Caractérisation clinique et génétique d’une nouvelle forme d’ataxie autosomique récessive dans la population québécoise

Les ataxies autosomiques récessives sont un groupe de troubles neurologiques hétérogènes caractérisés par une incoordination brute des mouvements musculaires impliquant le dysfonctionnement nerveux du cervelet qui coordonne le mouvement. Plusieurs formes héréditaires ont été décrites dont la plus connue : l’ataxie de Friedriech. Dans cette thèse nous rapportons l'identification et la caractérisation d’une nouvelle forme dans la population québécoise. L’ataxie récessive spastique avec leucoencéphalopathie (ARSAL; aussi connue comme l’ataxie autosomique récessive spastique de type 3 (SPAX3); OMIM 611390) est la deuxième ataxie spastique décrite dans la population canadienne française. En effet, près de 50 % de nos cas sont originaires de la région de Portneuf. En 2006, nous avons décrit les caractéristiques cliniques de cette nouvelle forme d’ataxie. Un premier criblage du génome entier, constitué de plus de 500 marqueurs microsatellites, a permis la localisation du locus sur le chromosome 2q33-34. Suite au séquençage de plus de 37 gènes candidats et afin de rétrécir cet intervalle candidat, nous avons utilisé une micro-puce d’ADN constituée de marqueurs SNP «single nucleotide polymorphism» et nous avons identifié un deuxième intervalle candidat de 0.658Mb au locus 2q33 dans lequel se trouvent moins de 9 gènes. L’identification et la caractérisation de ces mutations a nécessité l’utilisation de diverses technologies de pointe. Trois mutations (une délétion et deux réarrangements complexes) dans le gène mitochondrial tRNA-synthetase (MARS2) ont été identifiées dans notre cohorte. Nous émettons l’hypothèse que la nature des mutations complexes est responsable d’un dérèglement de la transcription du gène, ce qui a un impact néfaste sur la fonction mitochondriale et le tissu neuronal.Autosomal Recessive Ataxias are a group of heterogeneous neurological disorders consisting of gross incoordination of muscle movements implying dysfunction of parts of the nervous system that coordinate movement such as the cerebellum. Several hereditary forms exist for these patterns of neurological dysfunction. In this thesis we reported the identification and characterization of a new form in the French-Canadian population. Autosomal Recessive Spastic Ataxia with frequent Leukoencephalopathy (ARSAL; also referred to as Autosomal Recessive Spastic Ataxia type 3 (SPAX3); OMIM 611390) is the second recessive spastic ataxia originally described in the French-Canadian population. Furthermore, close to 50% of our cases share a Portneuf region origin. In 2006 we described the cardinal features of this new form of ataxia. A first genome wide scan was performed on three informative families with microsatellite (single tandem repeat) markers and a parametric linkage analysis. This allowed us to identify a candidate interval on chromosome 2q33-34. Sequencing of more than 37 genes did not uncover the putative mutation. In order to refine this candidate interval, we have a second genome scan using single nucleotide polymorphism SNP markers and we have identified a smaller candidate interval of 0.658 Mb in which there are nine genes. The use of a multimodal approach was required to uncover and characterize the three mutations (one deletion and two complex rearrangements) in the mitochondrial met-tRNA synthetase gene (MARS2). We suggest that complex rearrangement of the 5’ region of the gene has a great impact on the gene transcription regulation, which affect its mitochondrial function and impair the neuronal tissue

Neuropathological characterization of the cavitating leukoencephalopathy caused by COA8 cytochrome c oxidase deficiency: a case report

COA8-related leukoencephalopathy is a recently described rare cavitating leukoencephalopathy caused by biallelic variants in the COA8 gene. Clinically, it presents heterogeneously and usually follows a bi-phasic clinical course with a period of acute onset and regression, followed by stabilization, and in some cases, even subtle improvement. We present a 4-year-old boy with a homozygous 2.5 kilobase pair deletion in the COA8 gene following a severe neurological deterioration resulting in death weeks after onset. Brain MRI revealed a distinctive pattern of cavitating leukodystrophy predominantly involving the posterior cerebral white matter which improved upon a follow-up MRI a month later. Brain pathology displayed overall white matter destruction with gliosis and infiltration by macrophages. There was preservation of astrocytes around blood vessels and axons around the zones of demyelination. This study is the first neuropathological examination of COA8-related leukoencephalopathy and provides further characterization of the clinical and MRI phenotype

Multi-model approach to integrate climate change impact on carbon sequestration potential of afforestation scenarios in Quebec, Canada

Afforestation of unproductive or currently non-forested territories can increase carbon land sinks and thus contribute to mitigate climate change. However, investments on large-scale afforestation could be risky because of the predicted effect of climate change on forest productivity of newly created plantations. The aim of this study was to assess the carbon sequestration and mitigation potential of afforestation scenarios with different species (Picea mariana, Picea glauca, Pinus banksiana, Pinus resinosa and Populus spp) on open woodlands and abandoned farmlands in the Province of Quebec (Canada) under different radiative forcing projections. We modelled carbon dynamics in these lands under three Representative Concentration Pathways projections (RCP 2.6, RCP 4.5, and RCP 8.5) over the 2021–2100 period. The forest gap model PICUS was used to model tree growth of afforested species as a function of the Representative Concentration Pathways 2.6, 4.5 and 8.5; these data were then used as input in the Carbon Budget Model – Canadian Forest Sector 3 to simulate the evolution of ecosystem carbon stocks and fluxes as a function of forest management and climate. Carbon transfer to harvested wood products, and carbon fluxes associated with product life cycles and substitution effects on markets, were also included in the analyses. Results showed that Pinus species responded more strongly to variations in radiative forcing than for the other simulated species. Overall, aboveground biomass was particularly altered by increased radiative forcing, which in turn reduced harvesting yield and transfers to wood processing and products. At the end of the simulation, despite the expected impacts of radiative forcing on ecosystems, afforestation scenarios on open woodlands with black spruce, white spruce, and jack pine can deliver carbon mitigation of 32% – 70% over the baseline scenario and 4% – 12% for red pine on abandoned farmlands and, hence, contribute to efforts to reduce GHG emissions, especially over the long term. Although climate change is expected to impact the growth of newly planted areas as part of afforestation efforts, the results of our study suggest that the choice of species to plant and the selected forest management strategy have a greater impact on carbon stocks than climate change itself. This study provides a better understanding of the dynamics of afforestation under climate change and whether investments in plantation can contribute to GHG reduction targets

Recessive mutations in POLR1C cause a leukodystrophy by impairing biogenesis of RNA polymerase III

A small proportion of 4H (Hypomyelination, Hypodontia and Hypogonadotropic Hypogonadism) or RNA polymerase III (POLR3)-related leukodystrophy cases are negative for mutations in the previously identified causative genes POLR3A and POLR3B. Here we report eight of these cases carrying recessive mutations in POLR1C, a gene encoding a shared POLR1 and POLR3 subunit, also mutated in some Treacher Collins syndrome (TCS) cases. Using shotgun proteomics and ChIP sequencing, we demonstrate that leukodystrophy-causative mutations, but not TCS mutations, in POLR1C impair assembly and nuclear import of POLR3, but not POLR1, leading to decreased binding to POLR3 target genes. This study is the first to show that distinct mutations in a gene coding for a shared subunit of two RNA polymerases lead to selective modification of the enzymes’ availability leading to two different clinical conditions and to shed some light on the pathophysiological mechanism of one of the most common hypomyelinating leukodystrophies, POLR3-related leukodystrophy

Solving inherited white matter disorder etiologies in the neurology clinic: Challenges and lessons learned using next-generation sequencing

IntroductionRare neurodevelopmental disorders, including inherited white matter disorders or leukodystrophies, often present a diagnostic challenge on a genetic level given the large number of causal genes associated with a range of disease subtypes. This study aims to demonstrate the challenges and lessons learned in the genetic investigations of leukodystrophies through presentation of a series of cases solved using exome or genome sequencing.MethodsEach of the six patients had a leukodystrophy associated with hypomyelination or delayed myelination on MRI, and inconclusive clinical diagnostic genetic testing results. We performed next generation sequencing (case-based exome or genome sequencing) to further investigate the genetic cause of disease.ResultsFollowing different lines of investigation, molecular diagnoses were obtained for each case, with patients harboring pathogenic variants in a range of genes including TMEM106B, GJA1, AGA, POLR3A, and TUBB4A. We describe the lessons learned in reaching the genetic diagnosis, including the importance of (a) utilizing proper multi-gene panels in clinical testing, (b) assessing the reliability of biochemical assays in supporting diagnoses, and (c) understanding the limitations of exome sequencing methods in regard to CNV detection and region coverage in GC-rich areas.DiscussionThis study illustrates the importance of applying a collaborative diagnostic approach by combining detailed phenotyping data and metabolic results from the clinical environment with advanced next generation sequencing analysis techniques from the research environment to increase the diagnostic yield in patients with genetically unresolved leukodystrophies

Instability of the mitochondrial alanyl-tRNA synthetase underlies fatal infantile-onset cardiomyopathy

Recessively inherited variants in AARS2 (NM_020745.2) encoding mitochondrial alanyl-tRNA synthetase (mt-AlaRS) were first described in patients presenting with fatal infantile cardiomyopathy and multiple oxidative phosphorylation defects. To date, all described patients with AARS2-related fatal infantile cardiomyopathy are united by either a homozygous or compound heterozygous c.1774C>T (p.Arg592Trp) missense founder mutation that is absent in patients with other AARS2-related phenotypes. We describe the clinical, biochemical and molecular investigations of two unrelated boys presenting with fatal infantile cardiomyopathy, lactic acidosis and respiratory failure. Oxidative histochemistry showed cytochrome c oxidase-deficient fibres in skeletal and cardiac muscle. Biochemical studies showed markedly decreased activities of mitochondrial respiratory chain complexes I and IV with a mild decrease of complex III activity in skeletal and cardiac muscle. Using next-generation sequencing, we identified a c.1738C>T (p.Arg580Trp) AARS2 variant shared by both patients that was in trans with a loss-of-function heterozygous AARS2 variant; a c.1008dupT (p.Asp337*) nonsense variant or an intragenic deletion encompassing AARS2 exons 5-7. Interestingly, our patients did not harbour the p.Arg592Trp AARS2 founder mutation. In silico modelling of the p.Arg580Trp substitution suggested a deleterious impact on protein stability and folding. We confirmed markedly decreased mt-AlaRS protein levels in patient fibroblasts, skeletal and cardiac muscle, although mitochondrial protein synthesis defects were confined to skeletal and cardiac muscle. In vitro data showed that the p.Arg580Trp variant had a minimal effect on activation, aminoacylation or misaminoacylation activities relative to wild-type mt-AlaRS, demonstrating that instability of mt-AlaRS is the biological mechanism underlying the fatal cardiomyopathy phenotype in our patients.Peer reviewe

Functional validation of novel compound heterozygous variants in B3GAT3 resulting in severe osteopenia and fractures: expanding the disease phenotype

Background A new disease class of syndromes, described as linkeropathies, which are derived from defects in the glycosaminoglycan-linker region as well as glycosaminoglycan-side chains of proteoglycans is increasingly being recognized as a cause of human disease. Proteoglycans are an essential component of the extracellular matrix. Defects in the enzymatic process of proteoglycan synthesis broadly occur due to the incorrect addition of side chains. Previously, homozygous missense variants within the B3GAT3 gene encoding beta 1,3 glucuronyltransferase 3(GlcAT-I) responsible for the biosynthesis of glycosaminoglycans have been described in 7 individuals. Case presentation In this study, a 4-year-old patient with a severe phenotype of osteoporosis, hypotonia, joint laxity, fractures, scoliosis, biscuspid aortic valve and myopia was referred for next generation sequencing after extensive negative clinical testing. Whole exome sequencing was performed on the proband and his unaffected parents to identify the molecular basis of his disease. Sequencing revealed compound heterozygous variants in B3GAT3: c.1A > G (p.Met1?) and c.671 T > A (p.L224Q). Clinical and in vitro functional studies were then completed to verify the pathogenicity of the genotype and further characterize the functional basis of the patient’s disease demonstrating the patient had a decrease both in the protein level of B3GAT3 and in the glucuronyltransferase activity when compared to control samples. Independent in vitro assessment of each variant confirmed the B3GAT3: c.1A > G (p.Met1?) variant is functionally null and the c.671 T > A (p.L224Q) missense variant has significantly reduced glucuronyltransferase activity (~3% of control). Conclusions This is the first report of a patient with compound heterozygosity for a null variant in trans with a missense in B3GAT3 resulting in a severe phenotype, expanding both the genotypic and phenotypic spectrum of B3GAT3-related disease

IFT74 variants cause skeletal ciliopathy and motile cilia defects in mice and humans

Motile and non-motile cilia play critical roles in mammalian development and health. These organelles are composed of a 1000 or more unique proteins, but their assembly depends entirely on proteins synthesized in the cell body and transported into the cilium by intraflagellar transport (IFT). In mammals, malfunction of non-motile cilia due to IFT dysfunction results in complex developmental phenotypes that affect most organs. In contrast, disruption of motile cilia function causes subfertility, disruption of the left-right body axis, and recurrent airway infections with progressive lung damage. In this work, we characterize allele specific phenotypes resulting from IFT74 dysfunction in human and mice. We identified two families carrying a deletion encompassing IFT74 exon 2, the first coding exon, resulting in a protein lacking the first 40 amino acids and two individuals carrying biallelic splice site mutations. Homozygous exon 2 deletion cases presented a ciliary chondrodysplasia with narrow thorax and progressive growth retardation along with a mucociliary clearance disorder phenotype with severely shorted cilia. Splice site variants resulted in a lethal skeletal chondrodysplasia phenotype. In mice, removal of the first 40 amino acids likewise results in a motile cilia phenotype but with little effect on primary cilia structure. Mice carrying this allele are born alive but are growth restricted and developed hydrocephaly in the first month of life. In contrast, a strong, likely null, allele of Ift74 in mouse completely blocks ciliary assembly and causes severe heart defects and midgestational lethality. In vitro studies suggest that the first 40 amino acids of IFT74 are dispensable for binding of other IFT subunits but are important for tubulin binding. Higher demands on tubulin transport in motile cilia compared to primary cilia resulting from increased mechanical stress and repair needs could account for the motile cilia phenotype observed in human and mice

IFT74 variants cause skeletal ciliopathy and motile cilia defects in mice and humans

Motile and non-motile cilia play critical roles in mammalian development and health. These organelles are composed of a 1000 or more unique proteins, but their assembly depends entirely on proteins synthesized in the cell body and transported into the cilium by intraflagellar transport (IFT). In mammals, malfunction of non-motile cilia due to IFT dysfunction results in complex developmental phenotypes that affect most organs. In contrast, disruption of motile cilia function causes subfertility, disruption of the left-right body axis, and recurrent airway infections with progressive lung damage. In this work, we characterize allele specific phenotypes resulting from IFT74 dysfunction in human and mice. We identified two families carrying a deletion encompassing IFT74 exon 2, the first coding exon, resulting in a protein lacking the first 40 amino acids and two individuals carrying biallelic splice site mutations. Homozygous exon 2 deletion cases presented a ciliary chondrodysplasia with narrow thorax and progressive growth retardation along with a mucociliary clearance disorder phenotype with severely shorted cilia. Splice site variants resulted in a lethal skeletal chondrodysplasia phenotype. In mice, removal of the first 40 amino acids likewise results in a motile cilia phenotype but with little effect on primary cilia structure. Mice carrying this allele are born alive but are growth restricted and developed hydrocephaly in the first month of life. In contrast, a strong, likely null, allele of Ift74 in mouse completely blocks ciliary assembly and causes severe heart defects and midgestational lethality. In vitro studies suggest that the first 40 amino acids of IFT74 are dispensable for binding of other IFT subunits but are important for tubulin binding. Higher demands on tubulin transport in motile cilia compared to primary cilia resulting from increased mechanical stress and repair needs could account for the motile cilia phenotype observed in human and mice