Genetic testing: Weigh benefits and risks

If you're considering genetic testing, take time to weigh the pros and cons.

Not long ago, genetic testing existed mainly in science fiction. Now, however, a variety of genetic tests may provide hints of your medical future.

Want to take advantage of the progress in genetic testing? First, take some time to consider the pros and cons of genetic testing in your particular situation.

Genetic testing may raise more questions than it answers. It can't tell you if you'll definitely develop breast cancer or when you'll start to notice symptoms of Alzheimer's disease. Sometimes, genetic testing can alleviate anxiety — if, for example, you find out you have a below-average likelihood of developing some frightening disease. But if your risk of an unusual disease turns out to be slightly above average, you might worry needlessly.

Your genes and your health

Your genes direct all your body's functions and define you as a living human being. In the language of molecular biology, genes are sequences of four chemical compounds known as nucleotides. The nucleotides line up two by two to form the double-stranded DNA molecule in the nucleus of every living cell. Each pair of nucleotides is called a base pair.

In human cells, billions of base pairs are organized along 23 pairs of rod-like structures called chromosomes. You inherit one set of 22 numbered chromosomes from each of your parents, plus an X chromosome from your mother and an X or Y chromosome from your father. The X or Y you get from your father determines your sex; sons inherit a Y chromosome and daughters, an X.

An organism's complete DNA sequence is its genome. The human genome contains between 20,000 and 25,000 genes, each holding the molecular recipe for a gene product — a protein produced by a cell. Genes with missing, duplicated or misplaced nucleotides function improperly. They may not produce any of the needed protein, or they may produce the protein, but in an ineffective amount or form. The result is known as a single-gene disease.

Single-gene diseases — devastating as they often are — account for only a small proportion of medical conditions. That's because protein-coding genes make up only about 2 percent of your genome. But the other 98 percent of your DNA has profound effects on your health and life span as well. Science has barely begun to understand what this "nongenomic DNA" does, but answers are starting to emerge.

Types of genetic testing

Until recently, genetic testing was available only from a handful of laboratories affiliated with genetics departments at major medical centers. Your doctor might have referred you to a medical genetics department for genetic counseling and testing if you had a family history of a specific genetic disorder, if you were having repeated miscarriages, or if you had a high likelihood of having a baby with a genetic or chromosomal disorder.

Today, with over 1,100 genetic tests available from commercial genetics laboratories, the situation is quite different. Doctors everywhere can now send samples of their patients' blood, saliva or tissue to a commercial or academic genetics laboratory for a rapidly increasing number of tests. It's even possible to order some types of genetic testing on your own.

Diagnostic testing
Many genetic disorders, such as adult polycystic kidney disease and iron overload (hemochromatosis), appear in adolescence or later. If you have signs and symptoms of an adult-onset genetic disorder, genetic testing may confirm the diagnosis. Couples having difficulty conceiving also may undergo genetic testing for disorders that cause infertility. Also, if a hereditary cancer syndrome runs in your family, you can be tested for the genetic variations associated with the syndrome. If the test shows that your likelihood of eventually developing hereditary cancer is high, you can have the appropriate cancer-screening tests, such as colonoscopy, more often than generally recommended, and starting at an earlier age. More intensive screening may detect cancers at their earliest stages. Genetic testing may also affect your options for medical treatment of hereditary cancer syndromes.

Carrier testing
Couples with family histories of single-gene disorders may undergo genetic testing and genetic counseling before starting a family. In these circumstances, genetic testing determines whether either parent also carries a copy of the altered gene and is, as a result, at risk of having a child with the condition. Among the most common of these disorders are sickle cell anemia (affecting one in 400 African-Americans in the United States) and cystic fibrosis (affecting one in 2,500 descendants of Northern Europeans).

Prenatal genetic testing
For more than 25 years, doctors have been able to diagnose chromosomal abnormalities, such as Down syndrome, by doing prenatal genetic testing on fetal cells, which are obtained through diagnostic tests such as amniocentesis. Genetic testing is also an option when other prenatal screening tests suggest a genetic disorder, or when one or both parents have a family history of a single-gene disorder.

Preimplantation genetic testing
Couples conceiving through in vitro fertilization may benefit from genetic testing of the embryo before it's implanted in the woman's uterus. Preimplantation genetic tests detect the same conditions as prenatal tests.

Newborn genetic screening
Some babies are born with gene abnormalities causing specific diseases. With relatively inexpensive technology, it's now possible to test all newborns for many genetic conditions, some of which can be corrected with ongoing treatment started early in life. This type of genetic testing is required in most states, but the exact conditions tested for vary from state to state.

Genome scanning
Another type of genetic testing detects DNA variations at a more minute level, known as a single nucleotide polymorphism (SNP, pronounced "snip"). In a SNP, one nucleotide is out of sequence among the billions of base pairs in your genome.

Although many SNPs are inconsequential, some have powerful effects on disease risk and vulnerability to disease-causing agents. In clinical settings, for example, single tests for specific SNPs are sometimes used to predict how you'll respond to certain drugs or whether there's a chance you'll have a serious adverse reaction to a particular drug.

Analysis of SNPs is proceeding rapidly. In the past few years, specific SNP variations have been linked to diabetes, high blood pressure, breast cancer, prostate cancer, Crohn's disease, multiple sclerosis and rheumatoid arthritis.

Still, there's a lot of ground to cover: By one estimate, the genome contains about 10 million SNPs, and exactly how they affect disease risk is still unknown. In spite of this uncertainty, some companies are marketing whole-genome scans — with or without supplementary genetic counseling — directly to consumers. If you're tempted to get tested, make sure you find out exactly what you're buying and how you can use it.

Privacy and genetic testing

When you apply for health, disability or life insurance, you may have to disclose your complete medical history, including results of genetic tests, or risk losing your benefits if anything you omit comes to light. Genetic testing results held in your medical record may be available to your current insurer without your disclosure as well. Until privacy and anti-discrimination law catches up to genetic technology, you may not have much protection against denials of coverage based on your known genetic risks. A recently signed federal law, the Genetic Information Nondiscrimination Act (GINA) prohibits health insurers from discriminating against healthy individuals who are genetically predisposed to develop diseases. Under GINA, employment discrimination based on genetic risk also is illegal.

Regardless of the type of genetic testing you're considering, talk with your doctor or genetics professional about all the pros and cons.