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Review
. 2012 Jan 5;366(1):64-73.
doi: 10.1056/NEJMra1105043.

Genomics and perinatal care

Joann Bodurtha  1 , Jerome F Strauss 3rd
Affiliations
Review

Genomics and perinatal care

Joann Bodurtha et al. N Engl J Med. .
No abstract available

PubMed Disclaimer

Figures

Figure 1
Figure 1. Methods for Preimplantation Genetic Screening and Diagnosis
Material is obtained by microsurgical removal of polar bodies, blastomeres, or trophectoderm. The biopsy procedures generally do not affect the viability of the fertilized eggs or embryos. Depending on the specimen and analytic procedure, results can be obtained quickly and the selected embryos transferred without the need for cryopreservation while the genetic analysis is completed. Maternal genetic contributions (polar bodies, with evaluation of both bodies to determine the genetic status of the egg) and the genetic status of the embryo (blastomere and trophectoderm biopsy) can be analyzed by several different methods that provide varying amounts of information about sex chromosomes, chromosome copy number, and structural changes. These methods include fluorescence in situ hybridization (FISH), 24-chromosome single-nucleotide-polymorphism (SNP) arrays, and array comparative genomic hybridization. FISH methods detect a limited number of different chromosomes. Array comparative genomic hybridization and SNP arrays detect chromosome copy number as well as copy-number variations. In addition, SNP arrays can identify clinically significant uniparental disomy, consanguinity, and balanced translocations. Specific gene mutations may be identified with the use of methods based on polymerase-chain-reaction (PCR) assay. For mendelian disorders, a diagnosis can be obtained in 80% or more of samples. An inability to obtain a diagnosis is usually due to contamination or amplification failure. When a diagnosis is obtained for single-gene defects, it is highly accurate.
Figure 2
Figure 2. Noninvasive Prenatal Diagnosis with the Use of Plasma Cell-free Fetal RNA or DNA in Maternal Blood Derived from Dying Trophoblast Cells of the Placenta
For cell-free fetal RNA, target RNA molecules containing SNPs are quantified with the use of a PCR assay. The allelic ratio, which is determined on quantitative PCR, is used to determine the chromosome copy number when the fetus is heterozygous for the SNP. A 1:1 ratio of amplified allelic variants is expected in the euploid state, whereas a ratio of 2:1 indicates trisomy. Cell-free fetal DNA in plasma can be sequenced directly because it is smaller than maternal cell-free DNA; it can also be enriched by means of size fractionation before DNA sequence analysis. The abundance of specific chromosome sequences can be compared with normal reference samples or with another chromosome as a specified denominator in the sample to determine variation in chromosome copy number or structural changes in chromosomes. Additional methods of analysis of cell-free fetal DNA to determine chromosome copy number have been described, including methods to quantify differentially methylated regions of specific fetal chromosomes with the use of PCR. These methods await validation.
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References

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