The mitochondrial 13513G > A mutation is most frequent in Leigh syndrome combined with reduced complex I activity, optic atrophy and/or Wolff-Parkinson-White

The m.13513G > A transition in the mitochondrial gene encoding the ND5 subunit of respiratory chain complex I, can cause mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) and has been reported to be a frequent cause of Leigh syndrome (LS). We determined the frequency of the mutation in a cohort of 123 patients with reduced complex I activity in muscle (n = 113) or fibroblast (n = 10) tissue. We describe a Pyrosequencing assay for rapid detection and quantification of the m.13513G > A mutation. Two patients with the mutation were identified; both had LS, optical atrophy and a Wolff-Parkinson-White Syndrome (WPWS)-like cardiac conduction defect. The clinical presentation of the m.13513G > A mutation is discussed. We conclude that the m.13513G > A mutation seems not as frequent as previously suggested and is most likely to be present in patients with Leigh (-like) syndrome combined with a complex I deficiency, optic atrophy and/ or WPWS. In addition, we confirmed that the adjacent m.13514A > G mutation is a rare cause of LS or MELAS since no cases with this transition were found.     

The mitochondrial 13513G>A mutation is associated with Leigh disease phenotypes independent of complex I deficiency in muscle

The mitochondrial 13513G>A (D393N) mutation in the ND5 subunit of the respiratory chain complex I was initially described in association with MELAS syndrome. Recent observations have linked this mutation to Leigh disease. We screened for the 13513G>A mutation in a cohort of 265 patients with Leigh and Leigh-like disease. The mutation was found in a total of 5 patients. An additional patient who had clinical presentation consistent with a Leigh-like phenotype but with a normal brain MRI was added to the cohort. None of an additional 88 patients meeting MELAS disease criteria, nor 56 patients with respiratory chain deficiency screened for the 13513G>A were found positive for the mutation. The most frequent clinical manifestations in our patients were hypotonia, ocular and cerebellar involvement. Low mutation heteroplasmy in the range of 20–40% was observed in all 6 patients. This observation is consistent with the previously reported low heteroplasmy of this mutation in some patients with the 13513G>A mutation and complex I deficiency. However, normal complex I activity was observed in two patients in our cohort. As most patients with Leigh-like disease and the 13513G>A mutation have been described with complex I deficiency, this report adds to the previously reported subset of patients with normal respiratory complex function. We conclude that in any patient with Leigh or Leigh-like disease, testing for the 13513G>A mutation is clinically relevant and low mutant loads in blood or muscle may be considered pathogenic, in the presence of normal respiratory chain enzyme activities.     

Mutations in mitochondrially encoded complex I enzyme as the second common cause in a cohort of Chinese patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes

The mutation pattern of mitochondrial DNA (mtDNA) in mainland Chinese patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) has been rarely reported, though previous data suggested that the mutation pattern of MELAS could be different among geographically localized populations. We presented the results of comprehensive mtDNA mutation analysis in 92 unrelated Chinese patients with MELAS (85 with classic MELAS and 7 with MELAS/Leigh syndrome (LS) overlap syndrome). The mtDNA A3243G mutation was the most common causal genotype in this patient group (79/92 and 85.9%). The second common gene mutation was G13513A (7/92 and 7.6%). Additionally, we identified T10191C (p.S45P) in ND3, A11470C (p. K237N) in ND4, T13046C (p.M237T) in ND5 and a large-scale deletion (13025-13033:14417-14425) involving partial ND5 and ND6 subunits of complex I in one patient each. Among them, A11470C, T13046C and the single deletion were novel mutations. In summary, patients with mutations affecting mitochondrially encoded complex I (MTND) reached 12.0% (11/92) in this group. It is noteworthy that all seven patients with MELAS/LS overlap syndrome were associated with MTND mutations. Our data emphasize the important role of MTND mutations in the pathogenicity of MELAS, especially MELAS/LS overlap syndrome.     

Leigh syndrome caused by mitochondrial DNA G13513A mutation: frequency and clinical features in Japan

The mitochondrial DNA (mtDNA) G13513A mutation in the ND5 subunit gene has been recently reported as a common cause of some phenotypes of mitochondrial myopathy. Until now, the prevalence and characteristics of this mutation in Leigh syndrome (LS) has not been determined. We screened 84 patients with Leigh syndrome (LS) and found the mutation in six (7%) of them. The proportions of mutant mtDNA in muscles were relatively low (42–70%). The onset of symptoms for patients with this mutation was from 9 months to 5 years. It should be noted that five patients had cardiac conduction abnormalities, particularly Wolff-Parkinson-White (WPW) syndrome (three patients). This study suggests that G13513A mutation is a frequent cause of LS and that patients with this mutation may have a characteristic clinical course.     

线粒体ND5基因13513G＞A突变导致Leigh综合征

目的 分析1个母系遗传的Leigh综合征家系的线粒体突变.方法 描述该家系患者的临床和实验室特点,应用DNA芯片结合直接测序对该家系患者进行全线粒体DNA测序.结果 该Leigh综合征家系患者临床以生长发育延迟、精神运动迟滞、呼吸节律异常、颅神经麻痹、小脑共济失调、抽搐为主要特征.神经影像学显示中脑、双侧大脑脚、导水管周围灰质、小脑齿状核及双侧丘脑受累.伴乳酸、丙酮酸代谢异常.突变分析证实该Leigh综合征家系由线粒体ND5*13513G＞A突变所致.结论 ND5突变导致的母系遗传的Leigh综合征有某些特点,ND5*13513G＞A突变可作为Leigh综合征患者的高频候选位点.     

Mutations in the ND5 subunit of complex I of the mitochondrial DNA are a frequent cause of oxidative phosphorylation disease

Background

Detection of mutations in the mitochondrial DNA (mtDNA) is usually limited to common mutations and the transfer RNA genes. However, mutations in other mtDNA regions can be an important cause of oxidative phosphorylation (OXPHOS) disease as well.

Objective

To investigate whether regions in the mtDNA are preferentially mutated in patients with OXPHOS disease.

Methods

Screening of the mtDNA for heteroplasmic mutations was performed by denaturing high‐performance liquid chromatography analysis of 116 patients with OXPHOS disease but without the common mtDNA mutations.

Results

An mtDNA sequence variant was detected in 15 patients, 5 of which were present in the ND5 gene. One sequence variant was new and three were known, one of which was found twice. The novel sequence variant m.13511A→T occurred in a patient with a Leigh‐like syndrome. The known mutation m.13513G→A, associated with mitochondrial encephalomyopathy lactic acidosis and stroke‐like syndrome (MELAS) and MELAS/Leigh/Leber hereditary optic neuropathy overlap syndrome, was found in a relatively low percentage in two patients from two different families, one with a MELAS/Leigh phenotype and one with a MELAS/chronic progressive external ophthalmoplegia phenotype. The known mutation m.13042G→A, detected previously in a patient with a MELAS/myoclonic epilepsy, ragged red fibres phenotype and in a family with a prevalent ocular phenotype, was now found in a patient with a Leigh‐like phenotype. The sequence variant m.12622G→A was reported once in a control database as a polymorphism, but is reported in this paper as heteroplasmic in three brothers, all with infantile encephalopathy (Leigh syndrome) fatal within the first 15 days of life. Therefore, a causal relationship between the presence of this sequence variant and the onset of mitochondrial disease cannot be entirely excluded at this moment.

Conclusions

Mutation screening of the ND5 gene is advised for routine diagnostics of patients with OXPHOS disease, especially for those with MELAS‐ and Leigh‐like syndrome with a complex I deficiency.Mitochondria are key for many cellular processes. One of the most important mechanisms is oxidative phosphorylation (OXPHOS) resulting in the production of cellular energy in the form of ATP. The OXPHOS system consists of five multiprotein complexes (I–V) and two mobile electron carriers (coenzyme q and cytochrome c) embedded in the lipid bilayer of the mitochondrial inner membrane.^1,2 The mitochondrial genome encodes 13 essential polypeptides of the OXPHOS system and the necessary RNA machinery (two ribosomal RNAs and 22 transfer RNAs (tRNA)). The remaining structural proteins and proteins involved in import, assembly and mitochondrial DNA (mtDNA) replication are encoded by the nucleus and specifically targeted to the mitochondria. OXPHOS disease is characterised by a wide variety of clinical symptoms, in which one or more organs can be involved, and by genetic and clinical heterogeneity.^2,3 With an estimated total number of about 1500 nuclear mitochondrial genes of which 600 have been identified so far,⁴ this complicates the process of identification of the underlying genetic defect. Although mutations in the mtDNA tRNA genes have been reported far more often than other mutations in mtDNA protein‐coding genes,² this figure is highly biased by a preferential screening of these genes.In this study, the complete mtDNA was screened for heteroplasmic mutations using denaturing high‐performance liquid chromatography (DHPLC) analysis in a group of 116 unrelated patients suspected for OXPHOS disease but without the common mutations for mitochondrial encephalomyopathy, lactic acidosis and stroke‐like syndrome (MELAS) m.3243A→G, myoclonic epilepsy, ragged red fibres (MERRF) m.8344A→G, Leigh/neuropathy, ataxia and retinitis pigmentosa m.8993T→G/C or large deletions. For this group of patients, we report that the ND5 gene is a commonly mutated gene.     

LHON/MELAS overlap syndrome associated with a mitochondrial MTND1 gene mutation

Pathogenic point mutations in the mitochondrial MTND1 gene have previously been described in association with two distinct clinical phenotypes -- Leber hereditary optic neuropathy (LHON) and mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS). Here we report the first heteroplasmic mitochondrial DNA (mtDNA) point mutation (3376G>A) in the MTND1 gene associated with an overlap syndrome comprising the clinical features of both LHON and MELAS. Muscle histochemistry revealed subtle mitochondrial abnormalities, while biochemical analysis showed an isolated complex I deficiency. Our findings serve to highlight the growing importance of mutations in mitochondrial complex I structural genes in MELAS and its associated overlap syndromes.     

Respiratory chain complex I deficiency caused by mitochondrial DNA mutations

Defects of the mitochondrial respiratory chain are associated with a diverse spectrum of clinical phenotypes, and may be caused by mutations in either the nuclear or the mitochondrial genome (mitochondrial DNA (mtDNA)). Isolated complex I deficiency is the most common enzyme defect in mitochondrial disorders, particularly in children in whom family history is often consistent with sporadic or autosomal recessive inheritance, implicating a nuclear genetic cause. In contrast, although a number of recurrent, pathogenic mtDNA mutations have been described, historically, these have been perceived as rare causes of paediatric complex I deficiency. We reviewed the clinical and genetic findings in a large cohort of 109 paediatric patients with isolated complex I deficiency from 101 families. Pathogenic mtDNA mutations were found in 29 of 101 probands (29%), 21 in MTND subunit genes and 8 in mtDNA tRNA genes. Nuclear gene defects were inferred in 38 of 101 (38%) probands based on cell hybrid studies, mtDNA sequencing or mutation analysis (nuclear gene mutations were identified in 22 probands). Leigh or Leigh-like disease was the most common clinical presentation in both mtDNA and nuclear genetic defects. The median age at onset was higher in mtDNA patients (12 months) than in patients with a nuclear gene defect (3 months). However, considerable overlap existed, with onset varying from 0 to >60 months in both groups. Our findings confirm that pathogenic mtDNA mutations are a significant cause of complex I deficiency in children. In the absence of parental consanguinity, we recommend whole mitochondrial genome sequencing as a key approach to elucidate the underlying molecular genetic abnormality.     

A novel recurrent mitochondrial DNA mutation in ND3 gene is associated with isolated complex I deficiency causing Leigh syndrome and dystonia

Defects in NADH:ubiquinone oxidoreductase (complex I), the largest complex of the mitochondrial respiratory chain, account for most cases of respiratory chain deficiency in human. Complex I contains at least 45 subunits, 7 of which are encoded by mitochondrial DNA (mtDNA). Here we report a novel 10197G>A mutation of the ND3 gene in three unrelated families with Leigh syndrome (LS) or dystonia. Variable degrees of heteroplasmy were found in all tissues tested and a high percentage of mutant mtDNA was observed in muscle. The 10197G>A mutation modifies a hydrophobic alanine residue into a hydrophilic threonine (A47T) in a highly conserved domain of ND3 subunit. Furthermore, this defect could be transferred along with the mutant mtDNAs to rho degrees lymphoblastoid cells in cybrid experiments. However, nuclear modifier genes may also play a role in the phenotypic expression and severity of the 10197G>A mutation. The association of the 10197G>A ND3 mutation with an isolated biochemical defect involving complex I and the discovery of the 10197G>A mutation with a similar phenotype in three unrelated families establish its pathogenicity and demonstrate that the amino acid position A47 is important for the function of complex I. These results show that the 10197G>A mutation in the mitochondrial ND3 gene should be considered as a common mtDNA mutation responsible for LS and dystonia.     

Clinical features of MELAS and its relation with A3243G gene point mutation

Mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS) mostly occur in children. The point mutation A3243G of mitochondrial DNA (mtDNA) may work as a specific bio-marker for mitochondrial disorders. The related clinical features, however, may vary among individuals. This study therefore investigated the relation between MELAS clinical features and point mutation A3243G of mtDNA, in an attempt to provide further evidences for genetic diagnosis of MELAS. Children with MELAS-like syndromes were tested for both blood lactate level and point mutation A3243G of mtDNA. Further family study was performed by mtDNA mutation screening at the same loci for those who had positive gene mutation at A3243G loci. Those who were negative for A3243G point mutation were examined by muscle biopsy and genetic screening. Both clinical and genetic features were analyzed. In all 40 cases with positive A3243G mutation, 36 children fitted clinical diagnosis of MELAS. In other 484 cases with negative mutation, only 8 children were clinically diagnosed with MELAS. Blood lactate levels in both groups were all elevated (P>0.05). In a further genetic screening of 28 families, 10 biological mothers and 8 silbings of MELAS children had positive A3243G point mutations but without any clinical symptoms. Certain difference existed in the clinical manifestations between children who were positive and negative for A3243G mutation of mtDNA but without statistical significance. MELAS showed maternal inheritance under most circumstances.     

Phenotypic variability of mitochondrial disease caused by a nuclear mutation in complex II

We report a patient with relatively mild Leigh syndrome and mitochondrial respiratory chain complex II deficiency caused by a homozygous G555E mutation in the nuclear encoded flavoprotein subunit of succinate dehydrogenase. This mutation has previously been reported in a lethal-infantile presentation of complex II deficiency. Such marked phenotypic heterogeneity, although typical of heteroplasmic mutations in the mitochondrial genome, is unusual for nuclear mutations. Comparable activities and stability of mitochondrial respiratory chain enzymes were demonstrated in both patients, so other reasons for the phenotypic variability are considered.     

Two mtDNA mutations 14487T>C (M63V,ND6) and 12297T>C (tRNA Leu) in a Leigh syndrome family

Mitochondrial cytopathies are characterized by a large variability of clinical phenotypes and severity. The 14487T>C mutation in mtDNA has been recently described to be associated with Leigh syndrome. The 12297T>C mutation has been described in isolated dilated cardiomyopathy patients. Here, we report a family with multiple members who harbor both mutations, with only a few individuals who are affected with Leigh syndrome. Mitochondrial whole genome sequencing analysis in the proband’s muscle specimen detected two nearly homoplasmic mutations: 14487T>C (M63V in ND6) and 12297T>C in the tRNA ^Leu (CUN) gene. These two mutations were also detected in the blood, urine sediments, hair follicles, and buccal swab samples of all matrilineal relatives tested. All individuals tested were nearly homoplasmic for the 12297T>C mutation, but had variable degrees of heteroplasmy for 14487T>C. We also screened for the frequency of these two mutations. Of 268 patients with Leigh or Leigh-like disease, one case was found to harbor the 14487T>C mutation (0.3%), and one had the 12297T>C mutation (0.3%). Neither mutation was detected in the 88 patients meeting MELAS syndrome criteria nor in the 56 patients with respiratory chain complex I or I + III deficiency. In conclusion, the 14487T>C mutation appears as the primary etiology of Leigh syndrome in this family, demonstrating the high level of heteroplasmy needed for a clinically significant phenotype with this mutation. The 12297T>C mutation was not associated with dilated cardiomyopathy for the family members who were clinically evaluated and who were shown by testing to be nearly homoplasmic for that mutation.     

Mutant NDUFS3 subunit of mitochondrial complex I causes Leigh syndrome

Respiratory chain complex I deficiency represents a genetically heterogeneous group of diseases resulting from mutations in mitochondrial or nuclear genes. Mutations have been reported in 13 of the 14 subunits encoding the core of complex I (seven mitochondrial and six nuclear genes) and these result in Leigh or Leigh-like syndromes or cardiomyopathy. In this study, a combination of denaturing high performance liquid chromatography and sequence analysis was used to study the NDUFS3 gene in a series of complex I deficient patients. Mutations found in this gene (NADH dehydrogenase iron-sulphur protein 3), coding for the seventh and last subunit of complex I core, were shown to cause late onset Leigh syndrome, optic atrophy, and complex I deficiency. A biochemical diagnosis of complex I deficiency on cultured amniocytes from a later pregnancy was confirmed through the identification of disease causing NDUFS3 mutations in these cells. While mutations in the NDUFS3 gene thus result in Leigh syndrome, a dissimilar clinical phenotype is observed in mutations in the NDUFV2 and NDUFS2 genes, resulting in encephalomyopathy and cardiomyopathy. The reasons for these differences are uncertain.     

Segregation of mtDNA throughout human embryofetal development: m.3243A>G as a model system

Mitochondrial DNA (mtDNA) mutations cause a wide range of serious diseases with high transmission risk and maternal inheritance. Tissue heterogeneity of the heteroplasmy rate ("mutant load") accounts for the wide phenotypic spectrum observed in carriers. Owing to the absence of therapy, couples at risk to transmit such disorders commonly ask for prenatal (PND) or preimplantation diagnosis (PGD). The lack of data regarding heteroplasmy distribution throughout intrauterine development, however, hampers the implementation of such procedures. We tracked the segregation of the m.3243A>G mutation (MT-TL1 gene) responsible for the MELAS syndrome in the developing embryo/fetus, using tissues and cells from eight carrier females, their 38 embryos and 12 fetuses. Mutant mtDNA segregation was found to be governed by random genetic drift, during oogenesis and somatic tissue development. The size of the bottleneck operating for m.3243A>G during oogenesis was shown to be individual-dependent. Comparison with data we achieved for the m.8993T>G mutation (MT-ATP6 gene), responsible for the NARP/Leigh syndrome, indicates that these mutations differentially influence mtDNA segregation during oogenesis, while their impact is similar in developing somatic tissues. These data have major consequences for PND and PGD procedures in mtDNA inherited disorders.     

Acquisition of the wobble modification in mitochondrial tRNALeu(CUN) bearing the G12300A mutation suppresses the MELAS molecular defect

The A3243G mutation in the mitochondrial gene for human mitochondrial (mt) tRNA(Leu(UUR)), responsible for decoding of UUR codons, is associated with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS). We previously demonstrated that this mutation causes defects in 5-taurinomethyluridine (taum(5)U) modification at the anticodon first (wobble) position of the mutant mt tRNA(Leu(UUR)), leading to a UUG decoding deficiency and entraining severe respiratory defects. In addition, we previously identified a heteroplasmic mutation, G12300A, in the other mt leucine tRNA gene, mt tRNA(Leu(CUN)), which functions as a suppressor of the A3243G respiratory defect in cybrid cells containing A3243G mutant mtDNA. Although the G12300A mutation converts the anticodon sequence of mt tRNA(Leu(CUN)) from UAG to UAA, this tRNA carrying an unmodified wobble uridine still cannot decode the UUG codon. Mass spectrometric analysis of the suppressor mt tRNA(Leu(CUN)) carrying the G12300A mutation from the phenotypically revertant cells revealed that the wobble uridine acquires de novo taum(5)U modification. In vitro translation confirmed the functionality of the suppressor tRNA for decoding UUG codons. These results demonstrate that the acquisition of the wobble modification in another isoacceptor tRNA is critical for suppressing the MELAS mutation, and they highlight the primary role of the UUG decoding deficiency in the molecular pathogenesis of MELAS syndrome.     

Molecular-clinical correlations in a family with variable tissue mitochondrial DNA T8993G mutant load

Unlike many pathogenic mitochondrial DNA mutations, the T8993G mutation associated with Leigh syndrome (LS) and neurogenic muscle weakness, ataxia, retinitis pigmentosa (NARP) typically shows little variation in mutant load between different tissue types. We describe the molecular and clinical findings in a family with variable disease severity and tissue T8993G mutant loads. Real-time ARMS qPCR testing showed that two brothers with features of NARP and LS had high mutant loads (>90%) in all tissues tested, similar to previously reported cases. Their sister, who has mild speech delay but attends normal school, was found to have a relatively high mutant load (mean 93%) in tissues derived from endoderm (buccal mucosa) and mesoderm (blood and skin fibroblasts). However, in tissue derived from ectoderm (hair bulbs), she carried a considerably lower proportion of mutant mtDNA. Because both surface ectoderm, which gives rise to outer epithelia and hair, and neuroectoderm, which gives rise to the central nervous system, are derived from ectoderm, it is tempting to speculate that the mutant load detected in the oligosymptomatic sister's hair bulbs is a reflection of the brain mutant load. We conclude that significant variation in tissue mutant load may occur in at least some individuals that harbor the T8993G mutation. This adds additional complexity to genetic counseling and prenatal diagnosis in such instances. Given the shared embryonic origin of hair bulbs and brain, we recommend performing hair bulb mtDNA analysis in asymptomatic or oligosymptomatic individuals that have high blood mutant loads in order to understand better the genotype-phenotype correlations related to the T8993G mutation.     

Mitochondrial disorders: genetics, counseling, prenatal diagnosis and reproductive options

Most patients with mitochondrial disorders are diagnosed by finding a respiratory chain enzyme defect or a mutation in the mitochondrial DNA (mtDNA). The provision of accurate genetic counseling and reproductive options to these families is complicated by the unique genetic features of mtDNA that distinguish it from Mendelian genetics. These include maternal inheritance, heteroplasmy, the threshold effect, the mitochondrial bottleneck, tissue variation, and selection. Although we still have much to learn about mtDNA genetics, it is now possible to provide useful guidance to families with an mtDNA mutation or a respiratory chain enzyme defect. We describe a range of current reproductive options that may be considered for prevention of transmission of mtDNA mutations, including the use of donor oocytes, prenatal diagnosis (by chorionic villus sampling or amniocentesis), and preimplantation genetic diagnosis, plus possible future options such as nuclear transfer and cytoplasmic transfer. For common mtDNA mutations associated with mitochondrial cytopathies (such as NARP, Leigh Disease, MELAS, MERRF, Leber's Hereditary Optic Neuropathy, CPEO, Kearns-Sayre syndrome, and Pearson syndrome), we summarize the available data on recurrence risk and discuss the relative advantages and disadvantages of reproductive options.     

Point mutation of the mitochondrial tRNA(Leu) gene (A 3243 G) in maternally inherited hypertrophic cardiomyopathy, diabetes mellitus, renal failure, and sensorineural deafness.

The A 3243 G mutation of the mitochondrial tRNA(Leu) gene was found to segregate with maternally inherited diabetes mellitus, sensorineural deafness, hypertrophic cardiomyopathy, or renal failure in a large pedigree of 35 affected members in four generations. Presenting symptoms almost consistently involved deafness and recurrent attacks of migraine-like headaches, but the clinical course of the disease varied within and across generations. The A 3243 G mutation has been previously reported in association with the mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episode syndrome (MELAS) and with diabetes mellitus and deafness. To our knowledge, however, hypertrophic cardiomyopathy is not a common feature in people with the A 3243 G mutation and renal failure has not been hitherto reported in association with this mutation. The present observation gives additional support to the variable clinical expression of mtDNA mutations in humans.     

Rapid and noninvasive screening of patients with mitochondrial myopathy

In recent years, several point mutations in the mitochondria! genome have been associated with human disease. PCR Polymerase Chain Reaction/restriction endonuclease based techniques provide a reliable method for screening large numbers of specimens for many of the reported mutations. Muscle tissue usually carries the mutations and has been used in earlier studies. We describe a technique for analysis of mtDNA derived from hair follicles for a range of mutations. Both the 3243 A→G MELAS and 8344 A→G MERRF mutations were detected in mtDNA from hair follicles. In patients where both muscle and hair were screened, the mutation load was apparently higher in muscle. Furthermore, in patients positive for a given mutation, all the hair follicles analysed were shown to harbour the mutation, although the proportion of wild type to mutant mtDNA was found to somewhat vary. The advantages of this method are (1: six hair follicles provide sufficient mtDNA for analysis of at least 20 different mutations, and (2: specimen collection and transport to a central laboratory are easier than for other tissues. Our studies show that hair follicles constitute a reliable specimen for mitochondrial mutation screening at a diagnostic level. © 1994 Wiley-Liss, Inc.