Biallelic HMGXB4 loss-of-function variant causes intellectual disability, developmental delay, and dysmorphic features.

Background: HMGXB4 (additionally known as HMG2L1) is a non-histone DNA-binding protein that contains a single HMG-box domain. HMGXB4 was originally described in Xenopus where it was seen to negatively regulate the Wnt/ß-catenin signaling pathway. Materials and methods: In this study, we conducted a genetic and clinical evaluation of a single family with three affected individuals suffering from intellectual disability (ID), global developmental delay (GDD) and dysmorphic facial features.Whole genome sequencing (WGS) and Sanger sequencing were performed on the affected individuals' DNA to identify genetic variations. Additionally, a reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was used to assess gene expression in both the affected and unaffected individuals in the family. Result: WGS identified a homozygous frameshift variant c.1193_1196del p. (Lys398Argfs × 25) in exon 5 of the HMGXB4 gene (OMIM 604702), which completely segregated the disease phenotype in the family. Furthermore, RT-qPCR revealed a substantial decrease in the HMGXB4 gene expression in the affected individuals as compared to the unaffected individuals of the family. Conclusions: The current study is the first evidence linking a genetic variant in the HMGXB4 gene to ID, GDD, and dysmorphic facial features. Therefore, it is possible that HMGXB4 contributes significantly to developmental milestones and may be responsible for neurodevelopmental disorders in humans.

Mutated neuron navigator 3 as a candidate gene for a rare neurodevelopmental disorder.

BACKGROUND: Neuron navigator 3 (NAV3) is characterized as one of the neuron navigator family (NAV1, NAV2, NAV3) proteins predominantly expressed in the nervous system. The NAV3-encoded protein comprises a conserved AAA and coiled-coil domains characteristic of ATPases, which are associated with different cellular activities. METHODS: We describe a Saudi proband presenting a complex recessive neurodevelopmental disorder (NDD). Whole exome sequencing (WES) followed by Sanger sequencing, 3D protein modeling and RT-qPCR was performed. RESULTS: WES revealed a bi-allelic frameshift variant (c.2604_2605delAG; p.Val870SerfsTer12) in exon 12 of the NAV3 gene. Furthermore, RT-qPCR revealed a significant decrease in the NAV3 mRNA expression in the patient sample, and 3D protein modeling revealed disruption of the overall secondary structure. CONCLUSION: For the time, we associate a bi-allelic variant in the NAV3 gene causing NDD in humans.

Mutated RAP1GDS1 causes a new syndrome of dysmorphic feature, intellectual disability & speech delay.

BACKGROUND: RAP1GDS1 (RAP1, GTP-GDP dissociation stimulator 1), also known as SmgGDS, is a guanine nucleotide exchange factor (GEF) that regulates small GTPases, including, RHOA, RAC1, and KRAS. RAP1GDS1 was shown to be highly expressed in different tissue types including the brain. However, mutations in the RAP1GDS1 gene associated with human diseases have not previously been reported. METHODS: We report on four affected individuals, presenting intellectual disability, global developmental delay (GDD), and hypotonia. The probands' DNA was subjected to whole-genome sequencing, revealing a homozygous splice acceptor site mutation in the RAP1GDS1 gene (1444-1G > A). Sanger sequencing was performed to confirm the segregation of the variant in two Saudi families. The possible aberrant splicing in the patients' RNA was investigated using RT-PCR and changes in mRNA expression of the patients were confirmed using qRT-PCR. RESULTS: The identified splice variant was found to segregate within the two families. RT-PCR showed that the mutation affected RAP1GDS1 gene splicing, resulting in the production of aberrant transcripts in the affected individuals. Quantitative gene expression analysis demonstrated that the RAP1GDS1 mRNA expression in all the probands was significantly decreased compared to that of the control, and Sanger sequencing of the probands' cDNA revealed skipping of exon 13, further strengthening the pathogenicity of this variant. CONCLUSION: We are the first to report the mutation of the RAP1GDS1 gene as a potential cause of GDD and hypotonia. However, further investigations into the molecular mechanisms involved are required to confirm the role of RAP1GDS1 gene in causing GDD and hypotonia.