Alternative titles; symbols
Other entities represented in this entry:
HGNC Approved Gene Symbol: TNXB
Cytogenetic location: 6p21.33-p21.32 Genomic coordinates (GRCh38): 6:32,041,153-32,109,338 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 6p21.33-p21.32 | Ehlers-Danlos syndrome, classic-like, 1 | 606408 | Autosomal recessive | 3 |
| Vesicoureteral reflux 8 | 615963 | Autosomal dominant | 3 |
The TNXB gene encodes tenascin-XB, a glycoprotein of the extracellular matrix predominantly located in the outer reticular lamina of the basement membrane (summary by Penisson-Besnier et al., 2013).
The tenascins are a family of extracellular matrix proteins (ECMs); see 187380. The first member, termed tenascin or hexabrachion (TNC; 187380), attracted attention because of its prominent expression during tissue interactions in embryogenesis and its overexpression in many tumors.
By screening a fetal adrenal cDNA library with a CYP21 (613815) probe, Morel et al. (1989) obtained a partial cDNA corresponding to a gene on the opposite strand of CYP21. Northern blot analysis revealed expression of 3.5- and 1.8-kb transcripts in adult adrenal and a Leydig cell adenoma. Xu and Doolittle (1990) determined that tenascin and the gene product identified by Morel et al. (1989) have the same type of fibronectin (135600) repeats. Gitelman et al. (1992) determined that the functional gene, which they termed XB, is localized opposite CYP21B and not the pseudogene CYP21A.
Matsumoto et al. (1992) also identified a tenascin-like gene in the human major histocompatibility complex (MHC) class III region. Chromosome walking and sequencing in the region centromeric of CYP21 disclosed a cluster of fibronectin type III repeats. Such repeats consist of approximately 90 amino acids and exist in a wide range of protein species. Homology searches in protein databases showed that the repeats had the highest homology with those of human tenascin.
By screening genomic libraries and multiple cDNA libraries, RT-PCR, and 5-prime RACE, Bristow et al. (1993) cloned the XB gene. Sequence analysis of the estimated 3,816-amino acid XB gene product, which the authors termed TNX, predicted the presence of 5 N-linked glycosylation sites and multiple EFG and fibronectin type III repeats. RNase protection analysis detected variable expression in all tissues tested, with highest levels in fetal testis and fetal smooth, striated, and cardiac muscle. In situ hybridization demonstrated homogeneous expression of TNX in adult adrenal cortex.
Matsumoto et al. (1994) isolated a cDNA encoding mouse Tnx. They found that the subunit molecular size of Tnx is approximately 500 kD, suggesting that the protein may contain up to 40 fibronectin type III repeats, making it the largest tenascin family member. The Tnx mRNA and protein were predominantly expressed in heart and skeletal muscle, but low levels of the mRNA were found in most tissues. Immunostaining showed that the Tnx protein is associated with the extracellular matrix of the muscle tissues and with blood vessels in all tissues analyzed. Although the gene lies in the MHC class III region, it is not expressed in lymphoid organs, except for the staining around blood vessels.
Tee et al. (1995) stated that in the compact region of the human class III major histocompatibility locus on 6p21.3, genes for C4A (120810), C4B (120820), and steroid 21-hydroxylase occur in 1 transcriptional orientation, whereas the TNX gene overlaps the last exon of the CYP21 gene on the opposite strand of DNA in the opposite transcriptional orientation. This complex locus is duplicated into A and B loci, so that the orientation is 5-prime-C4A-21A-TNXA-C4B-21B-TNXB-3-prime. Although a duplication event truncated the 65-kb TNXB gene to a 4.5-kb TNXA gene, the TNXA gene is transcriptionally active in the adrenal cortex. To examine the basis of the tissue-specific expression of TNXA and C4B, Tee et al. (1995) cloned the 1763-bp region that lies between the cap sites for TNXA and C4B and analyzed its promoter activity in both the TNXA and the C4 orientations. Transcriptionally active, liver-specific sequences lie within the first 75 to 138 bp from the C4B cap site, and weaker transcriptional elements lie within 128 bp of the TNXA cap site that function in both liver and adrenal cells. Because these elements 128 bp upstream from the TNXA cap site are perfectly preserved in the TNXB gene, Tee et al. (1995) sought to determine whether a transcript similar to TNXA arises within the TNXB gene. RNase protection assays, cDNA cloning, and RT-PCR showed that adrenal cells contain a novel transcript, termed short XB (XB-S) by them, which has the same open reading frame as the gene encoding tenascin-XA. Cell-free translation and immunoblotting showed that this transcript encodes a novel 74-kD XB-S protein that is identical to the C-terminal 673 residues of tenascin-X. Because this protein consists solely of fibronectin type III repeats and a fibrinogen-like domain, it appears to correspond to an evolutionary precursor of the tenascin family of extracellular matrix proteins.
Speek et al. (1996) identified a TNX promoter specific for transcription in fetal adrenal and muscle tissue, as well as 2 promoters further upstream specific for transcription in adrenocortical carcinoma cells. Unlike TNC and TNR (601995), no alternative splicing in the fibronectin-like domains could be detected by 5-prime RACE and RNase protection analysis.
Ikuta et al. (1998) determined that the mouse and human TNX proteins are 73% identical. Mouse Tnx has an N-terminal domain containing cysteine residues and 4 heptad repeats, followed by 18.5 EGF repeats, 31 fibronectin III repeats, and a C-terminal fibrinogen-like motif. Ikuta et al. (1998) established that the human TNX protein contains 4,267 amino acids and has 33 fibronectin III repeats.
Gbadegesin et al. (2013) found expression of the TNXB gene in transitional epithelial cells in the bladder at the vesicoureteral junction in human tissue.
By sequencing phage and cosmid clones and cDNA fragments, Bristow et al. (1993) estimated that the TNX gene contains at least 39 exons and spans 65 kb. Speek et al. (1996) identified an additional exon 10 kb upstream from the previously known exons. They noted that the TNX gene appears to be unique in having both its 5-prime and 3-prime ends buried in other genes.
In a comparative analysis of the mouse Tnx gene and the human TNX gene, Ikuta et al. (1998) determined that mouse and human introns 1, 4, and 6 are highly conserved. The mouse Tnx gene has 43 exons.
Tee et al. (1995) stated that in the compact region of the human class III major histocompatibility locus on 6p21.3, genes for C4 and steroid 21-hydroxylase occur in 1 transcriptional orientation, while the TNX gene overlaps the last exon of the CYP21 gene on the opposite strand of DNA in the opposite transcriptional orientation. This complex locus is duplicated into A and B loci, so that the orientation is 5-prime-C4A-21A-TNXA-C4B-21B-TNXB-3-prime.
Ehlers-Danlos Syndrome, Classic-Like, 1
Burch et al. (1996) described a 25-year-old male with congenital adrenal hyperplasia due to 21-hydroxylase deficiency, associated with a classic Ehlers-Danlos syndrome-like phenotype (EDSCLL1; 606408), consisting of hyperextensible skin and joints, patellar chondromalacia, and easy bruising. The patient was studied for a possible contiguous gene deletion syndrome inasmuch as the TNX gene is encoded by a gene overlapping the opposite strand of the 21-hydroxylase B gene, CYP21. Western blotting of heparin-Sepharose concentrated fibroblast-conditioned medium with anti-human TNX antiserum showed no TNX in samples from the proband and reduced amounts of TNX in conditioned medium from both parents compared to controls. A 30-kb deletion (600985.0001), found in the proband and his father, resulted in loss of the CYP21 gene and creation of a hybrid gene between TNX and the partially duplicated TNX gene (called XA by them) with early termination of TNX translation. The nature of the molecular lesion in the proband's second TNX and CYP21 alleles was unknown. Burch et al. (1996) concluded that the patient's Ehlers-Danlos syndrome phenotype was due to loss of TNX and represented the first tenascin-related disease.
In their full report, Burch et al. (1997) stated that atypical histologic findings suggested a novel mechanism of disease in their proband: the most striking findings were abnormal elastin (130160) bodies beneath the dermal-epidermal junction, a diffuse increase in perivascular matrix, and uneven packing of the myelin sheaths of peripheral nerves. TNX is a 100-kb gene overlapping the 3-prime untranslated region of the CYP21B gene. The truncated XA gene is a partially duplicated TNX that occurred in the primordial duplication event involving this region of 6p. XA and TNX are at least 99% identical, but XA is transcribed solely in the adrenal gland and contains a 121-bp deletion that prematurely closes the reading frame that corresponds to TNX. Since the coding regions of TNX and CYP21B do not overlap, single point mutations are unlikely to disrupt the function of both genes. Homologous recombination frequently produces deletion of the CYP21B locus, but no deletion extending into TNX had been described. However, recombination between the 5-prime end of XA and the homologous point in TNX would delete CYP21B and create a TNX/XA fusion gene that carries the internal deletion normally found in XA, thereby truncating the TNX open reading frame. Erickson (1997) pointed out that, unlike the tenascin gene knockout mice, which showed no obvious defect in connective tissue or in wound healing (Saga et al., 1992), the human 'experiment of nature' indicates a vital role for TNX.
To investigate the role of tenascins in Ehlers-Danlos syndrome, Schalkwijk et al. (2001) screened serum samples from 151 patients with the classic (see 130000), hypermobility (130020), or vascular (130050) types of Ehlers-Danlos syndrome for the presence of tenascin-X and tenascin-C by enzyme-linked immunosorbent assay. The same assays were done in 75 patients with psoriasis, 93 patients with rheumatoid arthritis, and 21 healthy persons. In all subjects the authors examined the expression of tenascins and type V collagen in skin by immunohistochemical methods, and the TNX gene was sequenced. Absence of tenascin-X from the serum was found in 5 unrelated patients, all of whom had Ehlers-Danlos syndrome. Expression of tenascin-C and type V collagen was normal in these 5 patients. All 5 had hypermobile joints, hyperelastic skin, and easy bruising, without atrophic scarring. The authors identified mutations in the TNX gene in all 5 of these patients: 1 had a homozygous deletion, 1 was heterozygous for the same deletion, and 3 others were homozygous for truncating point mutations, confirming a causative role for tenascin-X in Ehlers-Danlos syndrome and a recessive pattern of inheritance. None of the parents of the 5 patients with tenascin-X deficiency were related, and none of the 4 parents available for study had clinical signs of Ehlers-Danlos syndrome.
The gene for steroid 21-hydroxylase deficiency, the adjacent complement C4 gene, and parts of the flanking genes serine/threonine protein kinase-19 (STK19; 604977) and TNXB constitute a tandemly duplicated arrangement. Koppens et al. (2002) determined that apparent large-scale gene conversions accounted for the defect in 9 of 77 chromosomes in a group of patients with congenital adrenal hyperplasia due to steroid 21-hydroxylase deficiency. They further showed that 4 of 9 'conversions' extended into TNXB. This implies that approximately 1 in every 10 steroid 21-hydroxylase deficiency patients is a carrier of tenascin-X deficiency.
Zweers et al. (2003) demonstrated that haploinsufficiency of the TNXB gene, caused by heterozygosity for a 30-kb deletion (600985.0001), results in hypermobile joints, often associated with joint subluxations and chronic musculoskeletal pain. Patients with haploinsufficiency do not have skin hyperextensibility and lack the easy bruising seen in patients with TNXB deficiency.
Vesicoureteral Reflux 8
In affected members of a 5-generation family with autosomal dominant vesicoureteral reflux-8 (VUR8; 615963), Gbadegesin et al. (2013) identified a heterozygous missense mutation in the TNXB gene (T3257I; 600985.0006). The mutation was found by a combination of linkage analysis and whole-exome sequencing. Screening of the TNXB gene in 11 probands with VUR identified 1 patient with a heterozygous missense mutation (G1331R; 600985.0007).
Associations Pending Confirmation
For discussion of a possible association between variation in the TNXB gene and systemic lupus erythematosus (SLE), see 152700.
Because TNXB is the first Ehlers-Danlos syndrome gene that does not encode a fibrillar collagen or collagen-modifying enzyme, Mao and Bristow (2001) suggested that tenascin-X may regulate collagen synthesis or deposition. To test this hypothesis, Mao et al. (2002) inactivated Tnxb in mice. Tnxb -/- mice showed progressive skin hyperextensibility, similar to that of individuals with Ehlers-Danlos syndrome. Biomechanical testing confirmed increased deformability and reduced tensile strength of their skin. The skin of Tnxb -/- mice was histologically normal, but its collagen content was significantly reduced. At the ultrastructural level, collagen fibrils of Tnxb -/- mice were of normal size and shape, but the density of fibrils in their skin was reduced, commensurate with the reduction in collagen content. Studies of cultured dermal fibroblasts showed that although synthesis of collagen I by Tnxb -/- and wildtype cells was similar, Tnxb -/- fibroblasts failed to deposit collagen I into cell-associated matrix. This study confirmed a causative role for TNXB in human Ehlers-Danlos syndrome and suggested that tenascin-X is an essential regulator of collagen deposition by dermal fibroblasts.
Burch et al. (1996, 1997) described a patient with Ehlers-Danlos syndrome (EDSCLL1; 606408) combined with congenital adrenal hyperplasia resulting from a 30-kb deletion in 6p21.3, which removed the functional 21-hydroxylase gene (CYP21; 613815) and produced partial duplication of the TNX gene, resulting in a nonfunctional fusion gene. The patient described by Burch et al. (1996, 1997) was referred to as the index patient by Schalkwijk et al. (2001) and was heterozygous for the 30-kb deletion. Patient 3 of a series of 5 patients with Ehlers-Danlos syndrome identified by Schalkwijk et al. (2001) was homozygous for this deletion, which explained both the presence of Ehlers-Danlos syndrome and congenital adrenal hyperplasia. Both her parents and 2 sibs were heterozygous for the deletion and were clinically normal, providing evidence of recessive inheritance in this family. Patient 2 of Schalkwijk et al. (2001) was heterozygous for the 30-kb deletion and did not have congenital adrenal hyperplasia. They were unable to identify a second TNX mutation in patient 2 and suggested that this patient, like the index patient (Burch et al., 1997), may have had a mutation in factors, not yet defined, that regulate the tenascin-X gene expression.
Zweers et al. (2003) demonstrated that haploinsufficiency of TNXB resulting from heterozygosity for the 30-kb deletion results in Ehlers-Danlos syndrome. They measured serum TNX levels by ELISA in an unselected cohort of 80 patients with hypermobility-type EDS who were recruited through a Dutch organization for EDS patients. In all patients, the diagnosis was made by a medical specialist, and approximately 90% were female. In 6 of these patients (7.5%), all female, serum TNX levels were more than 2.5 SD below the mean for unaffected individuals. Clinically, patients with reduced TNX levels showed hypermobile joints, often associated with joint subluxations and chronic musculoskeletal pain. Patients with haploinsufficiency did not have skin hyperextensibility and lacked the easy bruising seen in patients with complete TNX deficiency. In addition, TNXB haploinsufficiency was autosomal dominant. Zweers et al. (2003) found that 1 of these 6 patients had the 30-kb deletion, which created a fusion gene of TNXB and XA, a partial duplicate of TNXB. The XA gene has an internal deletion that truncates its open reading frame, rendering it and the fusion gene nonfunctional (Gitelman et al., 1992). The deleted allele also lacked CYP21 (613815).
In a patient with Ehlers-Danlos syndrome (EDSCLL1; 606408), Schalkwijk et al. (2001) identified a homozygous 2-bp deletion, 56063_56064delAA, in exon 8 of the TNXB gene. The 2-bp deletion altered the open reading frame, affecting amino acids 1184 through 1230, after which a premature stop codon was encountered. The patient's clinically normal father was heterozygous for the deletion; the mother could not be studied. One sister carried the deletion. An unrelated patient was also homozygous for this mutation, but her parents were unavailable for study.
In a patient with a hypermobility type of Ehlers-Danlos syndrome, Zweers et al. (2003) identified heterozygosity for the 56063_56064delAA mutation.
In a patient with autosomal recessive Ehlers-Danlos syndrome (EDSCLL1; 606408), Schalkwijk et al. (2001) identified a homozygous 2-bp insertion, 44906_44907insGT, in exon 3 of the TNXB gene. The insertion of GT replaced the glutamic acid residue at position 707 with a stop codon. Additional family members were not available for study.
In a patient with Ehlers-Danlos syndrome (EDSCLL1; 606408) and normal TNX serum levels, Zweers et al. (2005) identified a heterozygous 3583A-G transition in the TNXB gene, resulting in a val1195-to-met (V1195M) substitution. The V1195M mutation occurs in a highly conserved region and was not identified in 96 control individuals. Skin biopsy of the patient showed a significant increase in elastic fiber length. The authors concluded that missense mutations in the TNXB gene are also disease-causing.
In a French man with Ehlers-Danlos syndrome (EDSCLL1; 606408) with a predominantly myopathic phenotype, Penisson-Besnier et al. (2013) identified compound heterozygous mutations in the TNXB gene: a c.12214C-T transition, resulting in an arg4072-to-cys (R4072C) substitution at a highly conserved residue in the C-terminal globular fibrinogen-like domain, and the common 30-kb deletion (600985.0001). The R4072C mutation was absent from 100 control individuals. The deletion was not found in the patient's unaffected father or brother, but each carried the R4072C mutation. Functional studies of the variants were not performed.
In affected members of a family with vesicoureteral reflux-8 (VUR8; 615963), Gbadegesin et al. (2013) identified a heterozygous c.9770C-T transition in exon 29 of the TNXB gene, resulting in a thr3257-to-ile (T3257I) substitution at a highly conserved residue in the linker region between the 23rd and 24th fibronectin type III domain. The mutation, which was found by a combination of linkage analysis and whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not present in the dbSNP or 1000 Genomes Project databases, or in over 800 control chromosomes. Two affected mutation carriers who were examined showed signs of joint hypermobility. Compared to controls, patient fibroblasts showed significantly impaired migration in a wound-healing assay; this was associated with decreased expression of phosphorylated FAK (600758). These findings suggested a defect in the focal adhesions that link the cell cytoplasm to the extracellular matrix, with persistent and enhanced cell adhesion. Gbadegesin et al. (2013) postulated a gain-of-function effect.
In a Caucasian boy with vesicoureteral reflux-8 (VUR8; 615963), Gbadegesin et al. (2013) identified a heterozygous c.3991G-A transition in exon 10 of the TNXB gene, resulting in a gly1331-to-arg (G1331R) substitution at a highly conserved residue in the 5th fibronectin type III domain. The mutation was not present in the 1000 Genomes Project database or in 178 controls. The patient's sister had a history of recurrent urinary tract infections, but DNA samples and imaging were not available. The proband was 1 of 11 probands with VUR who underwent sequencing of coding exons of the TNXB gene.
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