Biallelic PTPMT1 variants disrupt cardiolipin metabolism and lead to a neurodevelopmental syndrome.

Primary mitochondrial diseases (PMDs) are among the most common inherited neurological disorders. They are caused by pathogenic variants in mitochondrial or nuclear DNA that disrupt mitochondrial structure and/or function, leading to impaired oxidative phosphorylation (OXPHOS). One emerging subcategory of PMDs involves defective phospholipid (PL) metabolism. Cardiolipin (CL), the signature PL of mitochondria, resides primarily in the inner mitochondrial membrane, where it is biosynthesised and remodelled via multiple enzymes and is fundamental to several aspects of mitochondrial biology. Genes that contribute to CL biosynthesis have recently been linked with PMD. However, the pathophysiological mechanisms that underpin human CL-related PMDs are not fully characterised. Here, we report six individuals, from three independent families, harbouring biallelic variants in PTPMT1, a mitochondrial tyrosine phosphatase required for de novo CL biosynthesis. All patients presented with a complex, neonatal/infantile onset neurological and neurodevelopmental syndrome comprising developmental delay, microcephaly, facial dysmorphism, epilepsy, spasticity, cerebellar ataxia and nystagmus, sensorineural hearing loss, optic atrophy, and bulbar dysfunction. Brain MRI revealed a variable combination of corpus callosum thinning, cerebellar atrophy, and white matter changes. Using patient-derived fibroblasts and skeletal muscle tissue, combined with cellular rescue experiments, we characterise the molecular defects associated with mutant PTPMT1 and confirm the downstream pathogenic effects that loss of PTPMT1 has on mitochondrial structure and function. To further characterise the functional role of PTPMT1 in CL homeostasis, we established a zebrafish ptpmt1 knockout model associated with abnormalities in body size, developmental alterations, decreased total CL levels, and OXPHOS deficiency. Together, these data indicate that loss of PTPMT1 function is associated with a new autosomal recessive PMD caused by impaired CL metabolism, highlight the contribution of aberrant CL metabolism towards human disease, and emphasise the importance of normal CL homeostasis during neurodevelopment.

Primary mitochondrial diseases.

Primary mitochondrial diseases (PMDs) are a heterogeneous group of hereditary disorders characterized by an impairment of the mitochondrial respiratory chain. They are the most common group of genetic metabolic disorders, with a prevalence of 1 in 4,300 people. The presence of leukoencephalopathy is recognized as an important feature in many PMDs and can be a manifestation of mutations in both mitochondrial DNA (classic syndromes such as mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; myoclonic epilepsy with ragged-red fibers [RRFs]; Leigh syndrome; and Kearns-Sayre syndrome) and nuclear DNA (mutations in maintenance genes such as POLG, MPV17, and TYMP; Leigh syndrome; and mitochondrial aminoacyl-tRNA synthetase disorders). In this chapter, PMDs associated with white matter involvement are outlined, including details of clinical presentations, brain MRI features, and elements of differential diagnoses. The current approach to the diagnosis of PMDs and management strategies are also discussed. A PMD diagnosis in a subject with leukoencephalopathy should be considered in the presence of specific brain MRI features (for example, cyst-like lesions, bilateral basal ganglia lesions, and involvement of both cerebral hemispheres and cerebellum), in addition to a complex neurologic or multisystem disorder. Establishing a genetic diagnosis is crucial to ensure appropriate genetic counseling, multidisciplinary team input, and eligibility for clinical trials.

A diagnostic framework to identify vestibular involvement in multi-sensory neurological disease.

BACKGROUND AND PURPOSE: Identifying vestibular causes of dizziness and unsteadiness in multi-sensory neurological disease can be challenging, with problems typically attributed to central or peripheral nerve involvement. Acknowledging vestibular dysfunction as part of the presentation provides an opportunity to access targeted vestibular rehabilitation, for which extensive evidence exists. A diagnostic framework was developed and validated to detect vestibular dysfunction, benign paroxysmal positional vertigo or vestibular migraine. The specificity and sensitivity of the diagnostic framework was tested in patients with primary mitochondrial disease. METHODS: Adults with a confirmed diagnosis of primary mitochondrial disease were consented, between September 2020 and February 2022. Participants with and without dizziness or unsteadiness underwent remote physiotherapy assessment and had in-person detailed neuro-otological assessment. The six framework question responses were compared against objective neuro-otological assessment or medical notes. The output was binary, with sensitivity and specificity calculated. RESULTS: Seventy-four adults completed the study: age range 20-81 years (mean 48 years, ±SD 15.05 years); ratio 2:1 female to male. The framework identified a vestibular diagnosis in 35 participants, with seven having two diagnoses. The framework was able to identify vestibular diagnoses in adults with primary mitochondrial disease, with a moderate (40-59) to very high (90-100) sensitivity and positive predictive value, and moderate to high (60-74) to very high (90-100) specificity and negative predictive value. CONCLUSIONS: Overall, the clinical framework identified common vestibular diagnoses with a moderate to very high specificity and sensitivity. This presents an opportunity for patients to access effective treatment in a timely manner, to reduce falls and improve quality of life.

Studies to Assess the Utility of Serum Neurofilament Light Chain as a Biomarker in Chemotherapy-Induced Peripheral Neuropathy.

Chemotherapy-induced peripheral neuropathy (CIPN) is one of the most common and disabling dose-limiting toxicities of chemotherapy. We report here the results of two separate non-interventional studies (49 patients), which evaluated blood neurofilament light chain (NfL) as a biomarker of CIPN in breast cancer patients treated with paclitaxel. All patients underwent a standard treatment protocol that was established independently of the present studies. NfL was measured in serum using an ultrasensitive single-molecule array and compared with the self-administered European Organization for Research and Treatment of Cancer Quality of Life Questionnaire-CIPN twenty-item scale (CIPN20) and Total Neuropathy Score clinical version (TNSc), a clinician-reported measure of neuropathy progression. The TNSc increased with cumulative dose compared with baseline, and the NfL concentrations were also strongly associated with the cumulative dose of chemotherapy. The analysis showed a correlation between TNSc and NfL. Both TNSc and NfL showed weak to moderate associations with CIPN20 subscores, with a better association for the CIPN20 sensory compared with motor and autonomic subscores. Data from the two studies provide evidence that serum NfL has the potential to be used as a biomarker to monitor and mitigate CIPN. However, studies with additional patients planned in the ongoing clinical trial will determine the universal application of NfL as a biomarker in CIPN.

Enhanced mitochondrial genome analysis: bioinformatic and long-read sequencing advances and their diagnostic implications.

INTRODUCTION: Primary mitochondrial diseases (PMDs) comprise a large and heterogeneous group of genetic diseases that result from pathogenic variants in either nuclear DNA (nDNA) or mitochondrial DNA (mtDNA). Widespread adoption of next-generation sequencing (NGS) has improved the efficiency and accuracy of mtDNA diagnoses; however, several challenges remain. AREAS COVERED: In this review, we briefly summarize the current state of the art in molecular diagnostics for mtDNA and consider the implications of improved whole genome sequencing (WGS), bioinformatic techniques, and the adoption of long-read sequencing, for PMD diagnostics. EXPERT OPINION: We anticipate that the application of PCR-free WGS from blood DNA will increase in diagnostic laboratories, while for adults with myopathic presentations, WGS from muscle DNA may become more widespread. Improved bioinformatic strategies will enhance WGS data interrogation, with more accurate delineation of mtDNA and NUMTs (nuclear mitochondrial DNA segments) in WGS data, superior coverage uniformity, indirect measurement of mtDNA copy number, and more accurate interpretation of heteroplasmic large-scale rearrangements (LSRs). Separately, the adoption of diagnostic long-read sequencing could offer greater resolution of complex LSRs and the opportunity to phase heteroplasmic variants.

Mitochondria generate our bodies' energy, and they contain their own circular DNA molecules. Changes in this mitochondrial DNA can cause a wide range of genetic diseases. Improved computer processing of the sequence of this DNA and new techniques that can read the full DNA sequence in one experiment may enhance our ability to understand these genetic variants.

Clinical Benefits of Therapeutic Interventions Targeting Mitochondria in Parkinson's Disease Patients.

Parkinson's disease is the second most common neurodegenerative disease. Mitochondrial dysfunction has been associated with neurodegeneration in Parkinson's disease, and several treatments targeting mitochondria have been tested in these patients to delay disease progression and tackle disease symptoms. Herein, we review available data from randomised, double-blind clinical studies that have investigated the role of compounds targeting mitochondria in idiopathic Parkinson's disease patients, with a view of providing patients and clinicians with a comprehensive and practical paper that can inform therapeutic interventions in this group of people. A total of 9 compounds have been tested in randomized clinical trials, but only exenatide has shown some promising neuroprotective and symptomatic effects. However, whether this evidence can be translated into daily clinical practice still needs to be confirmed. In conclusion, targeting mitochondrial dysfunction in Parkinson's disease is a promising therapeutic approach, although only one compound has shown a positive effect on Parkinson's disease progression and symptoms. New compounds have been investigated in animal models, and their efficacy needs to be confirmed in humans through robust, randomised, double-blind clinical trials.

Factors associated with the severity of COVID-19 outcomes in people with neuromuscular diseases: Data from the International Neuromuscular COVID-19 Registry.

BACKGROUND AND PURPOSE: Clinical outcome information on patients with neuromuscular diseases (NMDs) who have been infected with SARS-CoV-2 is limited. The aim of this study was to determine factors associated with the severity of COVID-19 outcomes in people with NMDs. METHODS: Cases of NMD, of any age, and confirmed/presumptive COVID-19, submitted to the International Neuromuscular COVID-19 Registry up to 31 December 2021, were included. A mutually exclusive ordinal COVID-19 severity scale was defined as follows: (1) no hospitalization; (2) hospitalization without oxygenation; (3) hospitalization with ventilation/oxygenation; and (4) death. Multivariable ordinal logistic regression analyses were used to estimate odds ratios (ORs) for severe outcome, adjusting for age, sex, race/ethnicity, NMD, comorbidities, baseline functional status (modified Rankin scale [mRS]), use of immunosuppressive/immunomodulatory medication, and pandemic calendar period. RESULTS: Of 315 patients from 13 countries (mean age 50.3 [±17.7] years, 154 [48.9%] female), 175 (55.5%) were not hospitalized, 27 (8.6%) were hospitalized without supplemental oxygen, 91 (28.9%) were hospitalized with ventilation/supplemental oxygen, and 22 (7%) died. Higher odds of severe COVID-19 outcomes were observed for: age ≥50 years (50-64 years: OR 2.4, 95% confidence interval [CI] 1.33-4.31; >64 years: OR 4.16, 95% CI 2.12-8.15; both vs. <50 years); non-White race/ethnicity (OR 1.81, 95% CI 1.07-3.06; vs. White); mRS moderately severe/severe disability (OR 3.02, 95% CI 1.6-5.69; vs. no/slight/moderate disability); history of respiratory dysfunction (OR 3.16, 95% CI 1.79-5.58); obesity (OR 2.24, 95% CI 1.18-4.25); ≥3 comorbidities (OR 3.2, 95% CI 1.76-5.83; vs. ≤2; if comorbidity count used instead of specific comorbidities); glucocorticoid treatment (OR 2.33, 95% CI 1.14-4.78); and Guillain-Barré syndrome (OR 3.1, 95% CI 1.35-7.13; vs. mitochondrial disease). CONCLUSIONS: Among people with NMDs, there is a differential risk of COVID-19 outcomes according to demographic and clinical characteristics. These findings could be used to develop tailored management strategies and evidence-based recommendations for NMD patients.

Specialist multidisciplinary input maximises rare disease diagnoses from whole genome sequencing.

Diagnostic whole genome sequencing (WGS) is increasingly used in rare diseases. However, standard, semi-automated WGS analysis may overlook diagnoses in complex disorders. Here, we show that specialist multidisciplinary analysis of WGS, following an initial 'no primary findings' (NPF) report, improves diagnostic rates and alters management. We undertook WGS in 102 adults with diagnostically challenging primary mitochondrial disease phenotypes. NPF cases were reviewed by a genomic medicine team, thus enabling bespoke informatic approaches, co-ordinated phenotypic validation, and functional work. We enhanced the diagnostic rate from 16.7% to 31.4%, with management implications for all new diagnoses, and detected strong candidate disease-causing variants in a further 3.9% of patients. This approach presents a standardised model of care that supports mainstream clinicians and enhances diagnostic equity for complex disorders, thereby facilitating access to the potential benefits of genomic healthcare. This research was made possible through access to the data and findings generated by the 100,000 Genomes Project: http://www.genomicsengland.co.uk .

Designing clinical trials for rare diseases: unique challenges and opportunities.

Orphan drug development is a rapidly expanding field. Nevertheless, clinical trials for rare diseases can present inherent challenges. Optimal study design and partnerships between academia and industry are therefore required for the successful development, delivery and clinical approval of effective therapies in this group of disorders.

COVID-19-Related Outcomes in Primary Mitochondrial Diseases: An International Study.

BACKGROUND AND OBJECTIVES: To identify factors associated with severe coronavirus disease 2019 (COVID-19), defined by hospitalization status, in patients with primary mitochondrial diseases (PMDs), thereby enabling future risk stratification and informed management decisions. METHODS: We undertook a cross-sectional, international, registry-based study. Data were extracted from the International Neuromuscular COVID-19 Database and collected between May 1, 2020, and May 31, 2021. The database included subjects with (1) PMD diagnosis (any age), clinically/histopathologically suspected and/or genetically confirmed; and (2) COVID-19 diagnosis classified as "confirmed", "probable", or "suspected" based on World Health Organization definitions. The primary outcome was hospitalization because of COVID-19. We collected demographic information, smoking status, coexisting comorbidities, outcomes after COVID-19 infection, and PMD genotype-phenotype. Baseline status was assessed using the modified Rankin scale (mRS) and the Newcastle Mitochondrial Disease Adult Scale (NMDAS). RESULTS: Seventy-nine subjects with PMDs from 10 countries were included (mean age 41.5 ± 18 years): 25 (32%) were hospitalized, 48 (61%) recovered fully, 28 (35%) improved with sequelae, and 3 (4%) died. Statistically significant differences in hospitalization status were observed in baseline status, including the NMDAS score (p = 0.003) and mRS (p = 0.001), presence of respiratory dysfunction (p < 0.001), neurologic involvement (p = 0.003), and more than 4 comorbidities (p = 0.002). In multivariable analysis, respiratory dysfunction was independently associated with COVID-19 hospitalization (odds ratio, 7.66; 95% CI, 2-28; p = 0.002). DISCUSSION: Respiratory dysfunction is an independent risk factor for severe COVID-19 in PMDs while high disease burden and coexisting comorbidities contribute toward COVID-19-related hospitalization. These findings will enable risk stratification and informed management decisions for this vulnerable population.

Mitochondrial Strokes: Diagnostic Challenges and Chameleons.

Mitochondrial stroke-like episodes (SLEs) are a hallmark of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). They should be suspected in anyone with an acute/subacute onset of focal neurological symptoms at any age and are usually driven by seizures. Suggestive features of an underlying mitochondrial pathology include evolving MRI lesions, often originating within the posterior brain regions, the presence of multisystemic involvement, including diabetes, deafness, or cardiomyopathy, and a positive family history. The diagnosis of MELAS has important implications for those affected and their relatives, given it enables early initiation of appropriate treatment and genetic counselling. However, the diagnosis is frequently challenging, particularly during the acute phase of an event. We describe four cases of mitochondrial strokes to highlight the considerable overlap that exists with other neurological disorders, including viral and autoimmune encephalitis, ischemic stroke, and central nervous system (CNS) vasculitis, and discuss the clinical, laboratory, and imaging features that can help distinguish MELAS from these differential diagnoses.

Phenotype and genotype of 197 British patients with McArdle disease: An observational single-centre study.

McArdle disease is caused by recessive mutations in PYGM gene. The condition is considered to cause a "pure" muscle phenotype with symptoms including exercise intolerance, inability to perform isometric activities, contracture, and acute rhabdomyolysis leading to acute renal failure. This is a retrospective observational study aiming to describe phenotypic and genotypic features of a large cohort of patients with McArdle disease between 2011 and 2019. Data relating to genotype and phenotype, including frequency of rhabdomyolysis, fixed muscle weakness, gout and comorbidities, inclusive of retinal disease (pattern retinal dystrophy) and thyroid disease, were collected. Data from 197 patients are presented. Seven previously unpublished PYGM mutations are described. Exercise intolerance (100%) and episodic rhabdomyolysis (75.6%) were the most common symptoms. Fixed muscle weakness was present in 82 (41.6%) subjects. Unexpectedly, ptosis was observed in 28 patients (14.2%). Hyperuricaemia was a common finding present in 88 subjects (44.7%), complicated by gout in 25% of cases. Thyroid dysfunction was described in 30 subjects (15.2%), and in 3 cases, papillary thyroid cancer was observed. Pattern retinal dystrophy was detected in 15 out of the 41 subjects that underwent an ophthalmic assessment (36.6%). In addition to fixed muscle weakness, ptosis was a relatively common finding. Surprisingly, dysfunction of thyroid and retinal abnormalities were relatively frequent comorbidities. Further studies are needed to better clarify this association, although our finding may have important implication for patient management.

Mitochondrial DNA Analysis from Exome Sequencing Data Improves Diagnostic Yield in Neurological Diseases.

A rapidly expanding catalog of neurogenetic disorders has encouraged a diagnostic shift towards early clinical whole exome sequencing (WES). Adult primary mitochondrial diseases (PMDs) frequently exhibit neurological manifestations that overlap with other nervous system disorders. However, mitochondrial DNA (mtDNA) is not routinely analyzed in standard clinical WES bioinformatic pipelines. We reanalyzed 11,424 exomes, enriched with neurological diseases, for pathogenic mtDNA variants. Twenty-four different mtDNA mutations were detected in 64 exomes, 11 of which were considered disease causing based on the associated clinical phenotypes. These findings highlight the diagnostic uplifts gained by analyzing mtDNA from WES data in neurological diseases. ANN NEUROL 2021;89:1240-1247.

First presentation of LPIN1 acute rhabdomyolysis in adolescence and adulthood.

LPIN1 mutations are a known common cause of autosomal recessive, recurrent and life-threatening acute rhabdomyolysis of childhood-onset. The first episode of rhabdomyolysis usually happens in nearly all cases before the age of 5 and death is observed in 1/3 of patients. Here we present two cases of acute rhabdomyolysis with a milder phenotype caused by LPIN1 mutation presenting in adolescence (11 years old) and adulthood (40 years old) after Parvovirus infection and metabolic stress, respectively. In our opinion, the mutation types, epigenetic factors, the environment exposition to triggers or the existence of proteins with a similar structure of LPIN1, may have a role in modulating the onset of rhabdomyolysis. LPIN1 should be included on a panel of genes analysed in the investigation of adult individuals with rhabdomyolysis. Metabolic and viral stressors should be included in the list of possible rhabdomyolysis precipitant.

Cortical visuomotor interactions in Freezing of Gait: A TMS approach.

OBJECTIVES: Altered cortical visuomotor integration has been involved in the pathophysiology of freezing of gait (FoG) in parkinsonism. The aim of this study was to assess the connections between the primary visual (V1) and motor (M1) areas with a paired-pulse, twin-coil transcranial magnetic stimulation (TMS) technique in patients with FoG. METHODS: Twelve Parkinson's disease (PD) patients suffering from levodopa-responsive-FoG (off-FoG) were compared with 12 PD patients without FoG and 12 healthy subjects of similar age/sex. In the "off" condition, visuomotor connections (VMCs) were assessed bilaterally. A conditioning stimulus over the V1 phosphene hotspot was followed at interstimulus intervals (ISIs) of 18 and 40ms by a test stimulus over M1, to elicit motor evoked potentials (MEPs) in the contralateral first dorsal interosseous muscle. RESULTS: Significant (P<0.01), bilateral effects due to VMCs were detected in all three groups, consisting of a MEP suppression at ISI 18 and 40ms. However, in PD patients with FoG, the MEP suppression was significantly (P<0.05) enhanced, both at ISI 18-40ms, in comparison with the other two groups. The phenomenon was limited to the right hemisphere. CONCLUSIONS: PD patients with FoG showed an excessive inhibitory response of the right M1 to inputs travelling from V1 at given ISIs. Right-sided alterations of the cortical visuomotor integration may contribute to the pathophysiology of FoG.

Nerve conduction, circulating osteopontin and taxane-induced neuropathy in breast cancer patients.

OBJECTIVE: Chemotherapy-induced peripheral neuropathy (CIPN) is a disabling complication related to taxanes. Underlying mechanisms are not completely understood and no specific treatment exists. We investigated the role of nerve conduction studies (NCS) and of serum osteopontin (OPN) measurement as a means to stratify the risk of developing taxane-induced neuropathy (TIN). METHODS: We enrolled 50 women with breast cancer treated with taxanes (docetaxel or paclitaxel) in a 3-month prospective study. They were evaluated before chemotherapy (time-point T0) and followed up at 1 (T1) and 3 (T2) months with clinical examinations/scales, quality of life (QoL) questionnaires, NCS, and serum OPN dosages. RESULTS: A reduction of sural and superficial peroneal sensory action potentials was seen at T1, with a progression at T2 (P<0.001). In contrast, a significant impact of neuropathic symptoms on QoL only occurred at T2 (P<0.01). OPN levels at T0 inversely correlated to axonal loss in the sural nerve (T0-T2, P<0.01). OPN levels at T0 were lower in the intermediate and poor outcome patient subgroups, compared to the good outcome subgroup, as specifically defined (P<0.05). CONCLUSION: Lower limb NCS changes occurred earlier than the detrimental effects of TIN on patients' QoL. Low serum OPN levels before chemotherapy may represent a novel biomarker of TIN risk.

Defective interhemispheric inhibition in drug-treated focal epilepsies.

BACKGROUND: Focal epilepsies (FEs) arise from a lateralized network, while in generalized epilepsies (GEs) there is a bilateral involvement from the outset. Intuitively, the corpus callosum is the anatomical substrate for interhemispheric spread. OBJECTIVE: We used transcranial magnetic stimulation (TMS) to explore whether there are any physiological differences in the corpus callosum of drug-treated patients with FE and those with genetic GE (GGE), compared to healthy subjects (HS). METHODS: TMS was used to measure the interhemispheric inhibition (IHI) from right-to-left primary motor cortex (M1) and viceversa in 16 patients with FE, 17 patients with GGE and 17 HS. A conditioning stimulus (CS) was given to one M1 10 and 50 ms before a test stimulus delivered to the contralateral M1. Motor evoked potentials (MEPs) were analysed both as a function of the side of stimulation and of the epileptic focus (left-right). RESULTS: In HS, IHI was reproducible with suppression of MEPs at ISIs of 10 and 50 ms. Similar effects occurred in GGE patients. FE patients behaved differently, since IHI was significantly reduced bilaterally. When FE patients were stratified according to the side of their epileptic focus, the long-ISI IHI (=50 ms) appeared to be defective only when the CS was applied over the "focal" hemisphere. CONCLUSIONS: FE patients had a defective inhibitory response of contralateral M1 to inputs travelling from the "focal" hemisphere that was residual to the drug action. Whilst IHI changes would not be crucial for the GGE pathophysiology, they may represent one key factor for the contralateral spread of focal discharges, and seizure generalization.

Abnormal motor cortex plasticity in juvenile myoclonic epilepsy.

PURPOSE: Abnormal cortical plasticity has been hypothesized to play a crucial role in the pathogenesis of juvenile myoclonic epilepsy (JME). To study the motor cortical plasticity we used paired associative stimulation (PAS). When a repetitive electrical stimulus to the median nerve is paired with a transcranial magnetic stimulus (TMS) pulse over the controlateral motor cortex with at an interstimulus interval (ISI) of 21.5-25ms, a long term potentiation (LTP)-like synaptic plasticity is induced in the corticospinal system. Aim of this study was to investigate the motor cortex LTP-like synaptic plasticity by means of PAS in patients with JME. METHODS: Twelve adult patients with JME were compared with 13 healthy subjects of similar age and sex. PAS consisted of 180 electrical stimuli of the right median nerve paired with a single TMS over the hotspot of right abductor pollicis brevis (APB) at an ISI of 25ms (PAS25). We measured motor evoked potentials (MEPs) before and after each intervention for up to 30min. RESULTS: In healthy subjects the PAS25 protocol was followed by a significant increase of the MEP amplitude (p<0.001). On the contrary, in patients with JME, the MEP amplitude did not change. CONCLUSION: Defective motor cortex plasticity is likely involved in the pathogenesis of JME.

Progression of muscle histopathology but not of spliceopathy in myotonic dystrophy type 2.

Myotonic dystrophy type 2 (DM2) is an autosomal dominant progressive disease involving skeletal and cardiac muscle and brain. It is caused by a tetranucleotide repeat within the first intron of the CNBP gene that leads to an alteration of the alternative splicing of several genes. To understand the molecular mechanisms that play a role in DM2 progression, the evolution of skeletal muscle histopathology and biomolecular findings in successive biopsies have been studied. Biceps brachii biopsies from 5 DM2 patients who underwent two successive biopsies at different years of age have been used. Muscle histopathology has been assessed on sections immunostained with fast or slow myosin. FISH in combination with MBNL1-immunofluorescence has been performed to evaluate ribonuclear inclusion and MBNL1 foci dimensions in myonuclei. Gene and protein expression and alteration of alternative splicing of several genes have been evaluated over time. All DM2 patients examined show a worsening of muscle histopathology and an increase of foci dimensions over time. The progressive worsening of myotonia in DM2 patients may be due to the decrease of CLCN1 mRNA observed in all patients examined. However, a worsening of alternative splicing alterations has not been evidenced over time. The data obtained in this study confirm that DM2 is a slow progression disease since histological and biomolecular alterations observed in skeletal muscle are minimal even after 10-year interval. The data indicate that muscle morphological alterations evolve more rapidly over time than the molecular changes thus indicating that muscle biopsy is a more sensitive tool than biomolecular markers to assess disease progression at muscle level.