Alternative titles; symbolsTRANSMEMBRANE PROTEIN 8C; TMEM8CHGNC Approved Gene Symbol: MYMKCytogenetic location: 9q34.2 Genomic coordinates (GRCh38): 9:133,51...
Alternative titles; symbols
HGNC Approved Gene Symbol: MYMK
Cytogenetic location: 9q34.2 Genomic coordinates (GRCh38): 9:133,514,585-133,524,958 (from NCBI)
The MYMK gene encodes myomaker, a transmembrane protein that mediates the fusion of mononuclear myoblasts to form multinucleate myocyte syncytia during skeletal muscle development (summary by Di Gioia et al., 2017).
TMEM8C is a transmembrane protein required for the fusion of skeletal muscle myoblasts into multinucleated myofibers (Millay et al., 2013).
▼ Cloning and Expression
Millay et al. (2013) cloned mouse Tmem8c, which they called 'myomaker.' The deduced 221-amino acid protein has a number of hydrophobic regions and a C-terminal CAAX isoprenylation motif. Orthologs of myomaker were detected in several vertebrate species, including human. Northern blot analysis of 12 mouse tissues at embryonic day 19 (E19) detected myomaker in tongue and limb muscle only. Quantitative real-time PCR detected weak myomaker expression also in liver, but little to no expression was detected in other tissues. Myomaker was expressed in the myotomal compartment of somites in early mouse embryos, in limb buds and axial skeletal muscle later in development, and was downregulated upon completion of muscle formation. Epitope-tagged mouse myomaker was expressed on the surface of transfected C2C12 cells and in intracellular vesicles.
Di Gioia et al. (2017) found that Mymk was not expressed in mouse sciatic nerve.
▼ Gene Function
Millay et al. (2013) found that expression of myomaker increased sharply during differentiation and fusion in C2C12 mouse myoblasts, and after cardiotoxin injury in regenerating embryonic mouse muscle. Overexpression of myomaker enhanced fusion of C2C12 myoblasts into multinucleated myotubes without increasing expression of skeletal muscle differentiation markers. Cell mixing experiments revealed that expression of myomaker alone in mouse fibroblasts was insufficient to result in fibroblast fusion; however, myomaker-expressing fibroblasts were able to fuse with wildtype mouse myoblasts and with C2C12 cells.
Hartz (2013) mapped the TMEM8C gene to chromosome 9q34.2 based on an alignment of the TMEM8C sequence (GenBank NM_001080483) with the genomic sequence (GRCh37).
▼ Molecular Genetics
In 8 patients from 5 unrelated families with Carey-Fineman-Ziter syndrome (CFZS; 254940), Di Gioia et al. (2017) identified homozygous or compound heterozygous mutations in the MYMK gene (615345.0001-615345.0005). Seven patients shared a missense mutation on 1 allele (P91T; 615345.0001), and haplotype analysis suggested a founder effect for that mutation. The mutations in the first 3 families were found by whole-exome sequencing and confirmed by Sanger sequencing; mutations in the 2 other families were found by sequencing the MYMK gene in over 300 additional probands with similar phenotypes. All mutations segregated with the disorder in the families. Western blot analysis of HeLa cells transfected with each of the mutations showed absence of the mutant proteins, suggesting protein instability and degradation and consistent with a loss of function. However, immunostaining, cellular expression studies, and rescue experiments in mutant zebrafish showed that 2 of the mutations (P91T, 615345.0001 and I154T, 615345.0005) were hypomorphic, showing some expression at the cell membrane and some residual ability to induce myoblast fusion. In contrast, the other 3 mutations (C185R, 615345.0002; G100S, 615345.0003; and c.2T-A, 615345.0004) formed cytoplasmic aggregates and had no fusogenic activity, consistent with being null alleles. Di Gioia et al. (2017) concluded that recessive mutations in MYMK cause CFZS through a combination of 1 hypomorphic and 1 null allele, or 2 hypomorphic alleles that reduce MYMK function and myoblast fusion below a threshold, but not to zero. The craniofacial abnormalities in CFZS patients likely reflect selective vulnerability of specific muscle groups to reduced levels of MYMK, as craniofacial muscles have specific developmental pathways and influence bone development. Mymk expression was not found in mouse sciatic nerve, thus excluding a neurogenic basis for the disorder. The findings enabled classification of CFZS as a congenital myopathy.
Using whole-exome sequencing in a 69-year-old British man with CFZS, Alrohaif et al. (2018) identified compound heterozygosity for previously identified mutations in the MYMK gene (615345.0001 and 613545.0002).
▼ Animal Model
Millay et al. (2013) found that deletion of myomaker in mice was perinatal lethal. Hearts of full-term myomaker -/- embryos were beating, but the animals were paralyzed and kyphotic with flaccid limbs. In myomaker -/- muscle, expression of muscle-specific transcription factors and differentiation markers was normal, and muscle cell precursors were organized appropriately; however, myomaker -/- muscle failed to form multinucleated myofibers. In culture, myomaker -/- myoblasts lacked the ability to fuse, but they were able to fuse with wildtype mouse myoblasts and C2C12 cells, and with myomaker-expressing mouse fibroblasts.
Di Gioia et al. (2017) found that zebrafish mymk is exclusively expressed in multinucleated type II fast-twitch muscle fibers, where it plays a role in differentiation and fusion. This expression pattern is in contrast to human and mouse muscle, where Mymk localizes to both slow type I and fast type II fibers. Di Gioia et al. (2017) used CRISPR-Cas9 technology to create zebrafish mutants with a truncating mutation and showed that they lacked fast-twitch myoblast fusion and had variably-sized hypotrophic fibers with fatty infiltration in the muscle, as well as misplaced myonuclei. Mutant zebrafish were viable, likely due to preservation of slow-twitch fibers. Adult mutant fish showed craniofacial deformities, including retrognathia and weakened jaw muscles.
▼ ALLELIC VARIANTS ( 5 Selected Examples):
.0001 CAREY-FINEMAN-ZITER SYNDROME
MYMK, PRO91THR (rs776566597)
In 7 patients from 4 unrelated families with Carey-Fineman-Ziter syndrome (CFZS; 254940), Di Gioia et al. (2017) identified compound heterozygous missense mutations in the MYMK gene: all patients carried a heterozygous c.271C-A transversion in exon 3, resulting in a pro91-to-thr (P91T) substitution in transmembrane domain (TM) 4 on 1 allele. Haplotype analysis suggested a founder effect. The mutation on the other allele differed in each family. Family 1 had a c.553T-C transition in exon 5, resulting in a cys185-to-arg (C185R; 615345.0002) substitution at a conserved residue in transmembrane domain 7; this family was originally reported by Carey et al. (1982). Affected members of families 2 and 3 had a c.298G-A transition in exon 3, resulting in a gly100-to-ser (G100S; 615345.0003) substitution in TM4 on the other allele. The G100S mutation, which occurred at a CpG dinucleotide, arose without a founder effect. The patient from family 4 had a c.2T-A transversion in exon 1 (615345.0004), resulting in disruption of the methionine initiation codon (p.M1?; 615345.0004). The mutations in the first 3 families were found by whole-exome sequencing and confirmed by Sanger sequencing; mutations in the other family were found by sequencing the MYMK gene in over 300 additional probands with similar phenotypes. All mutations segregated with the disorder in the families. The variants were filtered against various databases, including the 1000 Genomes Project, Exome Variant Server, and ExAC databases. Only P91T was found at a low frequency (0.0013) in the ExAC database: this mutation was reported in the homozygous state in 1 individual with no declared phenotype who was not recontactable. In vitro functional expression studies suggested that the P91T mutant protein was hypomorphic and able to induce fusion in transfected cells almost as well as the wildtype protein; however, expression of the mutant protein was decreased. The 3 other mutations were demonstrated to be null alleles.
By whole-exome sequencing in a 69-year-old British man with CFZS, Alrohaif et al. (2018) identified compound heterozygosity for the P91T and C185R mutations in the MYMK gene.
.0002 CAREY-FINEMAN-ZITER SYNDROME
For discussion of the c.553T-C transition in exon 5 of the MYMK gene, resulting in a cys185-to-arg (C185R) substitution, that was found in compound heterozygous state in 2 sibs with Carey-Fineman-Ziter syndrome (CFZS; 254940) by Di Gioia et al. (2017), see 615345.0001.
In a 69-year-old British man with CFZS, Alrohaif et al. (2018) identified compound heterozygosity for the P91T (615345.0001) and C185R mutations in the MYMK gene.
.0003 CAREY-FINEMAN-ZITER SYNDROME
For discussion of the c.298G-A transition in exon 3 of the MYMK gene, resulting in a gly100-to-ser (G100S) substitution, that was found in compound heterozygous state in affected members of 2 unrelated families with Carey-Fineman-Ziter syndrome (CFZS; 254940) by Di Gioia et al. (2017), see 615345.0001.
.0004 CAREY-FINEMAN-ZITER SYNDROME
For discussion of the c.2T-A transversion in exon 3 of the MYMK gene, resulting in failure to initiate translation, that was found in compound heterozygous state in a patient with Carey-Fineman-Ziter syndrome (CFZS; 254940) by Di Gioia et al. (2017), see 615345.0001.
.0005 CAREY-FINEMAN-ZITER SYNDROME
In a 28-year-old Brazilian woman, born of consanguineous parents, with Carey-Fineman-Ziter syndrome (CFZS; 254940), Di Gioia et al. (2017) identified a homozygous c.461T-C transition in exon 5 of the MYMK gene, resulting in an ile154-to-thr (I154T) substitution at a highly conserved residue in the TM6 domain. She had an affected brother who was not available for study. The mutation was found by sequencing the MYMK gene in over 300 probands with phenotypes similar to previously identified families. In vitro functional expression studies suggested that the I154T mutant protein was hypomorphic and able to induce fusion in transfected cells, although not to the extent of the wildtype protein.