Delineating the genotypic and phenotypic spectrum of HECW2-related neurodevelopmental disorders

A De Novo HECW2 Variant in a Patient with Acetazolamide-Responsive Episodic Ataxia

To the best of our knowledge, this is the first case to address episodic ataxia (EA) as a possible phenotypic feature of HECW2-related disorder. This single case study describes a 26-year-old female born at term with mild intellectual disability, neonatal hypotonia, and a history of febrile seizures who presented with paroxysmal events since the age of 2. These episodes include frequent falls due to imbalance, dilated pupils, vertigo, diaphoresis, nausea, vomiting, and nystagmus. Brain imaging was normal. A prolonged electroencephalogram (EEG) revealed interictal epileptiform discharges but failed to capture her clinical events. For several years, she was treated for presumed focal seizures with preserved awareness and trialed on adequate dosing of several antiepileptic medications without improvement. After 25 years, given the more prolonged nature of her episodes and the mild interictal cerebellar signs, empiric treatment with acetazolamide was initiated for a presumed diagnosis of EA. Acetazolamide treatment led to a dramatic reduction in event frequency and severity. The initial EA genetic panel was negative. Clinical exome sequence analysis revealed a novel pathogenic de novo missense variant in the HECW2 gene [c.3829 T > C;(p.Tyr1277His)], located in the HECT domain. HECW2 variants are associated with neurodevelopmental delay, hypotonia, and epilepsy. This study expands the genetic and clinical spectrum of HECW2-related disorder and adds EA to the phenotypic spectrum in affected individuals.

A novel HECW2 variant in an infant with congenital long QT syndrome

Pathogenic variants of HECW2 have been reported in cases of neurodevelopmental disorder with hypotonia, seizures, and absent language (NDHSAL; OMIM #617268). A novel HECW2 variant (NM_001348768.2:c.4343 T > C,p.Leu1448Ser) was identified in an NDHSAL infant with severe cardiac comorbidities. The patient presented with fetal tachyarrhythmia and hydrops and was postnatally diagnosed with long QT syndrome. This study provides evidence that HECW2 pathogenic variants can cause long QT syndrome along with neurodevelopmental disorders.

First splicing variant in HECW2 with an autosomal recessive pattern of inheritance and associated with NDHSAL

We report the clinical and genetic features of a Caucasian girl who presented a severe neurodevelopmental disorder with drug-resistant epilepsy, hypotonia, severe gastro-esophageal reflux and brain MRI anomalies. WES uncovered a novel variant in homozygosis (g.197092814_197092824delinsC) in HECW2 gene that encodes the E3 ubiquitin-protein ligase HECW2. This protein induces ubiquitination and is implicated in the regulation of several important pathways involved in neurodevelopment and neurogenesis. Furthermore, de novo heterozygous missense variants in this gene have been associated with NDHSAL. The homozygous variant of our patient disrupts the splice donor site of intron 22 and causes the elimination of exon 22 (r.3766_3917+1del) leading to an in-frame deletion of the protein (p.Leu1256_Trp1306del). Functional studies showed a two-fold increase of its RNA expression, while the protein expression level was reduced by 60%, suggesting a partial LOF mechanism of pathogenesis. Thus, this is the first patient with NDHSAL caused by an autosomal recessive splicing variant in HECW2. This article is protected by copyright. All rights reserved.

A homozygous nonsense HECW2 variant is associated with neurodevelopmental delay and intellectual disability

Intellectual disability is characterized by a significant impaired intellectual and adaptive functioning, affecting approximately 1-3% of the population, which can be caused by a variety of environmental and genetic factors. In this respect, de novo heterozygous HECW2 variants were associated recently with neurodevelopmental disorders associated to hypotonia, seizures, and absent language. HECW2 encodes an E3 ubiquitin-protein ligase that stabilizes and enhances transcriptional activity of p73, a key factor regulating proliferation, apoptosis, and neuronal differentiation, which are together essential for proper brain development. Here, using whole exome sequencing, we identified a homozygous nonsense HECW2 variant: c.736C > T; p.Arg246* in a proband from a Moroccan consanguineous family, with developmental delay, intellectual disability, hypotonia, generalized tonico-clonic seizures and a persistent tilted head. Thus this study describes the first homozygous HECW2 variant, inherited as an autosomal recessive pattern, contrasting with former reported de novo variants found in HECW2 patients.

The Roles of NEDD4 Subfamily of HECT E3 Ubiquitin Ligases in Neurodevelopment and Neurodegeneration

The ubiquitin pathway regulates the function of many proteins and controls cellular protein homeostasis. In recent years, it has attracted great interest in neurodevelopmental and neurodegenerative diseases. Here, we have presented the first review on the roles of the 9 proteins of the HECT E3 ligase NEDD4 subfamily in the development and function of neurons in the central nervous system (CNS). We discussed their regulation and their direct or indirect involvement in neurodevelopmental diseases, such as intellectual disability, and neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease or Amyotrophic Lateral Sclerosis. Further studies on the roles of these proteins, their regulation and their targets in neurons will certainly contribute to a better understanding of neuronal function and dysfunction, and will also provide interesting information for the development of therapeutics targeting them.