Causes and Genetics
It is associated with defects in Keratin 17. The condition is inherited in an autosomal dominant manner. This indicates that the defective gene responsible for a disorder is located on an autosome, and only one copy of the defective gene is sufficient to cause the disorder, when inherited from a parent who has the disorder.
However, a solitary case can also emerge in a family with no prior history of the disorder due to the occurrence of a new mutation (often referred to as a sporadic or spontaneous mutation).
Steatocystoma multiplex occurs as either a sporadic or autosomal dominant inherited condition characterized by benign sebaceous gland tumors. Lesions consist of a nevoid formation of abortive hair follicles at the site where sebaceous glands attach. Electron microscopy studies demonstrate cyst wall cells undergoing trichilemmal keratinization similar to that of the isthmus portion of the outer hair sheath. The relationship of steatocystoma multiplex to the development of sebaceous glands and common presentation at puberty suggest a hormonal trigger for lesion growth.
In the familial form of steatocystoma multiplex, mutations are localized to the keratin 17 (K17) gene in areas identical to mutations found in patients with pachyonychia congenita type 2 (PC-2). Pachyonychia congenita type 2, an autosomal dominant inherited disorder, is characterized by hypertrophic nail dystrophy, focal keratoderma, multiple pilosebaceous cysts, and a variety of conditions associated with ectodermal dysplasia. Keratin 17 is expressed in several epithelial structures, most notably in sebaceous glands, the outer root sheath of hair follicles, and the nail bed; its expression correlates well to the clinical phenotypic expression of both steatocystoma multiplex and pachyonychia congenita type 2. To date, 14 mutations have been described in patients with either steatocystoma multiplex or pachyonychia congenita type 2, all of which are localized to the helix initiation domain (1A domain) of the K17 gene.
Some authors propose that steatocystoma multiplex is simply a variant of pachyonychia congenita type 2 because they both share the same underlying etiology. Sporadic forms of steatocystoma multiplex have not been shown to be associated with K17 mutations. In previous reports, specific mutations were attributed to early-onset cyst formation in pachyonychia congenita type 2 and steatocystoma multiplex; however, more recent reports suggest that the age of onset is multifactorial.
Steatocystoma multiplex is often associated with eruptive vellus hair cysts (EVHCs). Both diseases share overlapping clinical features, including age of onset, location, appearance of lesions, and mode of inheritance. Reports of hybrid lesions showing histological features of both steatocystoma multiplex and eruptive vellus hair cysts exist. Given these similarities, some postulate that steatocystoma multiplex and eruptive vellus hair cysts are, in fact, variants of the same disease. However, major differences in keratin expression patterns between steatocystoma multiplex and eruptive vellus hair cysts have been elucidated, leading others to believe that they are 2 distinct disease entities. In steatocystoma multiplex associated with eruptive vellus hair cyst, no K17 mutation has been found.
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