Twin spots

Special form of cutaneous mosaicism. Twin spots (or didymosis; from didymos = twin) can be defined as the paired occurrence of mutated areas. These differ from each other as well as from the surrounding heterozygous tissue. The occurrence of twin spots is well documented in plants and animals. In humans, however, molecular evidence for the existence of twin spots is still lacking.

If an embryo is heterozygous for two different recessive genes that are located opposite each other on a homologous chromosome pair, i.e. at approximately the same allele locus, then somatic recombination (this process is also known as sister chromatid exchange) can result in an exchange of alleles in such a way that the two daughter cells each become homozygous for one of the two mutations. These are the stem cells of a didymosis. In allelic twin spots, the two mutations are located opposite each other at the same gene locus. Possible examples are vascular twin spots, cutis tricolor (paired hyperpigmented and depigmented spots on a normally pigmented background), coincidence of hyperplastic and hypoplastic anomalies in Proteus syndrome and paired segments of markedly affected and normal skin in Darier's disease. Other possible examples of non-allelic didymosis are phacomatosis pigmentovascularis (extensive pigmented nevus paired with a telangiectatic nevus) and phacomatosis pigmentokeratotica (nevus spilus paired with nevus sebaceus).

Possible examples of twin spots are:

• a combined occurrence of a nevus teleangiectaticus and anaemicus (twin vascular spot) in the same lesion.
• Pigmentary Vascular Phacomatosis (Ota)
• phacomatosis pigmentokeratotica
• Cutis tricolor
• didymosis capillaris
• Paired segments of excessively affected and healthy skin in Darier disease
• Paired segments of excessively affected and healthy skin in epidermolytic ichthyosis type Brocq.

1. Happle R (2004) Cutaneous mosaics: patterns and molecular mechanisms. Dtsch Arztebl 101: A-1886 / B-1575 / C-1511

2. Itin P (2018) Basics of genetics. In: G Plewig G et al (eds) Braun-Falco's Dermatology, Venereology and Allergology. Springer Reference Medicine p 35-43