This technique may be used for rapid identiication of a speciic chromosome abnormality and for veriication of suspected microdeletion or duplication syndromes, such as the 22q 11.2 microdeletion described earlier (p. 260). Because of its 1-to 2-day turnaround time, FISH is often selected for cases in which indings may alter pregnancy management. To perform FISH, cells are ixed onto a glass slide, and fluorescent-labeled probes are hybridized to the ixed chromosomes (Figs. 13-11 and 13-12). Each probe is a DNA sequence that is complementary to a region of the chromosome or gene being investigated. If the DNA sequence is present, hybridization is detected as a bright signal visible by microscopy. The number of signals indicates the number of chromosomes or genes of that type in the cell being analyzed. Findings are probe-speciic. Namely, FISH does not provide information on the entire chromosomal complement but merely the chromosomal or gene region of interest.
he most common prenatal application of FISH involves testing interphase chromosomes with DNA sequences speciic to chromosomes 21, 18, 13, X, and Y. Figure 13-12 shows an example of interphase FISH using a-satellite probes for chromosomes 18, X, and Y to conirm trisomy 18. In a review of
FIGURE 13-1 1 Steps in fluorescence in situ hybridization (FISH).
more than 45,000 samples, the concordance between FISH analysis and standard cytogenetic karyotyping was 99.S percent (Tepperberg, 2001). he American College of Obstetricians and Gynecologists (20 16b) recommends that clinical decision-making based on FISH incorporate clinical information consistent with the suspected diagnosis, such as an abnormal aneuploidy screening test result or sonographic inding, or incorporate a conirmatory diagnostic test such as karyotyping or CMA.
his test is 100 times more sensitive than standard karyotyping and detects microduplications and microdeletions as small as
FIGURE 13-12 Interphase fluorescence in situ hybridization (FISH) using a-satellite probes for chromosomes 18, X, and Y. In this case, the three light blue signals, two green signals, and absence of red signals indicate that this is a female fetus with trisomy 18. (Used with permission from Dr. Frederick Elder.)
Fluorescent probe illuminates chromosome region of interest 50 to 100 kilobases. Direct CMA can yield results in 3 to 5 days, whereas if cultured cells are required, results may take 10 to 14 days (American College of Obstetricians and Gynecologists, 2016b). Microarrays use either a comparative genomic hybridization (CGH) platform, a single-nucleotide polymorphism (SNP) platform, or a combination of the two. he CGH microarray platform compares test specimen DNA with a normal control sample. Shown in Figure 13-13, the CGH chip contains reference DNA fragments of known sequenceoligonucleotides. Fetal DNA from the amniocentesis or chorionic villus sampling specimen is labeled with a fluorescent dye and then hybridized to the DNA on the chip. Normal control DNA is labeled with a diferent probe and also hybridized to the chip. hen, the intensity of the luorescent signals from the two samples is compared. With an SNP array, the chip contains known DNA sequence variants-single-nucleotide polymorphisms. When fetal DNA is labeled and hybridized to the chip, the luorescent signal intensity indicates copy number variation.