On-line Ed Kit

Genetic Testing and Gene-based Medicine

Examining how a particular gene is spelled in an individual can serve quite a few uses:

Diagnosis Genetic analysis now can classify some conditions, like colon cancer and skin cancer, into finer categories. This is important since classifying diseases more precisely can suggest more appropriate treat- ments. The same approach will soon be possible for heart disease, schizophrenia, and many other medical conditions, as the genetic underpinnings for these diseases become more completely understood.

Pharmacogenomics is a new word that scientists and drug developers use. It describes the idea of tailoring drugs for patients, whose individual response can be predicted by genetic fingerprinting. For example, cancer patients facing chemotherapy may experience fewer side effects and improve their prognoses by first getting a genetic fingerprint of their tumor. This fingerprint can reveal which chemotherapy choices are most likely to be effective. Better understanding of genetics promises a future of precise, customized medical treatments.

Prognosis Diagnosing ailments more precisely will lead to more reliable predictions about the course of a disease. For example, a genetic workup can inform a patient with high cholesterol levels how damaging that condition is likely to be. And doctors treating prostate cancer will be able to predict how aggressive a tumor will be. For many diseases, such genetic information will help patients and doctors weigh the risks and benefits of different treatments.

Prevention Once scientists figure out what DNA sequence changes in a gene can cause disease, healthy people can be tested to see whether they risk developing conditions like heart disease, diabetes, or prostate cancer later in life. In many cases, this advance warning can be a cue to start a vigilant screening program, to take preventive medicines, or to make diet or lifestyle changes that might prevent the disease altogether.

For example, those at risk for colon cancer could undergo frequent colonoscopies; those with hereditary hemochromatosis, a common disorder of iron metabolism, could donate blood periodically to remove excess iron and prevent damage to the body. Some women at risk for breast cancer could benefit from tamoxifen; a young person at risk for developing lung cancer may become particularly motivated to quit smoking; those with familial hypercholesterolemia could begin treatment to lower their cholesterol levels and prevent heart attacks and strokes. Unfortunately, our ability to predict a disease sometimes precedes our ability to prevent or treat it. For example, a genetic test has been available for Huntington disease for years, but no treatment is available yet. As a result, only a minority of people at risk have chosen to be tested.

Newborn screening A particular form of predictive testing, newborn screening can sometimes help a great deal. For example, babies in the United States and a few other countries are routinely screened for phenylketonuria (PKU), a metabolic disorder that prevents the breakdown of phenylalanine, one of the building blocks of proteins and a component of the artificial sweetener Aspartame. Excess phenylalanine in the body is toxic to the nervous system. In the past, children with the condition became severely mentally retarded, but the screening program identifies children with the enzyme deficiency, allowing them to grow normally on a diet that strictly avoids phenylalanine.

Carrier screening For some genetic conditions, people who will never be ill themselves can pass a disease to their children. Some couples choose to be tested for this risk before they marry, especially in communities where a feared childhood disease is particularly common. For example, carrier testing for Tay-Sachs disease, which kills young children and is particularly common in some Jewish and Canadian populations, has been available and widely used for years.

Gene therapy Replacing a misspelled gene with a functional gene has long been an appealing idea. Small groups of patients have undergone gene therapy in clinical trials for more than a decade, but this remains an experimental treatment. Eventually, it likely will become a common treatment for some conditions.

Gene-based therapy Great medical benefit likely will derive from drug design that's guided by an understanding of how genes work and what exactly happens at the molecular level to cause disease. For example, the causes of adult-onset diabetes and the resulting complications remain difficult to decipher and, so, to treat. But researchers are optimistic that a more precise understanding of the underlying causes will lead to better therapies. In many cases, instead of trying to replace a gene, it will be more effective and simpler to replace the protein the gene would give rise to. Alternatively, it may be possible to administer a small molecule that interacts with the proteinas many drugs doand changes its behavior.

One of the first examples of such a rationally-designed drug targets the genetic flaw that causes chronic myelogenous leukemia, a form of leukemia that mostly affects adults. An unusual joining of chromosomes 9 and 22 produces an abnormal protein that spurs the uncontrolled growth of white blood cells. Scientists have designed a drug that specifically attaches to the abnormal protein and blocks its activity. In preliminary tests, blood counts returned to normal in all patients treated with the drug. And, compared with other forms of cancer treatment, the patients experienced very mild side effects.

Instead of having to rely on chance and screening thousands of molecules to find an effective drug, which is how most drugs we use today were found, scientists will begin the process of drug discovery with a clearer notion of what they're looking for. And because rationally designed drugs are more likely to act very specifically, they will be less likely to have damaging side effects.










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