Primary mitochondrial disorders and mimics: Insights from a large French cohort.

OBJECTIVE: The objective of this study was to evaluate the implementation of NGS within the French mitochondrial network, MitoDiag, from targeted gene panels to whole exome sequencing (WES) or whole genome sequencing (WGS) focusing on mitochondrial nuclear-encoded genes. METHODS: Over 2000 patients suspected of Primary Mitochondrial Diseases (PMD) were sequenced by either targeted gene panels, WES or WGS within MitoDiag. We described the clinical, biochemical, and molecular data of 397 genetically confirmed patients, comprising 294 children and 103 adults, carrying pathogenic or likely pathogenic variants in nuclear-encoded genes. RESULTS: The cohort exhibited a large genetic heterogeneity, with the identification of 172 distinct genes and 253 novel variants. Among children, a notable prevalence of pathogenic variants in genes associated with oxidative phosphorylation (OXPHOS) functions and mitochondrial translation was observed. In adults, pathogenic variants were primarily identified in genes linked to mtDNA maintenance. Additionally, a substantial proportion of patients (54% (42/78) and 48% (13/27) in children and adults, respectively), undergoing WES or WGS testing displayed PMD mimics, representing pathologies that clinically resemble mitochondrial diseases. INTERPRETATION: We reported the largest French cohort of patients suspected of PMD with pathogenic variants in nuclear genes. We have emphasized the clinical complexity of PMD and the challenges associated with recognizing and distinguishing them from other pathologies, particularly neuromuscular disorders. We confirmed that WES/WGS, instead of panel approach, was more valuable to identify the genetic basis in patients with "possible" PMD and we provided a genetic testing flowchart to guide physicians in their diagnostic strategy.

Plasma lysosphingolipids in GRN-related diseases: Monitoring lysosomal dysfunction to track disease progression.

GRN mutations are among the main genetic causes of frontotemporal dementia (FTD). Considering the progranulin involvement in lysosomal homeostasis, we aimed to evaluate if plasma lysosphingolipids (lysoSPL) are increased in GRN mutation carriers, and whether they might represent relevant fluid-based biomarkers in GRN-related diseases. We analyzed four lysoSPL levels in plasmas of 131 GRN carriers and 142 non-carriers, including healthy controls and patients with frontotemporal dementias (FTD) carrying a C9orf72 expansion or without any mutation. GRN carriers consisted of 102 heterozygous FTD patients (FTD-GRN), three homozygous patients with neuronal ceroid lipofuscinosis-11 (CLN-11) and 26 presymptomatic carriers (PS-GRN), the latter with longitudinal assessments. Glucosylsphingosin d18:1 (LGL1), lysosphingomyelins d18:1 and isoform 509 (LSM18:1, LSM509) and lysoglobotriaosylceramide (LGB3) were measured by electrospray ionization-tandem mass spectrometry coupled to ultraperformance liquid chromatography. Levels of LGL1, LSM18:1 and LSM509 were increased in GRN carriers compared to non-carriers (p < 0.0001). No lysoSPL increases were detected in FTD patients without GRN mutations. LGL1 and LSM18:1 progressively increased with age at sampling, and LGL1 with disease duration, in FTD-GRN. Among PS-GRN carriers, LSM18:1 and LGL1 significantly increased over 3.4-year follow-up. LGL1 levels were associated with increasing neurofilaments in presymptomatic carriers. This study evidences an age-dependent increase of ß-glucocerebrosidase and acid sphingomyelinase substrates in GRN patients, with progressive changes as early as the presymptomatic phase. Among FTD patients, plasma lysoSPL appear to be uniquely elevated in GRN carriers, and thus might serve as suitable non-invasive disease-tracking biomarkers of progression, specific to the pathophysiological process. Finally, this study might add lysoSPL to the portfolio of fluid-based biomarkers, and pave the way to disease-modifying approaches based on lysosomal function rescue in GRN diseases.

Neurofilaments: a key new biomarker for clinicians. Part 1: Importance of neurofilaments in the management of neurodegenerative diseases / Les neurofilaments : un nouveau biomarqueur clé pour les cliniciens. Partie 1 : Intérêt des neurofilaments dans la prise en charge des maladies neurodégénératives

Neurological biomarkers are of great use for clinicians, as they can be used for numerous purposes: guiding clinical diagnosis, estimating prognosis, assessing disease stage and monitoring progression or response to treatment. This field of neurology has evolved considerably in recent years due to analytical improvements in assay methods, now allowing the detection of biomarkers not only in cerebrospinal fluid (CSF) but also in blood. This progress greatly facilitates the repeated quantification of biomarkers, the collection of blood being much less invasive than that of CSF. Among the various informative biomarkers of neurological disorders, neurofilaments light chains (NfL) have proven to be particularly attractive in many contexts, in particular for the diagnosis and prognosis of neurodegenerative diseases (which this review will present), but also in other contexts of neurological disorders (which will be detailed in part 2). We further address the added value of NfL compared to other biomarkers commonly used to monitor the diseases described in this review.

Les biomarqueurs neurologiques sont d'une grande utilité, car ils peuvent être utilisés à de nombreuses fins : orienter le diagnostic clinique, estimer le pronostic, évaluer le stade de la maladie et surveiller la progression ou la réponse au traitement. Ce domaine de la neurologie a considérablement évolué ces dernières années grâce à l'amélioration des méthodes de dosage, permettant désormais la détection de biomarqueurs non seulement dans le liquide cérébro-spinal (LCS) mais aussi dans le sang. Ce progrès facilite la quantification répétée des biomarqueurs, le prélèvement de sang étant beaucoup moins invasif que celui du LCS. Parmi les différents biomarqueurs informatifs des troubles neurologiques, la chaîne légère des neurofilaments (NfL) s'est révélée particulièrement intéressante dans de nombreux contextes, notamment pour le diagnostic et le pronostic des maladies neurodégénératives (que cette revue présentera), mais aussi dans d'autres contextes de troubles neurologiques (qui seront détaillés dans la partie 2). La valeur ajoutée du NfL par rapport aux autres biomarqueurs couramment utilisés est analysée.

Neurofilaments: a key new biomarker for clinicians. Part 2: Neurofilaments, an asset beyond neurodegenerative diseases / Les neurofilaments : un nouveau biomarqueur clé pour les cliniciens. Partie 2 : Neurofilaments, un intérêt au-delà des maladies neurodégénératives.

Neurofilaments (Nf) are proteins selectively expressed in the cytoskeleton of neurons, and their increase is a marker of neuronal damage. The potential utility of neurofilament light chain (NfL) has recently increased considerably, well beyond neurodegenerative diseases, due to analytical advances that allow measurement of their concentrations (even low ones) in cerebrospinal fluid and blood. This article completes the first part, in which we presented the interest of NfL in the context of neurodegenerative diseases. Here we focus our review on other clinical contexts of neurological injury (such as traumatic brain injury, multiple sclerosis, stroke, and cancer) and present the potential value of NfL assay in the management of these patients, for both diagnosis and prognosis. We also discuss the added value of the NfL assay compared to other biomarkers commonly used in the described clinical situations.

Les neurofilaments (Nf) sont des protéines sélectivement exprimées dans le cytosquelette des neurones, dont l'augmentation est un marqueur de dommages neuronaux. L'utilité potentielle de la chaîne légère des neurofilaments (NfL) s'est récemment considérablement accrue, bien au-delà des maladies neurodégénératives, grâce aux progrès analytiques permettant de mesurer leurs niveaux (mêmes faibles) dans le liquide cérébro-spinal et le sang. Cet article complète la première partie, dans laquelle nous avions présenté l'intérêt des NfL dans le contexte des maladies neurodégénératives. Nous axons ici notre revue sur d'autres contextes cliniques de lésions neurologiques (tels que les traumatismes crâniens, la sclérose en plaques, les accidents vasculaires cérébraux et le cancer) et présentons l'intérêt potentiel du dosage des NfL pour la prise en charge de ces patients, tant au niveau diagnostique que pronostique. Nous discutons également de la plus-value du dosage des NfL par rapport aux autres biomarqueurs couramment utilisés dans les contextes cliniques décrits.

Neurofilaments contribution in clinic: state of the art.

Neurological biomarkers are particularly valuable to clinicians as they can be used for diagnosis, prognosis, or response to treatment. This field of neurology has evolved considerably in recent years with the improvement of analytical methods, allowing the detection of biomarkers not only in cerebrospinal fluid (CSF) but also in less invasive fluids like blood. These advances greatly facilitate the repeated quantification of biomarkers, including at asymptomatic stages of the disease. Among the various informative biomarkers of neurological disorders, neurofilaments (NfL) have proven to be of particular interest in many contexts, such as neurodegenerative diseases, traumatic brain injury, multiple sclerosis, stroke, and cancer. Here we discuss these different pathologies and the potential value of NfL assay in the management of these patients, both for diagnosis and prognosis. We also describe the added value of NfL compared to other biomarkers currently used to monitor the diseases described in this review.

Clinico-biological markers for the prognosis of status epilepticus in adults.

Prediction of mortality, functional outcome and recovery after status epilepticus (SE) is a challenge. Biological and clinical markers have been proposed to reflect the brain injury or to monitor critical ill patients' severity. The aim of this study was to characterize short-term and long-term prognostic factors for SE patients hospitalized in intensive care unit. Patient's outcome was assessed using the modified Rankin Scale at discharge and after 6-12 months. We first assessed the univariate prognosis significance of 51 clinical, demographic or biochemical markers. Next, we built multivariate clinico-biological models by combining most important factors. Statistical models' performances were compared to those of two previous published scales STESS and mSTESS. Eighty-one patients were enrolled. Thirty-five patients showed a steady state while 46 patients clinically worsened at discharge: 14 died, 14 had persistent disability at 6-12 months and 18 recovered. Logistic regression analysis revealed that clinical markers (SE refractoriness, SE duration, de novo SE) were significant independent predictors of worsening while lipids markers and progranulin better predicted mortality. The association of clinico-biological variables allowed to accurately predict worsening at discharge (AUC > 0.72), mortality at discharge (AUC 0.83) and recovery at long-term (AUC 0.89). Previous scales provided lower prediction for worsening (AUC 0.63, STESS; 0.53, mSTESS) and mortality (AUC 0.56, STESS; 0.62, mSTESS) (p < 0.001). We proposed new clinico-biological models with a strong discrimination power for prediction of short- and long-term outcome of hospitalized status epilepticus patients. Their implementation in electronic devices may enhance their clinical liability.

Neuron Specific Enolase, S100-beta protein and progranulin as diagnostic biomarkers of status epilepticus.

Status epilepticus (SE) is a life-threatening prolonged epileptic seizure. A rapid diagnosis is fundamental to initiate antiepileptic treatment and to prevent the development of neurological sequels. Several serum and cerebrospinal fluid biomarkers have been proposed to help in the diagnosis of SE. Nevertheless, previous studies were conducted on too small patient cohorts, precluding the utilization of interesting biomarkers for the SE diagnosis. Here, we aimed to assess the ability of Neuron Specific Enolase (NSE), S100-beta protein (S100B) and progranulin to help in the diagnosis of SE in a large cohort of patients (36 control patients, 56 patients with pharmacoresistant epilepsy and 82 SE patients). Blood NSE, S100B and progranulin levels were higher in SE patients when compared with control patients or patients with pharmacoresistant epilepsy. Both NSE and progranulin levels were higher in cerebrospinal fluid from SE patients when compared with control patients. The receiver-operating characteristics curves revealed good accuracy at detecting SE for serum S100B (AUC 0.748) and plasma progranulin (AUC 0.756). The performances were lower for serum NSE (AUC 0.624). Eighty-four percent of patients with serum S100B levels above 0.09 ng/mL presented with a SE, whereas 90% of patients without SE had serum S100B levels lower than 0.09 ng/mL. Serum S100B levels were not significantly different according to SE etiology, SE semiology or SE refractoriness. Our results confirm that NSE, S100B and progranulin levels are increased after SE. We suggest that serum S100B levels might be added to clinical evaluation and electroencephalogram to identify difficult-to-diagnose form of SE.

Diagnostic approach in adult-onset neurometabolic diseases.

Neurometabolic diseases are a group of individually rare but numerous and heterogeneous genetic diseases best known to paediatricians. The more recently reported adult forms may present with phenotypes strikingly different from paediatric ones and may mimic other more common neurological disorders in adults. Furthermore, unlike most neurogenetic diseases, many neurometabolic diseases are treatable, with both conservative and more recent innovative therapeutics. However, the phenotypical complexity of this group of diseases and the growing number of specialised biochemical tools account for a significant diagnostic delay and underdiagnosis. We reviewed all series and case reports of patients with a confirmed neurometabolic disease and a neurological onset after the age of 10 years, with a focus on the 36 treatable ones, and classified these diseases according to their most relevant clinical manifestations. The biochemical diagnostic approach of neurometabolic diseases lays on the use of numerous tests studying a set of metabolites, an enzymatic activity or the function of a given pathway; and therapeutic options aim to restore the enzyme activity or metabolic function, limit the accumulation of toxic substrates or substitute the deficient products. A quick diagnosis of a treatable neurometabolic disease can have a major impact on patients, leading to the stabilisation of the disease and cease of repeated diagnostic investigations, and allowing for familial screening. For the aforementioned, in addition to an exhaustive and clinically meaningful review of these diseases, we propose a simplified diagnostic approach for the neurologist with the aim to help determine when to suspect a neurometabolic disease and how to proceed in a rational manner. We also discuss the place of next-generation sequencing technologies in the diagnostic process, for which deep phenotyping of patients (both clinical and biochemical) is necessary for improving their diagnostic yield.

Covichem: A biochemical severity risk score of COVID-19 upon hospital admission.

Clinical and laboratory predictors of COVID-19 severity are now well described and combined to propose mortality or severity scores. However, they all necessitate saturable equipment such as scanners, or procedures difficult to implement such as blood gas measures. To provide an easy and fast COVID-19 severity risk score upon hospital admission, and keeping in mind the above limits, we sought for a scoring system needing limited invasive data such as a simple blood test and co-morbidity assessment by anamnesis. A retrospective study of 303 patients (203 from Bordeaux University hospital and an external independent cohort of 100 patients from Paris Pitié-Salpêtrière hospital) collected clinical and biochemical parameters at admission. Using stepwise model selection by Akaike Information Criterion (AIC), we built the severity score Covichem. Among 26 tested variables, 7: obesity, cardiovascular conditions, plasma sodium, albumin, ferritin, LDH and CK were the independent predictors of severity used in Covichem (accuracy 0.87, AUROC 0.91). Accuracy was 0.92 in the external validation cohort (89% sensitivity and 95% specificity). Covichem score could be useful as a rapid, costless and easy to implement severity assessment tool during acute COVID-19 pandemic waves.

Improved detection of mitochondrial DNA instability in mitochondrial genome maintenance disorders.

PURPOSE: Diseases caused by defects in mitochondrial DNA (mtDNA) maintenance machinery, leading to mtDNA deletions, form a specific group of disorders. However, mtDNA deletions also appear during aging, interfering with those resulting from mitochondrial disorders. METHODS: Here, using next-generation sequencing (NGS) data processed by eKLIPse and data mining, we established criteria distinguishing age-related mtDNA rearrangements from those due to mtDNA maintenance defects. MtDNA deletion profiles from muscle and urine patient samples carrying pathogenic variants in nuclear genes involved in mtDNA maintenance (n = 40) were compared with age-matched controls (n = 90). Seventeen additional patient samples were used to validate the data mining model. RESULTS: Overall, deletion number, heteroplasmy level, deletion locations, and the presence of repeats at deletion breakpoints were significantly different between patients and controls, especially in muscle samples. The deletion number was significantly relevant in adults, while breakpoint repeat lengths surrounding deletions were discriminant in young subjects. CONCLUSION: Altogether, eKLIPse analysis is a powerful tool for measuring the accumulation of mtDNA deletions between patients of different ages, as well as in prioritizing novel variants in genes involved in mtDNA stability.

[SARS-CoV-2 and neurological disorders: the revelance of biomarkers?] / SARS-CoV-2 et atteintes neurologiques : quelle est la place des biomarqueurs ?

Soon after the pandemic, numerous publications described cases of neurological disorders associated with the SARS-CoV-2 infection. The range of neurological symptoms is becoming increasingly more extensive as the pandemic progresses. However, it is not yet well established whether the manifestations are due to direct viral damage to the nervous system or indirect consequences of the infection. This review presents an inventory of the biochemical markers studied in the context of neurological disorders related to SARS-CoV-2. By reflecting various physiopathological mechanisms, these biomarkers allow both a better understanding of the pathophysiology of Covid-19 and a contribution to the diagnosis of neurologic troubles; they could participate in the prognostic evaluation of patients.

[Usefulness of combined sequencing of the mitochondrial genome and a targeted panel of nuclear genes involved in mitochondrial diseases]. / Intérêt du séquençage combiné du génome mitochondrial et d'un panel ciblé de gènes nucléaires impliqués dans les maladies mitochondriales.

The molecular study of mitochondrial diseases, essential for diagnosis, is special due to the dual genetic origin of these pathologies: mitochondrial DNA and nuclear DNA. Complete mtDNA sequencing still remains the first line diagnostic test followed if negative, by resequencing panels of several hundred mitochondrially-encoded nuclear genes. This strategy, with an initial entire mtDNA sequencing, is currently justified by the presence of nuclear mitochondrial DNA sequences (NUMTs) in the nuclear genome. We designed a resequencing panel combining the mtDNA and 135 nuclear genes which was evaluated compared to the performances of the standard mtDNA sequencing. Method validation was performed on the reading depth and reproducibility of the results. Thirty patients were analyzed by both methods. We were able to demonstrate that NUMTs did not impact the mtDNA sequencing quality, as the identified variants and mutant loads were identical with the reference mtDNA sequencing method. Reading depths were higher than the recommendations of the MitoDiag French diagnostic network, for the entire mtDNA for muscle and for 70% of the mtDNA for blood. These results highlight the usefulness of combining both mtDNA and mitochondrially nuclear-encoded genes and thus obtain more complete results and faster turnaround time for mitochondrial disease patients.

Homoplasmic deleterious MT-ATP6/8 mutations in adult patients.

To address the frequency of complex V defects, we systematically sequenced MT-ATP6/8 genes in 512 consecutive patients. We performed functional analysis in muscle or fibroblasts for 12 out of 27 putative homoplasmic mutations and in cybrids for four. Fibroblasts, muscle and cybrids with known deleterious mutations underwent parallel analysis. It included oxidative phosphorylation spectrophotometric assays, western blots, structural analysis, ATP production, glycolysis and cell proliferation evaluation. We demonstrated the deleterious nature of three original mutations. Striking gradation in severity of the mutations consequences and differences between muscle, fibroblasts and cybrids implied a likely under-diagnosis of human complex V defects.

Homoplasmic mitochondrial tRNAPro mutation causing exercise-induced muscle swelling and fatigue.

OBJECTIVE: To demonstrate the causal role in disease of the MT-TP m.15992A>T mutation observed in patients from 5 independent families. METHODS: Lactate measurement, muscle histology, and mitochondrial activities in patients; PCR-based analyses of the size, amount, and sequence of muscle mitochondrial DNA (mtDNA) and proportion of the mutation; respiration, mitochondrial activities, proteins, translation, transfer RNA (tRNA) levels, and base modification state in skin fibroblasts and cybrids; and reactive oxygen species production, proliferation in the absence of glucose, and plasma membrane potential in cybrids. RESULTS: All patients presented with severe exercise intolerance and hyperlactatemia. They were associated with prominent exercise-induced muscle swelling, conspicuous in masseter muscles (2 families), and/or with congenital cataract (2 families). MRI confirmed exercise-induced muscle edema. Muscle disclosed severe combined respiratory defect. Muscle mtDNA had normal size and amount. Its sequence was almost identical in all patients, defining the haplotype as J1c10, and sharing 31 variants, only 1 of which, MT-TP m.15992A>T, was likely pathogenic. The mutation was homoplasmic in all tissues and family members. Fibroblasts and cybrids with homoplasmic mutation had defective respiration, low complex III activity, and decreased tRNAPro amount. Their respiratory complexes amount and tRNAPro aminoacylation appeared normal. Low proliferation in the absence of glucose demonstrated the relevance of the defects on cybrid biology while abnormal loss of cell volume when faced to plasma membrane depolarization provided a link to the muscle edema observed in patients. CONCLUSIONS: The homoplasmic MT-TP m.15992A>T mutation in the J1c10 haplotype causes exercise-induced muscle swelling and fatigue.

Pigments test strips: A rapid companion test to exclude sub-arachnoid haemorrhage.

OBJECTIVES: Subarachnoid haemorrhage (SAH) is characterised by 25% of mortality or induces long-term care. It needs immediate diagnosis with computed tomography (CT) scan. For the inconclusive CT scans, the detection of haem pigments can be performed in the cerebrospinal fluid (CSF). The reference method is spectrophotometry but it requires a large volume of CSF, and specific equipment. Sometimes, urine test strips are used as an alternative method for haem pigments detection. However, this method needs validation in SAH context. The aim of the study was to compare the performance of Multistix® urine test strips for haem pigments detection to the reference spectrophotometry and the final clinical SAH diagnosis. METHODS: We collected 136 CSFs sampled for suspected SAH. We detected haem pigments with urine test strips and spectrophotometry and compared performances for 100 samples. RESULTS: Urine tests strips displayed a high sensitivity (0.97) as compared to the reference spectrophotometry for haem pigments detection. Interestingly, absence of haem pigments fully correlated with absence of SAH. CONCLUSIONS: Negative Multistix® urine test strips could help to exclude SAH diagnosis in combination with clinical data when a spectrophotometer is not available, or as a bedside diagnosis test.

Ganglionopathies Associated with MERRF Syndrome: An Original Report.

Neuropathies in Myoclonic Epilepsy with Ragged Red Fibers (MERRF) syndrome are frequent but ganglionopathies have never been reported. We retrospectively identified 24 patients with MERRF mutations in the neuromuscular center Nord/Est/Ile de France (Pitié-Salpêtrière, Paris, France). Seventeen nerve conduction studies (NCS) were available. Five patients had MERRF syndrome and ganglionopathy, a pure sensory neuropathy. All of them displayed ataxia and mild clinical sensory abnormalities. Ganglionopathies have been reported in mitochondrial diseases but never in MERRF syndrome. We suggest that patients presenting with ganglionopathy, especially if associated with myopathy, lipomatosis or epilepsy, should be screened for MERRF mutations.

Plasma progranulin levels for frontotemporal dementia in clinical practice: a 10-year French experience.

GRN mutations are frequent causes of familial frontotemporal degeneration. Although there is no clear consensual threshold, plasma progranulin levels represent an efficient biomarker for predicting GRN mutations when decreased. We evaluated plasma levels to determine whether it could also predict age at onset, clinical phenotype, or disease progression in 160 GRN carriers. Importantly, progranulin levels were influenced by gender, with lower levels in male than in female patients in our study. Although we found no correlation with age at onset or with clinical phenotype, we confirmed that decreased level predicts GRN mutations, even in presymptomatic carriers more than four decades before disease onset. We also provided first evidence for the stability of levels throughout longitudinal trajectory in carriers, over a 4-year time span. Finally, we confirmed that progranulin levels constitute a reliable, cost-effective marker, suitable as a screening tool in patients with familial frontotemporal degeneration, and more broadly in patients without family history or with atypical presentations who are less likely to be referred for molecular diagnosis.

The missense p.Trp7Arg mutation in GRN gene leads to progranulin haploinsufficiency.

GRN null mutations are among the main genetic causes of frontotemporal dementia through progranulin haploinsufficiency. Most missense mutations are considered not pathogenic. The p.Trp7Arg substitution is localized within the signal peptide domain and no formal evidence for its pathogenicity has yet been provided. We identified the p.Trp7Arg substitution in 3 carriers with low plasma progranulin levels. This evidences that this missense mutation leads to functional haploinsufficiency and should thus be considered pathogenic. Assessing the pathogenicity of variants of unknown significance has significant implications for clinical practice, genetic counseling, and future therapeutic interventions.

Homozygous GRN mutations: new phenotypes and new insights into pathological and molecular mechanisms.

Homozygous mutations in the progranulin gene (GRN) are associated with neuronal ceroid lipofuscinosis 11 (CLN11), a rare lysosomal-storage disorder characterized by cerebellar ataxia, seizures, retinitis pigmentosa, and cognitive disorders, usually beginning between 13 and 25 years of age. This is a rare condition, previously reported in only four families. In contrast, heterozygous GRN mutations are a major cause of frontotemporal dementia associated with neuronal cytoplasmic TDP-43 inclusions. We identified homozygous GRN mutations in six new patients. The phenotypic spectrum is much broader than previously reported, with two remarkably distinct presentations, depending on the age of onset. A childhood/juvenile form is characterized by classical CLN11 symptoms at an early age at onset. Unexpectedly, other homozygous patients presented a distinct delayed phenotype of frontotemporal dementia and parkinsonism after 50 years; none had epilepsy or cerebellar ataxia. Another major finding of this study is that all GRN mutations may not have the same impact on progranulin protein synthesis. A hypomorphic effect of some mutations is supported by the presence of residual levels of plasma progranulin and low levels of normal transcript detected in one case with a homozygous splice-site mutation and late onset frontotemporal dementia. This is a new critical finding that must be considered in therapeutic trials based on replacement strategies. The first neuropathological study in a homozygous carrier provides new insights into the pathological mechanisms of the disease. Hallmarks of neuronal ceroid lipofuscinosis were present. The absence of TDP-43 cytoplasmic inclusions markedly differs from observations of heterozygous mutations, suggesting a pathological shift between lysosomal and TDP-43 pathologies depending on the mono or bi-allelic status. An intriguing observation was the loss of normal TDP-43 staining in the nucleus of some neurons, which could be the first stage of the TDP-43 pathological process preceding the formation of typical cytoplasmic inclusions. Finally, this study has important implications for genetic counselling and molecular diagnosis. Semi-dominant inheritance of GRN mutations implies that specific genetic counselling should be delivered to children and parents of CLN11 patients, as they are heterozygous carriers with a high risk of developing dementia. More broadly, this study illustrates the fact that genetic variants can lead to different phenotypes according to their mono- or bi-allelic state, which is a challenge for genetic diagnosis.