Prenatal cytogenetic diagnosis: gone FISHing, BAC soon!
The Karolinska Institute, Department of Molecular Medicine, Clinical Genetic Unit and Center for Fetal Medicine, Karolinska University Hospital, SE-17176 Stockholm, Sweden (e-mail:firstname.lastname@example.org)
Cytogenetic analysis is an important component of invasive prenatal diagnosis as chromosomal abnormalities are detected in about 1 in 200 newborns and constitute a major cause of mental retardation and congenital malformations1 . Microscopic chromosome analysis of cultured cells has been regarded as the standard method for prenatal cytogenetic diagnosis sinceits ﬁrst application to prenatal testing in 19662 and the routine use of chromosome banding analysis (karyotyping) in the early 1970s3,4 . Karyotyping has proved to be highly reliable for diagnosis of numerical chromosome abnormalities (aneuploidy) and large structural rearrangements (> 5–10 million base pairs, Mb) in fetal cells obtained invasively by either amniocentesis in the second trimesterof pregnancy or chorionic villus sampling (CVS) in the ﬁrst trimester since the early 1980s. The diagnostic accuracy of karyotyping cultured amniotic ﬂuid cells has been found to be 99.4–99.8% and that of CVS 97.5–99.6%5 – 9 . However, the main limitation of karyotyping remains the requirement for cell culture, resulting in a delay of 10–14 days for test results in many clinical geneticlaboratories10 . In the early 1980s, as better ultrasonographic imaging became available, less traumatic access to pure fetal blood than the fetoscopic approach was obtained from about 18 weeks’ gestation by percutaneous umbilical blood sampling, also called cordocentesis11 , allowing rapid karyotyping of phytohemagglutinin-stimulated lymphocytes and results within 48–72 hours. However, this procedure isgenerally only available at referral centers due to the need for proximity to a genetics laboratory. It is also associated with a higher risk for complications than are the other invasive tests and, hence, only performed on selected cases12 . With the evolution and widespread use of screening methods and individualized risk estimates for Down syndrome based on maternal serum analytes andultrasonographic ﬁndings13 – 16 and the detection of most structural abnormalities at routine fetal anomaly scan17 – 19 or third-trimester ultrasound20 , the pattern of referral for invasive cytogenetic testing has radically
changed during the last 35 years. As a result, the need for more rapid testing methods which do not require cell culture has been recognized by all parties to improve pregnancymanagement and alleviate parental anxiety21 .
Rapid aneuploidy diagnosis
Interphase ﬂuorescence in situ hybridization (FISH), quantitative ﬂuorescence polymerase chain reaction (QF-PCR) and several other new techniques under investigation, collectively referred to as rapid aneuploidy detection or diagnosis (RAD), have been introduced to answer speciﬁc diagnostic questions (reviewed by Shaffer andBui22 ). These technologies allow rapid detection (1 or 2 days) of the most common aneuploidies: the autosomal trisomies for chromosome 13 (Patau syndrome), chromosome 18 (Edwards syndrome) and chromosome 21 (Down syndrome); aneuploidy of the sex chromosomes such as Turner syndrome (monosomy X) and Klinefelter syndrome (XXY); and triploidy, that together account for more than 80% of clinicallysigniﬁcant chromosomal abnormalities diagnosed in the prenatal period. Mosaicism and structural rearrangements
Copyright 2007 ISUOG. Published by John Wiley & Sons, Ltd.
Bui Quantitative ﬂuorescence polymerase chain reaction (QF-PCR) An alternative approach to interphase FISH is a QF-PCR multiplex assay, which also permits the detection of major numerical chromosome...