenatal diagnosis
• Alternatives.

Even the possibility of prenatal diagnosis can plunge a couple into conflictual areas. In many cases prenatal diagnosis can provide reassurance. However, only in a small number of cases can pathology be effectively treated by early intrauterine or neonatal treatment. The detection of significant disease or impairment can be grounds for termination of pregnancy. According to article 218 a paragraph 2 StGB (Strafgesetzbuch, the German Criminal Law), termination of pregnancy by a medical practitioner with the consent of the pregnant woman is legal when: „... after consideration of the present and future terms of living of the pregnant woman it is medically established that, in order to prevent danger to the life or danger of severe impairment of the physical or mental health of the mother, and this danger cannot be prevented by any other means acceptable to her“. In this conflict between the parents’ wish for a healthy child and the basic recognition of protection for the unborn child, termination of pregnancy following the detection of severe fetal illness or impairment represents „an incomplete attempt to end a fundamentally insoluble conflict“ (1).

The principle of non-directiveness must be applied in all genetic counselling including prenatal diagnosis. In this context it should be made clear that a pathological result in no way automatically implies termination of pregnancy. In addition to genetic counselling as part of prenatal diagnosis, psychosocial counselling can be offered. This may help the couple face and resolve the conflicts discussed, via exploration of the possible consequences of diagnosis, and offer support in dealing with serious fetal anomaly. This is particularly advisable in the case of serious anomaly. The new pregnancy conflict act which took force on January 1, 2010 stipulates that women considering legal termination of pregnancy must receive advice about psychosocial implications. Likewise, the woman must be informed of her right to appropriate psychosocial counselling and other specialist medical advice such as that of a specialized pediatrician. The responsibility for communication of this information lies with the doctor who confirms the indication for termination of pregnancy. In addition, a three day “cooling off period” is required following communication of a diagnosis before the formal indication for termination of pregnancy can be confirmed (eBox 7 gif ppt).

Acknowledgement
We wish to thank Prof. P. Propping (Bonn) for critical discussion.

Conflict of interest statement
The authors declare no conflict of interest in the terms of the guidelines of the International Committee of Medical Journal Editors.

Manuscript submitted on 17 November 2009, revised version accepted on 11 February 2010.

Corresponding author
Prof. Dr. med. Peter Wieacker
Institut für Humangenetik, Vesaliusweg 12–14,
48149 Münster, Germany

@For eReferences please refer to:
www.aerzteblatt-international.de/ref4810

eFigure (gif ppt) and eBoxes available at:
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