Kristoffer Haugarvoll

α-Synuclein and Parkinson disease susceptibility

Background: Mutations in the α-synuclein (SNCA) gene have been shown to be responsible for a rare familial form of Parkinson disease (PD). Furthermore, polymorphic variants in multiple regions of the gene have been associated with susceptibility to idiopathic PD in different populations. Objective: To evaluate and to confirm the role of SNCA variants in PD pathogenesis. Methods: We included 667 subjects (397 cases with idiopathic PD and 270 healthy, ethnically matched controls) of Northern Central and Southeastern European origin. We analyzed genotypes at 14 markers spanning the SNCA locus and its major haplotype blocks and conducted a haplotype analysis for four promoter markers including the microsatellite marker Rep1. Results: The three single nucleotide polymorphisms (SNPs) of the promoter region (rs2583988, rs2619364, rs2619363) and a SNP in the 3′UTR (rs356165) of the SNCA gene showed the greatest evidence for an association with PD (p ≤ 0.003), with significant pairwise values for linkage disequilibrium (D′ ≥ 0.74, r 2 ≥ 0.29). The promoter haplotype “261-T-G-T” (Rep1-rs2583988-rs2619364-rs2619363) was associated with disease (p = 0.032). The most significant association with PD was generated by excluding Rep1 (p = 0.008). This association remained significant when analyzing the Serbian patients separately and was of borderline significance for the German patients. Conclusions: Our findings confirm that genetic variability within the SNCA locus is associated with susceptibility to idiopathic Parkinson disease (PD). We found evidence for disease association with single nucleotide polymorphisms at both the 5′ and the 3′ end of the gene with pairwise linkage disequilibrium between them. The association was independent of the Rep1 status, and one major SNCA promoter haplotype class seems to be associated with PD susceptibility. GLOSSARY: SNCA = α-synuclein; CI = confidence interval; HT = haplotype-tagging; LD = linkage disequilibrium; OR = odds ratio; PD = Parkinson disease; SNP = single nucleotide polymorphism.

Lrrk2 R1441C parkinsonism is clinically similar to sporadic Parkinson disease

Objective: Leucine-rich repeat kinase 2 (LRRK2) mutations are the most common cause of Parkinson disease (PD). Several dominantly inherited pathogenic substitutions have been identified in different domains of the Lrrk2 protein. Herein, we characterize the clinical and genetic features associated with Lrrk2 p.R1441C. Methods: We identified 33 affected and 15 unaffected LRRK2 c.4321C>T (p.R1441C) mutation carriers through an international consortium originating from three continents. The age-specific cumulative incidence of PD was calculated by Kaplan-Meier analysis. Results: The clinical presentation of Lrrk2 p.R1441C carriers was similar to sporadic PD and Lrrk2 p.G2019S parkinsonism. The mean age at onset for parkinsonism was 60 years, range 30–79 years; fewer than 20% of the patients had symptoms before the age 50 years, while by 75 years >90% of them had developed symptoms. Haplotype analysis suggests four independent founders for the p.R1441C mutation. Conclusions: The distribution in age at onset and clinical features in Lrrk2 p.R1441C patients are similar to idiopathic and Lrrk2 p.G2019S parkinsonism. Several independent founders of the p.R1441C substitution suggest this site is prone to recurrent mutagenesis. GLOSSARY: COR = C-terminal of Roc; GTPase = guanosine triphosphatase; LBD = Lewy body disease; PD = Parkinson disease; SNPs = single nucleotide polymorphisms.

Dominant Mutations in the Autoimmune Regulator AIRE Are Associated with Common Organ-Specific Autoimmune Diseases.

The autoimmune regulator (AIRE) gene is crucial for establishing central immunological tolerance and preventing autoimmunity. Mutations in AIRE cause a rare autosomal-recessive disease, autoimmune polyendocrine syndrome type 1 (APS-1), distinguished by multi-organ autoimmunity. We have identified multiple cases and families with mono-allelic mutations in the first plant homeodomain (PHD1) zinc finger of AIRE that followed dominant inheritance, typically characterized by later onset, milder phenotypes, and reduced penetrance compared to classical APS-1. These missense PHD1 mutations suppressed gene expression driven by wild-type AIRE in a dominant-negative manner, unlike CARD or truncated AIRE mutants that lacked such dominant capacity. Exome array analysis revealed that the PHD1 dominant mutants were found with relatively high frequency (>0.0008) in mixed populations. Our results provide insight into the molecular action of AIRE and demonstrate that disease-causing mutations in the AIRE locus are more common than previously appreciated and cause more variable autoimmune phenotypes.

Novel Pathogenic LRRK2 p.Asn1437His Substitution in Familial Parkinson's Disease

Genealogical investigation of a large Norwegian family (F04) with autosomal dominant parkinsonism has identified 18 affected family members over four generations. Genetic studies have revealed a novel pathogenic LRRK2 mutation c.4309 A>C (p.Asn1437His) that co‐segregates with disease manifestation (LOD = 3.15, θ = 0). Affected carriers have an early age at onset (48 ± 7.7 SD years) and are clinically asymmetric and levodopa responsive. The variant was absent in 623 Norwegian control subjects. Further screening of patients from the same population identified one additional affected carrier (1 of 692) with familial parkinsonism who shares the same haplotype. The mutation is located within the Roc domain of the protein and enhances GTP‐binding and kinase activity, further implicating these activities as the mechanisms that underlie LRRK2‐linked parkinsonism.

MRI characterisation of adult onset alpha-methylacyl-coA racemase deficiency diagnosed by exome sequencing

BackgroundCorrect diagnosis is pivotal to understand and treat neurological disease. Herein, we report the diagnostic work-up utilizing exome sequencing and the characterization of clinical features and brain MRI in two siblings with a complex, adult-onset phenotype; including peripheral neuropathy, epilepsy, relapsing encephalopathy, bilateral thalamic lesions, type 2 diabetes mellitus, cataract, pigmentary retinopathy and tremor.MethodsWe applied clinical and genealogical investigations, homozygosity mapping and exome sequencing to establish the diagnosis and MRI to characterize the cerebral lesions.ResultsA recessive genetic defect was suspected in two siblings of healthy, but consanguineous parents. Homozygosity mapping revealed three shared homozygous regions and exome sequencing, revealed a novel homozygous c.367 G>A [p.Asp123Asn] mutation in the α-methylacyl-coA racemase (AMACR) gene in both patients. The genetic diagnosis of α-methylacyl-coA racemase deficiency was confirmed by demonstrating markedly increased pristanic acid levels in blood (169 μmol/L, normal <1.5 μmol/L). MRI studies showed characteristic degeneration of cerebellar afferents and efferents, including the dentatothalamic tract and thalamic lesions in both patients.ConclusionsMetabolic diseases presenting late are diagnostically challenging. We show that appropriately applied, homozygosity mapping and exome sequencing can be decisive for establishing diagnoses such as late onset α-methylacyl-coA racemase deficiency, an autosomal recessive peroxisomal disorder with accumulation of pristanic acid. Our study also highlights radiological features that may assist in diagnosis. Early diagnosis is important as patients with this disorder may benefit from restricted dietary phytanic and pristanic acid intake.

Genomewide Association, Parkinson Disease, and PARK10

The authors thank the patients and families for their participation in this study. Mayo Clinic Jacksonville is an M. K. Udall Parkinson’s Disease Research Center of Excellence (National Institute of Neurological Disorders and Stroke grant P01 NS40256), and the authors thank all collaborators at the Udall Center. This study was also supported by the National Institutes of Health grant R01 NS36960, the Research Council of Norway grant 153487/V50, and Reberg’s Legacy. The Ireland research consortium was supported by a Programme for Research in Third-Level Institutions neurosciences award and by the Research and Development Office of the Health and Personal Social Services. O.A.R. and M.T. are partly funded by National Parkinson Foundation and Parkinson’s Disease Foundation fellowships, respectively. We thank Minnie Schreiber for technical assistance.

Severe nigrostriatal degeneration without clinical parkinsonism in patients with polymerase gamma mutations

The role of mitochondria in the pathogenesis of neurodegeneration is an area of intense study. It is known that defects in proteins involved in mitochondrial quality control can cause Parkinsons disease, and there is increasing evidence linking mitochondrial dysfunction, and particularly mitochondrial DNA abnormalities, to neuronal loss in the substantia nigra. Mutations in the catalytic subunit of polymerase gamma are among the most common causes of mitochondrial disease and owing to its role in mitochondrial DNA homeostasis, polymerase gamma defects are often considered a paradigm for mitochondrial diseases generally. Yet, despite this, parkinsonism is uncommon with polymerase gamma defects. In this study, we investigated structural and functional changes in the substantia nigra of 11 patients with polymerase gamma encephalopathy. We characterized the mitochondrial DNA abnormalities and examined the respiratory chain in neurons of the substantia nigra. We also investigated nigrostriatal integrity and function using a combination of post-mortem and in vivo functional studies with dopamine transporter imaging and positron emission tomography. At the cellular level, dopaminergic nigral neurons of patients with polymerase gamma encephalopathy contained a significantly lower copy number of mitochondrial DNA (depletion) and higher levels of deletions than normal control subjects. A selective and progressive complex I deficiency was seen and this was associated with a severe and progressive loss of the dopaminergic neurons of the pars compacta. Dopamine transporter imaging and positron emission tomography showed that the degree of nigral neuronal loss and nigrostriatal depletion were severe and appeared greater even than that seen in idiopathic Parkinsons disease. Despite this, however, none of our patients showed any signs of parkinsonism. The additional presence of both thalamic and cerebellar dysfunction in our patients suggested that these may play a role in counteracting the effects of basal ganglia dysfunction and prevent the development of clinical parkinsonism.

FGF20 and Parkinson's disease: No evidence of association or pathogenicity via α‐synuclein expression

Genetic variation in fibroblast growth factor 20 (FGF20) has been associated with risk of Parkinsons disease (PD). Functional evidence suggested the T allele of one SNP, rs12720208 C/T, altered PD risk by increasing FGF20 and α‐synuclein protein levels. Herein we report our association study of FGF20 and PD risk in four patient‐control series (total: 1,262 patients and 1,881 controls), and measurements of FGF20 and α‐synuclein protein levels in brain samples (nine patients). We found no evidence of association between FGF20 variability and PD risk, and no relationship between the rs12720208 genotype, FGF20 and α‐synuclein protein levels.

Haplotype analysis of Lrrk2 R1441H carriers with parkinsonism

The Roc domain of the Lrrk2 protein harbors two pathogenic mutations which cause autosomal dominant parkinsonism (R1441C and R1441G). A third putatively pathogenic variant (R1441H) has been identified in four probands of diverse ethnicity with parkinsonism. Herein we show that the R1441H substitutions lie on different haplotypes within our patients, confirming this codon as a mutational hotspot. The absence of this variant in control subjects and the presence of two other pathogenic variants at this amino acid position collectively support the contention that R1441H is a pathogenic substitution.

Defective mitochondrial DNA homeostasis in the substantia nigra in Parkinson disease

Increased somatic mitochondrial DNA (mtDNA) mutagenesis causes premature aging in mice, and mtDNA damage accumulates in the human brain with aging and neurodegenerative disorders such as Parkinson disease (PD). Here, we study the complete spectrum of mtDNA changes, including deletions, copy-number variation and point mutations, in single neurons from the dopaminergic substantia nigra and other brain areas of individuals with Parkinson disease and neurologically healthy controls. We show that in dopaminergic substantia nigra neurons of healthy individuals, mtDNA copy number increases with age, maintaining the pool of wild-type mtDNA population in spite of accumulating deletions. This upregulation fails to occur in individuals with Parkinson disease, however, resulting in depletion of the wild-type mtDNA population. By contrast, neuronal mtDNA point mutational load is not increased in Parkinson disease. Our findings suggest that dysregulation of mtDNA homeostasis is a key process in the pathogenesis of neuronal loss in Parkinson disease.