What is Karyotyping is useful in diagnosis of- and explain in detail?
A karyotype is a photographic representation of a stained metaphase spread in which the chromosomes are arranged in order of decreasing length.A variety of techniques for staining chromosomes have been developed. With the widely used Giemsa stain (G banding) technique, each chromosome set can be seen to possess a distinctive pattern of alternating light and dark bands of variable widths . The use of banding techniques allows identification of each chromosome, and can detect and localize structural abnormalities. Karyotype analy- sis of chromosomes by G banding remains the classic approach for identifying changes at the chromosomal level; however, as might be expected, the resolution with this technique is fairly low. In order to identify subchromo- somal alterations, both focused analysis of chromosomal regions by FISH and global genomic approaches such as comparative genomic hybridization (CGH) have become popular. Fluorescence in Situ Hybridization (FISH) FISH utilizes DNA probes that recognize sequences spe- cific to chromosomal regions of greater than 100 kilobases in size, which defines the limit of resolution with this tech- nique for identifying chromosomal changes. Such probes are labeled with fluorescent dyes and applied to metaphase spreads or interphase nuclei. The probe hybridizes to its complementary sequence on the chromosome and thus labels the specific chromosomal region that can then be visualized under a fluorescence microscope. The ability of FISH to circumvent the need for dividing cells is invaluable when a rapid diagnosis is warranted (e.g., in a critically ill infant suspected of having an underlying genetic disorder). Such analysis can be performed on prenatal samples (e.g., cells obtained by amniocentesis, chorionic villus biopsy, or umbilical cord blood), peripheral blood lymphocytes, and even archival tissue sections. FISH has been used for detection of numeric abnormalities of chromosomes (aneuploidy) (Fig. 6-37, A); for the demonstration of subtle microdeletions (Fig. 6-37, B) or complex transloca- tions not detectable by routine karyotyping; for analysis of gene amplification (e.g., NMYC amplification in neuroblastomas); and for mapping newly isolated genes of interest to their chromosomal loci. Array-Based Genomic Hybridization It is obvious from the preceding discussion that FISH requires previous knowledge of the one or few specific chromosomal regions suspected of being altered in the test sample. However, chromosomal abnormalities may also be detected without previous knowledge of what these aber- rations may be, using a global strategy known as array- based CGH. Here the test DNA and a reference (normal) DNA are labeled with two different fluorescent dyes (most commonly, Cy5 and Cy3, which fluoresce red and green, respectively). The differentially labeled samples are then hybridized to an array of segments of genomic DNA "spotted" on a solid matrix, usually a glass slide (Fig. 6-38, A). These segments of DNA are representations of the human genome at regularly spaced intervals, and cover all 22 autosomes and the sex chromosome (Fig. 6-38, A). Amplifications and deletions in the test sample produce an increase or decrease in signal relative to the normal DNA that can be detected down to a 10-kilobase (kb) resolution (Fig. 6-38, B). Newer generations of microarrays using single-nucleotide polymorphisms (SNPs) (see fuher on) provide even higher resolution (with more than 1 million SNPs from the human genome on a single microarray!) and are currently being used to uncover copy number abnor- malities in a variety of diseases, from cancer to autism. (Robbins Basic Pathology, 9 th edition. page : 263)