Mosaic Skin Conditions

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51

Mosaic Skin Conditions

• A mosaic organism is composed of ≥2 genetically distinct cell populations derived from a homogeneous zygote.

– Genomic mosaicism results from alteration in the DNA sequence (affecting genes or chromosomes).

– 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’.

• The accessibility of the skin allows visualization of mosaic patterns.

– Blaschko’s lines are streaks and swirls that represent pathways of epidermal cell (e.g. keratinocyte or melanocyte) migration during embryonic development (Fig. 51.1).

Fig. 51.1 Blaschko’s lines. Note the S-shape of lines on the abdomen, the V-shape on the central back, perpendicular lines on the face, and swirls on the posterior scalp.

– Block-like, segmental, and dermatomal patterns can also reflect cutaneous mosaicism, typically involving melanocytes, mesodermal cells, and nerve cells, respectively.

• Types of cutaneous lesions that can follow Blaschko’s lines or have a block-like/segmental pattern are outlined in Tables 51.1 and 51.2, respectively (Fig. 51.2).

Table 51.1

Skin findings that can occur along Blaschko’s lines.

Inflammation
Examples	Clinical Clues
Lichen striatus	• Pink to hypopigmented flat-topped papules; common in children (see Chapter 9)
Linear lichen planus (Fig. 51.2A)	• Violaceous flat-topped papules with Wickham’s striae; often in adults
‘Blaschkitis’	• Pruritic erythematous papulovesicles in multiple streaks on the trunk; usually develops in adults and often recurs
Inflammatory linear verrucous epidermal nevus (ILVEN) (Fig. 51.2B)	• Scaly, erythematous psoriasiform plaques with prominent pruritus and lack of response to therapy; onset usually by childhood
Linear psoriasis (Fig. 51.2C)	• Psoriasiform plaques, often responding to therapy; ± psoriasis elsewhere
Other conditions resembling lichen planus: linear GVHD, lupus erythematous > dermatomyositis, drug eruptions Other conditions with a keratotic and/or vesiculobullous component: • Variable onset: linear Darier (Fig. 51.2D) and Hailey–Hailey (Fig. 51.2E) diseases • Early onset: linear porokeratosis (see Fig. 89.7E), PEODDN, IP* stages 1–2 (see Figs. 28.9 and 51.7A), epidermolytic epidermal nevus, Conradi–Hünermann–Happle syndrome (see Fig. 46.13)
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.

Table 51.2

Skin findings that can have a block-like or segmental pattern that reflects mosaicism.

Hypopigmentation – e.g. nevus depigmentosus, segmental vitiligo (see Chapter 54)

Hyperpigmention ± hypertrichosis – e.g. CALM, Becker’s nevus/smooth muscle hamartoma (see Table 50.3 and Chapter 55)

Hypertrichosis – e.g. X-linked congenital generalized hypertrichosis (female ‘carriers’)

Vascular lesions – e.g. port wine stain,^† CMTC, unilateral nevoid telangiectasia, venous malformation,*

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