悪性腫瘍関連筋炎

Malignancy-associated Myositis

はじめに

　筋炎は自己免疫性炎症機序により筋組織が破壊される病態であり，古典的には皮膚症状の有無から大きく多発筋炎（polymyositis：PM），皮膚筋炎（dermatomyositis：DM）に分類されるが^1），臨床像と組織所見の検討^2）や，合併膠原病のタイプおよび出現する自己抗体の検討^3）から，さまざまな病態機序を背景に持つheterogeneousな疾患であることが明らかにされつつある。一方，1世紀近く前より，筋炎の一部の症例に悪性腫瘍が合併し特徴的な臨床像を示すとして多くの報告がなされてきた。しかしながら，両者の関連を明確にする疫学検討がなされたのは，近年になってからである。また，最近，悪性腫瘍関連筋炎症例に高頻度で出現する抗体として，抗p155/p140抗体（抗p155抗体）が発見された。本抗体は悪性腫瘍を伴う特発性筋炎の診断に役立つばかりでなく，本疾患の病態機序を考えるうえでも重要である。本稿では，悪性腫瘍関連筋炎の疫学および臨床的な特徴について概説し，現時点での本疾患の病態機序に関する仮説を紹介する。

Abstract

　Inflammatory myopathies,also referred to as myositis,are a heterogeneous group of chronic inflammatory muscle diseases characterized by various clinical features and histological changes; in addition,patient with these disease exhibit positivity for autoantibodies as well as progressive inflammatory muscle damage and experienced weakness.　Although it has been well known for a century that myositis,particularly dermatomyositis,can be associated with malignancy,it was not until recently that the result of robust epidemiological studies confirmed this association.　Malignancy-associated myositis differs from primary myositis in many aspects.　Prognosis and life expectancy for patients are determined on the basis of the underlying malignancy.　Therefore,patient-specific examinations to detect an underlying cancer are important for the management of these patients.　Recently,a novel myositis-specific autoantibody (anti-p155 or p155/p140 antibody) was identified in malignancy-associated myositis.　The discovery of this autoantibody is important not only for an early diagnosis of adult-onset myositis patients with a higher risk of malignancy but also for a better understanding of the pathogenesis of paraneoplastic myositis.　In addition,it has also been recently found that both regenerating cells in myositis muscles and several cancers known to be associated with myositis express high levels of myositis-specific autoantigens.　Therefore,a model of paraneoplasia focusing on the expression of common autoantigens expression and immuno-targeting between cancer and muscle tissues in myositis has been proposed.　In this review,we aim to describe epidemiological evidence for an association between myositis and malignancy,and to describe the clinical features of malignancy-associated myositis.　We also aim to focus on a recently proposed model to understand the development of malignancy-associated myositis.