Common Issues in Prenatal Diagnosis

Introduction to Common Issues in Prenatal Diagnosis


The topics in this chapter bring together common issues in genetic counseling and the delivery of prenatal diagnosis services such as the interpretation of family and medical histories, parental consanguinity, non-paternity, and infertility.


A core concept in genetic counseling is the collection of accurate and complete family and maternal histories, and the utilization of that information in the risk assessment of birth defects and genetic disorders. Accurate risk assessment is dependent upon a correct diagnosis. Errors in diagnosis and hence risk assessment may occur when genetic disorders with different patterns of inheritance have overlapping clinical presentations, or when a genetic disorder masquerades as a condition that is usually explained by multifactorial or environmental factors.


Consanguinity raises concerns about the risk of birth defects and genetic disorders. Assessment of the degree of relatedness of the parents is required to establish whether a consanguineous mating places a future child at increased risk for autosomal recessive and multifactorial conditions. The cultural aspects of consanguineous marriages, which are common in some parts of the world, are also important to consider.


Non-paternity may be inadvertently detected as a result of prenatal diagnosis and may complicate the interpretation of results. The discovery of non-paternity may raise ethical questions about the risks and benefits of disclosure to a patient and the impact of that information on the integrity of relationships among family members.


The widespread accessibility of assisted reproductive technologies has resulted in an increasing number of pregnancies conceived via in vitro fertilization and related procedures. Infertility or subfertility is the most common reason for use of these technologies, and in some instances the etiology may be unknown yet still due to an inherited cause. Utilization of assisted reproductive technologies may circumvent natural barriers to reproduction, resulting in the transmission of genetic abnormalities and an increased risk of birth defects and genetic disorders.


Infertility


Case 1 A healthy 27-year-old woman and her husband are referred for genetic counseling. She has no children and has had three first trimester miscarriages. Her family history includes a sister who had multiple congenital abnormalities and died in the neonatal period. No further information about this individual is available. Her mother also had an early miscarriage and a stillbirth at 35 weeks’ gestation. The patient has two other sisters.


Around 10 to 15% of couples have fertility problems. Infertility, or early pregnancy loss that mimics infertility, can occur because of a balanced chromosomal rearrangement in one of the parents. Balanced chromosomal rearrangements, i.e., translocations and inversions, are found in a few percent of phenotypically normal individuals who have experienced recurrent spontaneous pregnancy loss. When a woman has had two or three miscarriages, chromosomal analysis of both members of the couple should be performed.


In this woman’s situation, the information that she had a sibling with multiple birth defects further increases the chance that the woman carries a chromosomal rearrangement.


Chromosomal analysis of both members of the couple reveals that the woman has an apparently balanced reciprocal translocation [46,XX, t(3;18)(q28;q12.2)]. Her husband has a normal karyotype.


In this reciprocal translocation, a small segment of the long arm of chromosome 3 has exchanged locations with about three-quarters of the long arm of chromosome 18. Because the translocation has been identified in a healthy adult woman and no cytogenetic material is missing at this level of resolution, it is unlikely to have important health implications for her. Nonetheless, there is potential disruption of genes situated at or near the translocation breakpoints and this issue should be reviewed every few years as more information about these genes becomes available.


The reciprocal translocation results in the production of abnormal gametes which can lead to infertility, recurrent early loss of chromosomally unbalanced embryos, or viable unbalanced fetuses who can survive postnatally with significant functional and structural abnormalities. The likelihood of a viable unbalanced fetus depends, in part, on the size of the imbalance. Unbalanced translocations with large partial trisomies or monosomies are usually lost very early in gestation. Survival into later pregnancy or the neonatal period is more likely when small imbalances are present or when full trisomy for the chromosome involved in the translocation is compatible with long-term survival, as is the case for chromosome 18. In this woman’s situation, her translocation places her at a significant risk for an unbalanced fetus who inherits three copies of part of the long arm of chromosome 18 and may survive into the second trimester or beyond. The pregnancy history of the woman’s mother raises suspicions that she or the woman’s father also carries the balanced translocation and that some chromosomally unbalanced fetuses may survive into late pregnancy or beyond.


Fetuses who inherit the woman’s balanced translocation would be expected to be normal. However, there are circumstances in which two family members with the same translocation have different phenotypes due to a variety of reasons. These include the inheritance of two copies of a recessive gene whereas the parent had only a single copy, small duplications or deletions of genetic material at or close to the translocation breakpoints, and epigenetic phenomena as discussed in more detail in Chapter 1 in the section on reciprocal translocations and structural abnormalities.


Prenatal diagnosis with chorionic villus sampling or amniocentesis is one option for this couple. Preimplantation genetic diagnosis could be utilized to identify embryos with unbalanced translocations and introduce only chromosomally normal or balanced embryos to the womb. The woman’s sisters and other at-risk relatives should be informed of their increased chance of carrying the chromosomal translocation.


Case 2 A non-consanguineous couple is referred for genetic counseling after a fertility evaluation revealed severe oligospermia. They are now considering in vitro fertilization and intracytoplasmic sperm injection. Both members of the couple are in their mid-thirties.


About 10 to 15% of couples have fertility problems. In about half of couples with infertility, sperm production is abnormal, either qualitatively or quantitatively; some of these men have an underlying genetic abnormality.


Intracytoplasmic sperm injection (ICSI) is the principal treatment for male factor infertility. Even in the setting of severe oligospermia or azoospermia, immature spermatozoa can be retrieved via an epididymal aspiration or testicular biopsy. Thus, ICSI circumvents effective biological mechanisms for sperm selection and improves the biological fitness of some men who have an underlying genetic abnormality.


The three most common genetic causes of male infertility are constitutional chromosomal abnormalities, microdeletions on the Y chromosome, and mutations in the cystic fibrosis (CFTR) gene located on chromosome 7. Less common explanations include androgen insensitivity, androgen receptor mutations, congenital adrenal hyperplasia, Kallmann syndrome, Noonan syndrome, and Kartagener syndrome and other primary ciliary dyskinesias.


In azoospermia and severe oligospermia an underlying constitutional chromosomal abnormality will be identified in the peripheral blood karyotype in ∼12% and 5% of men, respectively. Klinefelter syndrome (47,XXY), variants of Klinefelter syndrome (e.g., 46,XY/47,XXY), or mosaicism for other chromosomal abnormalities involving aberrations in the number of X or Y chromosomes (e.g., 46,XY/45,X) account for the great majority of chromosomal abnormalities. Chromosomal rearrangements including translocations, inversions, or marker chromosomes are present in a few percent or less. Most of these men have minor or no other phenotypic abnormalities other than abnormal sperm parameters.


Deletions in AZF or DAZ gene regions of the long arm of the Y chromosome are present in about 15% of azoospermic and severely oligospermic men. The more severe the spermatogenic defect, the higher is the likelihood of a Yq microdeletion; almost all large deletions in these regions of the Y chromosome are associated with azoospermia.


Mutations in the cystic fibrosis transmembrane regulator (CFTR) gene are often seen in healthy men who have abnormal sperm parameters and/or non-obstructive azoospermia. Among healthy men with congenital bilateral absence of the vas deferens, about 20% are compound heterozygotes for two CFTR gene mutations, or compound heterozygotes for a CFTR mutation and a gene variant which is associated with decreased transcription of the normal gene. Another 47% carry one cystic fibrosis mutation. These data suggest that the CFTR gene may play a critical role in spermatogenesis or sperm physiology. Among men with abnormal sperm parameters of any kind, as many as 1 in 5 are reported to carry one cystic fibrosis mutation.


The husband should be offered peripheral blood karyotyping, chromosome Yq microdeletion testing, and cystic fibrosis mutation screening. If he carries a cystic fibrosis mutation, his wife should be offered cystic fibrosis mutation screening and/or gene sequencing.


If the husband has a chromosomal abnormality or a Yq deletion or if both members of the couple carry identifiable CFTR gene mutations, preimplantation genetic diagnosis could be utilized. If the husband has a chromosome Yq microdeletion each of the couple’s sons would also have the microdeletion. Current information suggests that gene deletions on the Y chromosome are not medically important other than for effects on fertility.


If the husband carries a chromosomal abnormality, his siblings and other close relatives may also have the same finding. The husband should be encouraged to inform these individuals of their increased risk of fertility problems and possible risk of children with chromosomal abnormalities.


Case 3 A 35-year-old man and his wife are referred by his urologist because an evaluation for azoospermia showed that he carries an apparently balanced chromosomal translocation: 46,XY,t(11;19)(p11.2;q13.3). His past medical history included two recent hospitalizations for pneumonia and multiple episodes of bronchitis during childhood. An episode of hemoptysis occurred a few months previously and led to a chest X-ray which revealed a right-sided heart and aortic arch with the stomach bubble located under the right hemidiaphragm. A biopsy of respiratory mucosa and examination by electron microscopy showed characteristic ultrastructural ciliary defects which are usually present in individuals affected by primary ciliary dyskinesia.


Primary ciliary dyskinesia (PCD) is a ciliopathy, a class of genetic disorders characterized by abnormal ciliary structure and function. PCD is an autosomal recessive disorder manifested by abnormal clearance of mucus from the respiratory tract leading to chronic lung, sinus, and ear infections, which are a consequence of defective or absent cilia. About 50% of individuals with PCD also have situs inversus totalis in which there is mirror image reversal of the thoracic and abdominal organs without clinical consequences. Another 8% of affected individuals have heterotaxy in which the positions of the organs of the abdomen and chest are abnormally placed and which is associated with a high risk of congenital heart disease and other malformations. Infertility is common in males with PCD due to immotile sperm or impaired sperm motility caused by defects in the dynein arms present in the sperm tails resulting in abnormal flagellar structure.


Considerable locus heterogeneity for PCD is suspected. Mutations in two genes, DNAH11 and DNAH5, are the basis for about 40% of cases. Several other genes which have not yet been identified at the time of this writing are thought to account for the remaining cases based on candidate gene analysis, positional cloning, model organism analysis, and proteomic analysis. A quantitative defect in sperm, which is present in this man, is not typical of PCD.


The man undergoes genetic testing and does not have an identifiable mutation in either his DNAH11 or DNAH5 genes, the only loci for which clinically testing is currently available.


One intriguing possibility for his azoospemia is that his chromosomal translocation disrupts a gene which is critical for normal functioning and structure of cilia. If the homolog of that gene, by chance, also has a mutation associated with PCD, this would result in disease expression. Historically, studies of individuals with an abnormal phenotype and balanced chromosomal translocations have played an important role in identifying candidate disease genes. In fact, one of the loci that has previously been implicated in PCD is at chromosome 19q13.3, which is also the site of one of the translocation breakpoints in this man. Sequencing of the breakpoints of this man’s translocation might identify a nucleotide change or gene deletion that is the underlying basis for some cases of PCD. On a research basis, analysis of DNA from this man may provide important insights into the underlying molecular basis of the ciliopathies. Array comparative genomic hybridization (array CGH) would also have utility in determining whether the man’s chromosomal translocation is associated with deletion or duplication which was not detected by routine metaphase analysis.


If in vitro fertilization and intracytoplasmic sperm injection result in a successful pregnancy, the couple faces an increased risk of having a child with an unbalanced chromosomal translocation. Given that the couple will need to use assisted reproductive technology to achieve a pregnancy, preimplantation genetic diagnosis for unbalanced translocations should also be considered.


The couple is concerned about the risk of a child with PCD, presuming they are able to conceive.


The risk of PCD in the couple’s children depends on the carrier frequency of the gene in the general population. If the incidence of a disorder due to a specific gene is between 1 in 10 000 and 1 in 30 000, using the Hardy–Weinberg equilibrium, the disease incidence can be used to calculate a heterozygote frequency which ranges from 1 in 50 to about 1 in 90. Using 1/50 in the risk calculation, the risk of an affected child would be about 1/100 [1 (the chance that the husband carries a mutant gene) × 1/50 (the chance that the wife carries the same mutant gene) × ½ (the chance that she transmits that gene to the child)]. This risk calculation assumes no genetic heterogeneity for PCD, an assumption that is incorrect. The actual risk for a child with PCD is much smaller because at least twelve genetic loci have been implicated in PCD. A risk of disease in the child would only exist if the wife’s PCD mutation was at the same locus as her husband’s. The wife’s chance of being a carrier of a mutation at the same locus as her husband is smaller than the overall risk of her being a carrier of mutation in at least one locus. In the absence of an identifiable disease causing mutation in the husband or his wife, preimplantation or prenatal genetic diagnosis by molecular analysis is not possible. Ultrasonographic examination could be used to establish the fetal situs, but only half of individuals with PCD have situs inversus totalis.


Case 4 A couple is referred by their fertility specialist. Semen analysis had revealed azoospermia which prompted a chromosomal analysis for the husband. Ninety-five percent of his cells had a 46,XX karyotype and the remaining cells had a 46,XY karyotype. Testicular biopsy revealed normal-appearing testicular tissue. Sperm were noted in small quantities in one of the biopsy specimens. Physical examination of the husband revealed normal-appearing male external genitalia.


There is a wide range of effects for an individual with a mixture of XX and XY cells, ranging from normal or near normal male, to normal or near normal female, to individuals with ambiguous sexual organs. Individuals with a significant mixture of XX and XY cells have a high chance of abnormalities related to sexual development and/or sex cell development which are of variable severity. Although the husband has a high percentage of white blood cells with an XX chromosome complement, he likely has a significant percentage of cells with an XY chromosomal complement in other body tissues given his normal male appearance. The presence of the XX cell line is the most likely explanation for his impaired ability to make sperm.


Abdominal and pelvic imaging for the husband is indicated to look for evidence of any sex organ structures which would be related to having an XX cell line. If such structures were present, they may be associated with an increased risk of malignancy and endocrine and surgical consultation would be recommended.


The husband’s chromosomal complement could have resulted from the early fusion of XX and XY embryos in what was originally a twin gestation, or an XXY embryo which lost an X chromosome in some cells and a Y chromosome in others to generate the XX and XY cell lines. The latter possibility could be associated with some of the husband’s tissues containing some cells with a 47,XXY chromosomal complement.


The likelihood of a 47,XXY cell line being present in other tissues that we cannot easily study is unknown. If 47,XXY cells were present in the husband, this might be associated with a small increased risk of offspring with sex chromosome abnormalities (i.e., 47,XXX and 47,XXY).


There is almost no literature about the reproductive experience of individuals with the husband’s chromosomal complement. Although his sperm appear normally formed, they are few in number and whether they are functionally normal and can result in successful fertilization is not known. If pregnancy is achieved via in vitro fertilization and ICSI using sperm obtained from testicular biopsy, and the pregnancy proceeds normally with normal ultrasonographic imaging, one could be reasonably optimistic about the outcome.


Further Reading



1. Chodhari R, Mitchison HM, Meeks, M. (2004) Cilia, primary ciliary dyskinesia and molecular genetics. Pediatric Respiratory Reviews 5:69–76.


2. De Braekeleer M, Dao T-N (1990) Cytogenetic studies in couples experiencing repeated pregnancy losses. Human Reproduction 5:519–528.


3. Dohle GR, Halley DJ, Van Hemel JO et al. (2002) Genetic risk factors in infertile men with severe oligozoospermia and azoospermia. Human Reproduction 17 (1):13–16.


4. Hansen M, Bower C, Milne E et al. (2005) Assisted reproductive technologies and the risk of birth defects – a systematic review. Human Reproduction 20 (2):328–338.


5. Bhasin S (2007) Approach to the infertile man. Journal of Clinical Endocrinology and Metabolism 92 (6):1995–2004.


6. Gardner RJM, Sutherland GR (2004) Chromosome Abnormalities and Genetic Counseling, 3rd edition. Oxford University Press.


7. Sugawara N, Tokunaga Y, Maeda M et al. (2005) A successful pregnancy outcome using frozen testicular sperm from a chimeric infertile male with a 46,XX/46,XY karyotype: Case report. Human Reproduction 20 (1):147–148.


8. Walsh TJ, Pera RR, Turek PJ (2009) The genetics of male infertility. Seminars in Reproductive Medicine 27 (2):124–136.

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Jul 31, 2016 | Posted by in OBSTETRICS | Comments Off on Common Issues in Prenatal Diagnosis
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