The use of the twin model to investigate the genetics and epigenetics of skin diseases with genomic, transcriptomic and methylation data.

Twins have always fascinated medical research even before the discovery of DNA and the understanding of the differences between identical and non-identical twins. Dermatology with the benefit of being able to visualize phenotypes was one of the first specialities reporting on the fascinating concordance in identical (MZ) twins in the 1920's. Over the last 20 years, the heritability of skin diseases using twins has been clearly demonstrated, across a wide variety of traits including melanoma, polymorphic light eruption, psoriasis, eczema and acne. Other rarer diseases have also been shown to have a significant genetic basis such as lupus, sarcoidosis and lichen sclerosus. Following evidence of heritability for many skin disease the next step was Genome-Wide Association Studies (GWAS) which are uncovering new genes in large twin cohorts. The twin model is also ideal for the new field of epigenetics, investigating subtle differences in DNA methylation within discordant MZ pairs for a disease, as well as differences in CNVs. Twins are also valuable for examining differences in gene function via RNA expression in twins discordant for a skin trait or disease.