Hypusine N
ε
-(4-amino-2-hydroxybutyl)lysine is a derivative of lysine that is formed post-translationally in the eukaryotic initiation factor 5A (eIF5A). Its occurrence at a single site in one ...cellular protein defines hypusine synthesis as one of the most specific post-translational modifications. Synthesis of hypusine involves two enzymatic steps: first, deoxyhypusine synthase (DHPS) cleaves the 4-aminobutyl moiety of spermidine and transfers it to the ε-amino group of a specific lysine residue of the eIF5A precursor protein to form an intermediate, deoxyhypusine N
ε
-(4-aminobutyl)lysine. This intermediate is subsequently hydroxylated by deoxyhypusine hydroxylase (DOHH) to form hypusine in eIF5A. eIF5A, DHPS, and DOHH are highly conserved in all eukaryotes, and both enzymes exhibit a strict specificity toward eIF5A substrates. eIF5A promotes translation elongation globally by alleviating ribosome stalling and it also facilitates translation termination. Hypusine is required for the activity of eIF5A, mammalian cell proliferation, and animal development. Homozygous knockout of any of the three genes,
Eif5a, Dhps,
or
Dohh
, leads to embryonic lethality in mice. eIF5A has been implicated in various human pathological conditions. A recent genetic study reveals that heterozygous germline
EIF5A
variants cause Faundes–Banka syndrome, a craniofacial–neurodevelopmental malformations in humans. Biallelic variants of
DHPS
were identified as the genetic basis underlying a rare inherited neurodevelopmental disorder. Furthermore, biallelic
DOHH
variants also appear to be associated with neurodevelopmental disorder. The clinical phenotypes of these patients include intellectual disability, developmental delay, seizures, microcephaly, growth impairment, and/or facial dysmorphisms. Taken together, these findings underscore the importance of eIF5A and the hypusine modification pathway in neurodevelopment in humans.
CTP:phosphoethanolamine cytidylyltransferase (ET), encoded by PCYT2, is the rate-limiting enzyme for phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway. Phosphatidylethanolamine is ...one of the most abundant membrane lipids and is particularly enriched in the brain. We identified five individuals with biallelic PCYT2 variants clinically characterized by global developmental delay with regression, spastic para- or tetraparesis, epilepsy and progressive cerebral and cerebellar atrophy. Using patient fibroblasts we demonstrated that these variants are hypomorphic, result in altered but residual ET protein levels and concomitant reduced enzyme activity without affecting mRNA levels. The significantly better survival of hypomorphic CRISPR-Cas9 generated pcyt2 zebrafish knockout compared to a complete knockout, in conjunction with previously described data on the Pcyt2 mouse model, indicates that complete loss of ET function may be incompatible with life in vertebrates. Lipidomic analysis revealed profound lipid abnormalities in patient fibroblasts impacting both neutral etherlipid and etherphospholipid metabolism. Plasma lipidomics studies also identified changes in etherlipids that have the potential to be used as biomarkers for ET deficiency. In conclusion, our data establish PCYT2 as a disease gene for a new complex hereditary spastic paraplegia and confirm that etherlipid homeostasis is important for the development and function of the brain.
The structure of proline prevents it from adopting an optimal position for rapid protein synthesis. Poly-proline-tract (PPT) associated ribosomal stalling is resolved by highly conserved eIF5A, the ...only protein to contain the amino acid hypusine. We show that de novo heterozygous EIF5A variants cause a disorder characterized by variable combinations of developmental delay, microcephaly, micrognathia and dysmorphism. Yeast growth assays, polysome profiling, total/hypusinated eIF5A levels and PPT-reporters studies reveal that the variants impair eIF5A function, reduce eIF5A-ribosome interactions and impair the synthesis of PPT-containing proteins. Supplementation with 1 mM spermidine partially corrects the yeast growth defects, improves the polysome profiles and restores expression of PPT reporters. In zebrafish, knockdown eif5a partly recapitulates the human phenotype that can be rescued with 1 µM spermidine supplementation. In summary, we uncover the role of eIF5A in human development and disease, demonstrate the mechanistic complexity of EIF5A-related disorder and raise possibilities for its treatment.
Kabuki syndrome (KS) is a clinically recognisable syndrome in which 70% of patients have a pathogenic variant in
or
. Understanding the function of these genes opens the door to targeted therapies. ...The purpose of this report is to propose diagnostic criteria for KS, particularly when molecular genetic testing is equivocal.
An international group of experts created consensus diagnostic criteria for KS. Systematic PubMed searches returned 70 peer-reviewed publications in which at least one individual with molecularly confirmed KS was reported. The clinical features of individuals with known mutations were reviewed.
The authors propose that a definitive diagnosis can be made in an individual of any age with a history of infantile hypotonia, developmental delay and/or intellectual disability, and one or both of the following major criteria: (1) a pathogenic or likely pathogenic variant in
or
; and (2) typical dysmorphic features (defined below) at some point of life. Typical dysmorphic features include long palpebral fissures with eversion of the lateral third of the lower eyelid and two or more of the following: (1) arched and broad eyebrows with the lateral third displaying notching or sparseness; (2) short columella with depressed nasal tip; (3) large, prominent or cupped ears; and (4) persistent fingertip pads. Further criteria for a probable and possible diagnosis, including a table of suggestive clinical features, are presented.
As targeted therapies for KS are being developed, it is important to be able to make the correct diagnosis, either with or without molecular genetic confirmation.
Determining the clinical significance of germline and somatic KMT2D missense variants (MVs) in Kabuki syndrome (KS) and cancers can be challenging. We analysed 1920 distinct KMT2D MVs that included ...1535 germline MVs in controls (Control-MVs), 584 somatic MVs in cancers (Cancer-MVs) and 201 MV in individuals with KS (KS-MVs). The proportion of MVs likely to affect splicing was significantly higher for Cancer-MVs and KS-MVs than in Control-MVs (p = 0.000018). Our analysis identified significant clustering of Cancer-MVs and KS-MVs in the PHD#3 and #4, RING#4 and SET domains. Areas of enrichment restricted to just Cancer-MVs (FYR-C and between amino acids 3043-3248) or KS-MVs (coiled-coil#5, FYR-N and between amino acids 4995-5090) were also found. Cancer-MVs and KS-MVs tended to affect more conserved residues (lower BLOSUM scores, p < 0.001 and p = 0.007). KS-MVs are more likely to increase the energy for protein folding (higher ELASPIC ∆∆G scores, p = 0.03). Cancer-MVs are more likely to disrupt protein interactions (higher StructMAn scores, p = 0.019). We reclassify several presumed pathogenic MVs as benign or as variants of uncertain significance. We raise the possibility of as yet unrecognised 'non-KS' phenotype(s) associated with some germline pathogenic KMT2D MVs. Overall, this work provides insights into the disease mechanism of KMT2D variants and can be extended to other genes, mutations in which also cause developmental syndromes and cancer.
The G6PC3 gene encodes the ubiquitously expressed glucose-6-phosphatase enzyme (G-6-Pase β or G-6-Pase 3 or G6PC3). Bi-allelic G6PC3 mutations cause a multi-system autosomal recessive disorder of ...G6PC3 deficiency (also called severe congenital neutropenia type 4, MIM 612541). To date, at least 57 patients with G6PC3 deficiency have been described in the literature.G6PC3 deficiency is characterized by severe congenital neutropenia, recurrent bacterial infections, intermittent thrombocytopenia in many patients, a prominent superficial venous pattern and a high incidence of congenital cardiac defects and uro-genital anomalies. The phenotypic spectrum of the condition is wide and includes rare manifestations such as maturation arrest of the myeloid lineage, a normocellular bone marrow, myelokathexis, lymphopaenia, thymic hypoplasia, inflammatory bowel disease, primary pulmonary hypertension, endocrine abnormalities, growth retardation, minor facial dysmorphism, skeletal and integument anomalies amongst others. Dursun syndrome is part of this extended spectrum. G6PC3 deficiency can also result in isolated non-syndromic severe neutropenia. G6PC3 mutations in result in reduced enzyme activity, endoplasmic reticulum stress response, increased rates of apoptosis of affected cells and dysfunction of neutrophil activity.In this review we demonstrate that loss of function in missense G6PC3 mutations likely results from decreased enzyme stability. The condition can be diagnosed by sequencing the G6PC3 gene. A number of G6PC3 founder mutations are known in various populations and a possible genotype-phenotype relationship also exists. G6PC3 deficiency should be considered as part of the differential diagnoses in any patient with unexplained congenital neutropenia.Treatment with G-CSF leads to improvement in neutrophil numbers, prevents infections and improves quality of life. Mildly affected patients can be managed with prophylactic antibiotics. Untreated G6PC3 deficiency can be fatal. Echocardiogram, renal and pelvic ultrasound scans should be performed in all cases of suspected or confirmed G6PC3 deficiency. Routine assessment should include biochemical profile, growth profile and monitoring for development of varicose veins or venous ulcers.
Germline pathogenic variants in chromatin-modifying enzymes are a common cause of pediatric developmental disorders. These enzymes catalyze reactions that regulate epigenetic inheritance via histone ...post-translational modifications and DNA methylation. Cytosine methylation (5-methylcytosine 5mC) of DNA is the quintessential epigenetic mark, yet no human Mendelian disorder of DNA demethylation has yet been delineated. Here, we describe in detail a Mendelian disorder caused by the disruption of DNA demethylation. TET3 is a methylcytosine dioxygenase that initiates DNA demethylation during early zygote formation, embryogenesis, and neuronal differentiation and is intolerant to haploinsufficiency in mice and humans. We identify and characterize 11 cases of human TET3 deficiency in eight families with the common phenotypic features of intellectual disability and/or global developmental delay; hypotonia; autistic traits; movement disorders; growth abnormalities; and facial dysmorphism. Mono-allelic frameshift and nonsense variants in TET3 occur throughout the coding region. Mono-allelic and bi-allelic missense variants localize to conserved residues; all but one such variant occur within the catalytic domain, and most display hypomorphic function in an assay of catalytic activity. TET3 deficiency and other Mendelian disorders of the epigenetic machinery show substantial phenotypic overlap, including features of intellectual disability and abnormal growth, underscoring shared disease mechanisms.
Introduction: Although unmyelinated nerve fibers are affected in Charcot–Marie–Tooth type 1A (CMT1A) disease, they have not been studied in detail due to the invasive nature of the techniques needed ...to study them. We established alterations in C‐fiber bundles of the cornea in patients with CMT1A using non‐invasive corneal confocal microscopy (CCM). Methods: Twelve patients with CMT1A and 12 healthy control subjects underwent assessment of neuropathic symptoms and deficits, electrophysiology, quantitative sensory testing, corneal sensitivity, and corneal confocal microscopy. Results: Corneal sensitivity, corneal nerve fiber density, corneal nerve branch density, corneal nerve fiber length, and corneal nerve fiber tortuosity were significantly reduced in CMT1A patients compared with controls. There was a significant correlation between corneal sensation and CCM parameters with the severity of painful neuropathic symptoms, cold and warm thresholds, and median nerve CMAP amplitude. Conclusions: CCM demonstrates significant damage to C‐fiber bundles, which relates to some measures of neuropathy in CMT1A patients. Muscle Nerve, 2012
Variants in the chromodomain helicase DNA‐binding protein 8 (CHD8) have been associated with intellectual disability (ID), autism spectrum disorders (ASDs) and overgrowth and CHD8 is one of the ...causative genes for OGID (overgrowth and ID). We investigated 25 individuals with CHD8 protein truncating variants (PTVs), including 10 previously unreported patients and found a male to female ratio of 2.7:1 (19:7) and a pattern of common features: macrocephaly (62.5%), tall stature (47%), developmental delay and/or intellectual disability (81%), ASDs (84%), sleep difficulties (50%), gastrointestinal problems (40%), and distinct facial features. Most of the individuals in this cohort had moderate‐to‐severe ID, some had regression of speech (37%), seizures (27%) and hypotonia (27%) and two individuals were non‐ambulant. Our study shows that haploinsufficiency of CHD8 is associated with a distinctive OGID syndrome with pronounced autistic traits and supports a sex‐dependent penetrance of CHD8 PTVs in humans.
Variants in the chromodomain helicase DNA‐binding protein 8 (CHD8) have been associated with intellectual disability (ID), autism spectrum disorders (ASD) and overgrowth and CHD8 is one of the causative genes for OGID (overgrowth and ID). We investigated 25 individuals with CHD8 protein truncating variants (PTVs), including 10 previously unreported patients and found a male to female ratio of 2.7:1 (19:7) and a pattern of common features: macrocephaly (62.5%), tall stature (47%), developmental delay and/or intellectual disability (DD/ID, 81%), autism spectrum disorders (ASD, 84%), sleep difficulties (50%), gastrointestinal problems (40%), and distinct facial features. Most of the individuals in this cohort had moderate‐to‐severe intellectual disability, some had regression of speech (37%), seizures (27%) and hypotonia (27%) and two individuals were also non‐ambulant. Our study shows that haploinsufficiency of CHD8 is associated with a distinctive OGID syndrome with pronounced autistic traits and supports a sex‐dependent penetrance of CHD8 PTVs in humans.
ERBB4 encodes the tyrosine kinase receptor HER4, a critical regulator of normal cell function and neurodevelopmental processes in the brain. One of the key ligands of HER4 is neureglin-1 (NRG1), and ...the HER4-NRG1 signalling pathway is essential in neural crest cell migration, and neuronal differentiation. Pharmacological inactivation of HER4 has been shown to hasten the progression of epileptogenesis in rodent models, and heterozygous ERBB4 null mice are shown to have cognitive deficits and delayed motor development. Thus far there is only a single case report in the literature of a heterozygous ERBB4 deletion in a patient with intellectual disability (ID). We identified nine subjects from five unrelated families with chromosome 2q34 deletions, resulting in heterozygous intragenic loss of multiple exons of ERBB4, associated with either non-syndromic ID or generalised epilepsy. In one family, the deletion segregated with ID in five affected relatives. Overall, this case series further supports that haploinsufficiency of ERBB4 leads to non-syndromic intellectual disability or epilepsy.
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