Expanding the clinical phenotype of the ultra-rare Skraban-Deardorff syndrome: Two novel individuals with WDR26 loss-of-function variants and a literature review

Non-canonical substrate recognition by the human WDR26-CTLH E3 ligase regulates prodrug metabolism

The yeast glucose-induced degradation-deficient (GID) E3 ubiquitin ligase forms a suite of complexes with interchangeable receptors that selectively recruit N-terminal degron motifs of metabolic enzyme substrates. The orthologous higher eukaryotic C-terminal to LisH (CTLH) E3 complex has been proposed to also recognize substrates through an alternative subunit, WDR26, which promotes the formation of supramolecular CTLH E3 assemblies. Here, we discover that human WDR26 binds the metabolic enzyme nicotinamide/nicotinic-acid-mononucleotide-adenylyltransferase 1 (NMNAT1) and mediates its CTLH E3-dependent ubiquitylation independently of canonical GID/CTLH E3-family substrate receptors. The CTLH subunit YPEL5 inhibits NMNAT1 ubiquitylation and cellular turnover by WDR26-CTLH E3, thereby affecting NMNAT1-mediated metabolic activation and cytotoxicity of the prodrug tiazofurin. Cryoelectron microscopy (cryo-EM) structures of NMNAT1- and YPEL5-bound WDR26-CTLH E3 complexes reveal an internal basic degron motif of NMNAT1 essential for targeting by WDR26-CTLH E3 and degron mimicry by YPEL5's N terminus antagonizing substrate binding. Thus, our data provide a mechanistic understanding of how YPEL5-WDR26-CTLH E3 acts as a modulator of NMNAT1-dependent metabolism.

Skraban-Deardorff intellectual disability syndrome-associated mutations in WDR26 impair CTLH E3 complex assembly

Patients with Skraban-Deardorff syndrome (SKDEAS), a neurodevelopmental syndrome associated with a spectrum of developmental and intellectual delays and disabilities, harbor diverse mutations in WDR26, encoding a subunit of the multiprotein CTLH E3 ubiquitin ligase complex. Structural studies revealed that homodimers of WDR26 bridge two core-CTLH E3 complexes to generate giant, hollow oval-shaped supramolecular CTLH E3 assemblies. Additionally, WDR26 mediates CTLH E3 complex binding to subunit YPEL5 and functions as substrate receptor for the transcriptional repressor HBP1. Here, we mapped SKDEAS-associated mutations on a WDR26 structural model and tested their functionality in complementation studies using genetically engineered human cells lacking CTLH E3 supramolecular assemblies. Despite the diversity of mutations, 15 of 16 tested mutants impaired at least one CTLH E3 complex function contributing to complex assembly and interactions, thus providing first mechanistic insights into SKDEAS pathology.

Consensus reporting guidelines to address gaps in descriptions of ultra-rare genetic conditions

Genome-wide sequencing and genetic matchmaker services are propelling a new era of genotype-driven ascertainment of novel genetic conditions. The degree to which reported phenotype data in discovery-focused studies address informational priorities for clinicians and families is unclear. We identified reports published from 2017 to 2021 in 10 genetics journals of novel Mendelian disorders. We adjudicated the quality and detail of the phenotype data via 46 questions pertaining to six priority domains: (I) Development, cognition, and mental health; (II) Feeding and growth; (III) Medication use and treatment history; (IV) Pain, sleep, and quality of life; (V) Adulthood; and (VI) Epilepsy. For a subset of articles, all subsequent published follow-up case descriptions were identified and assessed in a similar manner. A modified Delphi approach was used to develop consensus reporting guidelines, with input from content experts across four countries. In total, 200 of 3243 screened publications met inclusion criteria. Relevant phenotypic details across each of the 6 domains were rated superficial or deficient in >87% of papers. For example, less than 10% of publications provided details regarding neuropsychiatric diagnoses and "behavioural issues", or about the type/nature of feeding problems. Follow-up reports (n = 95) rarely contributed this additional phenotype data. In summary, phenotype information relevant to clinical management, genetic counselling, and the stated priorities of patients and families is lacking for many newly described genetic diseases. The PHELIX (PHEnotype LIsting fiX) reporting guideline checklists were developed to improve phenotype reporting in the genomic era.

Whole exome sequencing and transcriptome analysis in two unrelated patients with novel SET mutations

The human SET nuclear proto-oncogene (SET) gene is a protein-coding gene that encodes proteins that affects chromatin remodeling and gene transcription. Mutations in the SET gene have been reported to cause intellectual disability (ID) and epilepsy. In this study, we collected and analyzed clinical, genetic, and transcript features of two unrelated Chinese patients with ID. Both patients were characterized by moderate intellectual disability. Whole-exome sequencing identified two novel heterozygous mutations in the SET gene: NM_001122821.1:c.532-3 T > A and NM_001122821.1:c.3 G > C (p.0?). Additionally, RNA sequencing revealed widespread dysregulation of genes involved in NF-kB signaling and neuronal system in these two patients. To our knowledge, this is the first report of SET mutations causing ID in the Chinese population, broadening the genetic and ethnic spectrum of SET-related disorders and highlighting the importance of screening for SET gene variants.

Two Novel Variants of WDR26 in Chinese Patients with Intellectual Disability

Skraban-Deardorff syndrome is a rare autosomal dominant genetic disease caused by variants in the WDR26 gene. Here, we report two Chinese patients diagnosed with Skraban-Deardorff syndrome caused by novel de novo, heterozygous pathogenic WDR26 variants c.977delA (p. 12 N326Ifs*2) and c.1020-2A>G (p. R340Sfs*29). Their clinical features were characterized by intellectual disability (ID), developmental delay, abnormal facial features and the absence of early-onset seizure, which expands the phenotype spectrum associated with Skraban-Deardorff syndrome. By comparing our cases with current reported cases of WDR26-related intellectual disability, we suggest that developmental delay, particularly in speech, and facial features including rounded palpebral fissures, depressed nasal root, full nasal tip and abnormal gums, represent the prominent clinical phenotypes for diagnosis of Skraban-Deardorff syndrome. Together, WDR26 variants and 1q41q42 deletions should feature prominently on the differential diagnosis of ID with distinctive facial features.