Search

We investigated the clinical, genetic, and pathological characteristics of a previously unknown severe juvenile brain disorder in several litters of Parson Russel Terriers. The disease started with epileptic seizures at 6 to 12 weeks of age and progressed rapidly to status epilepticus and death or euthanasia. Histopathological changes at autopsy were restricted to the brain. There was severe acute neuronal degeneration and necrosis diffusely affecting the grey matter throughout the brain with extensive intraneuronal mitochondrial crowding and accumulation of amyloid-β (Aβ). Combined homozygosity mapping and genome sequencing revealed an in-frame 6-bp deletion in the nuclear-encoded pitrilysin metallopeptidase 1 (PITRM1) encoding for a mitochondrial protease involved in mitochondrial targeting sequence processing and degradation. The 6-bp deletion results in the loss of two amino acid residues in the N-terminal part of PITRM1, potentially affecting protein folding and function. Assessment of the mitochondrial function in the affected brain tissue showed a significant deficiency in respiratory chain function. The functional consequences of the mutation were modeled in yeast and showed impaired growth in permissive conditions and an impaired respiration capacity. Loss-of-function variants in human PITRM1 result in a childhood-onset progressive amyloidotic neurological syndrome characterized by spinocerebellar ataxia with behavioral, psychiatric and cognitive abnormalities. Homozygous Pitrm1-knockout mice are embryonic lethal, while heterozygotes show a progressive, neurodegenerative phenotype characterized by impairment in motor coordination and Aβ deposits. Our study describes a novel early-onset PITRM1-related neurodegenerative canine brain disorder with mitochondrial dysfunction, Aβ accumulation, and lethal epilepsy. The findings highlight the essential role of PITRM1 in neuronal survival and strengthen the connection between mitochondrial dysfunction and neurodegeneration.

We establish cystatin B (CSTB) as a regulator of histone H3 tail clipping in murine neural progenitor cells (NPCs) and provide evidence suggesting that epigenetic dysregulation contributes to the early pathogenesis in brain disorders associated with deficient CSTB function. We show that NPCs undergo regulated cleavage of the N-terminal tail of histone H3 at threonine 22 (H3T22) transiently upon induction of differentiation. CSTB-deficient NPCs present premature activation of H3T22 clipping during self-renewal mediated by increased activity of cathepsins L and B. During differentiation, the proportion of immature committed neurons undergoing H3T22 clipping is significantly higher in CSTB-deficient than in wild-type NPCs, with no observable decline within 12 days post-differentiation. CSTB-deficient NPCs exhibit significant transcriptional changes highlighting altered expression of nuclear-encoded mitochondrial genes. These changes are associated with significantly impaired respiratory capacity of differentiating NPCs devoid of CSTB. Our data expand the mechanistic understanding of diseases associated with CSTB deficiency.

Progressive myoclonus epilepsies (PMEs) comprise a group of clinically and genetically heterogeneous rare diseases. Over 70% of PME cases can now be molecularly solved. Known PME genes encode a variety of proteins, many involved in lysosomal and endosomal function. We performed whole-exome sequencing (WES) in 84 (78 unrelated) unsolved PME-affected individuals, with or without additional family members, to discover novel causes. We identified likely disease-causing variants in 24 out of 78 (31%) unrelated individuals, despite previous genetic analyses. The diagnostic yield was significantly higher for individuals studied as trios or families (14/28) versus singletons (10/50) (OR = 3.9, p value = 0.01, Fisher's exact test). The 24 likely solved cases of PME involved 18 genes. First, we found and functionally validated five heterozygous variants in NUS1 and DHDDS and a homozygous variant in ALG10, with no previous disease associations. All three genes are involved in dolichol-dependent protein glycosylation, a pathway not previously implicated in PME. Second, we independently validate SEMA6B as a dominant PME gene in two unrelated individuals. Third, in five families, we identified variants in established PME genes; three with intronic or copy-number changes (CLN6, GBA, NEU1) and two very rare causes (ASAH1, CERS1). Fourth, we found a group of genes usually associated with developmental and epileptic encephalopathies, but here, remarkably, presenting as PME, with or without prior developmental delay. Our systematic analysis of these cases suggests that the small residuum of unsolved cases will most likely be a collection of very rare, genetically heterogeneous etiologies. Refereed/Peer-reviewed

International audience; Dysfunction of mitochondrial translation is an increasingly important molecular cause of human disease, but structural defects of mitochondrial ribosomal subunits are rare. We used next-generation sequencing to identify a homozygous variant in the mitochondrial small ribosomal protein 14 (MRPS14, uS14m) in a patient manifesting with perinatal hypertrophic cardiomyopathy, growth retardation, muscle hypotonia, elevated lactate, dysmorphy and mental retardation. In skeletal muscle and fibroblasts from the patient, there was biochemical deficiency in complex IV of the respiratory chain. In fibroblasts, mitochondrial translation was impaired, and ectopic expression of a wild-type MRPS14 cDNA functionally complemented this defect. Surprisingly, the mutant uS14m was stable and did not affect assembly of the small ribosomal subunit. Instead, structural modeling of the uS14m mutation predicted a disruption to the ribosomal mRNA channel.Collectively, our data demonstrate pathogenic mutations in MRPS14 can manifest as a perinatal-onset mitochondrial hypertrophic cardiomyopathy with a novel molecular pathogenic mechanism that impairs the function of mitochondrial ribosomes during translation elongation or mitochondrial mRNA recruitment rather than assembly.

Mitochondrial acyl-coenzyme A species are emerging as important sources of protein modification and damage. Succinyl-CoA ligase (SCL) deficiency causes a mitochondrial encephalomyopathy of unknown pathomechanism. Here, we show that succinyl-CoA accumulates in cells derived from patients with recessive mutations in the tricarboxylic acid cycle (TCA) gene succinyl-CoA ligase subunit-β (SUCLA2), causing global protein hyper-succinylation. Using mass spectrometry, we quantify nearly 1,000 protein succinylation sites on 366 proteins from patient-derived fibroblasts and myotubes. Interestingly, hyper-succinylated proteins are distributed across cellular compartments, and many are known targets of the (NAD+)-dependent desuccinylase SIRT5. To test the contribution of hyper-succinylation to disease progression, we develop a zebrafish model of the SCL deficiency and find that SIRT5 gain-of-function reduces global protein succinylation and improves survival. Thus, increased succinyl-CoA levels contribute to the pathology of SCL deficiency through post-translational modifications., The pathomechanism of succinyl-CoA ligase (SCL) deficiency, a hereditary mitochondrial disease, is not fully understood. Here, the authors show that increased succinyl-CoA levels contribute to SCL pathology by causing global protein hyper-succinylation.

Mutations of mitochondrial DNA (mtDNA) often underlie mitochondrial disease, one of the most common inherited metabolic disorders. Since the sequencing of the human mitochondrial genome and the discovery of pathogenic mutations in mtDNA more than 30 years ago, a movement towards generating methods for robust manipulation of mtDNA has ensued, although with relatively few advances and some controversy. While developments in the transformation of mammalian mtDNA have stood still for some time, recent demonstrations of programmable nuclease-based technology suggest that clinical manipulation of mtDNA heteroplasmy may be on the horizon for these largely untreatable disorders. Here we review historical and recent developments in mitochondrially targeted nuclease technology and the clinical outlook for treatment of hereditary mitochondrial disease.

Hartsfield syndrome is a rare clinical entity characterized by holoprosencephaly and ectrodactyly with the variable feature of cleft lip/palate. In addition to these symptoms patients with Hartsfield syndrome can show developmental delay of variable severity, isolated hypogonadotropic hypogonadism, central diabetes insipidus, vertebral anomalies, eye anomalies, and cardiac malformations. Pathogenic variants in FGFR1 have been described to cause phenotypically different FGFR1-related disorders such as Hartsfield syndrome, hypogonadotropic hypogonadism with or without anosmia, Jackson–Weiss syndrome, osteoglophonic dysplasia, Pfeiffer syndrome, and trigonocephaly Type 1. Here, we report three patients with Hartsfield syndrome from two unrelated families. Exome sequencing revealed two siblings harboring a novel de novo heterozygous synonymous variant c.1029G>A, p.Ala343Ala causing a cryptic splice donor site in exon 8 of FGFR1 likely due to gonadal mosaicism in one parent. The third case was a sporadic patient with a novel de novo heterozygous missense variant c.1868A>G, p.(Asp623Gly).

Mitochondrial dysfunction elicits stress responses that safeguard cellular homeostasis against metabolic insults. Mitochondrial integrated stress response (ISRmt) is a major response to mitochondrial (mt)DNA expression stress (mtDNA maintenance, translation defects), but the knowledge of dynamics or interdependence of components is lacking. We report that in mitochondrial myopathy, ISRmt progresses in temporal stages and development from early to chronic and is regulated by autocrine and endocrine effects of FGF21, a metabolic hormone with pleiotropic effects. Initial disease signs induce transcriptional ISRmt (ATF5, mitochondrial one-carbon cycle, FGF21, and GDF15). The local progression to 2nd metabolic ISRmt stage (ATF3, ATF4, glucose uptake, serine biosynthesis, and transsulfuration) is FGF21 dependent. Mitochondrial unfolded protein response marks the 3rd ISRmt stage of failing tissue. Systemically, FGF21 drives weight loss and glucose preference, and modifies metabolism and respiratory chain deficiency in a specific hippocampal brain region. Our evidence indicates that FGF21 is a local and systemic messenger of mtDNA stress in mice and humans with mitochondrial disease.

The methylation status of the O6-methyl guanine methyltransferase (MGMT) gene promoter is a prognostic biomarker for treatment with the alkylator, temozolomide (TMZ) in many solid tumors including gliomas and colorectal cancers. It is well established that patients with a methylated MGMT promoter (MGMT-) who are treated with the TMZ have a better overall survival than patients with an unmethylated MGMT promoter (MGMT+). The enzyme produced by the MGMT gene is responsible for removing cytotoxic O6-methylguanine (O6-meG) lesions formed by TMZ. In the MGMT- setting, the O6-meG lesion activates the mismatch repair (MMR) pathway which functions to remove the damage. Published work from our group reported differential activation of the ataxia telangiectasia and RAD3 related protein (ATR) in MGMT- and MGMT+ glioblastoma multiforme (GBM) cells in response to TMZ treatment, as demonstrated through the phosphorylation of CHK1. Though it is known that MMR proteins are involved in ATR activation, the specific MMR proteins required for ATR activation by TMZ-induced alkyl lesions remain unknown in the MGMT- setting. Here, we demonstrate that the MMR protein MSH2 plays a role in ATR activation in the presence of O6-meG lesions. We show that there is potent synergy with ATRi and TMZ in the MGMT- MSH2-proficient GBM cell line, which is abrogated in an shMSH2 MGMT- GBM cell line. Additionally, we observe decreased levels of pCHK1 in the shMSH2 MGMT- setting compared to the MGMT- MSH2-proficient cells, suggesting that MSH2 is integral in the activation of ATR upon TMZ treatment. This study elucidates a potential role for MSH2 in ATR activation. Mechanistic understanding of how the MMR system is involved in ATR activation by TMZ can ultimately be exploited for therapeutic gain. Citation Format: Sachita Ganesa, Amrita D. Sule, Christopher B. Jackson, Ranjini K. Sundaram, Ranjit S. Bindra. MSH2 is necessary for temozolomide-induced ATR activation in MGMT-methylated cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1179.

Isolated defects of the mitochondrial respiratory complex II (succinate dehydrogenase, SDH) are rare, accounting for approximately 2% of all respiratory chain deficiency diagnoses. Here, we report clinical and molecular investigations of three family members with a heterozygous mutation in the large flavoprotein subunit SDHA previously described to cause complex II deficiency. The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy. Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood. The daughter deceased at the age of 7 months due to cardiac insufficiency. The 30-year old son presents with cardiomyopathy and developed bilateral optic atrophy in adulthood. Of the four nuclear encoded proteins composing complex II (SDHA, SDHB, SDHC, SDHD) and currently known assembly factors SDHAF1 and SDHAF2 mainly recessively inherited mutations have been described in SDHA, SDHB, SDHD, and SDHAF1 to be causative for mitochondrial disease phenotypes. This is the second report presenting autosomal dominant inheritance of a SDHA mutation.© 2016 Wiley Periodicals, Inc.

We report a novel homozygous missense mutation in the ubiquinol-cytochrome c reductase synthesis-like (BCS1L) gene in two consanguineous Turkish families associated with deafness, Fanconi syndrome (tubulopathy), microcephaly, mental and growth retardation. All three patients presented with transitory metabolic acidosis in the neonatal period and development of persistent renal de Toni-Debré-Fanconi-type tubulopathy, with subsequent rachitis, short stature, microcephaly, sensorineural hearing impairment, mild mental retardation and liver dysfunction. The novel missense mutation c.142A>G (p.M48V) in BCS1L is located at a highly conserved region associated with sorting to the mitochondria. Biochemical analysis revealed an isolated complex III deficiency in skeletal muscle not detected in fibroblasts. Native polyacrylamide gel electrophoresis (PAGE) revealed normal super complex formation, but a shift in mobility of complex III most likely caused by the absence of the BCS1L-mediated insertion of Rieske Fe/S protein into complex III. These findings expand the phenotypic spectrum of BCS1L mutations, highlight the importance of biochemical analysis of different primary affected tissue and underline that neonatal lactic acidosis with multi-organ involvement may resolve after the newborn period with a relatively spared neurological outcome and survival into adulthood. CONCLUSION Mutation screening for BCS1L should be considered in the differential diagnosis of severe (proximal) tubulopathy in the newborn period. What is Known: • Mutations in BCS1L cause mitochondrial complex III deficiencies. • Phenotypic presentations of defective BCS1L range from Bjornstad to neonatal GRACILE syndrome. What is New: • Description of a novel homozygous mutation in BCS1L with transient neonatal acidosis and persistent de Toni-Debré-Fanconi-type tubulopathy. • The long survival of patients with phenotypic presentation of severe complex III deficiency is uncommon.

Mitochondrial dysfunction manifests as different neurological diseases, but the mechanisms underlying the clinical variability remain poorly understood. To clarify whether different brain cells have differential sensitivity to mitochondrial dysfunction, we induced mitochondrial DNA (mtDNA) depletion in either neurons or astrocytes of mice, by inactivating Twinkle (TwKO), the replicative mtDNA helicase. Here we show that astrocytes, the most abundant cerebral cell type, are chronically activated upon mtDNA loss, leading to early-onset spongiotic degeneration of brain parenchyma, microgliosis and secondary neurodegeneration. Neuronal mtDNA loss does not, however, cause symptoms until 8 months of age. Findings in astrocyte-TwKO mimic neuropathology of Alpers syndrome, infantile-onset mitochondrial spongiotic encephalopathy caused by mtDNA maintenance defects. Our evidence indicates that (1) astrocytes are dependent on mtDNA integrity; (2) mitochondrial metabolism contributes to their activation; (3) chronic astrocyte activation has devastating consequences, underlying spongiotic encephalopathy; and that (4) astrocytes are a potential target for interventions., Astrocytes in the brain are metabolically dynamic. Here, Ignatenko, Chilov and colleagues delete mitochondrial DNA (mtDNA) in a cell type specific manner, and show that inactivation of mtDNA helicase Twinkle in astrocytes leads to spongiotic encephalopathy.

Mutations in mitochondrial ATP synthase 6 (MT-ATP6) are a frequent cause of NARP (neurogenic muscle weakness, ataxia, and retinitis pigmentosa) or Leigh syndromes, especially a point mutation at nucleotide position 8993. M.8969G > A is a rare MT-ATP6 mutation, previously reported only in three individuals, causing multisystem disorders with mitochondrial myopathy, lactic acidosis, and sideroblastic anemia or IgA nephropathy. We present two siblings with the m.8969G > A mutation and a novel, substantially milder phenotype with lactic acidosis, poor growth, and intellectual disability. Our findings expand the phenotypic spectrum and show that mtDNA mutations should be taken account also with milder, stable phenotypes.

Optic atrophy 1 (OPA1) is a mitochondrial inner membrane protein that has an important role in mitochondrial fusion and structural integrity. Dysfunctional OPA1 mutations cause atrophy of the optic nerve leading to blindness. Here, we show that OPA1 has an important role in the innate immune system. Using conditional knockout mice lacking Opa1 in neutrophils (Opa1N∆), we report that lack of OPA1 reduces the activity of mitochondrial electron transport complex I in neutrophils. This then causes a decline in adenosine-triphosphate (ATP) production through glycolysis due to lowered NAD+ availability. Additionally, we show that OPA1-dependent ATP production in these cells is required for microtubule network assembly and for the formation of neutrophil extracellular traps. Finally, we show that Opa1N∆ mice exhibit a reduced antibacterial defense capability against Pseudomonas aeruginosa., Optic atrophy 1 (OPA1) is known to be important for mitochondrial fusion and structural integrity. Here, using OPA1 knockout mice, the authors show a detrimental effect on host defense capabilities against pathogen infections. This study reports a critical role for OPA1 in innate immunity.

We describe a novel frameshift mutation in the mitochondrial ATP6 gene in a 4-year-old girl associated with ataxia, microcephaly, developmental delay and intellectual disability. A heteroplasmic frameshift mutation in the MT-ATP6 gene was confirmed in the patient's skeletal muscle and blood. The mutation was not detectable in the mother's DNA extracted from blood or buccal cells. Enzymatic and oxymetric analysis of the mitochondrial respiratory system in the patients' skeletal muscle and skin fibroblasts demonstrated an isolated complex V deficiency. Native PAGE with subsequent immunoblotting for complex V revealed impaired complex V assembly and accumulation of ATPase subcomplexes. Whilst northern blotting confirmed equal presence of ATP8/6 mRNA, metabolic S-35-labelling of mitochondrial translation products showed a severe depletion of the ATP6 protein together with aberrant translation product accumulation. In conclusion, this novel isolated complex V defect expands the clinical and genetic spectrum of mitochondrial defects of complex V deficiency. Furthermore, this work confirms the benefit of native PAGE as an additional diagnostic method for the identification of OXPHOS defects, as the presence of complex V subcomplexes is associated with pathogenic mutations of mtDNA. (C) 2017 Elsevier Masson SAS. All rights reserved.

We report a sporadic case of chronic progressive external ophthalmoplegia associated with ragged red fibers. The patient presented with enlarged mitochondria with deranged internal architecture and crystalline inclusions. Biochemical studies showed reduced activities of complex I, III and IV in skeletal muscle. Molecular genetic analysis of all mitochondrial tRNAs revealed a G to A transition at nt 4308; the G is a highly conserved nucleotide that participates in a GC base-pair in the T-stem of mammalian mitochondrial tRNA(Ile). The mutation was detected at a high level (approx. 50%) in muscle but not in blood. The mutation co-segregated with the phenotype, as the mutation was absent from blood and muscle in the patient's healthy mother. Functional characterization of the mutation revealed a six-fold reduced rate of tRNA(Ile) precursor 3' end maturation in vitro by tRNAse Z. Furthermore, the mutated tRNA(Ile) displays local structural differences from wild-type. These results suggest that structural perturbations reduce efficiency of tRNA(Ile) precursor 3' end processing and contribute to the molecular pathomechanism of this mutation.

OBJECTIVE: Short-chain enoyl-CoA hydratase (ECHS1) is a multifunctional mitochondrial matrix enzyme that is involved in the oxidation of fatty acids and essential amino acids such as valine. Here, we describe the broad phenotypic spectrum and pathobiochemistry of individuals with autosomal-recessive ECHS1 deficiency. METHODS: Using exome sequencing, we identified ten unrelated individuals carrying compound heterozygous or homozygous mutations in ECHS1. Functional investigations in patient-derived fibroblast cell lines included immunoblotting, enzyme activity measurement, and a palmitate loading assay. RESULTS: Patients showed a heterogeneous phenotype with disease onset in the first year of life and course ranging from neonatal death to survival into adulthood. The most prominent clinical features were encephalopathy (10/10), deafness (9/9), epilepsy (6/9), optic atrophy (6/10), and cardiomyopathy (4/10). Serum lactate was elevated and brain magnetic resonance imaging showed white matter changes or a Leigh-like pattern resembling disorders of mitochondrial energy metabolism. Analysis of patients' fibroblast cell lines (6/10) provided further evidence for the pathogenicity of the respective mutations by showing reduced ECHS1 protein levels and reduced 2-enoyl-CoA hydratase activity. While serum acylcarnitine profiles were largely normal, invitro palmitate loading of patient fibroblasts revealed increased butyrylcarnitine, unmasking the functional defect in mitochondrial β-oxidation of short-chain fatty acids. Urinary excretion of 2-methyl-2,3-dihydroxybutyrate - a potential derivative of acryloyl-CoA in the valine catabolic pathway - was significantly increased, indicating impaired valine oxidation. INTERPRETATION: In conclusion, we define the phenotypic spectrum of a new syndrome caused by ECHS1 deficiency. We speculate that both the β-oxidation defect and the block in l-valine metabolism, with accumulation of toxic methacrylyl-CoA and acryloyl-CoA, contribute to the disorder that may be amenable to metabolic treatment approaches.

Mitochondrial dysfunction elicits various stress responses in different model systems, but how these responses relate to each other and contribute to mitochondrial disease has remained unclear. Mitochondrial myopathy (MM) is the most common manifestation of adult-onset mitochondrial disease and shows a multifaceted tissue-specific stress response: (1) transcriptional response, including metabolic cytokines FGF21 and GDF15; (2) remodeling of one-carbon metabolism; and (3) mitochondrial unfolded protein response. We show that these processes are part of one integrated mitochondrial stress response (ISRmt), which is controlled by mTORC1 in muscle. mTORC1 inhibition by rapamycin downregulated all components of ISRmt, improved all MM hallmarks, and reversed the progression of even late-stage MM, without inducing mitochondrial biogenesis. Our evidence suggests that (1) chronic upregulation of anabolic pathways contributes to MM progression, (2) long-term induction of ISRmt is not protective for muscle, and (3) rapamycin treatment trials should be considered for adult-type MM with raised FGF21.

Background/aim To investigate the underlying pathomechanism in a 33-year-old female Caucasian patient presenting with chronic progressive external ophthalmoplegia (CPEO) plus symptoms. Methods Histochemical anaylsis of skeletal muscle and biochemical measurements of individual oxidative phosphorylation (OXPHOS) complexes. Genetic analysis of mitochondrial DNA in various tissues with subsequent investigation of single muscle fibres for correlation of mutational load. Results The patient’s skeletal muscle showed 20% of cytochrome c oxidase -negative fibres and 8% ragged-red fibres. Genetic analysis of the mitochondrial DNA revealed a novel point mutation in the mitochondrial tRNA Ile ( MTTI ) gene at position m.4282G>A. The heteroplasmy was determined in blood, buccal cells and muscle by restriction fragment length polymorphism (RFLP) combined with a last fluorescent cycle. The total mutational load was 38% in skeletal muscle, but was not detectable in blood or buccal cells of the patient. The phenotype segregated with the mutational load as determined by analysis of single cytochrome c oxidase -negative/positive fibres by laser capture microdissection and subsequent LFC-RFLP. Conclusions We describe a novel MTTI transition mutation at nucleotide position m.4282G>A associated with a CPEO plus phenotype. The novel variant at position m.4282G>A disrupts the middle bond of the D-stem of the tRNA Ile and is highly conserved. The conservation and phenotype-genotype segregation strongly suggest pathogenicity and is in good agreement with the MTTI gene being frequently associated with CPEO. This novel variant broadens the spectrum of MTTI mutations causing CPEO.

Real-time PCR (qPCR) is the method of choice for quantification of mitochondrial DNA (mtDNA) by relative comparison of a nuclear to a mitochondrial locus. Quantitative abnormal mtDNA content is indicative of mitochondrial disorders and mostly confines in a tissue-specific manner. Thus handling of degradation-prone bioptic material is inevitable. We established a serial qPCR assay based on increasing amplicon size to measure degradation status of any DNA sample. Using this approach we can exclude erroneous mtDNA quantification due to degraded samples (e.g. long post-exicision time, autolytic processus, freeze-thaw cycles) and ensure abnormal DNA content measurements (e.g. depletion) in non-degraded patient material. By preparation of degraded DNA under controlled conditions using sonification and DNaseI digestion we show that erroneous quantification is due to the different preservation qualities of the nuclear and the mitochondrial genome. This disparate degradation of the two genomes results in over- or underestimation of mtDNA copy number in degraded samples. Moreover, as analysis of defined archival tissue would allow to precise the molecular pathomechanism of mitochondrial disorders presenting with abnormal mtDNA content, we compared fresh frozen (FF) with formalin-fixed paraffin-embedded (FFPE) skeletal muscle tissue of the same sample. By extrapolation of measured decay constants for nuclear DNA (λnDNA) and mtDNA (λmtDNA) we present an approach to possibly correct measurements in degraded samples in the future. To our knowledge this is the first time different degradation impact of the two genomes is demonstrated and which evaluates systematically the impact of DNA degradation on quantification of mtDNA copy number.