The study of the genetics of complex diseases: the example of systemic lupus erythematosus

Genetic diseases can be caused by pure genetic factors or a combination of genetic and environmental factors. Mutations can occur in the autosomal chromosomes (chromosome 1−22), the sex chromosomes (X or Y), or the mitochondrial genome. Genetic diseases are transmitted from parents to offspring and can be categorized into two groups; monogenic and complex diseases. Monogenic diseases are caused by mutations in a single gene and complex diseases are caused by several genes in interaction with each other as well as with environmental factors. Systemic lupus erythematosus, SLE, is a systemic autoimmune disorder. The disease is characterized by chronic inflammation in different organ systems and autoantibodies against intracellular components such as dsDNA. The disease primarily affects women with a female to male ratio of 9:1. SLE is considered to be a complex disease, caused by interaction between genetic and environmental factors. In complex diseases multiple genetic factors with unknown mode of inheritance contribute to the disease. The susceptibility genes are thought to work together to cause the disease where neither gene is necessary nor sufficient. This review deals with the basis of the studies on genetics of complex diseases and what is known to date on the genetics of systemic lupus erythematosus.