Genes are units of heredity which provide the blueprint for our physical body, determining not only how long we may live,but also the quality of that life.The extent and quality of life can be drastically altered by disease, and genetic disease is perhaps the purest illustration of the relationship between our genes and our health. In the case of a rare genetic disease such as cystic fibrosis, the inheritance of two mutated copies of a single gene, the cystic fibrosis transmembrane conductance regulator (CFTR) (ROMMENS et al., 1989),directly predisposes the individual to the disease. For common diseases, such as heart disease or cancer the situation is less clear.
Environment and lifestyle play essential roles in disease susceptibility, yet a genetic component may underpin most, if not all disease. Infectious disease for example, requires the host to be susceptible to infection by the disease agent. This susceptibility can be affected by diet, and the presence of the disease agent is most certainly environmental. As well, the expression of this susceptibility may rely on the presence or absence of certain alleles for genes that either predispose an individual to infection or provide some degree of resistance to a particular infectious agent.
For example, mutant alleles of the CFTR gene cause cystic fibrosis and predispose an individual to lung infections when present in two copies, but actually provide resistance to diarrheal diseases such as typhoid fever (JOSEFSON, 1998), when present in only one copy. This concept can also be extended to the genetic susceptibility to carcinogens in regard to cancer or to a predisposition for high-serum cholesterol in regard to coronary heart disease. Thus, all disease may have some genetic basis and the penetrance of that disease phenotype is modulated by a potent combination of environment, lifestyle, and the particular compliment of genes an individual has inherited.
The Human Genome Project promises to revolutionize medicine by providing the sequence of every human gene. It is hoped that this information will help determine the cause of many common diseases whose genetic components have been previously recalcitrant to dissection. The following chapter provides an overview of genetic disease and aims to aquaint the reader with the various classes of disease, the methods of determining their aetiology and common strategies for their treatment or management. The chapter coilcludes with observations on the current and future roles of academia and industry in genetic disease research.