Mosaic Skin Conditions
– Functional (epigenetic) mosaicism results from changes in gene expression (but not the DNA sequence) that are passed on during cellular replication; an important example is lyonization in female embryos, where random inactivation of one of the two X chromosomes occurs in each cell during early development.
• Clinical findings in mosaic skin conditions depend not only on the underlying genetic alteration but also on the timing of its origin (with earlier onset generally leading to more widespread involvement) and the cells or tissues affected (cutaneous ± extracutaneous).
– For example, mosaicism for an activating HRAS mutation can produce the combination of a nevus sebaceus (keratinocytes affected), speckled lentiginous nevus (melanocytes affected), and occasionally CNS abnormalities (nerve cells affected) in patients with phakomatosis pigmentokeratotica, a form of ‘twin spotting’.
Skin findings that can occur along Blaschko’s lines.
|Linear lichen planus (Fig. 51.2A)|
|Inflammatory linear verrucous epidermal nevus (ILVEN) (Fig. 51.2B)|
|Linear psoriasis (Fig. 51.2C)|
|Verrucous lesions – e.g. epidermal/sebaceous nevi (Table 51.3; Fig. 51.3), IP stage 2 (Fig. 51.7A,B)|
|Spines/comedones – e.g. nevus comedonicus (Table 51.3), PEODDN (favors palms/soles), linear lichen planopilaris (later onset; see Fig. 9.4H)|
|Hypopigmentation (see Table 54.3)|
|Hyperpigmentation (see Table 55.4)|
|Hairlessness – e.g. X-linked hypohidrotic ectodermal dysplasia (female ‘carriers’; Fig. 51.6), Goltz syndrome, IP stage 4 (Fig. 51.7C)|
|Atrophy – e.g. linear lichen sclerosus (epidermal wrinkling), linear atrophoderma of Moulin (hyperpigmented and depressed), Goltz syndrome (Fig. 51.8A,B), IP stage 4, Conradi–Hünermann–Happle syndrome (follicular atrophoderma)|
|Papulonodular lesions** – e.g. adnexal neoplasms (e.g. trichoepitheliomas), BCCs, basaloid follicular hamartomas|
* Inflammatory manifestations occur primarily during infancy.
** In addition to conditions with inflammatory or verrucous papulonodules noted above.
GVHD, graft-versus-host disease; IP, incontinentia pigmenti; PEODDN, porokeratotic eccrine ostial and dermal duct nevus.
Skin findings that can have a block-like or segmental pattern that reflects mosaicism.