Role of Molecular Cytogenetics in Obstetrics
Pornpimol Ruangvutilert MD*
Affiliation : * Department of Obstetrics and Gynecology, Faculty of Medicine, Siriraj Hospital, Mahidol University
Cytogenetics has become an important part of obstetric daily practice since prenatal diagnosis
became feasible a few decades ago. Information of chromosome constitution of the fetus can be obtained
through the prenatal diagnosis procedure. This consists of a collection of fetal samples such as chorion,
amniocytes, or fetal blood and the laboratory analysis of chromosomes. However, this usually takes time for
cell culture so that cells in metaphase are available for analysis and routine chromosome banding has
limitation in resolution.
Molecular cytogenetics has lately been developed as a result of tremendous improvement in molecu-
lar genetics both in knowledge and laboratory technology. The first one to develop was probably FISH
(fluorescent in situ hybridization). This technology has subsequently been modified to several techniques for
more applications or better quality of chromosome analysis.
FISH involves hybridization of segment (s) of DNA of interest onto the sample(1-3). The aforementioned
DNA segment is named “probe”. Probes are made to be specific for particular parts of particular chromo-
somes. They are then labeled with fluorescent dyes. When they are placed with the samples in certain condi-
tions, they will hybridize to those particular parts of chromosomes. Signals from the labeled fluorescent dyes
are then visualized under fluorescent microscope. If hybridized on to interphase cells, the number of signals of
a particular probe can be count for number of DNA segment the probe represents in each cell. This allows for
rapid diagnosis of numerical aberration of the studied chromosome in interphase cells as the sample can be
analyzed without culture. This also has a tremendous benefit on preimplantation genetic diagnosis as the cell
(s) biopsied from an embryo is in interphase. However, FISH has a drawback that not all chromosomes can be
analyzed and structural anomalies will be missed. A full karyotyping is needed later if feasible(4-7).
Chromosome painting. Another modification of in situ hybridization technique is to identify a chro-
mosome of interest on a sample. Sometimes parts of chromosome rearrangements are difficult to identified
using conventional chromosome banding. The altered position of chromosomes in question can be determined
using chromosome painting. Probes for the whole particular chromosomes are hybridized on to sample cells
in metaphase to locate parts of the rearranged chromosomes.
Comparative Genomic Hybridization (CGH). All the previous techniques have a drawback of limited
number of chromosomes that can be analyzed in a procedure due to limitation of fluorescent colors that can be
used. CGH is one of the techniques that try to circumvent this limitation. Instead of probe labeling, the sample
genome is labeled with a fluorescent color and a normal genome is labeled with another (for example, red
against green). Both genomes are then hybridized to a standard metaphase and the ratio of the fluorescent
intensity of both genomes from each chromosome can be analyzed using a computer program to see if there is
any deletion or duplication of a chromosome or parts of chromosomes.
M-FISH, spectral karyotyping (SKY), multicolor FISH(8-9). All these are analysis of all chromosomes
simultaneously using different combination of available fluorescent dyes. The analysis needs a special com-
puter program to visualize the subtle difference among each chromosome probe. The objective is to be able to
analyze all chromosomes simultaneously, especially to identify chromosomes involved in chromosome
rearrangement.
Therefore, molecular cytogenetics helps in some situations in obstetrics as follow:
1. in cases where rapid diagnosis for common trisomies is needed as a screening investigation such as in late prenatal diagnosis(10,11).
2. in cases where parts of chromosomes or a chromosome marker could not be identified using
conventional banding(10,12,13).
3. in cases of microdeletions or microduplications of chromosomes(10,12,14).
4. in cases where metaphase cannot be obtained such as in preimplantation genetic diagnosis(12).
However, as these techniques are very expensive and need expertise in the laboratory procedure, they
are still not available as routine service and most are still in research refinement. Hopefully, with further
development in other areas of prenatal sampling such as fetal cells in maternal circulation and preimplantation genetic diagnosis along with the laboratory technology, molecular cytogenetics will be more beneficial
to obstetrics in the future.
Keywords : Role of molecular cytogenetics, Obstetrics
