A recent retrospective analysis, the findings of which were published in the journal, Human Reproduction, reports that performing additional rounds of hybridization during fluorescence in situ hybridization (FISH) improves the accuracy of the technique in preimplantation genetic aneuploidy screening (PGS), thereby increasing the number of competent embryos available for transfer.
Pere Mir, from the Institut Universitari IVI-Valencia, Spain, and colleagues, investigated the influence of additional hybridization cycles on the accuracy of FISH during PGS. The researchers analyzed the FISH data obtained from 1,000 PGS cycles conducted between 2007 and 2008 for diverse indications. During the study, re-hybridization was performed for 2,477 embryos due to previous inconclusive study results and to confirm aneuploidies.
Study findings of re-hybridization for 1,496 embryos due to non-conclusive results were as follows:
• Rehybridization had validated 882 embryos (59%) as normal
• Abnormality was apparent in 600 embryos (40.1%)
• No specific information was obtained for remaining14 embryos (0.9%)
Analysis of 981 embryos with previously noted aneuploidies showed the following findings:
• Among the 890 previously detected monosomies, aneuploidy was confirmed in 716 embryos (80.5%), while 174 (19.6%) were validated as normal
• Re-hybridization contributed to the confirmation of 90 out of 91 previously noted trisomies (98.9%)
• Upon re-hybridization on day-3 and re-analysis on day-5, an increase in the rate of concordant diagnosis was noted (95% versus 82.7%), particularly for monosomy confirmation (82.8% versus 61.0%)
The researchers concluded that adopting the improved FISH technique (using additional probe for re-hybridization) as standard approach, could be beneficial in enhancing prenatal screening.
FISH preimplantation genetic diagnosis, which screens nine chromosomes in two consecutive hybridization rounds, is reported to detect around 85-90% of embryos having additional or missing chromosomes. Owing to the increased chance for misdiagnoses in 10-15% of the embryos screened, other techniques, including chorionic villus sampling or amniocentesis, are generally performed to confirm the FISH test results.
A technical update by the Society of Obstetricians and Gynaecologists of Canada (Journal of Obstetrics and Gynaecology Canada, 2008) stated FISH and quantitative fluorescence polymerase chain reaction (QF-PCR) as the currently validated techniques for rapid aneuploidy detection. The update also validated the specificity and sensitivity of these techniques being similar to that of full cytogenetic karyotyping for fetal aneuploidy screening. However, the main limitations pertaining to FISH, reported in the review, included increased cost when compared to PCR-based screening tools and inability to automate the technique. The update concluded on the need for further development of these molecular techniques to augment the detection of fetal chromosomal abnormalities as well as reduce the cost.
References
1. Mir P, Rodrigo L, Mateu E, et al. Improving FISH diagnosis for preimplantation genetic aneuploidy screening. Hum Reprod. 2010 25(7):1812-1817.
2. Sparkes R, Johnson JA, Langlois S, et al. New molecular techniques for the prenatal detection of chromosomal aneuploidy. J Obstet Gynaecol Can. 2008 Jul;30(7):617-21, 622-7.
