Myoclonus-dystonia in 18p deletion syndrome

Prenatal Diagnosis of a De Novo Partial Trisomy 6q and Partial Monosomy 18p Associated with Cephalocele: A Case Report

A 28-year-old woman underwent amniocentesis at 18 weeks’ gestation upon detection of increased fetal nuchal fold and parietal cephalocele on the second trimester ultrasound examination. Prenatal microarray showed a de novo unbalanced translocation resulting in a gain in 6q and loss in 18p. A female infant was delivered at 38 weeks’ gestation. At birth, cephalocele and webbed neck were noted as major dysmorphic features. The case presented here shows how a combination of different genetic studies is used to accurately elucidate a chromosomal anomaly in a prenatal setting.

Diagnosis and treatment of pediatric onset isolated dystonia

Isolated dystonia refers to a genetic heterogeneous group of progressive conditions with onset of symptoms during childhood or adolescence, progressive course with frequent generalization and marked functional impairment. There are well-known monogenic forms of isolated dystonia with pediatric onset such as DYT1 and DYT6 transmitted with autosomal dominant inheritance and low penetrance. Genetic findings of the past years have widened the etiological spectrum and the phenotype. The recently discovered genes (GNAL, ANO-3, KTM2B) or variant of already known diseases, such as Ataxia-Teleangectasia, are emerging as another causes of pediatric onset dystonia, sometimes with a more complex phenotype, but their incidence is unknown and still a considerable number of cases remains genetically undetermined. Due to the severe disability of pediatric onset dystonia treatment remains unsatisfactory and still mainly based upon oral pharmacological agents. However, deep brain stimulation is now extensively applied with good to excellent results especially when patients are treated early during the course of the disease.

Novel GNAL mutation with intra-familial clinical heterogeneity: Expanding the phenotype

INTRODUCTION

Mutations in GNAL have been associated with adult-onset cranio-cervical dystonia, but a limited number of cases have been reported so far and the clinical spectrum associated with this gene still needs to be fully characterized.

METHODS

We identified an Italian family with adult-onset, dominantly-inherited dystonia whose members presented with different combinations of dystonia affecting the cervical, oro-mandibular and laryngeal regions associated with prominent tremor in some cases. Pure asymmetric upper limb dystonic tremor was present in one of the members and jerky cervical dystonia was also observed. A dedicate dystonia gene panel (Illumina) was used to screen for dystonia-associated genes and Sanger sequencing was performed to confirm results obtained and to perform segregation analysis.

RESULTS

A novel single-base mutation in GNAL exon 9 (c.628G>A; p.Asp210Asn) leading to an aminoacidic substitution was identified and confirmed by Sanger sequencing. In silico prediction programmes as well as segregation analysis confirmed its pathogenicity. Clinically, no generalization of dystonia was observed after onset and DBS lead to an excellent motor outcome in two cases.

CONCLUSION

We report a novel GNAL mutation and expand the clinical spectrum associated with mutations in this gene to comprise pure asymmetric dystonic tremor and a jerky cervical phenotype partially mimicking DYT11 positive cases.

A review of 18p deletions

Since 18p- was first described in 1963, much progress has been made in our understanding of this classic deletion condition. We have been able to establish a fairly complete picture of the phenotype when the deletion breakpoint occurs at the centromere, and we are working to establish the phenotypic effects when each gene on 18p is hemizygous. Our aim is to provide genotype-specific anticipatory guidance and recommendations to families with an 18p- diagnosis. In addition, establishing the molecular underpinnings of the condition will potentially suggest targets for molecular treatments. Thus, the next step is to establish the precise effects of specific gene deletions. As we look forward to deepening our understanding of 18p-, our focus will continue to be on the establishment of robust genotype-phenotype correlations and the penetrance of these phenotypes. We will continue to follow our 18p- cohort closely as they age to determine the presence or absence of some of these diagnoses, including spinocerebellar ataxia (SCA), facioscapulohumeral muscular dystrophy (FSHD), and dystonia. We will also continue to refine the critical regions for other phenotypes as we enroll additional (hopefully informative) participants into the research study and as the mechanisms of the genes in these regions are elucidated. Mouse models will also be developed to further our understanding of the effects of hemizygosity as well as to serve as models for treatment development.

Whole arm deletions of 18p: medical and developmental effects

Deletions of the short arm of chromosome 18 have been well-described in case reports. However, the utility of these descriptions in clinical practice is limited by varied and imprecise breakpoints. As we work to establish genotype-phenotype correlations for 18p-, it is critical to have accurate and complete clinical descriptions of individuals with differing breakpoints. In addition, the developmental profile of 18p- has not been well-delineated. We undertook a thorough review of the medical histories of 31 individuals with 18p- and a breakpoint in the centromeric region. We collected developmental data using mailed surveys and questionnaires. The most common findings included neonatal complications; cardiac anomalies; hypotonia; MRI abnormalities; endocrine dysfunction; strabismus; ptosis; and refractive errors. Less common features included holoprosencephaly and its microforms; hearing loss; and orthopedic anomalies. The developmental effects of the deletion appear to be less severe than reported in the literature, as average IQ scores were in the range of borderline intellectual functioning. Based on responses to standardized questionnaires, it appears this population has marked difficulty with activities of daily living, though several young adults were able to live independent of their parents. This manuscript represents the most comprehensive description of a cohort of 18p- individuals with identical breakpoints. Despite identical breakpoints, a great deal of phenotype variability remained among this population, suggesting that many of the genes on 18p- cause low-penetrance phenotypes when present in a hemizygous state. Future efforts will focus on the clinical description of individuals with more distal breakpoints and the identification of critical regions and candidate genes.

Chromosome 18p deletion syndrome presenting holoprosencephaly and premaxillary agenesis: prenatal diagnosis and aCGH characterization using uncultured amniocytes

We present prenatal diagnosis of a de novo distal 18p deletion involving 14.06Mb at 18p11.32-p11.21 by aCGH using uncultured amniocytes in a pregnancy with fetal holoprosencephaly and premaxillary agenesis. QF-PCR analysis showed that distal 18p deletion was from maternal origin. Metaphase FISH analysis confirmed haploinsufficiency of TGIF. We discuss the functions of the genes that are deleted within this region. The present case shows the usefulness of applying aCGH on uncultured amniocytes for rapid aneuploidy diagnosis in cases with prenatally detected fetal structural abnormalities.

Role of Gα(olf) in familial and sporadic adult-onset primary dystonia

The vast majority of patients with primary dystonia are adults with focal or segmental distribution of involuntary movements. Although ~10% of probands have at least one first- or second-degree relative to dystonia, large families suited for linkage analysis are exceptional. After excluding mutations in known primary dystonia genes (TOR1A, THAP1 and CIZ1), whole-exome sequencing identified a GNAL missense mutation (c.682G>T, p.V228F) in an African-American pedigree with clinical phenotypes that include cervical, laryngeal and hand-forearm dystonia. Screening of 760 subjects with familial and sporadic primary dystonia identified three Caucasian pedigrees with GNAL mutations [c.591dupA (p.R198Tfs*13); c.733C>T (p.R245*); and c.3G>A (p.M1?)]. These mutations show incomplete penetrance. Our findings corroborate those of a recent study which used whole-exome sequencing to identify missense and nonsense GNAL mutations in Caucasian pedigrees of mixed European ancestry with mainly adult-onset cervical and segmental dystonia. GNAL encodes guanine nucleotide-binding protein G(olf), subunit alpha [Gα(olf)]. Gα(olf) plays a role in olfaction, coupling D1 and A2a receptors to adenylyl cyclase, and histone H3 phosphorylation. African-American subjects harboring the p.V228F mutation exhibited microsmia. Lymphoblastoid cell lines from subjects with the p.V228F mutation showed upregulation of genes involved in cell cycle control and development. Consistent with known sites of network pathology in dystonia, immunohistochemical studies indicated that Gα(olf) is highly expressed in the striatum and cerebellar Purkinje cells, and co-localized with corticotropin-releasing hormone receptors in the latter.

Mutations in GNAL cause primary torsion dystonia

Dystonia is a movement disorder characterized by repetitive twisting muscle contractions and postures^1,2. Its molecular pathophysiology is poorly understood, in part due to limited knowledge of the genetic basis of the disorder. Only three genes for primary torsion dystonia (PTD), TOR1A (DYT1)³, THAP1 (DYT6)⁴, and CIZ1⁵ have been identified. Using exome sequencing in two PTD families we identified a novel causative gene, GNAL, with a nonsense p.S293X mutation resulting in premature stop codon in one family and a missense p.V137M mutation in the other. Screening of GNAL in 39 PTD families, revealed six additional novel mutations in this gene. Impaired function of several of the mutations was shown by bioluminescence resonance energy transfer (BRET) assays.