special subjects
Genetic Diagnostic Technologies
Introduction
Genetic diagnostic technology is rapidly improving. A small amount of DNA can be amplified using the polymerase chain reaction (PCR) process, which can produce millions of copies of a gene or gene segments. RNA can be amplified by combining the reverse transcriptase (RT) enzyme with traditional PCR. (See also Overview of Genetics .) Gene probescan be used to locate specific segments of normal or mutated DNA. Different types of probes may investigate a broad range of sizes of DNA sequence. A known DNA segment may be cloned and then fluorescently tagged (using fluorescent in situ hybridization [FISH]); this segment is then combined with a test specimen. The tagged DNA binds to its complementary DNA segment and can be detected by measuring the amount and type of fluorescence. Gene probes can detect a number of disorders before and after birth. DNA microarraysare powerful tools that can be used to identify DNA mutations. A single microarray can test for millions of different DNA changes using only one sample. DNA microarrays can be used in genome-wide association studies (GWAS) to compare patient and control populations to identify DNA variants that can contribute to disease risk. Array comparative genomic hybridization (aCGH)is a type of microarray now routinely used to identify deleted or duplicated regions of DNA sequence in specific chromosomes on a genome-wide basis. DNA from a patient is compared to a reference genome using many oligonucleotide probes. Using aCGH, the entire genome can be tested (queried). Next-generation sequencing technologieshave dramatically changed the approach to genetic diagnosis. This technology involves breaking the entire genome into small segments, sequencing the segments, and then reassembling the sequences using intensive computational techniques to provide the base-by-base sequence of the entire genome or more limited regions, such as the expressed portion of the genome known as the exome. This process helps identify single or multiple nucleotide variations as well as areas of insertion or deletion. The costs of this technology have dramatically fallen and continue to fall. The equipment and computational methods also continue to improve. This revolutionary and rapidly evolving technology has moved a significant portion of the technical aspects of genetic diagnosis to next-generation sequencing and has become the mainstay of genetic diagnosis. However, the sheer volume of information generated by sequencing the exome or genome results in a variety of interpretive problems that complicate understanding of the results. Despite these issues, these techniques appear to be the technology of the future. 