We describe the molecular basis of a distinctive syndrome characterized by infantile stress-induced episodic weakness, ataxia, and sensorineural hearing loss, with permanent areflexia and optic nerve pallor. Whole exome sequencing identified a deleterious heterozygous c.2452 G>A, p.(E818K) variant in the ATP1A3 gene and structural analysis predicted its protein-destabilizing effect. This variant has not been reported in context with rapid-onset dystonia parkinsonism and alternating hemiplegia of childhood, the 2 main diseases associated with ATP1A3. The clinical presentation in the family described here differs categorically from these diseases in age of onset, clinical course, cerebellar over extrapyramidal movement disorder predominance, and peripheral nervous system involvement. While this paper was in review, a highly resembling phenotype was reported in additional patients carrying the same c.2452 G>A variant. Our findings substantiate this variant as the cause of a unique inherited autosomal dominant neurologic syndrome that constitutes a third allelic disease of the ATP1A3 gene.

Bi‐allelic TECPR2 variants have been associated with a complex syndrome with features of both a neurodevelopmental and neurodegenerative disorder. Here, we provide a comprehensive clinical description and variant interpretation framework for this genetic locus. Through an international collaboration, we identified 17 individuals from 15 families with bi‐allelic TECPR2‐variants. We systemically reviewed clinical and molecular data from this cohort and 11 cases previously reported. Phenotypes were standardized using Human Phenotype Ontology terms. A cross‐sectional analysis revealed global developmental delay/intellectual disability, muscular hypotonia, ataxia, hyporeflexia, respiratory infections and central/nocturnal hypopnea as core manifestations. A review of brain MRI scans demonstrated a thin corpus callosum in 52%. We evaluated 17 distinct variants. Missense variants in TECPR2 are predominantly located in the N‐ and C‐terminal regions containing β‐propeller repeats. Despite constituting nearly half of disease associated TECPR2 variants, classifying missense variants as (likely) pathogenic according to ACMG criteria remains challenging. We estimate a pathogenic variant carrier frequency of 1/1,221 in the general and 1/155 in the Jewish Ashkenazi populations. Based on clinical, neuroimaging and genetic data, we provide recommendations for variant reporting, clinical assessment, and surveillance/treatment of individuals with TECPR2‐associated disorder. This sets the stage for future prospective natural history studies.

Peroxisomal D-bifunctional protein (DBP) deficiency is an autosomal recessive disorder historically described as a Zellweger-like syndrome comprising neonatal seizures, retinopathy, hearing loss, dysmorphic features, and other complications. The HSD17B4 gene encodes DBP which is essential for oxidation of peroxisomal substrates. We describe 4 patients - 2 unrelated female girls and 2 monozygotic twin sisters - with DBP deficiency and phenotypic diversity. Patient 1 presented neonatally with hypotonia and seizures, and later on developed global developmental delay and regression, sensorineural hearing loss, nystagmus and cortical blindness. The brain MRI demonstrated bilateral peri-sylvian polymicrogyria. Whole exome sequencing revealed 2 mutations in the HSD17B4 gene (c.752G>A, p.(Arg251Gln); c.868 + 1delG). Patient 2 presented with hypotonia, motor delay, and sensorineural hearing loss in infancy, considerable developmental regression during her fourth year, nystagmus, and peripheral neuropathy. Brain MRI demonstrated cerebellar atrophy and abnormal basal ganglia and white matter signal, which appeared after the age of two years. Whole exome sequencing revealed 2 mutations in the HSD17B4 gene (c.14 T>G, p.(Leu5Arg); c.752G>A, p.(Arg251Gln)). Patients 3 and 4, two female monozygotic twins, presented with hypotonia, developmental delay, and macrocephaly from birth, and later on also sensorineural hearing loss, infantile spasms and hypsarrhythmia, and adrenal insufficiency. Brain MRI demonstrated delayed myelination, and an assay of peroxisomal beta oxidation suggested DBP deficiency. Sequencing of the HSD17B4 gene revealed the same 2 mutations as in patient 1. We describe 4 patients with variable and diverse clinical picture of DBP deficiency and particularly emphasize the clinical, biochemical, and neuroimaging characteristics. Interestingly, the clinical phenotype varied even between patients with the exact two mutations in the HSD17B4 gene. In addition, in two of the three patients in whom levels of VLCFA including phytanic acid were measured, the levels were within normal limits. This is expanding further the clinical spectrum of this disorder, which should be considered in the differential diagnosis of every patient with hypotonia and developmental delay especially if accompanied by polymicrogyria, seizures, sensorineural hearing loss, or adrenal insufficiency regardless of their VLCFA profile.

Pontocerebellar hypoplasia (PCH) represents a group of disorders characterized by cerebellum and pons hypoplasia, variable cerebral involvement, microcephaly, severe global developmental delay (GDD), and seizures. We sought the genetic cause of PCH in two siblings. Genetic workup was performed by whole-exome sequencing followed by Sanger validation. Morpholino-knockdown zebrafish embryos with human wild-type gene rescue were used to assess cerebellar development and motor function. Transfected mouse hippocampal cultures and electroporated mouse embryos were employed to assess functional effects on neuronal morphology and development. Both patients presented with profound GDD, severe microcephaly, cataracts, and variably seizures. Their MRIs demonstrated marked cerebellar and pontine hypoplasia. Both were homozygous for a c.416T > C, p.(Leu139Pro) MED29 variant which was predicted to be pathogenic. Locomotion and cerebellar GABAergic neurons development were both impaired in MED29 Morpholino-knockdown zebrafish and rescued by human wild-type gene expression. ShRNA-knockdown of MED29 in mouse hippocampal neurons decreased neurite length and arborization in vitro, and caused defective embryonic neuronal migration in vivo. Overexpression of MED29 p.(Leu139Pro) was consistent with a loss-of-function. Taken together, the Mediator complex regulates transcription processes, and defects in particular subunits are associated with distinct neurodevelopmental phenotypes involving PCH. We conclude that MED29 is a novel risk gene for PCH.