Oculodentodigital Dysplasia: A Hypomyelinating Leukodystrophy with a Characteristic MRI Pattern of Brain Stem Involvement

Deep neurological phenotyping in oculo-dento-digital syndrome

OBJECTIVES

Oculodentodigital dysplasia (ODDD) is a rare autosomal dominant congenital malformation syndrome characterized by high penetrance and great phenotypic heterogeneity. Neurological manifestations are thought to occur in about one third of cases, but systematic studies are not available. We performed deep neurological phenotyping of 10 patients in one ODDD pedigree.

METHODS

Retrospective case series. We analyzed in depth the neurological phenotype of a three-generation family segregating the heterozygous c.416 T > C, p.(Ile139Thr) in GJA1. Clinical and neuroradiological features were retrospectively evaluated. Brain MRI and visual evoked potentials were performed in 8 and 6 cases, respectively.

RESULTS

Central nervous system manifestations occurred in 5 patients, the most common being isolated ataxia either in isolation or combined with spasticity. Furthermore, sphincteric disturbances (neurogenic bladder and fecal incontinence) were recognized as the first manifestation in most of the patients. Subclinical electrophysiological alteration of the optic pathway occurred in all the examined patients. Neuroimaging was significant for supratentorial hypomyelination pattern and hyperintense superior cerebellar peduncle in all examined patients.

CONCLUSION

The neurological involvement in ODDD carriers is often missed but peculiar clinical and radiological patterns can be recognized. Deep neurological phenotyping is needed to help untangle ODDD syndrome complexity and find genotype-phenotype correlations.

Dental abnormalities in rare genetic bone diseases: Literature review

Currently, over 500 rare genetic bone disorders are identified. These diseases are often accompanied by dental abnormalities, which are sometimes the first clue for an early diagnosis. However, not many dentists are sufficiently familiar with phenotypic abnormalities and treatment approaches when they encounter patients with rare diseases. Such patients often need dental treatment but have difficulties in finding a dentist who can treat them appropriately. Herein we focus on major dental phenotypes and summarize their potential causes and mechanisms, if known. We discuss representative diseases, dental treatments, and their effect on the oral health of patients and on oral health-related quality of life. This review can serve as a starting point for dentists to contribute to early diagnosis and further investigate the best treatment options for patients with rare disorders, with the goal of optimizing treatment outcomes.

Rare mosaic variant of GJA1 in a patient with a neurodevelopmental disorder

GJA1 is the causative gene for oculodentodigital dysplasia (ODDD). A novel de novo GJA1 variant, NM 000165:c263C > T [p.P88L], was identified in a mosaic state in a patient with short stature, seizures, delayed myelination, mild hearing loss, and tooth enamel hypoplasia. Although the patient exhibited severe neurodevelopmental delay, other clinical features of ODDD, including limb anomalies, were mild. This may be due to differences in the mosaic ratios in different organs.

Connexins and Pannexins: Important Players in Neurodevelopment, Neurological Diseases, and Potential Therapeutics

Cell-to-cell communication is essential for proper embryonic development and its dysfunction may lead to disease. Recent research has drawn attention to a new group of molecules called connexins (Cxs) and pannexins (Panxs). Cxs have been described for more than forty years as pivotal regulators of embryogenesis; however, the exact mechanism by which they provide this regulation has not been clearly elucidated. Consequently, Cxs and Panxs have been linked to congenital neurodegenerative diseases such as Charcot-Marie-Tooth disease and, more recently, chronic hemichannel opening has been associated with adult neurodegenerative diseases (e.g., Alzheimer's disease). Cell-to-cell communication via gap junctions formed by hexameric assemblies of Cxs, known as connexons, is believed to be a crucial component in developmental regulation. As for Panxs, despite being topologically similar to Cxs, they predominantly seem to form channels connecting the cytoplasm to the extracellular space and, despite recent research into Panx1 (Pannexin 1) expression in different regions of the brain during the embryonic phase, it has been studied to a lesser degree. When it comes to the nervous system, Cxs and Panxs play an important role in early stages of neuronal development with a wide span of action ranging from cellular migration during early stages to neuronal differentiation and system circuitry formation. In this review, we describe the most recent available evidence regarding the molecular and structural aspects of Cx and Panx channels, their role in neurodevelopment, congenital and adult neurological diseases, and finally propose how pharmacological modulation of these channels could modify the pathogenesis of some diseases.

Functional Study of TMEM163 Gene Variants Associated with Hypomyelination Leukodystrophy

Hypomyelinating leukodystrophies (HLDs) are a rare group of heterogeneously genetic disorders characterized by persistent deficit of myelin observed on magnetic resonance imaging (MRI). To identify a new disease-associated gene of HLD, trio-based whole exome sequencing was performed for unexplained patients with HLD. Functional studies were performed to confirm the phenotypic effect of candidate protein variants. Two de novo heterozygous variants, c.227T>G p.(L76R) or c.227T>C p.(L76P) in TMEM163 were identified in two unrelated HLD patients. TMEM163 protein is a zinc efflux transporter localized within the plasma membrane, lysosomes, early endosomes, and other vesicular compartments. It has not been associated with hypomyelination. Functional zinc flux assays in HeLa cells stably-expressing TMEM163 protein variants, L76R and L76P, revealed distinct attenuation or enhancement of zinc efflux, respectively. Experiments using a zebrafish model with knockdown of tmem163a and tmem163b (morphants) showed that loss of tmem163 causes dysplasia of the larvae, locomotor disability and myelin deficit. Expression of human wild type TMEM163 mRNAs in morphants rescues the phenotype, while the TMEM163 L76P and L76R mutants aggravated the condition. Moreover, poor proliferation, elevated apoptosis of oligodendrocytes, and reduced oligodendrocytes and neurons were also observed in zebrafish morphants. Our findings suggest an unappreciated role for TMEM163 protein in myelin development and add TMEM163 to a growing list of genes associated with hypomyelination leukodystrophy.

Hypomyelinating leukodystrophies - unravelling myelin biology

Hypomyelinating leukodystrophies constitute a subset of genetic white matter disorders characterized by a primary lack of myelin deposition. Most patients with severe hypomyelination present in infancy or early childhood and develop severe neurological deficits, but the clinical presentation can also be mild with onset of symptoms in adolescence or adulthood. MRI can be used to visualize the process of myelination in detail, and MRI pattern recognition can provide a clinical diagnosis in many patients. Next-generation sequencing provides a definitive diagnosis in 80-90% of patients. Genes associated with hypomyelination include those that encode structural myelin proteins but also many that encode proteins involved in RNA translation and some lysosomal proteins. The precise pathomechanisms remain to be elucidated. Improved understanding of the process of myelination, the metabolic axonal support functions of myelin and the proposed contribution of myelin to CNS plasticity provide possible explanations as to why almost all patients with hypomyelination experience slow clinical decline after a long phase of stability. In this Review, we provide an overview of the hypomyelinating leukodystrophies, the advances in our understanding of myelin biology and of the genes involved in these disorders, and the insights these advances have provided into their clinical presentations and evolution.

Hypomyelinating leukodystrophies in adults: Clinical and genetic features

BACKGROUND AND PURPOSE

Little is known about hypomyelinating leukodystrophies (HLDs) in adults. The aim of this study was to investigate HLD occurrence, clinical features, and etiology among undefined leukoencephalopathies in adulthood.

METHODS

We recruited the patients with cerebral hypomyelinating magnetic resonance imaging pattern (mild T2 hyperintensity with normal or near-normal T1 signal) from our cohort of 62 adult index cases with undefined leukoencephalopathies, reviewed their clinical features, and used a leukoencephalopathy-targeted next generation sequencing panel.

RESULTS

We identified 25/62 patients (~40%) with hypomyelination. Cardinal manifestations were spastic gait and varying degree of cognitive impairment. Etiology was determined in 44% (definite, 10/25; likely, 1/25). Specifically, we found pathogenic variants in the POLR3A (n = 2), POLR1C (n = 1), RARS1 (n = 1), and TUBB4A (n = 1) genes, which are typically associated with severe early-onset HLDs, and in the GJA1 gene (n = 1), which is associated with oculodentodigital dysplasia. Duplication of a large chromosome X region encompassing PLP1 and a pathogenic GJC2 variant were found in two patients, both females, with early-onset HLDs persisting into adulthood. Finally, we found likely pathogenic variants in PEX3 (n = 1) and PEX13 (n = 1) and potentially relevant variants of unknown significance in TBCD (n = 1), which are genes associated with severe, early-onset diseases with central hypomyelination/dysmyelination.

CONCLUSIONS

A hypomyelinating pattern characterizes a relevant number of undefined leukoencephalopathies in adulthood. A comprehensive genetic screening allows definite diagnosis in about half of patients, and demonstrates the involvement of many disease-causing genes, including genes associated with severe early-onset HLDs, and genes causing peroxisome biogenesis disorders.

Heterozygous Variants in the Mechanosensitive Ion Channel TMEM63A Result in Transient Hypomyelination during Infancy

Mechanically activated (MA) ion channels convert physical forces into electrical signals. Despite the importance of this function, the involvement of mechanosensitive ion channels in human disease is poorly understood. Here we report heterozygous missense mutations in the gene encoding the MA ion channel TMEM63A that result in an infantile disorder resembling a hypomyelinating leukodystrophy. Four unrelated individuals presented with congenital nystagmus, motor delay, and deficient myelination on serial scans in infancy, prompting the diagnosis of Pelizaeus-Merzbacher (like) disease. Genomic sequencing revealed that all four individuals carry heterozygous missense variants in the pore-forming domain of TMEM63A. These variants were confirmed to have arisen de novo in three of the four individuals. While the physiological role of TMEM63A is incompletely understood, it is highly expressed in oligodendrocytes and it has recently been shown to be a MA ion channel. Using patch clamp electrophysiology, we demonstrated that each of the modeled variants result in strongly attenuated stretch-activated currents when expressed in naive cells. Unexpectedly, the clinical evolution of all four individuals has been surprisingly favorable, with substantial improvements in neurological signs and developmental progression. In the three individuals with follow-up scans after 4 years of age, the myelin deficit had almost completely resolved. Our results suggest a previously unappreciated role for mechanosensitive ion channels in myelin development.