Structure-based mutagenic analysis of mechanism and substrate specificity in mammalian glycosyltransferases: porcine ST3Gal-I.

Sialyltransferases (STs) play essential roles in signaling and in the cellular recognition processes of mammalian cells by selectively installing cell-surface sialic acids in an appropriate manner both temporally and organ-specifically. The availability of the first three-dimensional structure of a mammalian (GT29) sialyltransferase has, for the first time, allowed quantitative structure/function analyses to be performed, thereby providing reliable insights into the roles of key active site amino acids. Kinetic analyses of mutants of ST3Gal-I, in conjunction with structural studies, have confirmed the mechanistic roles of His302 and His319 as general acid and base catalysts, respectively, and have quantitated other interactions with the cytosine monophosphate-N-acetyl ß-neuraminic acid donor substrate. The contributions of side chains that provide key interactions with the acceptor substrate, defining its specificity, have also been quantitated. Particularly important transition-state interactions of 2.5 and 2.7 kcal mol(-1) are found between the acceptor axial 4-hydroxyl and the conserved side chains of Gln108 and Tyr269, respectively. These results provide a basis for the engineering of mammalian STs to accommodate non-natural substrate analogs that should prove valuable as chemical biological probes of sialyltransferase function.

X-linked Dystonia-Parkinsonism manifesting in a female patient due to atypical turner syndrome.

BACKGROUND: Recessive X-linked dystonia-parkinsonism almost exclusively affects men. We investigated the genetic mechanisms causing this disorder in a female patient. METHODS: We confirmed the presence of an X-linked dystonia-parkinsonism-specific change in our patient by sequencing. In addition, we employed quantitative real-time PCR and array comparative genomic hybridization to determine the patient's X-chromosome copy number. RESULTS: The patient's sequence electropherogram suggested a higher amount of the mutated allele compared with the wild-type allele. Subsequently, extensive gene dosage analyses revealed a copy number of the X chromosomes between 1 and 2, indicating loss of 1 X chromosome in a subset of cells. Phenotypic reevaluation of the patient showed several clinical features of Turner syndrome. CONCLUSIONS: Our female X-linked dystonia-parkinsonism patient suffered from an undiagnosed X-chromosome monosomy in a subset of cells (45,X/46,XX), suggesting an atypical Turner syndrome and contributing the first molecular explanation for the manifestation of an X-linked dystonia-parkinsonism phenotype in women. © 2013 Movement Disorder Society.

New insights into the genetics of X-linked dystonia-parkinsonism (XDP, DYT3).

X-linked recessive dystonia-parkinsonism is a rare movement disorder that is highly prevalent in Panay Island in the Philippines. Earlier studies identified seven different genetic alterations within a 427-kb disease locus on the X chromosome; however, the exact disease-causing variant among these is still not unequivocally determined. To further investigate the genetic cause of this disease, we sequenced all previously reported genetic alterations in 166 patients and 473 Filipino controls. Singly occurring variants in our ethnically matched controls would have allowed us to define these as polymorphisms, but none were found. Instead, we identified five patients carrying none of the disease-associated variants, and one male control carrying all of them. In parallel, we searched for novel single-nucleotide variants using next-generation sequencing. We did not identify any shared variants in coding regions of the X chromosome. However, by validating intergenic variants discovered via genome sequencing, we were able to define the boundaries of the disease-specific haplotype and narrow the disease locus to a 294-kb region that includes four known genes. Using microarray-based analyses, we ruled out the presence of disease-linked copy number variants within the implicated region. Finally, we utilized in silico analysis and detected no strong evidence of regulatory regions surrounding the disease-associated variants. In conclusion, our finding of disease-specific variants occurring in complete linkage disequilibrium raises new insights and intriguing questions about the origin of the disease haplotype, the existence of phenocopies and of reduced penetrance, and the causative genetic alteration in XDP.

Woman with x-linked recessive dystonia-parkinsonism: clue to the epidemiology of parkinsonism in Filipino women?

IMPORTANCE: Despite recessive inheritance, X-linked dystonia-parkinsonism (Lubag disease) has also been described in women presenting with a late-onset isolated parkinsonian syndrome. Interestingly, unlike in other populations, there is a slight female predominance in the prevalence of parkinsonism in the Philippines. OBSERVATIONS: In a Filipino woman with suspected Parkinson disease, we confirmed the presence of all changes specific for X-linked dystonia-parkinsonism in genomic DNA. Subsequently, we analyzed complementary DNA and evaluated the methylation status of the androgen receptor gene. Owing to extremely skewed (98%:2%) X-chromosome inactivation, the patient expressed almost solely the mutated allele in a disease-specific change, rendering her molecularly comparable with a hemizygously affected man. CONCLUSIONS AND RELEVANCE: Skewed X-chromosome inactivation is the likely cause of parkinsonism in this heterozygous mutation carrier. Because women carriers of the genetic changes specific for X-linked dystonia-parkinsonism are common in the Philippines, the epigenetic factor of nonrandom X-chromosome inactivation may contribute to the skewing of the sex prevalence of parkinsonism toward women in this country, warranting further investigation.

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.

Mutations in GNAL: a novel cause of craniocervical dystonia.

IMPORTANCE: Mutations in the GNAL gene have recently been shown to cause primary torsion dystonia. The GNAL-encoded protein (Gαolf) is important for dopamine D1 receptor function and odorant signal transduction. We sequenced all 12 exons of GNAL in 461 patients from Germany, Serbia, and Japan, including 318 patients with dystonia (190 with cervical dystonia), 51 with hyposmia and Parkinson disease, and 92 with tardive dyskinesia or acute dystonic reactions. OBSERVATIONS: We identified the following two novel heterozygous putative mutations in GNAL: p.Gly213Ser in a German patient and p.Ala353Thr in a Japanese patient. These variants were predicted to be pathogenic in silico, were absent in ethnically matched control individuals, and impaired Gαolf coupling to D1 receptors in a bioluminescence energy transfer (BRET) assay. Two additional variants appeared to be benign because they behaved like wild-type samples in the BRET assay (p.Ala311Thr) or were detected in ethnically matched controls (p.Thr92Ala). Both patients with likely pathogenic mutations had craniocervical dystonia with onset in the fifth decade of life. No pathogenic mutations were detected in the patients with hyposmia and Parkinson disease, tardive dyskinesias, or acute dystonic reactions. CONCLUSIONS AND RELEVANCE: Mutations in GNAL can cause craniocervical dystonia in different ethnicities. The BRET assay may be a useful tool to support the pathogenicity of identified variants in the GNAL gene.

Mortalin mutations are not a frequent cause of early-onset Parkinson disease.

Dysfunctional mitochondria and the mitochondrial chaperone mortalin (HSPA9, GRP75) have been implicated in the pathogenesis of Parkinson disease (PD). We screened 139 early-onset PD (EOPD) patients for mutations in mortalin revealing one missense change (p.L358P) that was absent in 279 control individuals. We also found one additional missense variant among the controls (p.T333K). Although both missense changes were predicted to be disease causing, we detected no differences in subcellular localization, mitochondrial morphology, or respiratory function between wild-type and mutant mortalin. These findings suggest that variants in mortalin (1) are not a major cause of EOPD; (2) occur in patients and controls; and (3) do not lead to functional impairment of mitochondria.