Alternative titles; symbolsGPH; GEPHKIAA1385Other entities represented in this entry:MLL/GPHN FUSION GENE, INCLUDEDHGNC Approved Gene Symbol: GPHNCytogenetic loc...
Alternative titles; symbols
Other entities represented in this entry:
HGNC Approved Gene Symbol: GPHN
Cytogenetic location: 14q23.3 Genomic coordinates (GRCh38): 14:66,507,933-67,181,804 (from NCBI)
The GPHN gene encodes gephyrin, an organizational protein that clusters and localizes the inhibitory glycine and GABA receptors to the microtubular matrix of the neuronal postsynaptic membrane (summary by Rees et al., 2003).
To integrate signals from the many synaptic connections on its cell body and dendrites rapidly and specifically, a neuron anchors high concentrations of receptors at postsynaptic sites, matching the correct receptor with the neurotransmitter released from the presynaptic terminal. Receptor-associated proteins are thought to be involved in forming these postsynaptic specializations, possibly by linking the receptor to the postsynaptic cytoskeleton (Kirsch et al., 1993). Gephyrin is essential for both the postsynaptic localization of inhibitory neurotransmitter receptors in the central nervous system and the biosynthesis of the molybdenum cofactor (MoCo) in different peripheral organs (Stallmeyer et al., 1999).
▼ Cloning and Expression
Prior et al. (1992) cloned the rat gene encoding a 93-kD protein that is associated with the mammalian inhibitory glycine receptor (see 138492). They designated this protein 'gephyrin,' from the Greek word meaning 'bridge,' because it binds with high affinity to polymerized tubulin, suggesting that it may serve as a receptor-microtubule linker.
Ramming et al. (2000) described gephyrin splice variants that were differentially expressed in nonneural tissues and different regions of the adult mouse brain. They found that the mouse gephyrin gene shows a highly mosaic organization, with 8 of its 29 exons corresponding to an alternatively spliced region identified by cDNA sequencing. The N- and C-terminal domains of gephyrin, encoded by exons 3-7 and 16-29, respectively, displayed sequence similarities to bacterial, invertebrate, and plant proteins involved in Moco biosynthesis, whereas the central exons 8, 13, and 14 encode motifs that may mediate oligomerization and tubulin binding. The data were consistent with the evolution of gephyrin from a Moco biosynthetic protein by insertion of protein interaction sequences.
By searching databases for sequences homologous to rat Geph, Reiss et al. (2001) identified a brain tissue cDNA containing the complete coding sequence of human GPHN.
Rees et al. (2003) isolated gephyrin cDNAs and by RT-PCR analysis of human tissues demonstrated the presence of 5 alternatively spliced GPHN exons concentrated in the central linker region of the gene. This region generated 11 distinct GPHN transcript isoforms, with 10 being specific to neuronal tissue.
▼ Gene Structure
Reiss et al. (2001) determined that the GPHN gene contains 22 exons spanning approximately 375 kb.
By genomic sequence analysis, Reiss et al. (2001) mapped the GPHN gene to chromosome 14.
▼ Gene Function
Kirsch et al. (1993) demonstrated that gephyrin is essential for localizing the inhibitory glycine receptor to presumptive postsynaptic plasma membrane specializations. Essrich et al. (1998) found that gephyrin is also required for clustering and postsynaptic localization of GABA(A) receptors. Sabatini et al. (1999) determined that gephyrin interacts with RAFT1 (FRAP; 601231) in mammalian cells. RAFT1 is an ATM (607585)-related protein that appears to participate in mitogen-stimulated signaling pathways that control mRNA translation. RAFT1 mutants that could not associate with gephyrin failed to signal to downstream molecules. Sabatini et al. (1999) concluded that gephyrin plays a role in signal transduction. They reported that all tissues examined, including a human embryonic kidney cell line, contained RAFT1 and gephyrin.
Prior et al. (1992) noted that the C-terminal region of rat gephyrin shares 36% amino acid identity with the E. coli ChlE (MoeA) protein, which is thought to be involved in bacterial molybdopterin biosynthesis. Stallmeyer et al. (1999) stated that the N-terminal region of gephyrin is homologous to MogA, a second E. coli molybdenum cofactor (MoCo) biosynthesis protein. They demonstrated that gephyrin binds with high affinity to molybdopterin, the metabolic precursor of Moco. Gephyrin expression reconstituted Moco biosynthesis in Moco-deficient bacteria, a molybdenum-dependent mouse cell line, and a Moco-deficient plant mutant. Stallmeyer et al. (1999) concluded that gephyrin plays a role in Moco biosynthesis.
Butler et al. (2000) identified high-titer autoantibodies directed against GPH in a patient with mediastinal cancer and clinical features of stiff-man syndrome (184850). Their findings provided evidence for a link between autoimmunity directed against components of inhibitory synapses and neurologic conditions characterized by chronic rigidity and spasms.
▼ Molecular Genetics
The sequence of gephyrin shares homology with the proteins necessary for the biosynthesis of MoCo: MoCo synthesis-1 (MOCS1; 603707) and MoCo synthesis-2 (MOCS2; 603708). Because gephyrin expression can rescue a MoCo-deficient mutation in bacteria, plants, and a murine cell line, it is clear that gephyrin also plays a role in MoCo biosynthesis. Human molybdenum cofactor deficiency is a fatal disease resulting in severe neurologic damage and death in early childhood. Most patients harbor MOCS1 mutations, which prohibit the formation of a precursor, or carry MOCS2 mutations, which abrogate precursor conversion to molybdopterin. In a patient with symptoms typical of molybdenum cofactor deficiency belonging to complementation group C (MOCODC; 615501), Reiss et al. (2001) identified a homozygous deletion in the GEPH gene (603930.0001). Biochemical studies of the patient's fibroblasts demonstrated that gephyrin catalyzes the insertion of molybdenum into molybdopterin and suggested that this novel form of molybdenum cofactor deficiency might be curable by molybdate supplementation.
In an Algerian girl with MOCODC, Reiss et al. (2011) identified a homozygous mutation in the GPHN gene (D580A; 603930.0002).
For discussion of a possible role of variation in the GPHN gene in hyperekplexia (see 149400), see 603930.0002.
The MLL/GPHN Fusion Gene
Eguchi et al. (2001) found that the gephyrin gene can partner with MLL (159555) in leukemia associated with the translocation t(11;14)(q23;q24). The child in whom this translocation was discovered showed signs of acute undifferentiated leukemia 3 years after intensive chemotherapy that included the topoisomerase II inhibitor VP16. The AT hook motifs and a DNA methyltransferase homology domain of the MLL gene were fused to the C-terminal half of the gephyrin gene, including the presumed tubulin-binding site and a domain homologous to the E. coli molybdenum cofactor biosynthesis protein. Eguchi et al. (2001) suggested that MLL-GPHN may have been generated by the chemotherapeutic agent, followed by error-prone DNA repair via nonhomologous end-joining.
The MLL (mixed lineage leukemia) gene forms chimeric fusions with a diverse set of partner genes as a consequence of chromosome translocations in leukemia. In several fusion partners, a transcriptional activation domain appears to be essential for conferring leukemogenic capacity on MLL protein. Other fusion partners, however, lack such domains. Eguchi et al. (2004) showed that gephyrin, a neuronal receptor assembly protein and rare fusion partner of MLL in leukemia, has the capacity as an MLL-GPHN chimera to transform hematopoietic progenitors, despite lack of transcriptional activity. They found that a small 15-amino acid tubulin-binding domain of GPHN is necessary and sufficient for this activity in vitro and in vivo. This domain also confers oligomerization capacity on MLL protein, suggesting that such activity may contribute critically to leukemogenesis. The transduction of MLL-GPHN into hematopoietic progenitor cells caused myeloid and lymphoid lineage leukemias in mice, suggesting that MLL-GPHN can target multipotent progenitor cells.
Possible Association With Neuropsychiatric Disorders
Lionel et al. (2013) presented evidence that heterozygous deletions of exons 3 to 5 of the GPHN gene may play a role in the risk for neurodevelopmental disorders, particularly autism spectrum disorders (ASD; see 209850) and schizophrenia (SCZD; see 181500). The GPHN gene was selected for study because of its functional links with several synaptic proteins that have been implicated in neurodevelopmental disorders, including NLGN4 (300427) and NRXN2 (600566), as well as its role in receptor stability at the synapse. Copy number variant analysis identified heterozygous deletions at chromosome 14q23.3 interrupting multiple exons of the GPHN gene in 5 of 5,384 individuals from cohorts of patients with ASD, schizophrenia, and seizure disorders. A sixth patient with schizophrenia and a heterozygous deletion affecting the GPHN gene was also included in the study; this patient had previously been reported (International Schizophrenia Consortium, 2008). The deletions ranged in size from 183 to 357 kb; 1 breakpoint was shared by 3 patients. No exonic deletions at the GPHN locus were reported in the Database of Genomic Variants, and CNVs at this locus were only found in 3 of 27,019 controls. The frequency of deletions was significantly greater in patients (6 of 8,775) compared to controls (3 of 27,019, p = 0.009). Three of the deletions were proven to occur de novo in patients with ASD, ASD with seizures, and schizophrenia, respectively. Parental information was not available from the fourth patient, who had seizures. A deletion found in a fifth patient, who had ASD, was inherited from a father with subclinical social skills; there was significant psychiatric history on both sides of the family. The sixth patient, who had schizophrenia, inherited the deletion from an unaffected mother whose mother reportedly had schizophrenia. The common region of overlap encompassed exons 3 to 5 of the GPHN gene, corresponding to the coding segment of the G domain, which is vital to the formation of gephyrin scaffolds. Lionel et al. (2013) pointed to the study of Forstera et al. (2010), who found expression of abnormally spliced GPHN mRNA in the hippocampus of patients with temporal lobe epilepsy (see 600512) in the absence of GPHN mutations. The splice variants lacked several exons corresponding to the G domain, and the aberrant protein variants were unable to form trimers. The abnormal variants acted in a dominant-negative manner, resulting in a depletion of GABA receptor cluster density and reduced GABAergic postsynaptic current amplitudes. Forstera et al. (2010) concluded that expression of these variant GPHN isoforms may reduce seizure threshold by reducing inhibitory currents under certain physiologic conditions.
▼ Animal Model
Feng et al. (1998) used gene targeting to disrupt the mouse gephyrin gene. Homozygous gephyrin-null mutant mice were born without apparent developmental abnormalities but died within 1 day. Neonatal mutant animals responded in an exaggerated way to a light touch on the skin, becoming rigid and hyperextended and having difficulty breathing. Using the mutant animals, the authors demonstrated that gephyrin is required both for synaptic clustering of glycine receptors in spinal cord and for molybdoenzyme activity in nonneural tissues. To determine whether the neurologic symptoms were due to disruption of glycinergic synapses or to a molybdenum cofactor deficiency, Feng et al. (1998) injected neonatal mice with strychnine, a specific antagonist of the inhibitory glycine receptor. The injection phenocopied the motor symptoms of gephyrin deficiency, consistent with the idea that the phenotype is primarily attributable to the failure of glycinergic synaptic activity. The mutant phenotype resembled that of human patients with hereditary molybdenum cofactor deficiency (see 615501) and hyperekplexia (see 149400), leading the authors to suggest that gephyrin function may be impaired in both diseases.
▼ ALLELIC VARIANTS ( 3 Selected Examples):
.0001 MOLYBDENUM COFACTOR DEFICIENCY, COMPLEMENTATION GROUP C
Reiss et al. (2001) studied the last of 3 affected infants born to a Danish mother and father who were cousins. All 3 died in the neonatal period (at day 12, 29, and 3, respectively), with symptoms identical to those of molybdenum cofactor (MoCo) deficiency (MOCODC; 615501). Three other pregnancies of the mother resulted in 2 healthy sibs and 1 spontaneous abortion. The first affected infant was a boy; the other 2 were girls. All showed hypotonia combined with hyperreflexia, as well as tonic-clonic convulsions. Fibroblasts of the third infant were used to verify molybdenum cofactor deficiency by biochemical and in vitro complementation assays and to isolate DNA for genetic analysis. Reiss et al. (2001) identified a deletion of exons 2 and 3 of the GPHN gene, resulting in a frameshift after only 21 codons of normal coding sequence.
.0002 VARIANT OF UNKNOWN SIGNIFICANCE
This variant is classified as a variant of unknown significance because its contribution to hyperekplexia has not been confirmed.
In 1 of 38 unrelated patients with hyperekplexia (see 149400), Rees et al. (2003) detected a heterozygous 28A-T transversion in exon 1 of the GPHN gene, resulting in an asn10-to-tyr (N10Y) substitution at the extreme N terminus. The N10Y variant was not found in 94 controls. The GPHN gene was chosen for sequencing after it was shown to interact with the GLRB (138492) subunit. The N10Y substitution is located 5 residues upstream from a putative region important for protein interactions; however, in vitro functional expression studies in HEK293 cells suggested that the variant did not affect the structural lattices formed by gephyrin or interrupt its interactions with GLRB. The variant protein did not interrupt cell surface clustering. Thus, the functional effect of the variant remained elusive.
.0003 MOLYBDENUM COFACTOR DEFICIENCY, COMPLEMENTATION GROUP C
In a girl, born of consanguineous Algerian parents, with molybdenum cofactor deficiency of complementation group C (MOCODC; 615501), Reiss et al. (2011) identified a homozygous c.1739A-C transversion in exon 18 of the GPHN gene, resulting in an asp580-to-ala (D580A) substitution at a highly conserved residue in the E domain. The unaffected parents were heterozygous for the mutation. The E domain is believed to hydrolyze adenylylated molybdopterin while inserting the molybdenum to yield active cofactor. Accordingly, sulfite oxidase activity in patient fibroblasts could not be detected even after incubation with molybdate. The patient presented as a neonate with poor feeding, hypotonia, and intractable seizures. At age 2 years, she had spasticity and lack of psychomotor development.