Prenatal diagnosis and therapy: from laboratory to bedside

The Karolinska Institute, Department of Molecular Medicine, Clinical Genetics Unit & Center for Fetal Medicine, Department of Obstetrics and Gynecology, Karolinska University Hospital, Stockholm, Sweden. Email: the.bui@karolinska.se.

Evolution of screening methods for Down syndrome and advances in fetal imaging (2D- and 4D ultrasound, rapid MRI, fetoscopy), invasive tests and laboratory diagnostic technologies have all played an important role in progresses made in the field of fetal diagnosis. The technologies developed for the Human Genome Project, the surge of available DNA sequences resulting from it and the increasing pace of gene discoveries have also contributed to new technologies and knowledge that have widened the spectrum of disorders that can be diagnosed prenatally. The importance of determining the disease-causing mutation in single gene disorders or the informativeness of linked genetic markers before embarking upon a DNA-based prenatal diagnosis is, however, still emphasized.

Microscopic chromosome analysis of cultured cells has been regarded as the standard method for prenatal cytogenetic diagnosis since its first application to prenatal testing in 1966 and the routine use of chromosome banding analysis (karyotyping) in the early 1970’s. Karyotyping has proved to be highly reliable for diagnosis of numerical chromosome abnormalities (aneuploidy) and large structural rearrangements (>5-10 Mb) in fetal cells obtained invasively by either amniocentesis in the second trimester of 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%. 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 genetic laboratories.

Aneuploidy screening is now in the domain of epidemiology, public health and health economics. With the evolution and widespread use of screening methods on large populations of pregnant women and individualised risk estimates for Down syndrome based on maternal serum analytes and ultrasonographic findings, and the detection of most structural abnormalities at fetal organ scan by ultrasound in the second or in the third trimester, 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 also been recognised by all parties to improve pregnancy management and alleviate parental anxiety.

Interphase FISH and the quantitative fluorescence polymerase chain reaction (QF-PCR) are efficient tools for the rapid prenatal diagnosis (RAD) of selected aneuploidies, the latter method being considered to be most cost-effective if analyses are performed on a large scale. There is an on-going debate surrounding whether RAD should be employed as an adjunct to karyotyping or whether it could be used as a stand-alone test for selected indications. Obviously, the diagnostic capability of interphase FISH and QF-PCR is limited by the choice of probes used: uncommon structural and numerical chromosome abnormalities other than those tested for will remain undetected. Different fluorescence in situ hybridization (FISH) technologies provide increased resolution for the elucidation of structural chromosome abnormalities that cannot be resolved by more conventional cytogenetic analyses, including microdeletion syndromes, cryptic or subtle duplications and translocations, complex rearrangements involving many chromosomes, and marker chromosomes. Use of micro-arrays for detection of genomic disorders in fetuses with malformations is now part of research protocols.

Preimplantation genetic diagnosis (PGD) is an alternative to more conventional prenatal diagnosis by CVS or amniocentesis for patients known to be at increased risk of transmitting a specific single gene disorder or chromosomal imbalance to their offspring. The issue of pregnancy termination is avoided as genetic tests are performed before embryo transfer, generally on one or two cells biopsied at the cleavage stage from embryos obtained after in vitro fertilization (IVF),