Does uncoupling protein 2 expression qualify as marker of disease status in LRRK2-associated Parkinson's disease?

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common known genetic cause of late-onset Parkinson's disease (PD). However, the penetrance of the disease is below 50% at 60 years of age. LRRK2 is associated with the mitochondrial membrane, and mutant forms impair the function of the organelle and autophagosome clearance in human cells, including induced pluripotent stem cell-derived neurons. Elevated expression of uncoupling proteins has been identified as the cause of mitochondrial depolarization in human fibroblasts with G2019S LRRK2. To identify factors that contribute to the penetrance of LRRK2 mutations, we studied respiratory chain function, markers of mitochondrial uncoupling, oxidative stress, and autophagy in fibroblasts from affected and unaffected carriers of the G2019S mutation. Independent of disease status, all mutation carriers showed reduced mitochondrial membrane potential, increased proton leakage, and more fragmented mitochondria. However, a significant increase in the expression of uncoupling protein 2 (UCP2) was only detected in affected individuals with the G2019S mutation in LRRK2. Since oxidative stress and autophagic markers were selectively increased in some of the PD patients, we hypothesize that UCP2 expression is upregulated in response to elevated reactive oxygen species generation in affected mutation carriers and that UCP2 mRNA levels might, therefore, serve as markers of disease status in LRRK2-associated PD.

Recessive dystonia-ataxia syndrome in a Turkish family caused by a COX20 (FAM36A) mutation.

DYTCA is a syndrome that is characterized by predominant dystonia and mild cerebellar ataxia. We examined two affected siblings with healthy, consanguineous, Turkish parents. Both patients presented with a combination of childhood-onset cerebellar ataxia, dystonia, and sensory axonal neuropathy. In the brother, dystonic features were most pronounced in the legs, while his sister developed torticollis. Routine diagnostic investigations excluded known genetic causes. Biochemical analyses revealed a mitochondrial respiratory chain complex IV and a coenzyme Q10 deficiency in a muscle biopsy. By exome sequencing, we identified a homozygous missense mutation (c.154A >C; p.Thr52Pro) in both patients in exon 2 of the COX20 (FAM36A) gene, which encodes a complex IV assembly factor. This variant was confirmed by Sanger sequencing, was heterozygous in both parents, and was absent from 427 healthy controls. The exact same mutation was recently reported in a patient with ataxia and muscle hypotonia. Among 128 early-onset dystonia and/or ataxia patients, we did not detect any other patient with a COX20 mutation. cDNA sequencing and semi-quantitative analysis were performed in fibroblasts from one of our homozygous mutation carriers and six controls. In addition to the exchange of an amino acid, the mutation led to a shift in splicing. In conclusion, we extend the phenotypic spectrum of a recently identified mutation in COX20 to a recessively inherited, early-onset dystonia-ataxia syndrome that is characterized by reduced complex IV activity. Further, we confirm a pathogenic role of this mutation in cerebellar ataxia, but this mutation seems to be a rather rare cause.

Whispering dysphonia (DYT4 dystonia) is caused by a mutation in the TUBB4 gene.

OBJECTIVE: A study was undertaken to identify the gene underlying DYT4 dystonia, a dominantly inherited form of spasmodic dysphonia combined with other focal or generalized dystonia and a characteristic facies and body habitus, in an Australian family. METHODS: Genome-wide linkage analysis was carried out in 14 family members followed by genome sequencing in 2 individuals. The index patient underwent a detailed neurological follow-up examination, including electrophysiological studies and magnetic resonance imaging scanning. Biopsies of the skin and olfactory mucosa were obtained, and expression levels of TUBB4 mRNA were determined by quantitative real-time polymerase chain reaction in 3 different cell types. All exons of TUBB4 were screened for mutations in 394 unrelated dystonia patients. RESULTS: The disease-causing gene was mapped to a 23cM region on chromosome 19p13.3-p13.2 with a maximum multipoint LOD score of 5.338 at markers D9S427 and D9S1034. Genome sequencing revealed a missense variant in the TUBB4 (tubulin beta-4; Arg2Gly) gene as the likely cause of disease. Sequencing of TUBB4 in 394 unrelated dystonia patients revealed another missense variant (Ala271Thr) in a familial case of segmental dystonia with spasmodic dysphonia. mRNA expression studies demonstrated significantly reduced levels of mutant TUBB4 mRNA in different cell types from a heterozygous Arg2Gly mutation carrier compared to controls. INTERPRETATION: A mutation in TUBB4 causes DYT4 dystonia in this Australian family with so-called whispering dysphonia, and other mutations in TUBB4 may contribute to spasmodic dysphonia. Given that TUBB4 is a neuronally expressed tubulin, our results imply abnormal microtubule function as a novel mechanism in the pathophysiology of dystonia.

ATP13A2 mutations impair mitochondrial function in fibroblasts from patients with Kufor-Rakeb syndrome.

Mutations in ATP13A2 cause autosomal-recessive parkinsonism (Kufor-Rakeb syndrome; KRS). Because several other parkinsonism-associated proteins have been connected to mitochondrial function and mitophagy, we studied the impact of endogenous mutations in ATPase type 13A2 (ATP13A2) on mitochondria in fibroblasts from KRS patients compared with controls. In patients, we detected decreased adenosine triphosphate (ATP) synthesis rates, increased mitochondrial DNA levels, a higher frequency of mitochondrial DNA lesions, increased oxygen consumption rates, and increased fragmentation of the mitochondrial network. Importantly, overexpression of wild-type ATP13A2 rescued the respiration phenotype. These findings collectively suggest that ATP13A2 contributes to the maintenance of a healthy mitochondrial pool, supporting the hypothesis that impaired mitochondrial clearance represents an important pathogenic mechanism underlying KRS.