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KEY POINTS
Differences in incidence and prevalence of disease in different racial and ethnic populations provide evidence for a genetic contribution to the etiology of a disorder.
Familial clustering of disease and more frequent concordance of disease in monozygotic (MZ) than dizygotic twins provide further support for a genetic component.
Less than 100% concordance in MZ twins as well as evidence for epigenetic modifications in disease support a strong environmental component in addition to the genetic contribution.
Genome-wide linkage and gene association studies provide evidence for involvement of genes at specific chromosomal regions.
Systemic lupus erythematosus (SLE), vitiligo, systemic sclerosis (SSc), sarcoidosis, and keloids are genetically complex, with multiple genetic loci and environmental triggers conferring risk for the disease and its severity.
Autoimmune diseases including SLE, SSc, sarcoidosis, and vitiligo share some susceptibility loci at human chromosomal region 6p21, which contains genes of the major histocompatibility complex as well as many non–human leukocyte antigen loci involved in lymphocyte activation (B and T-cells), cytokine pathways, and host–microbe interactions. Sharing among subsets of autoimmune disease and network analysis of the functions of candidate genes may provide important information about disease pathogenesis and therapeutic strategies.
In addition to shared genetic susceptibilities, some gene differences are observed mainly within a single disorder, within a single racial group, or within a group of patients with similar clinical symptoms.
Problems in replicating genetic findings and in elucidating the genetic contribution to the pathogenesis of complex diseases are currently being addressed by analyzing results from different racial group separately; admixture mapping to identify association of specific disease-related variation with ancestry; association studies using panels of single-nucleotide polymorphisms (SNPs) from different racial populations; using subsets of patients with similar clinical symptoms; and using ordered subset analysis to obtain evidence for gene interaction (epistasis) between different genetic loci. To further refine findings from genome-wide association studies, where significant SNP associations often implicate more than one gene, comprehensive analysis of all gene variation at associated loci and functional studies are needed to identify specific gene involvement and causal mutations.
The gene discovery process is an ongoing and dynamic one. Tables of specific genes and chromosomal regions require frequent updating, and the reader is referred to regularly updated resources including Online Mendelian Inheritance in Man (http://omim.org/) and the website for the National Human Genome Research Institute (http://www.genome.gov/) studies for the most current compilations.
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Multiple types of evidence support a genetic contribution to human disease. Differences in incidence and prevalence in different racial and ethnic populations provide strong suggestive evidence for a genetic contribution to the etiology of a disorder. Although socioeconomic, behavioral, and environmental factors confound the identification and contribution of genetic factors, dissimilar frequencies of rare alleles and polymorphisms that associate with disease in different racial groups support a role for genetic factors. Gene admixture complicates assignment of individuals to a single group. Studies in multiple populations, using panels of ancestry informative markers (AIMs) ...