The development of fluorescence in situ hybridization (FISH)- and polymerase chain reaction (PCR)-based assays for the detection of deletions of chromosome 22q11.2 has enabled the medical community to offer couples at risk prenatal diagnostic testing. Current indications for testing include a previous child with a 22q11.2 deletion or DiGeorge/velocardiofacial syndrome, an affected parent with a 22q11.2 deletion, and in utero detection of a conotruncal cardiac defect. Antenatal knowledge of the deletion status provides couples and clinicians with an accurate diagnosis, prognostic information, and recurrence risk, which may assist couples with their reproductive decisions. However, there are limitations to prenatal testing, which should be reviewed prior to testing.
The majority of patients with DiGeorge and velocardiofacial syndrome (DGS/VCFS) have large interstitial deletions of chromosomal region 22q11.2.1 In addition, several studies have demonstrated that a significant percentage of cardiac patients with conotruncal cardiac malformations have a 22q11.2 deletion.2–5 Although DGS/VCFS were initially considered rare disorders, recent studies suggest that the 22q11.2 deletion may occur as frequently as 1 in 4000 live births.6 Deletions of 22q11.2 have also been detected in patients with conotruncal anomaly face syndrome and in some patients with OpitzG/BBB and Cayler cardiofacial syndrome.7–9 These disorders, collectively referred to as the 22q11.2 deletion syndrome, are predominantly characterized by congenital cardiac defects, immune deficiencies secondary to aplasia or hypoplasia of the thymus, hypocalcemia due to small or absent parathyroid glands, palatal and speech abnormalities, and cognitive difficulties. Large clinical studies and case reports have shown that the phenotypic features seen in patients with the 22q11.2 deletion are much more variable and extensive than previously recognized, and include developmental problems, feeding difficulties, neurologic, ocular, psychiatric, renal, and skeletal abnormalities.10–13 Although the majority of deletions are de novo, the deletion can be transmitted as an autosomal dominant; hence, individuals with a 22q11.2 deletion have a 50% risk of having an affected offspring in each pregnancy. Familial deletions have been identified in 8–28% of probands.11,12 The development of fluorescence in situ hybridization (FISH)- and polymerase chain reaction (PCR)-based assays for the detection of deletions of chromosome 22q11.2 has enabled the medical community to offer couples at risk prenatal diagnostic testing.14–20
INDICATIONS FOR PRENATAL TESTING
Current indications for prenatal testing for the 2q11.2 deletion include (1) a previous child with a 22q11.2 deletion or DiGeorge/velocardiofacial syndrome, (2) an affected parent with a 22q11.2 deletion, and (3) in utero detection of a fetus with a conotruncal cardiac defect. The risk for unaffected parents of having another child with the 22q11.2 deletion is presumably low; however, prenatal testing for the deletion is offered because of the possibility of germline mosaicism. Germline mosaicism for intragenic deletions has been reported for several genetic disorders, including the 22q11.2 deletion.21 In cases for which a deceased child with features suggestive of DGS/VCFS was not tested for the deletion, a careful evaluation of the parents for the presence of mild phenotypic features and 22q11.2 deletion testing is recommended. These parents may elect prenatal testing with the understanding that the interpretation of negative test results is limited. The etiology of DGS/VCFS is heterogeneous; 10–15% of individuals with characteristic features do not have a 22q11.2 deletion.1 Therefore, prenatal testing for the deletion is not helpful for couples with affected offspring without evidence of a 22q11.2 deletion. In contrast, affected parents and patients have a 50% chance of transmitting the deletion to their offspring in each pregnancy and may proceed with prenatal testing to make informed reproductive decisions.
Since the 22q11.2 deletion has been shown to be one of the most commonly recognized causes of conotruncal cardiac defects, testing for the 22q11.2 deletion should be considered, in addition to a fetal karyotype, when a cardiac defect is detected in utero by fetal echocardiography. This is particularly true for cardiac lesions with a high frequency of the deletion including interrupted aortic arch (50–80%) and truncus arteriosus (35%). The deletion is less commonly found in patients with tetralogy of Fallot (15%) and rare in double outlet right ventricle and transposition of the great vessels.2–5 Several reports have demonstrated the utility of antenatal testing for the 22q11.2 deletion when a congenital heart defect is identified in utero.14–19,23 The majority of these fetuses had either an outflow tract or aortic arch malformation, although Raymond et al. also identified a 22q11.2 deletion in one fetus with an atrioventricular septal defect and double outlet right ventricle.16
Sonographic demonstration of associated anomalies, including cleft palate, polyhydramnios, renal, or skeletal anomalies, may raise the suspicion of a 22q11.2 deletion in a fetus with any type of cardiac defect. Although palatal abnormalities are a common feature in the 22q11.2 deletion syndrome, overt cleft palate or cleft lip, which may be detected antenatally by ultrasonography, occur in < 12% of patients.12 The 22q11.2 deletion has not been found to be a cause of nonsyndromic cleft palate; hence, prenatal testing is not recommended in the absence of other findings.22 Polyhydramnios can be associated with several anomalies common to the 22q11.2 deletion syndrome, including cardiac defects, cleft palate, and swallowing difficulties. Prenatal detection of a 22q11.2 deletion has been reported in a fetus with polyhydramnios and tetralogy of Fallot.23 Retrospectively, polyhydramnios of variable severity and onset was documented in 8 of 52 cases subsequently found to have 22q11.2 deletion. Six of the eight had structural anomalies, including cardiac defects, hydronephrosis, and cleft palate; all 8 developed severe postnatal feeding difficulties.23 Hence, the finding of polyhydramnios in a fetus with a 22q11.2 deletion should alert the medical team and family to the possibility of severe feeding difficulties. Skeletal anomalies of the extremities such as pre- and postaxial polydactyly, club hands and feet, and syndactyly occur in approximately 15% of patients. Renal anomalies, including renal agenesis, multicystic kidneys, hydroureter with hydronephrosis, have been reported in as high as 37% of deleted patients.11,12 Although these skeletal and renal anomalies may be recognized antenatally by sonographic examination, the incidence of the 22q11.2 deletion in patients with these anomalies in the absence of a cardiac defect remains to be determined but, is presumably low.24 Therefore, the decision to offer prenatal testing for the 22q11.2 deletion should be based on the finding of either a cardiac defect or two or more associated anomalies and family history.
Family history is very important in assessing the risk of a 22q11.2 deletion in the fetus. Recent studies have demonstrated that affected parents are often overlooked because of the mild presentation but, in fact, often report learning difficulties, hypernasal speech and have mild dysmorphic facial features consistent with the 22q11.2 deletion syndrome.25,26 Hence, close attention should be paid to the parent’s past school performance, need for speech therapy, evidence of hypernasal speech, and the presence of classic facial characteristics such as protuberant ears with overfolded helices, hypoplastic alae nasae, bulbous nasal tip, prominent nasal root, “hooded eyelids,” and long face with malar flattening (Fig. 1). Although the facial features can provide a clue to the diagnosis, they can also be quite subtle and variable.
The deletion has also been reported in an infant with nuchal translucency at 12–13 weeks gestation that was subsequently found at birth to have an interrupted aortic arch.27 Since nuchal translucency can be seen in association with cardiac defects and given the high frequency of cardiac defects in the 22q11.2 deletion syndrome, this is not an unexpected finding. At the present time, 22q11.2 deletion testing is not recommended until fetal echocardiography has been performed to document the presence of a cardiac defect. However, since chorionic villus sampling (CVS) or amniocentesis are often performed to diagnose fetal anueploidy when the fetus has nuchal translucency, the laboratory may want to consider saving the cultured cells for FISH analysis in the event that a cardiac defect is subsequently found.27
DETECTION OF THE 22q11.2 DELETION
The most widely utilized method for detecting the 22q11.2 deletion is FISH with probes from the DiGeorge chromosomal region (DGCR). Prenatal detection of the 22q11.2 deletion is usually performed on metaphase chromosomes from either cultured amniocytes obtained by amniocentesis after 15 weeks gestation or cultured chorionic villi obtained by CVS at 10–12 weeks gestation. Occasionally, cytogenetic analysis will demonstrate an unbalanced translocation or an interstitial deletion of 22q11.2. However, the sensitivity is low; therefore, most cytogenetic laboratories rely almost exclusively on FISH to evaluate the at-risk pregnancy. Preimplantation genetic diagnosis by FISH has also been reported in an at-risk mother with a 22q11.2 deletion.28 This may be an attractive option for some couples with a 50% risk of transmitting the deletion who wish to avoid having an affected child and would not consider prenatal testing and termination of an affected pregnancy.
Alternatively, DNA-based assays can be used to demonstrate failure to inherit a parental allele or hemizygosity in the deleted region. Restriction fragment length polymorphism (RFLP) analysis and quantitative hybridization have been used in the past.29 More recently, PCR assays using short tandem repeat polymorphisms (STRPs) within the DGCR have been developed and utilized for diagnostic purposes.30,31 PCR assays are attractive for prenatal testing, since genotyping can be performed rapidly using uncultured villi or amniotic fluid. Furthermore, they may be utilized by molecular diagnostic laboratories that lack the ability to perform FISH. However, a complete evaluation of the fetus with a cardiac malformation should include a karyotype to exclude aneuploidy or other chromosomal rearrangements; PCR may be used as a rapid adjunct to cytogenetic analysis.
Ultrasonography and fetal echocardiography may identify a fetus at risk for the 22q11.2 deletion. However, these are not considered diagnostic tests. Once a conotruncal cardiac defect with or without associated anomalies has been identified, deletion testing is recommended. Some of the anomalies reported in the 22q11.2 deletion syndrome may also be detected by ultrasonographic examination. However, these are nonspecific findings and if seen in combination with either a cardiac defect or other associated anomalies should prompt further evaluation for the presence of a 22q11.2 deletion.
BENEFITS OF PRENATAL TESTING FOR THE 22q11.2 DELETION
For unaffected parents with a previous child with a 22q11.2 deletion, prenatal testing usually provides reassurance. Couples at high risk for transmitting a familial deletion may base their decision whether to continue the pregnancy or the type of obstetric and pediatric care they require on prenatal test results. When a fetal cardiac defect is identified, antenatal knowledge of the fetal deletion status provides couples and clinicians with an accurate diagnosis. The finding of a 22q11.2 deletion enables clinicians to provide a couple with an accurate assessment of their recurrence risk and to determine whether other relatives are at risk. Furthermore, they can provide prognostic information, including cognitive and developmental profiles based on data collected from large clinical studies of individuals with the 22q11.2 deletion, which will enable the couple to make an informed reproductive decision.10–13,32,33 Couples and their physicians can prepare for the delivery of a child with the deletion and anticipate associated problems such as hypocalcemia, immune deficiencies, and feeding difficulties in the newborn period. Delivery at a tertiary care center should also be considered, especially when a complex cardiac lesion is detected antenatally. Infants and children with 22q11.2 deletions may benefit from early intervention based on the evaluation and long-term care by a multidisciplinary team of pediatric specialists aware of the wide spectrum of medical, neuropsychologic, and cognitive problems these children can encounter. Parent support groups are available in the United States, Canada, Europe, and Australia, as well as on-line and can provide families with additional information and support.
LIMITATIONS OF PRENATAL TESTING FOR THE 22q11.2 DELETION
FISH is a highly accurate test for detecting the standard 22q11.2 deletion when a couple has had a previous child with the deletion or a parent has the deletion. However, DGS/VCFS is a heterogeneous disorder and 10–15% of individuals with the characteristic features do not have the standard 22q11.2 deletion. Hence, prenatal testing for the detection of a 22q11.2 deletion is of limited value for couples who have had a previous affected child without a deletion. Several other genetic causes of DGS/VCFS have been identified in these individuals, including interstitial deletions of the short arm of chromosome 10 and long arm of chromosome 4, unbalanced chromosomal translocations, and atypical deletions of 22q11.2.34–40 These atypical deletions of 22q11 have not been detected by FISH using commercially available probes. Although numerous genes in the commonly deleted region of 22q11.2 have been identified, there has been only a single case report of a small intragenic deletion in UFDIL.41 At the present time, no laboratory has identified point mutations in any other gene in the region in nondeleted patients with DGS/VCFS.42
The etiology of congenital heart defects (CHD) is heterogeneous; in addition to the 22q11.2 deletion, other causes must be considered, including aneuploidy, other single gene defects, maternal disease states such as diabetes, and exposure to teratogens. The detection rate for the 22q11.2 deletion in a fetus with a conotruncal cardiac defect depends on the type of lesion and the accuracy of the fetal echocardiogram. Based on postnatal studies the detection rate for the 22q11.2 deletion in interrupted aortic arch and truncus arteriosus is high, 50–80% and 35%, respectively. The detection rate is lower for other types of conotruncal cardiac malformations, in particular, double outlet right ventricle and transposition of the great vessels. Furthermore, the sensitivity of detailed fetal echocardiography for the detection of a CHD is approximately 70% but, varies widely from 51 to 91%. The detection rate based on routine ultrasound examination is much lower, 10 to 20%.43 Couples should receive counseling regarding the potential causes of CHD, limitations of echocardiography, and the accuracy of deletion testing prior to consenting to the procedure and testing.
Another major limitation of prenatal testing is our inability to accurately predict the phenotype antenatally. Studies indicate that there is no correlation between the phenotype and the extent or size of the 22q11.2 deletion.44–46 The severity cannot be assessed prenatally. Studies of familial deletions demonstrate significant intrafamilial variability; hence, the parental phenotype is not a reliable predictor of outcome in the fetus.25,26 Ultrasonography and fetal echocardiography may be helpful in further defining the phenotype. The identification of a cardiac defect provides clinicians with information, which may be useful in the management of the pregnancy and the neonate. However, fetal echocardiography may not identify all structural lesions prenatally and abnormalities of the outflow tract and aberrant vessels commonly seen in association with the 22q11.2 deletion may be missed. Hence, a postnatal cardiac evaluation is recommended. The clinical presentation of the 22q11.2 deletion is highly variable and includes many features which cannot be evaluated prenatally but require long-term prospective evaluation by a multidisciplinary team of pediatric specialists.
Prenatal testing for the 22q11.2 deletion should be offered to couples based on their family history of a previous child with the deletion or features of DGS/VCFS, a parent with the deletion, or when a conotruncal cardiac defect is identified antenatally. Pretest counseling should include a risk assessment based on the family history or the frequency of the deletion in patients with conotruncal cardiac defects. Couples should also receive information about the 22q11.2 deletion syndrome and be informed of the variability and prognosis. Prior to testing for the deletion, couples should be counseled about the benefits and limitations of testing. Written informed consent should always be obtained from couples electing to undergo either CVS or amniocentesis. When a deletion is detected, couples benefit from post-test counseling to review the phenotypic variability, the implications of having a child with the 22q11.2 deletion, and the current testing limitations. Parents may benefit from speaking with a geneticist with expertise in the 22q11.2 deletion syndrome or with parents of children with the 22q11.2 deletion through the local or on-line support groups. This information may be helpful for couples in considering their pregnancy options.
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This work was supported in part by grants DC02027 and HL51533 from the National Institutes of Health.
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Cite this article
Driscoll, D. Prenatal diagnosis of the 22q11.2 deletion syndrome. Genet Med 3, 14–18 (2001). https://doi.org/10.1097/00125817-200101000-00004
- prenatal diagnosis
- 22q11.2 deletion
- conotruncal cardiac defect
- fetal echocardiography
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