SMFM consult: Chromosomal microarray for prenatal diagnosis
By Society for Maternal-Fetal Medicine (SMFM); Lorraine Dugoff, MD; Mary E Norton, MD; and Jeffrey A Kuller, MD
Chromosomal microarray analysis (CMA) is a high-resolution whole-genome screening technique that can identify most of the chromosomal imbalances detected by conventional cytogenetic analysis, as well as smaller submicroscopic deletions and duplications known as copy-number variants (CNVs). CNVs may cause a wide range of disorders, including neurodevelopmental disorders and congenital anomalies such as cardiac defects. CMA is recommended as the first-tier test in postnatal evaluation of congenital abnormalities and neurodevelopmental disorders. With accumulating experience over the last decade, and data demonstrating improved detection of chromosomal abnormalities compared to conventional karyotyping, CMA is proving to be a valuable diagnostic tool in the prenatal setting. CMA can be performed on uncultured DNA samples, including those obtained from chorionic villus sampling (CVS) and amniocentesis, which may lead to quicker turnaround than karyotyping.
Q: What are the different types of microarray?
Two major microarray platforms are used in prenatal diagnosis: single nucleotide polymorphism (SNP) arrays and comparative genomic hybridization (CGH) arrays. With SNP and CGH arrays, DNA from a fetal sample, such as CVS or amniocentesis, is hybridized to an array platform consisting of DNA probes on a solid surface, such as a microscope slide or a silicon chip.
CGH compares the fetal DNA sample with a normal reference DNA sample. An SNP is a variation at a single position in a DNA sequence among individuals. With SNP arrays, only the DNA test sample is hybridized to the array platform. While CGH arrays are able to detect only copy number variants, SNP arrays can also detect triploidy and regions on the 2 homologous chromosomes that are identical to each other, as occurs with uniparental disomy (UPD) and consanguinity. In the case of UPD, both copies of a chromosome are inherited from the same parent, instead of one from each parent. UPD has been associated with genetic disorders such as Prader-Willi syndrome, which can occur when both copies of chromosomes 15 are maternally inherited. SNP arrays can also detect some cases of maternal cell contamination and mosaicism.
Arrays may include probes that cover the whole genome, or may be targeted, with concentrated coverage in known disease-causing regions of the genome and more limited coverage of the rest of the genome.