Superimposed segmental dermatomyositis: an emerging new paradigm

ARTICLE

Auteur(s) : Rudolf Happle

Department of Dermatology, University Medical Center Freiburg, Hauptstr. 7, 79104 Freiburg, Germany

In this issue, Fleury et al. from Poitiers (France) describe a case of muscular calcinosis showing an unusually pronounced, unilateral distribution in a patient who had, in addition, classical nonsegmental lesions of dermatomyositis (DM) [1]. The calcified masses involved the patient's right shoulder and upper arm to such a degree that they had to be removed by a major surgical procedure.

I propose to explain this uncommon case in the following way. The patient carried several genes predisposing to DM. At an early stage of embryogenesis, either a postzygotic mutational event gave rise to loss of heterozygosity at one of these predisposing loci, or a somatic cell became heterozygous for a mutation at an additional predisposing gene locus [2, 3]. The pronounced unilateral calcinosis as documented by Fleury et al. [1] would reflect a clonal outgrowth of cells having undergone such additional mutational step.

By 2007, the concept of superimposed segmental manifestation had been applied to 11 common skin disorders with a polygenic background, including psoriasis, atopic dermatitis, lichen planus, and vitiligo [2]. In the meantime, lichen nitidus [3], chronic prurigo [4], lichen planopilaris [5], and giant melanocytic nevus [6] have been added to this list. Moreover, a case of pronounced linear calcinosis of the skin appearing in a 4-year-old boy was described by Boente et al. [7] and categorized as a possible example of superimposed segmental DM. A similar case supporting this inference was published by Lipsker and Lenormand [8].

Hence, this concept can be used as a powerful tool to understand exceptional cases of pronounced segmental DM involving the skin [7, 8] or deep tissues [9], or both. Accordingly, the case reported by Fleury et al. [1] would represent an impressive example of pronounced unilateral muscular involvement superimposed on classical nonsegmental lesions of DM.

To date, at least seven different gene loci predisposing to DM have been described [10]. Molecular analysis of unusual cases of superimposed segmental DM, as reported by Fleury et al. [1], may help elucidate further the polygenic basis of this autoimmune disorder.

Acknowledgements

References

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10 Online Mendelian Inheritance in Man (OMIM). Web site available at: http://www.ncbi.nlm.nih.gov/omim/. Accessed 25 May 2010.