FOUNDATIONS OF GENETIC DIAGNOSIS
Cytogenetics is the study of genetics at the chromosome level. Chromosomal anomalies occur in 0.4% of all live births and are a common cause of intellectual disabilities and congenital anomalies. The prevalence of chromosomal anomalies is much higher among spontaneous abortions and stillbirths.
Human chromosomes consist of DNA (the blueprint of genetic material), specific proteins forming the backbone of the chromosome (called histones), and other chromatin structural and interactive proteins. Chromosomes contain most of the genetic information necessary for growth and differentiation. The nuclei of all normal human cells, with the exception of gametes, contain 46 chromosomes, consisting of 23 pairs (Figure 37–1). Of these, 22 pairs are called autosomes. They are numbered according to their size; chromosome 1 is the largest and chromosome 22 the smallest. In addition, there are two sex chromosomes: two X chromosomes in females and one X and one Y chromosome in males. The two members of a chromosome pair are called homologous chromosomes. One homolog of each chromosome pair is maternal in origin; the second is paternal. The egg and sperm each contain 23 chromosomes (haploid cells). During formation of the zygote, they fuse into a cell with 46 chromosomes (diploid cell).
Normal male and female human karyotype. (Used with permission from Colorado Genetics Laboratory.)
A karyotype is the arrangement of chromosomes in homologous pairs in numerical order. There is a characteristic-banding pattern that is reproducible for each chromosome, allowing the chromosomes to be identified. High-resolution chromosome analysis is the study of more elongated chromosomes and can detect smaller imbalances than routine chromosome analysis (see Figure 37–1). Although the bands can be visualized in greater detail, subtle chromosomal rearrangements less than 5 million base pairs (5 Mb) can still be missed.
Fluorescence in situ hybridization (FISH) is a powerful technique that labels a known chromosome sequence with DNA probes attached to fluorescent dyes, thus enabling visualization of specific regions of chromosomes by fluorescent microscopy. FISH can detect submicroscopic structural rearrangements undetectable by classic cytogenetic techniques and can identify marker chromosomes. (For pictures of FISH studies, go to http://www.pathology.washington.edu/galleries/Cytogallery/main.php?file=fish_examples.)
Interphase FISH allows noncultured cells (lymphocytes, amniocytes) to be rapidly screened for numerical abnormalities such as trisomy 13, 18, or 21, and sex chromosome anomalies. However, because of the possible background or contamination of the signal, the abnormality must be confirmed by conventional chromosome analysis. Two hundred-cell FISH can also be used to ascertain mosaicism.
Chromosomal Microarray Analysis or Array Comparative Genomic Hybridization
Advances in technology and bioinformatics have led to the development of genetic testing using comparative genomic hybridization with microarray technique (aCGH). This ...