Canadian Guidelines for Hereditary Transthyretin Amyloidosis Polyneuropathy Management.

Hereditary transthyretin-mediated (hATTR) amyloidosis is a progressive disease caused by mutations in the TTR gene leading to multisystem organ dysfunction. Pathogenic TTR aggregation, misfolding, and fibrillization lead to deposition of amyloid in multiple body organs and frequently involve the peripheral nerve system and the heart. Common neurologic manifestations include: sensorimotor polyneuropathy (PN), autonomic neuropathy, small-fiber PN, and carpal tunnel syndrome. Many patients have significant progression due to diagnostic delays as hATTR PN is not considered within the differential diagnosis. Recently, two effective novel disease-modifying therapies, inotersen and patisiran, were approved by Health Canada for the treatment of hATTR PN. Early diagnosis is crucial for the timely introduction of these disease-modifying treatments that reduce impairments, improve quality of life, and extend survival. In this guideline, we aim to improve awareness and outcomes of hATTR PN by making recommendations directed to the diagnosis, monitoring, and treatment in Canada.

Lignes directrices sur la prise en charge de l'amylose héréditaire à transthyrétine, accompagnée de polyneuropathie, au Canada.L'amylose héréditaire à transthyrétine (ATTRh) est une maladie évolutive, causée par des mutations du gène de la transthyrétine (TTR), qui entraînent un dysfonctionnement plurisystémique. L'agrégation, le mauvais repliement et la fibrillisation pathogènes de la TTR aboutissent au dépôt de protéines amyloïdes dans plusieurs organes, et affectent souvent le système nerveux périphérique et le cœur. Les troubles neurologiques fréquents comprennent une polyneuropathie sensorimotrice (PN), une neuropathie autonome, une polyneuropathie des petites fibres et le syndrome du canal carpien. Chez bon nombre de patients, la maladie a connu une évolution importante en raison de la pose tardive du diagnostic, la PN-ATTRh ne faisant pas l'objet d'un diagnostic différentiel. Santé Canada a approuvé, depuis peu, deux nouveaux médicaments modificateurs de la PN-ATTRh et efficaces contre l'affection, soit l'inotersen et le patisiran. La pose précoce du diagnostic revêt une importance cruciale dans l'instauration, en temps opportun, de ces tout nouveaux traitements qui atténuent les troubles, améliorent la qualité de vie et prolongent la survie. Les auteurs, par l'élaboration de la nouvelle ligne directrice, espèrent sensibiliser la communauté médicale à la PN-ATTRh, et améliorer les résultats cliniques qui y sont associés, en formulant des recommandations sur le diagnostic et le traitement de la maladie au Canada ainsi que sur la surveillance de son évolution.

Long-term safety and efficacy of patisiran for hereditary transthyretin-mediated amyloidosis with polyneuropathy: 12-month results of an open-label extension study.

BACKGROUND: Hereditary transthyretin-mediated amyloidosis is a rare, inherited, progressive disease caused by mutations in the transthyretin (TTR) gene. We assessed the safety and efficacy of long-term treatment with patisiran, an RNA interference therapeutic that inhibits TTR production, in patients with hereditary transthyretin-mediated amyloidosis with polyneuropathy. METHODS: This multicentre, open-label extension (OLE) trial enrolled patients at 43 hospitals or clinical centres in 19 countries as of Sept 24, 2018. Patients were eligible if they had completed the phase 3 APOLLO or phase 2 OLE parent studies and tolerated the study drug. Eligible patients from APOLLO (patisiran and placebo groups) and the phase 2 OLE (patisiran group) studies enrolled in this global OLE trial and received patisiran 0·3 mg/kg by intravenous infusion every 3 weeks with plans to continue to do so for up to 5 years. Efficacy assessments included measures of polyneuropathy (modified Neuropathy Impairment Score +7 [mNIS+7]), quality of life, autonomic symptoms, nutritional status, disability, ambulation status, motor function, and cardiac stress, with analysis by study groups (APOLLO-placebo, APOLLO-patisiran, phase 2 OLE patisiran) based on allocation in the parent trial. The global OLE is ongoing with no new enrolment, and current findings are based on the interim analysis of the patients who had completed 12-month efficacy assessments as of the data cutoff. Safety analyses included all patients who received one or more dose of patisiran up to the data cutoff. This study is registered with ClinicalTrials.gov, NCT02510261. FINDINGS: Between July 13, 2015, and Aug 21, 2017, of 212 eligible patients, 211 were enrolled: 137 patients from the APOLLO-patisiran group, 49 from the APOLLO-placebo group, and 25 from the phase 2 OLE patisiran group. At the data cutoff on Sept 24, 2018, 126 (92%) of 137 patients from the APOLLO-patisiran group, 38 (78%) of 49 from the APOLLO-placebo group, and 25 (100%) of 25 from the phase 2 OLE patisiran group had completed 12-month assessments. At 12 months, improvements in mNIS+7 with patisiran were sustained from parent study baseline with treatment in the global OLE (APOLLO-patisiran mean change -4·0, 95 % CI -7·7 to -0·3; phase 2 OLE patisiran -4·7, -11·9 to 2·4). Mean mNIS+7 score improved from global OLE enrolment in the APOLLO-placebo group (mean change from global OLE enrolment -1·4, 95% CI -6·2 to 3·5). Overall, 204 (97%) of 211 patients reported adverse events, 82 (39%) reported serious adverse events, and there were 23 (11%) deaths. Serious adverse events were more frequent in the APOLLO-placebo group (28 [57%] of 49) than in the APOLLO-patisiran (48 [35%] of 137) or phase 2 OLE patisiran (six [24%] of 25) groups. The most common treatment-related adverse event was mild or moderate infusion-related reactions. The frequency of deaths in the global OLE was higher in the APOLLO-placebo group (13 [27%] of 49), who had a higher disease burden than the APOLLO-patisiran (ten [7%] of 137) and phase 2 OLE patisiran (0 of 25) groups. INTERPRETATION: In this interim 12-month analysis of the ongoing global OLE study, patisiran appeared to maintain efficacy with an acceptable safety profile in patients with hereditary transthyretin-mediated amyloidosis with polyneuropathy. Continued long-term follow-up will be important for the overall assessment of safety and efficacy with patisiran. FUNDING: Alnylam Pharmaceuticals.

Practice Guidelines for Canadian Neurophysiology Laboratories During the COVID-19 Pandemic.

The COVID-19 pandemic has had a major impact on clinical practice. Safe standards of practice are essential to protect health care workers while still allowing them to provide good care. The Canadian Society of Clinical Neurophysiologists, the Canadian Association of Electroneurophysiology Technologists, the Association of Electromyography Technologists of Canada, the Board of Registration of Electromyography Technologists of Canada, and the Canadian Board of Registration of Electroencephalograph Technologists have combined to review current published literature about safe practices for neurophysiology laboratories. Herein, we present the results of our review and provide our expert opinion regarding the safe practice of neurophysiology during the COVID-19 pandemic in Canada.

Epidemiological evidence for a hereditary contribution to myasthenia gravis: a retrospective cohort study of patients from North America.

OBJECTIVES: To approximate the rate of familial myasthenia gravis and the coexistence of other autoimmune disorders in the patients and their families. DESIGN: Retrospective cohort study. SETTING: Clinics across North America. PARTICIPANTS: The study included 1032 patients diagnosed with acetylcholine receptor antibody (AChR)-positive myasthenia gravis. METHODS: Phenotype information of 1032 patients diagnosed with AChR-positive myasthenia gravis was obtained from clinics at 14 centres across North America between January 2010 and January 2011. A critical review of the epidemiological literature on the familial rate of myasthenia gravis was also performed. RESULTS: Among 1032 patients, 58 (5.6%) reported a family history of myasthenia gravis. A history of autoimmune diseases was present in 26.6% of patients and in 28.4% of their family members. DISCUSSION: The familial rate of myasthenia gravis was higher than would be expected for a sporadic disease. Furthermore, a high proportion of patients had a personal or family history of autoimmune disease. Taken together, these findings suggest a genetic contribution to the pathogenesis of myasthenia gravis.

High rate of hypertension in patients with m.3243A>G MELAS mutations and POLG variants.

Animal studies suggest that decreased vascular mitochondrial DNA copy number can promote hypertension. We conducted a chart review of blood pressure and hemodynamics in patients with either mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS, n = 36) or individuals with variants in the mitochondrial DNA polymerase gamma (POLG, n = 26). The latter included both pathogenic variants and variants of unknown significance (VUS). Hypertension rates (MELAS 50%, POLG 50%) were elevated relative to Canadian norms in 20-39 (MELAS) and 40-59 (MELAS and POLG) years of age groups. Peripheral resistance was high in the hypertensive versus normotensive patients, potentially indicative of microvascular disease. Despite antihypertensive treatment, systolic blood pressure remained elevated in the POLG versus MELAS group. The risk of hypertension was not associated with MELAS heteroplasmy. Hypertension rates were not different between individuals with known pathogenic POLG variants and those with VUS, including common variants. Hypertension (HT) also did not differ between patients with POLG variants with (n = 17) and without chronic progressive external opthalmoplegia (n = 9) (CPEO). HT was associated with variants in all three functional domains of POLG. These findings suggest that both pathogenic variants and several VUS in the POLG gene may promote human hypertension and extend our past reports that increased risk of HT is associated with MELAS.

A National Spinal Muscular Atrophy Registry for Real-World Evidence.

BACKGROUND: Spinal muscular atrophy (SMA) is a devastating rare disease that affects individuals regardless of ethnicity, gender, and age. The first-approved disease-modifying therapy for SMA, nusinursen, was approved by Health Canada, as well as by American and European regulatory agencies following positive clinical trial outcomes. The trials were conducted in a narrow pediatric population defined by age, severity, and genotype. Broad approval of therapy necessitates close follow-up of potential rare adverse events and effectiveness in the larger real-world population. METHODS: The Canadian Neuromuscular Disease Registry (CNDR) undertook an iterative multi-stakeholder process to expand the existing SMA dataset to capture items relevant to patient outcomes in a post-marketing environment. The CNDR SMA expanded registry is a longitudinal, prospective, observational study of patients with SMA in Canada designed to evaluate the safety and effectiveness of novel therapies and provide practical information unattainable in trials. RESULTS: The consensus expanded dataset includes items that address therapy effectiveness and safety and is collected in a multicenter, prospective, observational study, including SMA patients regardless of therapeutic status. The expanded dataset is aligned with global datasets to facilitate collaboration. Additionally, consensus dataset development aimed to standardize appropriate outcome measures across the network and broader Canadian community. Prospective outcome studies, data use, and analyses are independent of the funding partner. CONCLUSION: Prospective outcome data collected will provide results on safety and effectiveness in a post-therapy approval era. These data are essential to inform improvements in care and access to therapy for all SMA patients.

Self-initiated lifestyle interventions lead to potential insight into an effective, alternative, non-surgical therapy for mitochondrial disease associated multiple symmetric lipomatosis.

BACKGROUND: A 56-year-old female, diagnosed as a carrier of the mitochondrial DNA mutation (MTTK c.8344A > G) associated with the MERRF (myoclonic epilepsy with ragged red fibers) syndrome, presented with a relatively uncommon but well-known phenotypic manifestation: severe multiple symmetric lipomatosis (MSL). After surgical resection of three kilograms of upper mid-back lipomatous tissue, the patient experienced a significant decline in her functional capacity and quality of life, which ultimately resulted in her placement on long-term disability. METHODS: Dissatisfied with the available treatment options centered on additional resection surgeries, given the high probability of lipoma regrowth, the patient independently researched and applied alternative therapies that centred on a carbohydrate-restricted diet and a supervised exercise program. RESULTS: The cumulative effect of her lifestyle interventions resulted in the reversal of her MSL and her previously low quality of life. She met all her personal goals by the one-year mark, including reduced size of the residual post-surgical lipomas, markedly enhanced exercise tolerance, and return to work. She continues to maintain her interventions and to experience positive outcomes at the two-year mark. INTERPRETATION: This case report documents the timing and nature of lifestyle interventions in relation to the reversal in growth pattern of her previously expanding and debilitating lipomas. The profound nature of the apparent benefit on lipoma growth demonstrates the intervention's potential as a new feasible non-surgical therapy for mitochondrial-disease-associated MSL, and justifies its systematic study. We also describe how this case has inspired the care team to re-examine its approach to involved patients.

Patisiran, an RNAi Therapeutic, for Hereditary Transthyretin Amyloidosis.

BACKGROUND: Patisiran, an investigational RNA interference therapeutic agent, specifically inhibits hepatic synthesis of transthyretin. METHODS: In this phase 3 trial, we randomly assigned patients with hereditary transthyretin amyloidosis with polyneuropathy, in a 2:1 ratio, to receive intravenous patisiran (0.3 mg per kilogram of body weight) or placebo once every 3 weeks. The primary end point was the change from baseline in the modified Neuropathy Impairment Score+7 (mNIS+7; range, 0 to 304, with higher scores indicating more impairment) at 18 months. Other assessments included the Norfolk Quality of Life-Diabetic Neuropathy (Norfolk QOL-DN) questionnaire (range, -4 to 136, with higher scores indicating worse quality of life), 10-m walk test (with gait speed measured in meters per second), and modified body-mass index (modified BMI, defined as [weight in kilograms divided by square of height in meters]×albumin level in grams per liter; lower values indicated worse nutritional status). RESULTS: A total of 225 patients underwent randomization (148 to the patisiran group and 77 to the placebo group). The mean (±SD) mNIS+7 at baseline was 80.9±41.5 in the patisiran group and 74.6±37.0 in the placebo group; the least-squares mean (±SE) change from baseline was -6.0±1.7 versus 28.0±2.6 (difference, -34.0 points; P<0.001) at 18 months. The mean (±SD) baseline Norfolk QOL-DN score was 59.6±28.2 in the patisiran group and 55.5±24.3 in the placebo group; the least-squares mean (±SE) change from baseline was -6.7±1.8 versus 14.4±2.7 (difference, -21.1 points; P<0.001) at 18 months. Patisiran also showed an effect on gait speed and modified BMI. At 18 months, the least-squares mean change from baseline in gait speed was 0.08±0.02 m per second with patisiran versus -0.24±0.04 m per second with placebo (difference, 0.31 m per second; P<0.001), and the least-squares mean change from baseline in the modified BMI was -3.7±9.6 versus -119.4±14.5 (difference, 115.7; P<0.001). Approximately 20% of the patients who received patisiran and 10% of those who received placebo had mild or moderate infusion-related reactions; the overall incidence and types of adverse events were similar in the two groups. CONCLUSIONS: In this trial, patisiran improved multiple clinical manifestations of hereditary transthyretin amyloidosis. (Funded by Alnylam Pharmaceuticals; APOLLO ClinicalTrials.gov number, NCT01960348 .).

Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations.

Choline acetyltransferase catalyzes the synthesis of acetylcholine at cholinergic nerves. Mutations in human CHAT cause a congenital myasthenic syndrome due to impaired synthesis of ACh; this severe variant of the disease is frequently associated with unexpected episodes of potentially fatal apnea. The severity of this condition varies remarkably, and the molecular factors determining this variability are poorly understood. Furthermore, genotype-phenotype correlations have been difficult to establish in patients with biallelic mutations. We analyzed the protein expression of phosphorylated ChAT of seven CHAT mutations, p.Val136Met, p.Arg207His, p.Arg186Trp, p.Val194Leu, p.Pro211Ala, p.Arg566Cys, and p.Ser694Cys, in HEK-293 cells to phosphorylated ChAT, determined their enzyme kinetics and thermal stability, and examined their structural changes. Three mutations, p.Arg207His, p.Arg186Trp, and p.Arg566Cys, are novel, and p.Val136Met and p.Arg207His are homozygous in three families and associated with severe disease. The characterization of mutants showed a decrease in the overall catalytic efficiency of ChAT; in particular, those located near the active-site tunnel produced the most seriously disruptive phenotypic effects. On the other hand, p.Val136Met, which is located far from both active and substrate-binding sites, produced the most drastic reduction of ChAT expression. Overall, CHAT mutations producing low enzyme expression and severe kinetic effects are associated with the most severe phenotypes.

A case of good syndrome presumed secondary to metastatic pancreatic thymoma in a patient presenting with a myasthenic crisis postthymectomy.

INTRODUCTION: Myasthenia gravis (MG) is an autoimmune disorder characterized by autoantibodies against the postsynaptic nicotinic acetylcholine receptors, muscle-specific tyrosine kinase, low-density lipoprotein receptor-related protein 4, and agrin. The incidence of thymoma in MG is reported as ∼10%-15%. The incidence of extrathoracic metastatic thymoma is exceedingly rare and may present years after resection. Associations between thymoma and immunodeficiency have also been described, including Good syndrome. METHODS AND RESULTS: We describe the clinical course, investigations, and treatments performed in a patient presenting with a myasthenic crisis in the setting of acetylcholine receptor antibody-positive generalized MG 10 years postthymectomy. Computed tomography imaging revealed 2 pancreatic lesions, but no residual thoracic thymoma. Biopsy confirmed metastatic pancreatic thymoma, which was successfully resected. His course was further complicated by cytomegalovirus retinitis with a depressed CD4 count and perniosis. DISCUSSION: This presentation was felt to be consistent with Good immunodeficiency syndrome.

Increased Prevalence of Hypertension in Young Adults with High Heteroplasmy Levels of the MELAS m.3243A>G Mutation.

BACKGROUND: The pathophysiology of hypertension in patients with mitochondrial diseases is different from that of the general population. Growing evidence exists linking mtDNA, its mutations, and mitochondrial dysfunction to the pathogenesis of hypertension. No reports on the prevalence of hypertension in late-onset mtDNA diseases have been described. METHODS: We performed a retrospective chart review of adult patients with late-onset mtDNA diseases between January 1999 and January 2012 at our center. We grouped them into age categories to allow comparison with previously reported Canadian Health Measures Survey (CHMS) prevalence data. RESULTS: Twenty-three subjects with hypertension were identified for a crude prevalence of 39.7 % (95 % CI 27-53 %) as compared to the CHMS age-predicted prevalence of 30.5 %. When analyzed by individual age group, there were no significant differences between the observed and the CHMS predicted prevalence rates in the 40 years and older cohorts (age category 40-59, p = 0.63; age category 60-79, p = 0.85). However, hypertension rates were significantly higher than predicted in the under 40 years cohort (55.6 vs. 2.8 %, p < 0.001, CI 21-86 %), in which hypertensive patients with the MELAS m.3243A>G mutation were significantly clustered (p < 0.01). This younger MELAS cohort (n = 4, mean age = 24 years) with hypertension had heteroplasmy levels (mean = 68 %) that were significantly higher than the levels found in the older non-hypertensive MELAS cohort (n = 8, mean age = 52 years, mean = 33 %) (p = 0.04). CONCLUSION: Relative to age, gender, and mtDNA disease subtype, young adults with high heteroplasmy levels of the MELAS m.3243A>G mutation demonstrate an increased prevalence of hypertension. Further prospective data are needed to confirm this initial finding, which has potentially important treatment implications.

Cardiac screening investigations in adult-onset progressive external ophthalmoplegia patients.

INTRODUCTION: Patients with mitochondrial myopathies may develop cardiac complications such as cardiomyopathy and/or cardiac conduction defects. To identify these potentially life-threatening and treatable conditions, it is common practice to screen patients intermittently with electrocardiography and echocardiography. The optimal time interval for such screening investigations is unknown. We developed this study to review our screening results in adult-onset patients with progressive external ophthalmoplegia (PEO). METHODS: This study was a retrospective review of PEO patients with 5 years or more of cardiac screening investigations who did not have any cardiac symptoms. RESULTS: Fifteen patients were included, and cardiomyopathy was identified on screening echocardiogram in 1 patient. Four patients had other abnormalities identified, which were unrelated to their mitochondrial myopathy. CONCLUSIONS: Only 1 patient in 15 developed cardiac complications related to mitochondrial disease during 5 years of follow-up. We suggest that a screening interval of 3-5 years is probably appropriate for adult-onset PEO patients who do not have cardiac symptoms.

Levator palpebrae biopsy and diagnosis of progressive external ophthalmoplegia.

BACKGROUND: Progressive external ophthalmoplegia (PEO) is a mitochondrial myopathy of ocular muscles. Diagnostic investigation usually involves limb skeletal muscle biopsy and molecular genetic studies, although diagnostic yield tends to be low. The purpose of this study was to evaluate the diagnostic yield obtained by analysis of levator palpebrae (LP) muscle tissue. METHODS: This is a clinicopathologic study of 8 patients with a diagnosis of PEO, who had LP muscle biopsies as part of oculoplastic procedures. Six of these patients also had limb muscle biopsies. Histopathology, electron microscopy and genetic studies were performed. RESULTS: Diagnostic histopathologic findings were present in 4/6 quadriceps biopsies, and 7/8 LP biopsies. Genetic testing on DNA extracted from LP muscle revealed abnormalities in 4 patients. CONCLUSION: In patients whose LP. muscle demonstrate both genetic defects and histopathological abnormalities, the diagnosis of PEO can be confirmed without limb muscle biopsy. Patients having LP resection during oculoplastics procedures for treatment of ptosis may therefore be able to avoid a separate procedure for limb muscle biopsy. Further study is required to determine the specificity of these findings.

Expanding the clinical phenotype of the mitochondrial m.13513G>A mutation with the first report of a fatal neonatal presentation.

Diagnosis of mitochondrial disease is often a challenge because of the extreme heterogeneity of the clinical phenotype and the variety of underlying gene defects. Insight into the range of clinical phenotypes associated with a particular mitochondrial DNA mutation will facilitate better recognition of patients at risk by focused gene testing. We present a family affected by the mitochondrial m.13513G>A (p.D393N, ND5) mutation, illustrating a previously unreported degree of clinical heterogeneity, varying from mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS) in a 10-year-old female, to a fatal neonatal course with metabolic acidosis and hypotonia in a younger sister, to absence of medical problems in the mother. The mutation loads ranging from 66% in the deceased neonate to 30% in the female with MELAS and 7% in the asymptomatic mother, correlated with severity of the clinical phenotype. The importance of proactive collection and storage of appropriate samples during the diagnostic work-up of an acutely ill or deceased neonate, allowing subsequent mitochondrial investigations, is hereby illustrated.

Multisystem disorder in late-onset chronic progressive external ophthalmoplegia.

INTRODUCTION: Chronic progressive external ophthalmoplegia (CPEO) is a mitochondrial syndrome on a disease spectrum with Kearns-Sayre syndrome (KSS). Clinical presentation is variable and our experience suggested that phenotypic differences exist in CPEO with onset after age 20. METHODS: This descriptive study is a retrospective chart review of 40 patients with late-onset CPEO. Clinical features, laboratory and neurophysiology results were reviewed. RESULTS: Multisystem dysfunction was very common in this series. Gastrointestinal dysfunction was more common than expected (60%) as was migraine headache (40%). Clinical characteristics on the KSS disease spectrum were uncommon in this series with only 2.5% having pigmentary retinopathy, 5% with cardiac conduction abnormality, and 22.5% having endocrinopathy (most often thyroid dysfunction rather than diabetes). Neurophysiology abnormalities included length-dependent axonal polyneuropathy in 44% (sometimes subclinical) and myopathic EMG changes in 26%. Exposure to sources of acquired mitochondrial toxicity including cigarette use and hepatitis C infection were more common than expected in this series. DISCUSSION: Phenotype was different in this late-onset series compared with previous reports in CPEO patients. In this series of late-onset patients, multi-organ dysfunction was more common than previously reported in CPEO, and some classical mitochondrial manifestations, such as pigmentary retinopathy were rare. We suggest that acquired mitochondrial toxicity may have a role in the pathogenesis of adult-onset CPEO.

Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations.

Mutations in POLG gene are responsible for a wide spectrum of clinical disorders with altered mitochondrial DNA (mtDNA) integrity, including mtDNA multiple deletions and depletion. Sensory ataxic neuropathy with ophthalmoparesis (SANDO) caused by mutations in POLG gene, fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis, has described in a few reports. Here we described five cases of adult onset autosomal recessive sensory ataxic neuropathy with ophthalmoplegia. All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO). The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients. However, deficiencies in the activities of mitochondrial respiratory chain enzyme complexes were not detected in any of the patients' muscle samples. Multiple deletions of mtDNA were detected in blood and muscle specimens but mtDNA depletion was not found. Due to these diagnostic difficulties, POLG-related syndromes are definitively diagnosed based on the presence of deleterious mutations in the POLG gene.