Mitochondrial pathology in inclusion body myositis

https://doi.org/10.1016/j.nmd.2014.12.010Get rights and content

Highlights

Abstract

Inclusion body myositis (IBM) is usually associated with a large number of cytochrome c oxidase (COX)-deficient muscle fibers and acquired mitochondrial DNA (mtDNA) deletions.

We studied the number of COX-deficient fibers and the amount of mtDNA deletions, and if variants in nuclear genes involved in mtDNA maintenance may contribute to the occurrence of mtDNA deletions in IBM muscle.

Twenty-six IBM patients were included. COX-deficient fibers were assayed by morphometry and mtDNA deletions by qPCR. POLG was analyzed in all patients by Sanger sequencing and C10orf2 (Twinkle), DNA2, MGME1, OPA1, POLG2, RRM2B, SLC25A4 and TYMP in six patients by next generation sequencing.

Patients with many COX-deficient muscle fibers had a significantly higher proportion of mtDNA deletions than patients with few COX-deficient fibers. We found previously unreported variants in POLG and C10orf2 and IBM patients had a significantly higher frequency of an RRM2B variant than controls. POLG variants appeared more common in IBM patients with many COX-deficient fibers, but the difference was not statistically significant.

We conclude that COX-deficient fibers in inclusion body myositis are associated with multiple mtDNA deletions. In IBM patients we found novel and also previously reported variants in genes of importance for mtDNA maintenance that warrants further studies.

Introduction

Sporadic inclusion body myositis (IBM) is an idiopathic inflammatory myopathy that causes progressive muscle weakness and atrophy, predominantly affecting quadriceps and long finger flexors, and dysphagia is common [1], [2], [3], [4]. IBM is the most common inflammatory muscle disease in patients over 50 years of age, one study showed a prevalence of 51 cases per million inhabitants in this age group [5]. However, the pathogenesis remains enigmatic, and no therapies have yet proven effective [1], [6], [7]. Morphological findings in IBM muscle fibers include atrophy, rimmed vacuoles, ragged red fibers, cytochrome c oxidase (COX) deficiency and infiltration of mononuclear inflammatory cells. Eventually the muscle is replaced by fat and fibrous connective tissue.

Occasional COX-deficient muscle fibers can be found in normal aging [8], but patients with IBM display a larger amount of such fibers than age-matched controls [9]. A recent study from Rygiel et al. showed that fibers with respiratory deficiency were more prone to be atrophic and therefore of pathogenic importance [10]. COX-deficient fibers are the second most common histopathological finding in IBM patients [11], being 100% sensitive and 73% specific for IBM in inflammatory myopathy without rimmed vacuoles [12] and they are associated with somatic deletions in mtDNA [8], [9], [10].

An accelerated aging process in muscle has been discussed as a part of the pathogenesis in IBM [9], [13], [14], [15]. Variants in genes encoding for proteins associated with mtDNA maintenance such as mitochondrial DNA polymerase gamma (POLG) [14] and Twinkle (C10orf2) (RJ Wiesner, personal communication) have been shown to induce a mitochondrial ageing phenotype in mice by causing somatic mtDNA mutations.

Mitochondrial DNA polymerase gamma is the only known polymerase to replicate and repair mtDNA, and the catalytical part is encoded by POLG. Many pathogenic POLG variants are characterized by multiple mtDNA deletions and disruption of mitochondrial function in post-mitotic tissues. POLG contains a CAG trinucleotide repeat, which normally consists of 10 or 11 repeats, encoding a poly-Q tract [16], [17]. It has been shown that abnormal length of the POLG CAG repeat is associated with Parkinson's disease in Sweden and Finland [16], [18], and a meta-analysis has confirmed an association between non-10Q-alleles and Parkinson's disease [17].

Variants in additional nuclear DNA (nDNA) genes, which regulate transcription, replication and maintenance of mtDNA, can cause accumulation of somatic mtDNA deletions leading to human disease. They include C10orf2 (Twinkle), DNA2 [19], MGME1 (earlier C20orf72) [20], POLG2 and OPA1 [21]. An imbalance in the mitochondrial deoxyribonucleotide (dNTP) pools, which may be caused by variants in TYMP, RRM2B [22] and SLC25A4 (ANT1) [23] can also result in mtDNA deletions and disease [22], [24].

In this study, we wished to examine if IBM is associated with sequence variants in these genes. We also aimed to confirm the association between COX-deficient muscle fibers and mtDNA deletions.

Section snippets

Patients and morphology

From a register of approximately 150 patients diagnosed with IBM at Sahlgrenska University Hospital in Gothenburg, Sweden, fresh frozen muscle specimens were examined regarding the amount of COX-deficient muscle fibers as previously described [25]. The patients were included in the study if they had typical clinical symptoms and morphology for IBM including inflammation, rimmed vacuoles and positive staining for p62/Sequestosome1, and either very few or very numerous COX-deficient fibers (Fig. 1

Analysis of mtDNA in muscle

LX-PCR analysis of mtDNA extracted from muscle tissue demonstrated multiple mtDNA deletions in the mtDNA major arc in thirteen of fifteen patients in the group with a large proportion of COX-deficient fibers, compared with one of eleven patients in the group with a low proportion (Supplementary Table S1). No patients disclosed deletions in the minor arc, and a portion of the ND1-gene could thus be used as a target to measure the total amount of mtDNA by qPCR.

The relative amount of mtDNA

Discussion

The pathogenesis of IBM is still unknown, but mitochondrial alterations are frequently encountered. In this study, we show that the percentage of COX-deficient muscle fibers correlates to the relative amount of mtDNA deletions. We further demonstrate that eight of the 26 studied IBM patients carry heterozygous POLG variants, including a previously not reported missense variant. There was no association between non-10/11 CAG repeats in POLG and IBM in our cohort. Furthermore, we found a

Acknowledgments

The authors are grateful to Gabriella Almén and Anna-Carin Ericson for skilled methodical and technical assistance and Kirsten Mehlig for statistical advice. The authors would like to thank the NHLBI GO Exome Sequencing Project and its ongoing studies which produced and provided exome variant calls for comparison: the Lung GO Sequencing Project (HL-102923), the WHI Sequencing Project (HL-102924), the Broad GO Sequencing Project (HL-102925), the Seattle GO Sequencing Project (HL-102926) and the

References (41)

  • BenvenisteO. et al.

    Long-term observational study of sporadic inclusion body myositis

    Brain

    (2011)
  • GreenbergS.A.

    Pathogenesis and therapy of inclusion body myositis

    Curr Opin Neurol

    (2012)
  • OldforsA. et al.

    Diagnosis, pathogenesis and treatment of inclusion body myositis

    Curr Opin Neurol

    (2005)
  • OldforsA. et al.

    Mitochondrial abnormalities in inclusion-body myositis

    Neurology

    (2006)
  • OldforsA. et al.

    Mitochondrial DNA deletions in inclusion body myositis

    Brain

    (1993)
  • RygielK.A. et al.

    Mitochondrial and inflammatory changes in sporadic inclusion body myositis

    Neuropathol Appl Neurobiol

    (2014)
  • BradyS. et al.

    A retrospective cohort study identifying the principal pathological features useful in the diagnosis of inclusion body myositis

    BMJ Open

    (2014)
  • OldforsA. et al.

    Mitochondrial DNA deletions in muscle fibers in inclusion body myositis

    J Neuropathol Exp Neurol

    (1995)
  • TrifunovicA. et al.

    Premature ageing in mice expressing defective mitochondrial DNA polymerase

    Nature

    (2004)
  • MoslemiA.R. et al.

    Analysis of multiple mitochondrial DNA deletions in inclusion body myositis

    Hum Mutat

    (1997)
  • Cited by (28)

    • Inclusion body myositis: The interplay between ageing, muscle degeneration and autoimmunity

      2022, Best Practice and Research: Clinical Rheumatology
      Citation Excerpt :

      Mitochondria are essential for ATP generation, and mitochondrial dysfunction has been linked to a large number of metabolic and muscle diseases. IBM muscle features mitochondrial abnormalities including a higher proportion of COX negative fibres, ragged red fibres and mitochondrial DNA (mtDNA) deletions compared to healthy aged-matched samples [7,29]. In many neurodegenerative diseases, cellular stress, such as that caused by protein aggregation, can lead to mitochondrial toxicity and activation of the mitochondrial permeability transition pore (mPTP) [30].

    • Mitochondrial DNA depletion in sporadic inclusion body myositis

      2019, Neuromuscular Disorders
      Citation Excerpt :

      MtDNA deletions in sIBM are similar to those seen in normal aging and in autosomally inherited external opthalmoplegia suggesting a similar mechanism [12]. There are also rare reports of mtDNA depletion and dysregulation of Krebs cycle and respiratory chain proteins [13] and variants in nuclear mitochondrial genes such as POLG and RRM2B have been recorded [10]. While there is considerable evidence of mitochondrial dysfunction and especially mtDNA abnormalities in sIBM, mtDNA deletions have not been seen in all patients [8].

    • Sporadic inclusion-body myositis: Recent advances and the state of the art in 2016

      2016, Revue Neurologique
      Citation Excerpt :

      At present, only a few of these efforts have identified genes potentially involved in sIBM pathogenesis [29], namely, genes located in MHC regions and those involved in mtRNA maintenance or protein homeostasis. A good example of the pathological course and entanglement of impaired genes is reflected by the correlation between the amount of COX-deficient fibers and proportion of deleted mtDNA [11,12]. In the years to come, studies of transcriptome profiling and expression quantitative trait loci (eQRLs) in sIBM may possibly be considered by application of RNA sequencing in muscle tissues and lymphoblast lines.

    • Idiopathic inflammatory myositis

      2016, Best Practice and Research: Clinical Rheumatology
      Citation Excerpt :

      A variable inflammatory response comprising CD8+ lymphocytes and macrophages invading major histocompatibility complex (MHC) class 1 immunolabelled myofibres is present. The number of myofibres deficient in cytochrome oxidase (COX negative) is greater than that expected for age [87], and it is associated with mitochondrial dysfunction, recently shown to be secondary to multiple acquired deletions in mitochondrial DNA [88]. Electron microscopy characteristically reveals 15–20-nm filamentous cytoplasmic (occasionally nuclear) inclusions, although the absence of these filaments does not exclude a diagnosis of IBM.

    View all citing articles on Scopus
    View full text