Melanocytic Naevus
Junctional, compound, and dermal naevi
Definition
Junctional, compound, and dermal naevi are benign localized neoplastic proliferations of naevus cells (a type of melanocyte). Like all melanocytic naevi, these lesions are defined by the presence of nests of melanocytes.
Synonyms
Common acquired naevi; banal naevi
Epidemiology
Acquired naevi appear during childhood and reach maximum number in adolescence, declining in number thereafter in cross-sectional studies. This finding may be due to a cohort effect, a process of senescence and disappearance over time, or both.
Etiology
The prevalence of naevi in various populations depends on phenotypic factors (in particular sun susceptibility), sun exposure patterns, and genetic susceptibility, resulting in mutations of a single oncogene in a single cell, which expands to form a clone.
Localization
Naevi are widely distributed over the body, with the most frequent site being
the trunk.
Clinical features
These are localized, generally symmetrical and uniformly pigmented lesions, typically <5 mm in diameter They have been assumed to evolve from initial junctional proliferations to compound and then dermal naevi with accompanying development of a papular component, loss of the junctional component, and loss of pigment; however, a more complex model of separate evolution has been proposed.
Histogenesis
These lesions are thought to evolve as junctional proliferations of melanocytes, related to the mutation of a single oncogene as a driver mutation in actinically altered skin {2390}, and to evolve through the stages of compound and dermal naevi as described above.
Genetic profile
Most acquired naevi have activating mutations of the oncogene BRAF (typically BRAF p.V600E) and less often mutations of NRAS. Different mutations are associated with distinct forms of naevi of other types, discussed in the respective sections. BAPI-inactivated melanocytic lesions must be distinguished from Spitz naevi and atypical Spitz tumours, and are more akin to naevi with partial transformation by a second genomic event (i.e. BAPI loss).They usually lack epidermal hyperplasia, are usually epithelioid rather than spindled, often include a second naevus cell component, lack BAPI expression, and are usually BRAF-mutant.
Genetic susceptibility
Several so-called naevus susceptibility genes have been described, but this information has not yet been translated into clinical practice. Prognosis and predictive factors Naevi have medical significance in relation to melanoma, as simulants, potential precursors, and risk markers (discussed in the sections on dysplastic naevi and melanomas). Briefly, individuals with an increased total number of naevi or an increased number of large naevi are at increased risk of melanoma as deter-mined in case—control and prospective cohort studies. It has been shown that about one third of melanomas arise in compound naevi. Although many melanomas are associated with precursor naevi, the risk of progression for individual lesions is very low.
Reference:
WHO classification of skin tumours, 4th edition (2018): 80-81.