Gene therapy is an experimental treatment for a variety of medical conditions, including heart disease. The American Heart Association has listed gene therapy as one of the most important areas of research, and it continues to be pursued by researchers from around the world.

Gene therapy focuses on repairing or replacing malfunctioning genes. In the center of most human cells are 46 chromosomes. They are arranged in 23 pairs, with one chromosome in each pair from each biological parent. Twenty-two of these pairs are called autosomes and are common to both males and females. The 23^rd pair is known as the sex chromosomes, which are responsible for determining gender (XY in males, XX in females).

Each chromosome contains long, tightly wound strands of DNA (deoxyribonucleic acid), which is the genetic blueprint for how the body is constructed and maintained. If the DNA strands within each chromosome were uncoiled and connected, the resulting strand of DNA would be over five feet long but only a tiny fraction of an inch in width.

A strand of DNA is composed of four types of chemicals called bases: adenine (A), cytosine (C), guanine (G) and thymine (T). Strands of DNA are linked together at specific points along their length. They are linked together such that the base (A, C, G or T) on one strand is paired with a matching base on the other strand. For example, A only pairs with T, and G only pairs with C. Segments of the DNA base pairs that have been identified as having specific effects on the body (e.g., manufacturing a protein) are called genes. Each gene forms the basis for some piece of genetic information, such as hair color, eye color or the presence of some disease.

The number of base pairs possessed by a living thing determines the size of its genome (the sum total of all its genetic information). The human genome is estimated to contain about three billion (3,000,000,000) base pairs and about 20,000 to 25,000 genes.

Identification of specific genes and their associated proteins has made gene therapy possible. Certain conditions are caused by a single defective gene. Scientists hypothesized that if they could replace a given defective gene with a functioning one, the disease or condition could be cured. In addition, they might also be able to treat certain defective genes of conditions where multiple genes were malfunctioning, possibly alleviating symptoms or lessening the severity of a condition.

A healthy gene is inserted into a cell to produce an enzyme or protein that is missing or defective. Scientists use certain types of viruses to transport genes. The disease-causing part of the virus is removed and the healthy gene is inserted. Different types of viruses have different advantages and disadvantages for gene transport and researchers continue to study their uses.

Gene therapy is still experimental. Researchers are conducting studies with animals and have some clinical trials underway with humans. Some trials have had problems by only getting short-term results from gene therapy. In addition, both the introduced gene and virus can trigger the body’s immune response, which tries to kill any invading cells. Recent research has focused on developing non-viral methods of gene introduction, which may reduce the risk of unwanted immune responses.