Mitochondrial DNA variants in inclusion body myositis

Abstract

Mitochondrial DNA variants have been shown to be associated with many diseases. Mutations at mitochondrial DNA nucleotide positions 3192, 3196, 3397 and 4336 have been described in association with late-onset Alzheimer's disease. The pathological similarities between inclusion body myositis and Alzheimer's disease prompted an analysis of the relationship between the reported mutations and sporadic inclusion body myositis. The 4336G variant was not significantly increased in patients with inclusion body myositis or Alzheimer's disease when compared to controls. None of the patients with inclusion body myositis carried mutations at nucleotide positions 3192, 3196 and 3397. A transition at nucleotide position 4580 was detected in some patients with inclusion body myositis and Alzheimer's disease but was not significantly higher in frequency when compared to controls. Phylogenetic analysis showed that the 4336G and 4580A variants clustered together in their respective group. A group of patients with inclusion body myositis also clustered together on a separate branch of the phylogenetic tree. Closer investigation of this group revealed a common polymorphism at nucleotide position 16311. The frequency of the 16311C variant was higher in inclusion body myositis than in Alzheimer's disease and controls, although when only caucasian patients were considered the increased frequency was not statistically significant. Further studies will be required to determine whether this variant plays a role in the pathogenesis of inclusion body myositis.

Introduction

Inclusion body myositis (IBM) is the most commonly diagnosed form of myositis in male patients over 50 years of age [1]. The disease has a distinctive set of clinical and pathological features [2], [3], [4]. Clinically it is characterized by insidious development of muscle weakness and atrophy of the distal and proximal muscles of the arms and legs and a poor response to corticosteroid or immunosuppressive therapy [4], [5]. Histopathological features include mononuclear inflammatory infiltrates consisting primarily of macrophages and CD8⁺ T cells [6], rimmed vacuoles, hyaline eosinophilic inclusions and atrophic angular muscle fibres. Electron microscopy reveals clusters of 15–21 nm tubulofilamentous structures. The vacuolated fibres are remarkable for their accumulation of a range of proteins which include the β-amyloid fragment (βA4) of the amyloid precursor protein, apolipoprotein E, ubiquitin, hyperphosphorylated tau, α1-antichymotrypsin and superoxide dismutase [1].

In biopsies of affected muscle from patients with sporadic IBM, ragged red and cytochrome c oxidase (COX) negative fibres are present at increased frequency when compared to age-matched controls and patients with other forms of myositis [7], [8], [9]. Further investigation of the mitochondrial DNA (mtDNA) in such fibres has revealed a high frequency of DNA deletions. These often encompass the COX sub-unit genes and/or other genes encoding proteins of complexes I and IV of the electron transport chain, as well as tRNA genes essential for intra-mitochondrial protein synthesis [8], [9], [10]. The relationship between the immune response, the vacuoles, the proteins deposited and the mitochondrial defects in IBM is unclear. The mtDNA deletions may occur as a consequence of the muscle damage caused by other factors such as oxidative stress or the accumulation of βA4 [1], [11]. Alternatively, mtDNA damage may be the initiating event and consequent mitochondrial malfunction may set up an environment which leads to the abnormal production of the proteins characteristic of IBM with subsequent muscle damage.

mtDNA deletions are common in normal, aged individuals [12], but in diseases such as autosomal dominant progressive external ophthalmoplegia (AD-PEO), mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) and mitochondrial myopathies these deletions are markedly increased in frequency [13], [14], [15]. This is true in IBM [8], [9], [10]. Genetic loci removed from the location of the mtDNA deletions have been associated with these aberrations. Two nuclear genomic loci have been associated with predisposition to an increased frequency of mtDNA replication errors and deletions in families with AD-PEO [16], [17], and mutations in the thymidine phosphorylase gene on chromosome 22 have been implicated in the mtDNA deletions in MNGIE [15]. A variant in the D-loop sequence of the mtDNA has also been associated with an increased rate of mtDNA deletions in a family with mitochondrial myopathy [18].

Analogies between Alzheimer's disease (AD) and IBM have been drawn previously [1], [19]. Many of the proteins deposited in IBM are also abnormally accumulated in the brain in AD. These include the βA4 protein, ubiquitin, hyperphosphorylated tau and apolipoprotein E [1], [4]. Both diseases have onset late in life and in both diseases characteristic paired helical filaments are detectable in the affected cells [1], [20]. In both diseases there is evidence of mtDNA aberrations. In AD a series of reports have described either inherited mtDNA mutations [21], [22], or somatic mutations in mtDNA derived from the brain [23], each of which has been implicated in disease development.

In our investigation of genetic factors which predispose to the development of sporadic IBM we have determined whether any of the mtDNA mutations previously reported to be associated with AD [21], [22] are increased in frequency in IBM. Furthermore we have conducted a phylogenetic analysis of the D-loop sequences of mtDNA in our patients with IBM to determine the relatedness of these sequences to each other. D-loop identity has been described in patients who share a mtDNA mutation elsewhere in the mtDNA genome [21], [22]. Any clustering of IBM patients based on D-loop sequences would suggest the presence of a common mutation elsewhere in the mitochondrial genome which may play a role in the development of IBM.