guanosine-triphosphate and Intellectual-Disability

Topics: Child; Guanosine Diphosphate; Guanosine Triphosphate; Humans; Intellectual Disability; Mutation; ral GTP-Binding Proteins; Signal Transduction

Mutations that alter signaling of RAS/MAPK-family proteins give rise to a group of Mendelian diseases known as RASopathies. However, among RASopathies, the matrix of genotype-phenotype relationships is still incomplete, in part because there are many RAS-related proteins and in part because the phenotypic consequences may be variable and/or pleiotropic. Here, we describe a cohort of ten cases, drawn from six clinical sites and over 16,000 sequenced probands, with de novo protein-altering variation in RALA, a RAS-like small GTPase. All probands present with speech and motor delays, and most have intellectual disability, low weight, short stature, and facial dysmorphism. The observed rate of de novo RALA variants in affected probands is significantly higher (p = 4.93 x 10(-11)) than expected from the estimated random mutation rate. Further, all de novo variants described here affect residues within the GTP/GDP-binding region of RALA; in fact, six alleles arose at only two codons, Val25 and Lys128. The affected residues are highly conserved across both RAL- and RAS-family genes, are devoid of variation in large human population datasets, and several are homologous to positions at which disease-associated variants have been observed in other GTPase genes. We directly assayed GTP hydrolysis and RALA effector-protein binding of the observed variants, and found that all but one tested variant significantly reduced both activities compared to wild-type. The one exception, S157A, reduced GTP hydrolysis but significantly increased RALA-effector binding, an observation similar to that seen for oncogenic RAS variants. These results show the power of data sharing for the interpretation and analysis of rare variation, expand the spectrum of molecular causes of developmental disability to include RALA, and provide additional insight into the pathogenesis of human disease caused by mutations in small GTPases.

Topics: Developmental Disabilities; Facies; Genotype; Guanosine Diphosphate; Guanosine Triphosphate; Humans; Intellectual Disability; Mitochondrial Proteins; Models, Molecular; Mutation; Mutation, Missense; Phenotype; Protein Conformation; Protein Interaction Domains and Motifs; ral GTP-Binding Proteins; ras Proteins

The Rab GTPase family comprises ∼70 GTP-binding proteins, functioning in vesicle formation, transport and fusion. They are activated by a conformational change induced by GTP-binding, allowing interactions with downstream effectors. Here, we report five individuals with two recurrent de novo missense mutations in RAB11B; c.64G>A; p.Val22Met in three individuals and c.202G>A; p.Ala68Thr in two individuals. An overlapping neurodevelopmental phenotype, including severe intellectual disability with absent speech, epilepsy, and hypotonia was observed in all affected individuals. Additionally, visual problems, musculoskeletal abnormalities, and microcephaly were present in the majority of cases. Re-evaluation of brain MRI images of four individuals showed a shared distinct brain phenotype, consisting of abnormal white matter (severely decreased volume and abnormal signal), thin corpus callosum, cerebellar vermis hypoplasia, optic nerve hypoplasia and mild ventriculomegaly. To compare the effects of both variants with known inactive GDP- and active GTP-bound RAB11B mutants, we modeled the variants on the three-dimensional protein structure and performed subcellular localization studies. We predicted that both variants alter the GTP/GDP binding pocket and show that they both have localization patterns similar to inactive RAB11B. Evaluation of their influence on the affinity of RAB11B to a series of binary interactors, both effectors and guanine nucleotide exchange factors (GEFs), showed induction of RAB11B binding to the GEF SH3BP5, again similar to inactive RAB11B. In conclusion, we report two recurrent dominant mutations in RAB11B leading to a neurodevelopmental syndrome, likely caused by altered GDP/GTP binding that inactivate the protein and induce GEF binding and protein mislocalization.

Topics: Adolescent; Amino Acid Sequence; Binding Sites; Cerebellar Vermis; Child; Child, Preschool; Corpus Callosum; Epilepsy; Female; Gene Expression; Guanosine Diphosphate; Guanosine Triphosphate; Humans; Intellectual Disability; Magnetic Resonance Imaging; Male; Models, Molecular; Muscle Hypotonia; Mutation; Optic Nerve Diseases; Phenotype; Protein Binding; rab GTP-Binding Proteins; White Matter

RAB39B is a member of the RAB family of small GTPases that controls intracellular vesicular trafficking in a compartment-specific manner. Mutations in the RAB39B gene cause intellectual disability comorbid with autism spectrum disorder and epilepsy, but the impact of RAB39B loss of function on synaptic activity is largely unexplained. Here we show that protein interacting with C-kinase 1 (PICK1) is a downstream effector of GTP-bound RAB39B and that RAB39B-PICK1 controls trafficking from the endoplasmic reticulum to the Golgi and, hence, surface expression of GluA2, a subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). The role of AMPARs in synaptic transmission varies depending on the combination of subunits (GluA1, GluA2 and GluA3) they incorporate. RAB39B downregulation in mouse hippocampal neurons skews AMPAR composition towards non GluA2-containing Ca(2+)-permeable forms and thereby alters synaptic activity, specifically in hippocampal neurons. We posit that the resulting alteration in synaptic function underlies cognitive dysfunction in RAB39B-related disorders.

Topics: Animals; Carrier Proteins; Chlorocebus aethiops; Cognition Disorders; COS Cells; Electrophysiology; Gene Expression Regulation; Glutathione Transferase; Glycosylation; Golgi Apparatus; Green Fluorescent Proteins; Guanosine Triphosphate; HEK293 Cells; Hippocampus; Humans; Intellectual Disability; Mice; Mutation; Neurons; Nuclear Proteins; Protein Structure, Tertiary; Protein Transport; rab GTP-Binding Proteins; Receptors, AMPA; Synapses; Synaptic Transmission; Two-Hybrid System Techniques

RAB18, RAB3GAP1, RAB3GAP2 and TBC1D20 are each mutated in Warburg Micro syndrome, a rare autosomal recessive multisystem disorder. RAB3GAP1 and RAB3GAP2 form a binary 'RAB3GAP' complex that functions as a guanine-nucleotide exchange factor (GEF) for RAB18, whereas TBC1D20 shows modest RAB18 GTPase-activating (GAP) activity in vitro. Here, we show that in the absence of functional RAB3GAP or TBC1D20, the level, localization and dynamics of cellular RAB18 is altered. In cell lines where TBC1D20 is absent from the endoplasmic reticulum (ER), RAB18 becomes more stably ER-associated and less cytosolic than in control cells. These data suggest that RAB18 is a physiological substrate of TBC1D20 and contribute to a model in which a Rab-GAP can be essential for the activity of a target Rab. Together with previous reports, this indicates that Warburg Micro syndrome can be caused directly by loss of RAB18, or indirectly through loss of RAB18 regulators RAB3GAP or TBC1D20.

Topics: Abnormalities, Multiple; Animals; Blotting, Western; Case-Control Studies; Cataract; Cells, Cultured; Cornea; Cytosol; Endoplasmic Reticulum; Fibroblasts; Flow Cytometry; Fluorescent Antibody Technique; Gene Expression Regulation; Guanosine Triphosphate; HeLa Cells; Humans; Hydrolysis; Hypogonadism; Intellectual Disability; Mice; Mice, Knockout; Microcephaly; Optic Atrophy; rab GTP-Binding Proteins; rab1 GTP-Binding Proteins; rab3 GTP-Binding Proteins; Real-Time Polymerase Chain Reaction; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger

Costello syndrome (CS) is a rare congenital disorder characterized by failure to thrive, craniofacial dysmorphisms, cardiac and skin abnormalities, mental retardation, and predisposition to malignancies. CS is caused by heterozygous gain-of-function mutations in HRAS that also occur as somatic alterations in human tumors. HRAS is one of the three classical RAS proteins and cycles between an active, GTP- and an inactive, GDP-bound conformation. We used primary human skin fibroblasts from patients with CS as a model system to study the functional consequences of HRAS mutations on endogenous signaling pathways. The GTP-bound form of HRAS was significantly enriched in CS compared with normal fibroblasts. Active HRAS is known to stimulate both the RAF-MEK-ERK and the PI3K-AKT signaling cascade. Phosphorylation of MEK and ERK was normal in CS fibroblasts under basal conditions and slightly prolonged after epidermal growth factor (EGF) stimulation. Interestingly, basal phosphorylation of AKT was increased yet more in CS fibroblasts. Moreover, AKT phosphorylation was diminished in the early and enhanced in the late phase of EGF stimulation. Taken together, these results document that CS-associated HRAS mutations result in prolonged signal flux in a ligand-dependent manner. Our data suggest that altered cellular response to growth factors rather than constitutive activation of HRAS downstream signaling molecules may contribute to some of the clinical features in patients with CS.

Topics: Abnormalities, Multiple; Cells, Cultured; Chromones; Craniofacial Abnormalities; Enzyme Inhibitors; Epidermal Growth Factor; Failure to Thrive; Fibroblasts; Guanosine Triphosphate; Humans; Immunoblotting; Intellectual Disability; MAP Kinase Kinase 1; MAP Kinase Kinase 2; Morpholines; Mutation; Phosphatidylinositol 3-Kinases; Phosphoinositide-3 Kinase Inhibitors; Phosphorylation; Proto-Oncogene Proteins c-akt; Proto-Oncogene Proteins p21(ras); Signal Transduction; Skin; Skin Abnormalities; Syndrome

Primary or nonspecific X-linked mental retardation (MRX) is a heterogeneous condition in which affected patients do not have any distinctive clinical or biochemical features in common apart from cognitive impairment. Although it is present in approximately 0.15-0.3% of males, most of the genetic defects associated with MRX, which may involve more than ten different genes, remain unknown. Here we report the characterization of a new gene on the long arm of the X-chromosome (position Xq12) and the identification in unrelated individuals of different mutations that are predicted to cause a loss of function. This gene is highly expressed in fetal brain and encodes a protein of relative molecular mass 91K, named oligophrenin-1, which contains a domain typical of a Rho-GTPase-activating protein (rhoGAP). By enhancing their GTPase activity, GAP proteins inactivate small Rho and Ras proteins, so inactivation of rhoGAP proteins might cause constitutive activation of their GTPase targets. Such activation is known to affect cell migration and outgrowth of axons and dendrites in vivo. Our results demonstrate an association between cognitive impairment and a defect in a signalling pathway that depends on a Ras-like GTPase.

Topics: Amino Acid Sequence; Animals; Central Nervous System; Chromosome Mapping; Cytoskeletal Proteins; Female; Frameshift Mutation; Gene Expression Regulation, Developmental; Genetic Linkage; GTP Phosphohydrolases; GTP-Binding Proteins; GTPase-Activating Proteins; Guanosine Triphosphate; Humans; In Situ Hybridization, Fluorescence; Intellectual Disability; Male; Molecular Sequence Data; Nuclear Proteins; Pedigree; Phosphoproteins; Polymerase Chain Reaction; Proteins; ras GTPase-Activating Proteins; Recombinant Fusion Proteins; Sequence Deletion; Translocation, Genetic; X Chromosome

A syndrome of hyperuricemia, sensorineural deafness, mild mental handicap and congenital disequilibrium in a four-year-old boy is probably inherited as a sex-linked condition since his mother has sensorineural deafness and similar biochemical abnormalities. There is evidence of a superactive PP-ribose-P synthetase, normal purine salvage enzymes, and severe depletion of nicotinamide adenine dinucleotide and guanine triphosphate in red cells.

Topics: Deafness; Erythrocytes; Female; Guanosine Triphosphate; Humans; Infant; Infant, Newborn; Intellectual Disability; Male; NAD; Purine-Pyrimidine Metabolism, Inborn Errors; Ribose-Phosphate Pyrophosphokinase; Syndrome; Uric Acid; X Chromosome