Gene Tests – Three Common Methods

Gene testing involves examining a person’s DNA--typically taken from cells in a sample of blood--for mutations linked to a disease or disorder.

Some genetic tests can identify changes in whole chromosomes. Others examine short stretches of DNA within genes. Yet others look for the protein products of genes.

Genetic tests serve many purposes. They are widely used to screen newborns for a variety of disorders. Often this information enables the doctors to minimize the damage caused by the mutation.

In oncology, doctors use gene testing to diagnose cancer, to classify cancer into subtypes, or to predict a patient’s responsiveness to new treatments.

Much of the excitement today centers on gene expression profiling that uses a technology called microarrays. A DNA microarray is a thin-sized chip that has been spotted at fixed locations with thousands of single-stranded DNA fragments corresponding to various genes of interest. A single microarray may contain 10,000 or more spots, each containing pieces of DNA from a different gene. Fluorescent-labeled probe DNA fragments are added to ask if there are any places on the microarray where the probe strands can match and bind. Complete patterns of gene activity can be captured with this technology.

Identifying genes associated with disease--indeed, tracking down every chemical base in each of the estimated 25,000 genes as well as the spaces between them, a process called mapping the human genome--has been accomplished successfully by an international collaboration known as the Human Genome Project.

Scientists have developed a consensus sequence, laying out the order in which all the human genes sit along the chromosomes. This information can be used to determine where gene mutations occur in specific diseases. For example, here is a chart of disease-linked genes located along the X chromosome.

To study specific genes, scientists can now use information from the Human Genome Project to locate a gene’s position along a chromosome.

Scientists clone or copy specific sites to obtain millions of identical copies, which are then stored. Researchers then retrieve the appropriate clone and use it for additional experiments.

Scientists clone or copy specific sites to obtain millions of identical copies, which are then stored. Researchers then retrieve the appropriate clone and use it for additional experiments.

Scientists searching for disease-linked genes often begin by studying DNA samples from “disease families” in which numerous relatives, over several generations, have developed the illness.

The scientists look for altered regions of the human genome--known DNA segments containing disease-linked mutations-- that are consistently inherited by persons with the disease and are not found in relatives who are disease free. Researchers use this information and tools from the Human Genome Project to zero in on the exact location of the altered gene or genes and characterize the specific base changes.

Gene tests are already available for some two dozen disorders, including life-threatening diseases such as cystic fibrosis and Tay Sachs disease.

Gene tests for some cancers are also available. Specific genetic mutations have been identified as linked to several types of cancer, and, for some cancer types, this information has been converted into clinical tests. For example, scientists identified gene mutations that are linked to an inherited tendency to develop colon or breast cancer, and tests for an inherited susceptibility to these cancers are commercially available.

Date: 2015-12-24; view: 1022