Novel mutations in the guanosine triphosphate cyclohydrolase 1 gene associated with DYT5 dystonia

Consensus guideline for the diagnosis and treatment of tetrahydrobiopterin (BH4) deficiencies

BACKGROUND

Tetrahydrobiopterin (BH4) deficiencies comprise a group of six rare neurometabolic disorders characterized by insufficient synthesis of the monoamine neurotransmitters dopamine and serotonin due to a disturbance of BH4 biosynthesis or recycling. Hyperphenylalaninemia (HPA) is the first diagnostic hallmark for most BH4 deficiencies, apart from autosomal dominant guanosine triphosphate cyclohydrolase I deficiency and sepiapterin reductase deficiency. Early supplementation of neurotransmitter precursors and where appropriate, treatment of HPA results in significant improvement of motor and cognitive function. Management approaches differ across the world and therefore these guidelines have been developed aiming to harmonize and optimize patient care. Representatives of the International Working Group on Neurotransmitter related Disorders (iNTD) developed the guidelines according to the SIGN (Scottish Intercollegiate Guidelines Network) methodology by evaluating all available evidence for the diagnosis and treatment of BH4 deficiencies.

CONCLUSION

Although the total body of evidence in the literature was mainly rated as low or very low, these consensus guidelines will help to harmonize clinical practice and to standardize and improve care for BH4 deficient patients.

Targeted gene capture sequencing in diagnosis of dystonia patients

BACKGROUND

Dystonia is a movement disorder with high clinical and genetic heterogeneity. Molecular diagnosis is important for an accurate diagnosis of dystonia. Targeted gene capture sequencing has been an effective method for screening multiple candidate genes simultaneously. This method, however, has been rarely reported to be used with dystonia patients.

OBJECTIVES AND METHODS

To assess the effectiveness of the targeted gene capture sequencing in dystonia, we performed custom target gene capture followed by next-generation sequencing in dystonia patients from China. Sanger sequencing was utilized to substantiate the findings. The effects of identified variants were classified according to the American College of Medical Genetics and Genomics (ACMG) standards and guidelines.

RESULTS

A total of 65 patients (34 female and 31 male) were recruited in this study. The mean age at onset was 22.7 ± 13.3 years ranging from 2 to 59 years. According to ACMG standards and guidelines, of 65 patients, 12 were identified with pathogenic variants (12/65, 18.5%) in gene TOR1A, PANK2 or ATP1A3, and another four were identified with likely-pathogenic variants (4/65, 6.2%) in gene PRRT2, GCH1 or THAP1. In total, 24.6% of patients in this cohort were detected to have a genetic cause of dystonia. Another four patients (4/65, 6.2%) were identified with variants which were considered to be VUS (Variants of Uncertain Significance) in gene SGCE, TH, ANO3 and ATP1A3 respectively. The most common detected gene was TOR1A, known to be causative for DYT1 (8/65, 12.3%).

CONCLUSIONS

The study demonstrates that targeted gene capture sequencing is an effective tool for identifying the genetic cause of heterogeneous dystonia patients.

Urinary neopterin and phenylalanine loading test as tools for the biochemical diagnosis of segawa disease

Background. The diagnosis of autosomal dominant GTP-cyclohydrolase deficiency relies on the examination of the GCH1 gene and/or pterins and neurotransmitters in CSF. The aim of the study was to assess the diagnostic value, if any, of pterins in urine and blood phenylalanine (Phe) and tyrosine (Tyr) under oral Phe loading test. Methods. We report on two new pedigrees with four symptomatic and four asymptomatic carriers whose pattern of urinary pterins and blood Phe/Tyr ratio under oral Phe loading pointed to GTP-cyclohydrolase deficiency. The study was then extended to 3 further patients and 90 controls. The diagnostic specificity and sensitivity of these metabolic markers were analysed by backwards logistic analysis. Results. Two genetic alterations segregated alternatively in Family 1 (c.631-632 del AT and c.671A > G), while exon 1 deletion was transmitted along three generations in Family 2. Neopterin and biopterin concentrations in urine clustered differently in controls under and over the age of 15. Therefore patients and controls were sub grouped according to this age. Neopterin was significantly reduced in GCH1 mutated subjects younger than 15, and both neopterin and biopterin in those older than 15. Moreover, the Phe/Tyr ratios at the second and third hour were both significantly higher in patients than in controls. Backwards logistic regression demonstrated the high diagnostic sensitivity and specificity of combined values of neopterin concentration and Phe/Tyr ratio at the second hour. Conclusions. Pterins in urine and Phe loading test are non-invasive and reliable tools for the biochemical diagnosis of GTP-cyclohydrolase deficiency.

Segawa syndrome due to mutation Q89X in the GCH1 gene: a possible founder effect in Córdoba (southern Spain)

Autosomal dominant guanosine triphosphate cyclohydrolase I deficiency is an inborn error of neurotransmitter metabolism, with a prevalence of 0.5 per million, caused by mutations/deletions in the GCH1 gene. The finding of the mutation Q89X in the GCH1 gene in 23 patients from two pedigrees in an area inhabited by a population of 800,000 prompted us to consider that our cohort may have descended from a single founder. Twelve Q89X mutation-positive cases belonging to two families and 100 unrelated control subjects from the same geographical region were studied. Six microsatellite markers located near GCH1 were analyzed to validate a possible mutation-related founder haplotype. Haplotype analysis revealed two different haplotypes for six microsatellite markers that segregated with the Q89X mutation. A common haplotype in 10 out of 12 mutation carriers studied was identified. Two subjects carried a second haplotype, most probably because of a recombination event. However, at least 186 different haplotypes were established in the control subjects. In contrast with the frequencies of 83.3% and 16.7%, respectively, found for both mutation-segregating haplotypes, the frequency of none of the control haplotypes exceeded 1.5%. Dystonia was the most frequent symptom in our series, and parkinsonism was present in five patients. The large number of Q89X mutation carriers in our community is because of a founder effect. The same mutation in GCH1 causes a wide phenotypic spectrum of clinical variability occurring in this population of affected patients.