Non-random X chromosome inactivation in Aicardi syndrome

Diverse childhood neurologic disorders and outcomes following fetal neurologic consultation

Fetal neurology encompasses the full spectrum of neonatal and child neurology presentations, with complex additional layers of diagnostic and prognostic challenges unique to the specific prenatal consultation. Diverse genetic and acquired etiologies with a range of potential outcomes may be encountered. Three clinical case presentations are discussed that highlight how postnatal phenotyping and longitudinal follow-up are essential to address the uncertainties that arise in utero, after birth, and in childhood, as well as to provide continuity of care.

A case of Aicardi syndrome associated with duplication event of Xp22 including SHOX

BACKGROUND

Aicardi syndrome is a neurodevelopmental disorder characterized by a triad of partial or complete agenesis of the corpus callosum, infantile spasms, and pathognomonic chorioretinal lacunae.

METHODS

Examination, multimodal imaging, and genetic testing were used to guide diagnosis.

RESULTS

We report a case of a pediatric patient who was initially diagnosed with refractory infantile spasms. The patient was unresponsive to conventional antiepileptic therapy, and genetic testing with whole exome and mitochondrial genome sequencing could not identify the underlying cause, so vigabatrin was initiated. The ophthalmic examination under anesthesia for vigabatrin toxicity screening revealed chorioretinal atrophy in the retinal periphery of both eyes, with two 3-disc diameter chorioretinal lacunae superotemporal and inferonasal to the optic nerve in the left eye. Given the neuroimaging findings of corpus callosum hypoplasia with polymicrogyria and ocular findings, the patient was diagnosed with Aicardi syndrome. Genetic testing revealed a novel duplication event at the Xp22 locus.

CONCLUSIONS

Aicardi syndrome, albeit a rare condition, should always be considered in the differential diagnosis when investigating a female child with refractory seizures in early childhood. Genetic testing may help further our understanding of AIS and the search for a genetic etiology.

Aicardi Syndrome Is a Genetically Heterogeneous Disorder

Aicardi Syndrome (AIC) is a rare neurodevelopmental disorder recognized by the classical triad of agenesis of the corpus callosum, chorioretinal lacunae and infantile epileptic spasms syndrome. The diagnostic criteria of AIC were revised in 2005 to include additional phenotypes that are frequently observed in this patient group. AIC has been traditionally considered as X-linked and male lethal because it almost exclusively affects females. Despite numerous genetic and genomic investigations on AIC, a unifying X-linked cause has not been identified. Here, we performed exome and genome sequencing of 10 females with AIC or suspected AIC based on current criteria. We identified a unique de novo variant, each in different genes: KMT2B, SLF1, SMARCB1, SZT2 and WNT8B, in five of these females. Notably, genomic analyses of coding and non-coding single nucleotide variants, short tandem repeats and structural variation highlighted a distinct lack of X-linked candidate genes. We assessed the likely pathogenicity of our candidate autosomal variants using the TOPflash assay for WNT8B and morpholino knockdown in zebrafish (Danio rerio) embryos for other candidates. We show expression of Wnt8b and Slf1 are restricted to clinically relevant cortical tissues during mouse development. Our findings suggest that AIC is genetically heterogeneous with implicated genes converging on molecular pathways central to cortical development.

Genetic heterogeneity in corpus callosum agenesis

The corpus callosum is the largest white matter structure connecting the two cerebral hemispheres. Agenesis of the corpus callosum (ACC), complete or partial, is one of the most common cerebral malformations in humans with a reported incidence ranging between 1.8 per 10,000 livebirths to 230–600 per 10,000 in children and its presence is associated with neurodevelopmental disability. ACC may occur as an isolated anomaly or as a component of a complex disorder, caused by genetic changes, teratogenic exposures or vascular factors. Genetic causes are complex and include complete or partial chromosomal anomalies, autosomal dominant, autosomal recessive or X-linked monogenic disorders, which can be either de novo or inherited. The extreme genetic heterogeneity, illustrated by the large number of syndromes associated with ACC, highlight the underlying complexity of corpus callosum development. ACC is associated with a wide spectrum of clinical manifestations ranging from asymptomatic to neonatal death. The most common features are epilepsy, motor impairment and intellectual disability. The understanding of the genetic heterogeneity of ACC may be essential for the diagnosis, developing early intervention strategies, and informed family planning. This review summarizes our current understanding of the genetic heterogeneity in ACC and discusses latest discoveries.

3D facial morphometry in Italian patients affected by Aicardi syndrome

Aicardi syndrome (AIC) is a rare congenital neurodevelopmental disorder of unknown etiology, that affects almost exclusively females, originally characterized by corpus callosum agenesis, chorioretinal lacunae, and infantile spasms. The current diagnostic criteria also include qualitative facial features (prominent premaxilla, upturned nasal tip, decreased nasal bridge angle, sparse lateral eyebrows, and microphthalmia) that still need quantification. A three-dimensional (3D) photogrammetric assessment of 11 Italian females, age 7-32 years, who satisfied AIC criteria, was performed. Linear distances and angles were computed from soft-tissue facial landmarks coordinates. The z-score values were calculated using data of 850 healthy reference females matched for age and compared by Mann-Whitney test (p < .01). Patients showed a shorter philtrum and right side orbital height (mean z-scores: -1.7, -0.9), shorter superior, middle, and inferior facial depths (mean z-scores: -1.3, -2.2, -2.3), and a smaller length of mandibular ramus (mean z-score: -2.1); conversely, they showed larger nasal and lower facial widths, and lower facial convexity (mean z-scores: 1.7, 1.4, 2.4). The inclinations of the orbit versus the true horizontal were increased bilaterally (mean z-scores: 1.8, 1.1). Some common facial abnormalities were quantified in AIC patients using a noninvasive instrument. They may help clinicians in performing a definite AIC diagnosis in atypical or doubt cases.

Aicardi syndrome, an unsolved mystery: Review of diagnostic features, previous attempts, and future opportunities for genetic examination

Aicardi syndrome is a rare, severe neurodevelopmental disorder classically characterized by the triad of infantile spasms, central chorioretinal lacunae, and agenesis of the corpus callosum. Aicardi syndrome only affects females, with the exception of a few males with a 47, XXY chromosome constitution. All cases are de novo and the only cases of definitive recurrence in families are in identical twins. It is now recognized that individuals with Aicardi syndrome commonly exhibit a variety of other neuronal migration defects, eye anomalies, and other somatic features, including skin, skeletal, and craniofacial systems. The etiology of Aicardi syndrome remains unknown despite an international effort exploring different genetic mechanisms. Although various technologies examining candidate genes, copy number variation, skewing of X-chromosome inactivation, and whole-exome sequences have been explored, no strong genetic candidates have been identified to date. New technologies that can detect low-level mosaicism and balanced rearrangements, as well as platforms examining changes at the DNA and chromatin level affecting regulatory regions are all potential avenues for future studies that may one day solve the mystery of the etiology of Aicardi syndrome.

Skewed X-chromosome inactivation and XIST locus methylation levels do not contribute to the lower prevalence of Parkinson's disease in females

Parkinson's disease (PD) is a degenerative disorder of the nervous system and the cause of the majority of sporadic cases is unknown. Females are relatively protected from PD as compared with males and linkage studies suggested a PD susceptibility locus on the X chromosome. To determine a putative association of skewed X-chromosome inactivation (XCI) and PD, we examined XCI patterns using a human androgen receptor gene-based assay (HUMARA) and did not identify any association of skewed or random X inactivation with clinical heterogeneity among female PD patients. In addition, we sought to determine methylation-specific changes at the X-inactive specific transcript (XIST) locus, which is known to be responsible for initiating X inactivation. We observed a trend towards hypomethylation in the gene body region of the XIST locus in PD females which did not reach significance. Furthermore, we extended our analysis of DNA methylation across the entire X-chromosome which revealed no methylation-specific differences between PD females and healthy controls. Thus, we propose that skewed XCI and methylation levels on the entire X chromosome did not reveal changes which could account for the decreased PD susceptibility in females or suitable to use as a biomarker.

Independent variant analysis of TEAD1 and OCEL1 in 38 Aicardi syndrome patients

Background

Aicardi syndrome is a severe neurodevelopmental disorder characterized by infantile spasms, typical chorioretinal lacunae, agenesis of the corpus callosum, and other neuronal migration defects. It has been reported recently that de novo variants in TEAD1 and OCEL1 each may cause Aicardi syndrome in a single individual of a small cohort of females with this clinical diagnosis. These data were interpreted to suggest that the clinical diagnosis of Aicardi syndrome may be genetically heterogeneous.

Methods

To investigate this further, we sequenced TEAD1 and OCEL1 coding regions using DNA from 38 clinically well‐characterized girls with Aicardi syndrome.

Results

We did not detect the previously reported or any other deleterious variants in any of the analyzed samples.

Conclusions

This suggests that the published variants represent either an extremely rare cause of Aicardi syndrome or an incidental finding.

X inactivation and reactivation in X-linked diseases

X chromosome inactivation (XCI) is the phenomenon by which mammals compensate for dosage of X-linked genes in females (XX) versus males (XY). XCI patterns can be random or show extreme skewing, and can modify the mode of inheritance of X-driven phenotypes, which contributes to the variability of human pathologies. Recent findings have shown reversibility of the XCI process, which has opened new avenues in the approaches used for the treatment of X-linked diseases.

Aicardi syndrome in a 47 XXY male - a variable developmental phenotype?

BACKGROUND

Aicardi syndrome (AS) is a rare neurodevelopmental disorder characterized by the triad of corpus callosum agenesis, chorioretinal lacunae, and infantile spasms. Most patients with AS also have intractable epilepsy, moderate to severe learning disability, and a reduced life expectancy. An X-linked dominant inheritance caused by de novo mutations pattern, lethal in males, is postulated, but the gene has not yet been isolated. There are three case reports of 47 XXY males with classic features of AS who all had severe developmental disability.

CASE REPORT

We report a case of a 3.5-year old 47 XXY male with the classic triad of Aicardi syndrome but with good seizure control and mild learning disability.

Ophthalmologic findings in Aicardi syndrome

BACKGROUND

Aicardi syndrome is a rare X-linked disorder that has been characterized classically by agenesis of the corpus callosum, seizures, and the finding of chorioretinal lacunae. This triad has been augmented more recently by central nervous system and ocular findings. The goal of this study was to determine how frequently other ophthalmologic findings are associated with Aicardi syndrome.

METHODS

A single ophthalmologist recorded the ocular and adnexal findings of 40 girls with this disorder at the annual meeting of an Aicardi syndrome family support group. For each subject, the examiner performed facial anthropometrics, portable biomicroscopy, and, where feasible, indirect ophthalmoscopy.

RESULTS

The most common findings were chorioretinal lacunae in 66 (88%) of 75 eyes and optic nerve abnormalities in 61 (81%) of 75 eyes. Other less common findings included persistent pupillary membrane in 4 (5%) of 79 eyes and anterior synechiae in 1 of 79 eyes (1%).

CONCLUSIONS

Although the ophthalmic hallmark and defining feature of Aicardi syndrome is the cluster of distinctive chorioretinal lacunae surrounding the optic nerve(s), the spectrum of ocular, papillary, and retinal anomalies varies widely, from nearly normal to dysplasia of the optic nerve and to severe microphthalmos.

Laterality of brain and ocular lesions in Aicardi syndrome

This study reports a large case series of children with Aicardi syndrome. A new severity scoring system is established to assess sidedness of ocular and brain lesions. Thirty-five children were recruited from Aicardi syndrome family conferences. All children received dilated ophthalmologic examinations, and brain magnetic resonance images (MRIs) were reviewed. Ocular and brain MRI Aicardi lesion severity scores were devised. A linear mixed model was used to compare each side for the ocular and brain MRI severity scores of Aicardi-associated disease. Twenty-six children met the inclusion criteria for the study. All subjects were female, ages 3 months to 19 years. Rates per child of optic nerve coloboma, severe lacunae, and microphthalmos in one or both eyes (among those with complete fundus examinations available) were 10/24 (42%), 8/22 (36%), and 7/26 (27%), respectively. Ocular and brain MRI asymmetry was found in 18% (4/22) and 58% (15/26) of subjects, respectively, with more right-sided brain lesions than left-sided ones (V = 52, P = 0.028). A significant correlation between sidedness of brain disease and microphthalmos was noted (T = 2.54, P = 0.02). This study substantiates the range and severity of Aicardi syndrome-associated ophthalmologic and brain MRI lesions from prior smaller case series.

A genome-wide screen for copy number alterations in Aicardi syndrome

Aicardi syndrome is a severe neurodevelopmental disorder that affects females or rarely males with a 47,XXY karyotype. Therefore, it is thought to be caused by heterozygous defects in an essential X-linked gene or by defects in an autosomal gene with sex-limited expression. Because all reported cases are sporadic with one exception, traditional linkage analysis to identify the mutant gene is not possible, and the de novo mutation rate must be high. As an alternative approach to localize the mutant gene, we screened the DNA of 38 girls with Aicardi syndrome by high-resolution, genome-wide array comparative genomic hybridization for copy number gains and losses. We found 110 copy number variants (CNVs), 97 of which are known, presumably polymorphic, CNVs; 8 have been seen before in unrelated studies in unaffected individuals. Four previously unseen CNVs on autosomes were each inherited from a healthy parent. One subject with Aicardi syndrome had a de novo loss of X-linked copy number in a region without known genes. Detailed analysis of this and flanking regions did not reveal CNVs or mutations in annotated genes in other affected subjects. We conclude that, in this study population of 38 subjects, Aicardi syndrome is not caused by CNVs detectable with the high-resolution array platform that was used.