Reply: MFN2 mutations cause compensatory mitochondrial DNA ...

Download PDF
1 downloads 0 Views 151KB Size Report
Comment
Apr 4, 2012 - ness, cerebellar ataxia and mitochondrial myopathy with ragged-red and cytochrome c oxidase (COX)-negative fibres. (Rouzier et al., 2012).
doi:10.1093/brain/aws052

Brain 2012: 135; 1–3

| e220

BRAIN A JOURNAL OF NEUROLOGY

LETTER TO THE EDITOR Reply: MFN2 mutations cause compensatory mitochondrial DNA proliferation Ce´cile Rouzier,1,2 Sylvie Bannwarth,1,2 Annabelle Chaussenot,1 Arnaud Chevrollier,3,4 Annie Verschueren,5 Nathalie Bonello-Palot,6 Konstantina Fragaki,1,2 Aline Cano,7 Jean Pouget,5 Jean-Franc¸ois Pellissier,8 Vincent Procaccio,3,4 Brigitte Chabrol7 and Ve´ronique Paquis-Flucklinger1,2 1 2 3 4 5 6 7 8

Department of Medical Genetics, National Centre for Mitochondrial diseases, Nice Teaching Hospital, 06202, France LBPG, CNRS UMR 6267/INSERM U998/UNS, School of Medicine, Nice Sophia-Antipolis University, 06107, France CNRS UMR 6214/INSERM U771, Angers, F-49000, France Department of Biochemistry and Genetics, Angers Teaching Hospital, 4900, France Department of Neurology, Timone Hospital, Marseille Teaching Hospital, 13500, France Department of Medical Genetics, Timone Hospital, Marseille Teaching Hospital, 13500, France Department of Neuropaediatrics, Timone Hospital, Marseille Teaching Hospital, 13500, France Department of Neuropathology, Timone Hospital, Marseille Teaching Hospital, 13500, France

Correspondence to: Prof Ve´ronique Paquis-Flucklinger, LBPG, CNRS UMR 6267/INSERM U998/UNS, School of Medicine, 28 av de Valombrose, 06107 Nice cedex 2, France E-mail: [email protected]

Sir, We recently reported in Brain a large Tunisian family with optic atrophy ‘plus’ phenotype including axonal neuropathy, deafness, cerebellar ataxia and mitochondrial myopathy with ragged-red and cytochrome c oxidase (COX)-negative fibres (Rouzier et al., 2012). This neurological disorder is linked to a novel heterozygous missense mutation in MFN2 whereas this gene is typically responsible for autosomal dominant axonal Charcot–Marie–Tooth disease (CMT2A). Furthermore, these Tunisian patients presented multiple mitochondrial DNA deletions in muscle associated with mitochondrial DNA repair deficiency after oxidative stress in fibroblasts, suggesting that MFN2 is involved in mitochondrial DNA maintenance. Interestingly, Sitarz et al. (2012) provide additional evidence to our study. They compared mitochondrial DNA copy number in blood from CMT2A patients with MFN2 mutations and age-matched controls. They found that mitochondrial DNA levels were significantly higher in the MFN2 group compared with controls. The 3-fold increase in mitochondrial DNA content detected in the CMT2A population was independent of the position of MFN2 mutations in the GTPase domain and blood cell count, which can affect the measurement of mitochondrial DNA copy number in leucocytes. Mitochondrial proliferation and increased mitochondrial DNA

copy number act as a compensatory mechanism to maintain an adequate level of ATP for normal cellular function. In mitochondrial myopathies, this process leads to the subsarcolemmal accumulation of mitochondria, corresponding to ragged-red fibres. By demonstrating a similar mechanism in blood of patients with CMT2A and ‘classical’ MFN2 mutations, Sitarz et al. (2012) show that these patients present an underlying bioenergetic defect despite the absence of obvious signs of respiratory chain dysfunction. MFN2 and OPA1 genes encode two dynamin-like GTPase proteins involved in the fusion of the mitochondrial membrane. OPA1 has been associated with autosomal dominant optic atrophy (DOA) but up to 20% of OPA1 carriers present a DOA + phenotype associated with sensorineural deafness, ataxia, axonal sensory motor polyneuropathy, chronic progressive external ophthalmoplegia and mitochondrial myopathy (Yu-Wai-Man et al., 2010a). DOA + patients also present ragged-red fibres and multiple mitochondrial DNA deletions in muscle supporting the idea that Mfn2 and OPA1 proteins are functionally similar. Microdissection of single skeletal muscle fibres from patients with OPA1 mutations revealed that COX-negative fibres are associated with increased mitochondrial DNA levels

Advance Access publication April 4, 2012 ß The Author (2012). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: [email protected]

e220

| Brain 2012: 135; 1–3

Letter to the Editor

Figure 1 (A) Pedigree of the family. Solid and hatched symbols represent clinically affected individuals. + = MFN2 wild-type allele; = MFN2 mutated allele (Rouzier et al., 2012). (B) Determination of mitochondrial DNA (mtDNA) copy number by quantitative real-time polymerase chain reaction in skeletal muscle of 19 age-matched control individuals and Patients I-1, II-8 and III-14 (left). Determination of mitochondrial DNA copy number by quantitative real-time polymerase chain reaction in skeletal muscle of 8 age-matched control individuals and Patient III-16 (right). Values are shown relative to the average of all control samples. Experiments were performed as described in Rouzier et al. (2012). (C) Determination of mitochondrial DNA copy number by quantitative real-time polymerase chain reaction in blood of 30 age-matched control individuals and Patients I-1, II-8 and III-14 (left). Determination of mitochondrial DNA copy number by quantitative real-time polymerase chain reaction in blood of 30 age-matched control individuals and Patient III-16. Values are shown relative to the average of all control samples. Experiments were performed as described in Rouzier et al. (2012).

(Yu-Wai-Man et al., 2010b). Unfortunately, we do not have enough material to do the same analysis in muscle fibres from patients bearing the MFN2 mutation responsible for optic atrophy ‘plus’ syndrome. Nevertheless, the determination of the relative mitochondrial DNA copy number revealed a high amount of mitochondrial DNA (25.52 versus 20, 128% of age-matched controls) in homogenate muscle extracts of the index case (Patient I-1, Fig. 1A and B), who had the highest level of ragged-red and COX-negative fibres (45%). In the two other adult patients (Patients II-8 and III-14), the values were 100% (20.1 versus 20) and 118% (23.6 versus 20), respectively. These results are evocative of mitochondrial DNA proliferation in muscle of affected individuals (Fig. 1B). Nevertheless, they are to be taken with caution because the number of patients is very limited. The last patient was a young male (6 years old) who presented an unusually severe phenotype with regression and psychomotor delay (Patient III-16). The amount of mitochondrial DNA in muscle was 91% (16 versus 17.57) of age-matched controls (Fig. 1B). Although we excluded mutations in nuclear genes known to be involved in mitochondrial DNA instability syndromes, it is possible that a variant in an unknown modifier gene could play a role in

the severity of the phenotype and compensatory associated mechanisms in this child. To go further in the comparison between patients with MFN2associated neuropathy and MFN2-associated optic atrophy ‘plus’ syndrome, we measured mitochondrial DNA levels in the blood of patients from the Tunisian family. Interestingly, no difference was found between patients and age-matched controls in adult individuals or in the young boy (Fig. 1C). Sitarz et al. (2012) clearly showed that MFN2 mutations in patients with CMT2A cause compensatory mitochondrial DNA proliferation in blood. Increased mitochondrial DNA levels have also been detected in muscle from OPA1 carriers (Yu-Wai-Man et al., 2010b). Although the number of affected individuals is very limited, our results suggest mitochondrial DNA proliferation in muscle of patients with MFN2-associated optic atrophy ‘plus’ syndrome but not in blood. It would be of great interest to know whether mitochondrial DNA proliferation is also present in blood from patients with OPA1 mutations and to compare DOA and DOA + individuals. Both OPA1 and MFN2 are responsible for classical and syndromic phenotypes. It has been suggested that the marked variability in disease severity may reflect the multiple

Letter to the Editor

Brain 2012: 135; 1–3

distinct roles played by OPA1 protein in normal cellular function (Davies and Votruba, 2006). Some OPA1 mutations could also lead to multi-systemic manifestations as a result of their more pronounced effects on mitochondrial DNA deletion formation, and thus proliferation (Yu-Wai-Man et al., 2010b). The reasons why the MFN2 mutation that we described leads to a syndromic phenotype are unknown but we can not exclude an effect dependent on cell type and its capacity for renewal. The accumulation of mitochondrial DNA deletions in post-mitotic tissues can be secondary to an increase in mitochondrial DNA damage, a defect in mitochondrial DNA repair and/or a failure to clear mitochondria with damaged DNA (Chen and Chan, 2010). The respective share of these different mechanisms may also play a role in the compensation process. Further studies are needed to understand the consequences of MFN2 and OPA1 mutations on mitochondrial DNA stability and the variability of clinical phenotypes. The identification of new patients with MFN2 mutations associated with syndromic phenotypes is an essential element for progress in this understanding.

Acknowledgements We thank Gae¨lle Auge´ for technical Ait-El-Mkadem for helpful discussion.

help

and

Samira

| e220

Funding Grants from the Association Franc¸aise contre les Myopathies (AFM, to V.P.-F.).

References Chen H, Chan D. Physiological functions in mitochondrial fusion. Ann NY Acad Sci 2010; 1201: 21–5. Davies V, Votruba M. Focus on molecules : the OPA1 protein. Exp Eye Res 2006; 83: 1003–4. Rouzier C, Bannwarth S, Chaussenot A, Chevrollier A, Verschuren A, Bonello-Palot N, et al. The MFN2 gene is responsible for mitochondrial DNA instability and optic atrophy ‘plus’ phenotype. Brain 2012; 135 (Pt 1): 23–34. Sitarz KS, Yu-Wai-Man P, Pyle A, Stewart JD, Rautenstrauss B, Seeman P, et al. MFN2 mutations cause compensatory mitochondrial DNA proliferation. Brain 2012; 135: e219. Yu-Wai-Man P, Griffiths P, Gorman G, Lourenco C, Wright A, Auer-Grumbach M, et al. Multi-system neurological disease is common in patients with OPA1 mutations. Brain 2010a; 133: 771–86. Yu-Wai-Man P, Sitarz K, Samuels D, Griffiths P, Reeve A, Bindoff L, et al. OPA1 mutations cause cytochrome c oxidase deficiency due to loss of wild-type mtDNA molecules. Hum Mol Genet 2010b; 19: 3043–52.