| *610285 | |||||||||||||||
| DOWNSTREAM OF TYROSINE KINASE 7; DOK7 | |||||||||||||||
| Alternative titles; symbols | |||||||||||||||
| CHROMOSOME 4 OPEN READING FRAME 25; C4ORF25 | |||||||||||||||
| HGNC Approved Gene Symbol: DOK7 | |||||||||||||||
| Cytogenetic location: 4p16.3 Genomic coordinates (GRCh37): 4:3,465,032 - 3,496,208 (from NCBI) | |||||||||||||||
| Gene Phenotype Relationships | |||||||||||||||
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| TEXT | |||||||||||||||
| Description | |||||||||||||||
| DOK7 is a muscle protein essential for neuromuscular synaptogenesis (Okada et al., 2006). | |||||||||||||||
| Cloning | |||||||||||||||
| By searching databases for a previously unidentified member of the DOK family of proteins (e.g., see 602919), Okada et al. (2006) identified DOK7 and cloned human cDNA encoding 504 amino acids. Like other members of this family, DOK7 contains a pleckstrin homology (PH) and PTB domain in the N-terminal portion and Src homology 2 (SH2) domain target motifs in the C-terminal region. Cloning of mouse and pufferfish Dok7 cDNA revealed a highly conserved structure. Like agrin (103320) and muscle-specific receptor kinase (MuSK; 601296), no ortholog was found in invertebrates such as fruit fly and nematode. Northern blot analysis of human tissues showed that DOK7 mRNA is preferentially expressed in skeletal muscle and heart, and immunoblot analysis identified a 55-kD DOK7 protein in thigh muscle, diaphragm, and heart but not in liver or spleen. Okada et al. (2006) found that immunostaining of mouse skeletal muscles, including the sternocleidomastoid, extensor digitorum longus, and gastrocnemius, with antiserum to Dok7 highlighted the accumulation of Dok7 at neuromuscular junctions. Immunostaining of denervated mouse gastrocnemius muscle confirmed postsynaptic localization of Dok7. Whole-mount in situ hybridization showed that Dok7 transcripts are expressed in the central region encompassing the endplate area of the diaphragm muscles at embryonic day 14.5, when acetylcholine receptors (AChRs) cluster in a nerve- and agrin-independent manner. | |||||||||||||||
| Gene Function | |||||||||||||||
| The formation of the neuromuscular synapse requires MuSK to orchestrate postsynaptic differentiation, including the clustering of receptors for the neurotransmitter acetylcholine. Upon innervation, neural agrin activates MuSK to establish the postsynaptic apparatus, although agrin-independent formation of neuromuscular synapses can also occur experimentally in the absence of neurotransmission. Okada et al. (2006) showed that DOK7 induces the autophosphorylation of MuSK, and that DOK7 binds to MuSK through the PTB domain in a manner dependent upon the tyrosine phosphorylation of its target motif in MuSK. They also showed that DOK7 is essential for MuSK activation in cultured myotubes. Okada et al. (2006) concluded that DOK7 is essential for neuromuscular synaptogenesis through its interaction with MuSK. | |||||||||||||||
| Gene Structure | |||||||||||||||
| Okada et al. (2006) determined that the DOK7 gene comprises 7 exons. | |||||||||||||||
| Molecular Genetics | |||||||||||||||
| Beeson et al. (2006) identified the same frameshift mutation (1124_1127dupTGCC; 610285.0001) in 16 of 21 patients with limb-girdle myasthenia (254300). Other mutations included a nonsense mutation (610285.0007) and a glycine-to-alanine substitution at codon 180 (610285.0006), involving an invariant glycine. Other cases were homozygous. Beeson et al. (2006) found that mutations in DOK7 affect the size and structure of the neuromuscular junction and underlie a congenital myasthenic syndrome with a characteristic pattern of weakness, particularly affecting the proximal muscle groups. Because the onset of symptoms resulting from DOK7 mutations is usually observed in early childhood and not at birth, the mutations probably have little pathogenic effect on initial synapse formation but instead exert their effects through aberrant synaptic maturation or maintenance. Beeson et al. (2006) found that the 1124_1127dupTGCC frameshift impairs the maturation of the postsynaptic structure in cultured myotubes. DOK7 that harbored a truncated C-terminal region induced MuSK activation during the early stages of the differentiation of C2C12 cells into myotubes but not when the myotubes were fully differentiated. Among 16 patients with limb-girdle congenital myasthenic syndrome, Selcen et al. (2008) identified 17 different mutations in the DOK7 gene, including 10 novel mutations (see, e.g., 610285.0009; 610285.0010). All of the mutations resulted in a termination codon or a frameshift, except for 3 that resulted in the in-frame deletion of 1 or more exons. The deleted exons in these latter cases removed domains necessary in the phosphorylation of MuSK. However, immunostained intercostal muscles of patients with either frameshift or exon-skipping mutations showed normal phosphorylation of AChR, suggesting that this activity is compensated. In vitro functional expression studies in C2C12 murine muscle cells showed that many of the mutations resulted in decreased axial length and density of AChR clusters at the endplate. Selcen et al. (2008) concluded that the pathogenesis of the disorder results from destruction and simplification of synaptic structures with resultant decrease in neuromuscular transmission. Vogt et al. (2009) identified a homozygous truncating mutation in the DOK7 gene (610285.0009) in 3 affected sibs with a severe lethal form of myasthenia resulting in fetal death. Clinical features consistent with a diagnosis of fetal akinesia deformation sequence (FADS; 208150), a heterogeneous disorder. Vogt et al. (2009) concluded that complete loss of DOK7 results in a lethal fetal akinesia phenotype that is at the more severe end of a spectrum of disorders involving acetylcholine receptors. | |||||||||||||||
| Animal Model | |||||||||||||||
| Okada et al. (2006) generated mice lacking Dok7 and observed that all homozygous Dok7-deficient mice were immobile at birth and died shortly thereafter. Alveoli of these mice were not expanded at birth, indicating a failure to breathe and suggesting a severe defect in neuromuscular transmission in skeletal muscles. Heterozygous-deficient littermates were normal. Okada et al. (2006) found that Dok7 homozygous mutants formed neither AChR clusters nor neuromuscular synapses. They concluded that neuromuscular synaptogenesis requires DOK7 within skeletal muscle. Muller et al. (2010) reported that Dok7 deficiency led to motility defects in zebrafish embryos and larvae. The relative importance of Dok7 at different stages of neuromuscular junction development varied; it was crucial for the earliest step, the formation of acetylcholine receptor (AChR) clusters in the middle of the muscle fiber prior to motor neuron contact. At later stages, presence of Dok7 was not absolutely essential, as focal and nonfocal synapses did form when Dok7 expression was downregulated. However, these contacts were smaller than in the wildtype zebrafish, reminiscent of the neuromuscular endplate pathology seen in patients with DOK7 mutations. Changes in slow muscle fiber arrangement were also observed. The authors suggested an additional role for Dok7 in muscle that is independent of the muscle-specific tyrosine kinase MuSK (601296), the binding partner of Dok7 at the neuromuscular junction. | |||||||||||||||
| ALLELIC VARIANTS (Selected Examples): | |||||||||||||||
| Table View | |||||||||||||||
| .0001 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, 4-BP DUP, 1124TGCC | |||||||||||||||
| In 16 of 21 probands with limb-girdle myasthenic syndrome (254300), Beeson et al. (2006) found homozygosity or compound heterozygosity for a 4-bp duplication between nucleotides 1124 and 1127 in exon 7 of the DOK7 gene, a duplication of TGCC (1124_1127dupTGCC). This frameshift resulted in a premature termination 30 basepairs further downstream (Pro376ProfsX30). Analysis of acetylcholine receptor clusters from CTC12 myotubes with the mutant DOK7 showed a significant reduction in the number of branched-type plaques, suggesting that the mutation attenuates the maturation of the synaptic structure. Selcen et al. (2008) identified the 4-bp duplication in 14 of 16 patients with congenital myasthenic syndrome. One patient was homozygous for the mutation, whereas the others were compound heterozygous with another pathogenic DOK7 mutation. | |||||||||||||||
| .0002 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, 1-BP INS, 1263C | |||||||||||||||
| In 4 of 21 patients with autosomal recessive limb-girdle myasthenia (254300), Beeson et al. (2006) identified compound heterozygosity for insertion of a C nucleotide at position 1263 of the DOK7 gene with the 1124_1127dupTGCC mutation (610285.0001). The insertion mutation resulted in frameshift and premature termination (Ser422LeufsX94). | |||||||||||||||
| .0003 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, 4-BP DEL, 548TCCT | |||||||||||||||
| In an Indian patient with limb-girdle myasthenia (254300), Beeson et al. (2006) found a 4-bp deletion in exon 5 of the DOK7 gene (548_551delTCCT), resulting in frameshift with premature termination (Phe183CysfsX61). The mutation was found in compound heterozygosity with the common 1124_1127dupTGCC frameshift (610285.0001). | |||||||||||||||
| .0004 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, 4-BP DUP, 1339CTGG | |||||||||||||||
| In 3 patients with limb-girdle myasthenia (254300), Beeson et al. (2006) identified a 4-bp duplication in exon 7 of the DOK7 gene, 1139_1342dupCTGG, that resulted in frameshift and premature termination (Gly447AlafsX70). This mutation was found in compound heterozygosity with the common 1124_1127dupTGCC mutation (610285.0001) in 2 cases and with a 1-bp insertion (610285.0005) in the third. | |||||||||||||||
| .0005 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, 1-BP INS, 1143C | |||||||||||||||
| In 3 Caucasian individuals with autosomal recessive limb-girdle myasthenia (254300) from the U.K., Beeson et al. (2006) identified homozygosity and compound heterozygosity for a 1143insC mutation in exon 7 of the DOK7 gene, resulting in a frameshift and premature termination (Glu382ArgfsX24). | |||||||||||||||
| .0006 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, GLY180ALA [dbSNP:rs118203994] | |||||||||||||||
| In a Caucasian patient from the U.K. with autosomal recessive limb-girdle myasthenia (254300), Beeson et al. (2006) identified a G-to-C transversion at nucleotide 539 in exon 5 of the DOK7 gene, resulting in a gly-to-ala substitution at codon 180 (G180A). This mutation was found in compound heterozygosity with the common 1124_1127dupTGCC frameshift mutation (610285.0001). | |||||||||||||||
| .0007 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, ARG201TER [dbSNP:rs118203995] | |||||||||||||||
| In a German patient with limb-girdle myasthenia (254300), Beeson et al. (2006) identified a C-to-T transition at nucleotide 601 in exon 5 of the DOK7 gene, resulting in an arg-to-stop substitution at codon 201 (R201X). This mutation occurred in compound heterozygosity with the common 1124_1127dupTGCC mutation in exon 7 (610285.0001). | |||||||||||||||
| .0008 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, IVS2, G-T, -1 | |||||||||||||||
| In a Spanish patient with limb-girdle myasthenia (254300), Beeson et al. (2006) identified a G-to-T substitution at the -1 position of intron 2 of the DOK7 gene. This mutation occurred in compound heterozygosity with the common 1124_1127dupTGCC mutation (610285.0001). | |||||||||||||||
| .0009 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| FETAL AKINESIA DEFORMATION SEQUENCE, INCLUDED | |||||||||||||||
| DOK7, IVS3DS, G-T, +1 | |||||||||||||||
| In a patient with limb-girdle congenital myasthenia (254300), Selcen et al. (2008) identified compound heterozygosity for 2 mutations in the DOK7 gene: a G-to-T transversion in intron 3, resulting the skipping of exon 3, and the common 4-bp duplication in exon 7 (610285.0001). The skipping of exon 3 removed the pleckstrin homology domain and interrupted phosphorylation of MuSK (601296). Vogt et al. (2009) identified a homozygous IVS3DS+1G-T mutation in the DOK7 gene in 3 sibs from a consanguineous Bengali family with a severe lethal form of myasthenia resulting in fetal death. The first fetus was stillborn at 32 weeks' gestation and had signs of a neuromuscular developmental abnormality. The second fetus miscarried spontaneously at 22 weeks' gestation. Postmortem examination showed downslanting palpebral fissures, short neck, small jaw, overlapping fingers, and rocker-bottom feet. The third fetus showed no fetal movements at 24 weeks' gestation. None had evidence of pterygia. Muscle biopsy in 1 fetus showed muscle denervation with immature irregularly shaped muscle cells. The findings were consistent with a clinical diagnosis of fetal akinesia deformation sequence (FADS; 208150), a heterogeneous disorder. Vogt et al. (2009) concluded that complete loss of DOK7 results in a lethal fetal akinesia phenotype that is at the more severe end of a spectrum of disorders involving acetylcholine receptors. | |||||||||||||||
| .0010 MYASTHENIA, LIMB-GIRDLE, FAMILIAL | |||||||||||||||
| DOK7, 1-BP INS, 1378C | |||||||||||||||
| In a patient with limb-girdle congenital myasthenia (254300), Selcen et al. (2008) identified compound heterozygosity for 2 mutations in the DOK7 gene: a 1-bp insertion (1378insC) in exon 7, resulting in a frameshift and premature truncation, and the common 4-bp duplication in exon 7 (610285.0001). In vitro functional expression studies in murine myotubes showed that the 1378insC mutation resulted in decreased axial length and density of AChR compared to wildtype. | |||||||||||||||
| REFERENCES | |||||||||||||||
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