From Single Cell Gene-based Diagnostics to Diagnostic Genomics: Current Applications and Future Perspectives

Clinical Laboratory Science, Fall 2005 by Zhao, Richard

Molecular-based genetic tests can be used for many different purposes including carrier screening, prenatal testing, cancer risk, or confirmatory cancer testing, and genetic identification. Carrier testing normally involves identification of an individual who carries one copy of a gene for a disease that requires two copies for the disease to be expressed. An example of such a carrier testing includes Ashkenazi genetic carrier screening for an array of genetic diseases including Tay-Sachs diseases, Gaucher disease, cystic fibrosis, Canavan disease, Niemann-Pick, and familial dysautonomia. For further introduction on the use of molecular-based methodologies in genetic testing for cystic fibrosis, see the article in the next issue of Clinical Laboratory Science (Winter 2006) by Dr Edward Highsmith and his colleague Timothy S UphofF from Mayo Clinic. Prenatal testing involves identification of gene alternations in the fetus for a specific genetic disease such as fragile X syndrome.

For cancer detection, molecular-based diagnosis has never been so close to reality in assisting physicians in detecting specific cancers or even in assisting in monitoring anti-cancer therapies. Dr Christopher Gocke from John Hopkins University provides specific information on molecular diagnosis and monitoring of hematological malignancies in the next issue (Winter 2006). Dr Gocke's article will provide detailed information on molecular diagnostics and monitoring of hematological malignancies.

Because of the genetic distinction of each individual, molecular-based testing can also be used for forensic or identity testing. Short tandem repeat (STR) assay has been used, for example, for paternity identification or criminal investigations. STR has also been implemented in the clinical setting to monitor efficacy of bone marrow transplantation of leukemia patients.

Altogether, gene-based molecular diagnostics provides us with tremendous power for clinical diagnosis of a broad range of diseases. With emergence of new and more powerful technologies, use of molecular diagnostics will only continue to grow. Implementation of more sensitive and specific tests with these new technologies in clinical diagnosis would allow us to continue improving patient care and thus saving more lives.

In this article, I will briefly describe principles of various gene-based amplification technologies including some of the most popular technologies such as Amplicor® (Roche), NucliSens® (OrganonTeknika), Quantiplex® (Bayer), as well as real-time PCR methods that are currently available in the market for clinical diagnostics. Other technologies such as LCR® (Abbott), Qâ-replicase® (Gene-Track), and SDA® (Becton Dickinson) will also be introduced. New emerging technologies such as immuno-PCR (IPCR), bio-barcode assay (BCA) and genomic/proteomic approaches, which have not yet been applied to clinical testing, but may prove to be even more powerful tools in molecular diagnostics, will be addressed.

Gene-based amplification typically consists of three major steps: primertemplate hybridization, synthesis, and amplification. The gene-based amplification methodologies may be categorized based on the means of amplification, i.e., 1) target-based amplification methods, which are designed to detect and amplify the target gene of interest; 2) probe-based amplification methods, which rely on amplification of the probes that are homologous to a specific gene target, and, 3) signal-based technologies that amplify the signal rather than the gene target sequence.