Alternative titles; symbols
HGNC Approved Gene Symbol: TUBB3
Cytogenetic location: 16q24.3 Genomic coordinates (GRCh38): 16:89,921,925-89,936,097 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 16q24.3 | Cortical dysplasia, complex, with other brain malformations 1 | 614039 | Autosomal dominant | 3 |
| Fibrosis of extraocular muscles, congenital, 3A | 600638 | Autosomal dominant | 3 |
Microtubules are dynamic polymeric structures consisting of heterodimers of alpha-tubulins (see 602529) and beta-tubulins, such as TUBB3, that are continuously incorporated and released. Microtubules function in mitosis, intracellular transport, neuron morphology, and ciliary and flagellar motility (Leandro-Garcia et al., 2010).
Sullivan and Cleveland (1986) isolated a partial TUBB3 cDNA, which they designated beta-4, from a human placenta cDNA library. Ranganathan et al. (1998) cloned cDNAs encoding human TUBB3 from a prostate carcinoma cell line.
Wang et al. (1986) cloned mouse Tubb3, which they called M-beta-3, from testis and bone marrow cDNA libraries. Northern blot analysis detected abundant Tubb3 expression in testis, where expression began around postnatal day 32. Much lower Tubb3 levels were detected in all other adult mouse tissues examined except brain, where expression was even lower.
Burgoyne et al. (1988) stated that the mouse homolog of human TUBB3, M-beta-6, is expressed primarily in neural tissue.
By RT-PCR analysis of a human brain cDNA library, Poirier et al. (2010) cloned TUBB3, which encodes a 450-amino acid protein. RT-PCR, immunocytochemical, and Western blot analyses showed that TUBB3 was significantly expressed in fibroblasts.
Using database analysis, Leandro-Garcia et al. (2010) identified 8 major beta-tubulins, including TUBB3. Quantitative RT-PCR of 21 normal human tissues detected highest TUBB3 expression in brain, much lower expression in small intestine, testis, and placenta, and little to no expression in other tissues.
Tischfield et al. (2010) noted that the TUBB3 gene maps to chromosome 16q24.3.
Antimicrotubule drugs are used in cancer chemotherapy. By RT-PCR and Western blot analyses, Ranganathan et al. (1998) found that drug treatment of prostate carcinoma cells selectively increased TUBB3 mRNA and protein levels. In brain tumor cell lines, the amount of TUBB3 protein was positively correlated with resistance to an antimicrotubule agent. Ranganathan et al. (1998) suggested that the selective increase in TUBB3, an isotype that imparts more dynamicity to microtubules, may provide a way for cells to overcome the effect of antimicrotubule agents.
Hari et al. (2003) transfected beta III tubulin into Chinese hamster ovary cells and demonstrated that beta III tubulin diminished microtubule assembly in a dose-dependent manner and was toxic when present at high levels. Cells expressing beta III tubulin at moderate levels were weakly resistant to paclitaxel, a commonly used chemotherapeutic agent.
Raspaglio et al. (2008) found that hypoxia induced TUBB3 expression in several human cell lines, but not in a paclitaxel-resistant cell line. RNA interference revealed that HIF1A (603348) mediated hypoxia-induced TUBB3 expression. Chromatin immunoprecipitation analysis showed that HIF1A bound an HIF1A-binding site in the 5-prime flanking region of the TUBB3 gene. Methylation of an enhancer in the 3-prime flanking region abolished hypoxia-induced TUBB3 expression.
Congenital Fibrosis of Extraocular Muscles 3A
Tischfield et al. (2010) identified 8 different heterozygous mutations in the TUBB3 gene (see, e.g., 602661.0001-602661.0005) in affected individuals with autosomal dominant or sporadic congenital fibrosis of extraocular muscles type 3A with or without extraocular involvement (CFEOM3A; 600638). In vitro functional expression studies, mutant mouse studies, and yeast studies showed that all of the mutations resulted in altered microtubule dynamics and stability, and some showed various loss of kinesin microtubule interactions. Most affected individuals had congenital fibrosis of extraocular muscles, but some showed additional defects, including developmental delay or learning disabilities associated with dysgenesis of the corpus callosum. Other variable features included facial weakness and peripheral axonal neuropathy, sometimes associated with wrist and finger contractures. There were some genotype/phenotype correlations: those with the E410K (602661.0005) and D417H (602661.0003) mutations had a severe phenotype with additional features, including facial weakness and learning disabilities, whereas those with the D417N (602661.0004) mutation had peripheral neuropathy and hypoplasia of the corpus callosum, and those with the common R262C mutation (602661.0001) had the mildest phenotype. Overall, the findings were consistent with defects in axonal guidance, suggesting that the TUBB3 gene is critical for normal microtubule dynamics, axonal guidance, and brain development.
Complex Cortical Dysplasia With Other Brain Malformations 1
In affected members of 7 unrelated families with complex cortical dysplasia with other brain malformations-1 (CDCBM1; 614039), Poirier et al. (2010) identified 6 different heterozygous missense mutations in the TUBB3 gene (see, e.g., 602661.0006-602661.0009). In vitro functional expression studies showed that most of the mutations resulted in impairment of the tubulin heterodimerization process or caused impaired microtubule stability. Affected individuals had mild to severe mental retardation, strabismus, axial hypotonia, and spasticity. Brain imaging showed variable malformations of cortical development, including polymicrogyria, gyral disorganization, and fusion of the basal ganglia, as well as thin corpus callosum, hypoplastic brainstem, and dysplastic cerebellar vermis. Extraocular muscles were not involved.
Three of the TUBB3 mutations identified by Poirier et al. (2010) in patients with complex cortical dysplasia and other brain malformations (G82R, T178M; 602661.0008, and M323V; 602661.0009) were localized to the surface of the tubulin molecule, on the opposite side of TUBB3 mutations reported by Tischfield et al. (2010) (R262C; 602661.0001, E410K; 602661.0005, and D417N; 602661.0004), suggesting that these 2 groups of mutations could impair microtubules and/or their interactions with proteins critical for microtubule function in different ways. Mutations associated with different phenotypes might reside in different functional areas of the protein. However, in the specific case of the ala302 residue, the nature of the variation seemed to influence the resulting phenotype, since the A302T (602661.0002) and A302V (602661.0008) mutations resulted in CFEOM3A and CDCBM syndromes, respectively.
Tischfield et al. (2010) showed that heterozygous mice carrying the R262C mutation (602661.0001) appeared healthy and had no external eye phenotypes. Homozygous mice failed to breathe normally and died within hours of birth. Homozygous mice showed defects in the guidance of commissural axons and cranial nerves, and some had agenesis of the corpus callosum; heterozygous mice had thin anterior commissures. Both homo- and heterozygous R262C mice had decreased levels of Tubb3 protein, but the Tubb3 protein resulted in increased microtubule stability with decreased Kif21a (608283) microtubule interactions.
In affected members of 11 unrelated pedigrees with congenital fibrosis of extraocular muscles-3A (CFEOM3A; 600638), Tischfield et al. (2010) identified a heterozygous 784C-T transition in exon 4 of the TUBB3 gene, resulting in an arg262-to-cys (R262C) substitution. Several of the pedigrees had previously been reported by Doherty et al. (1999), Gillies et al. (1995), Mackey et al. (2002), and Yamada et al. (2004). The ocular motility defects ranged from severe to mild. Brain MRI of affected members in 4 pedigrees showed hypoplasia of the oculomotor nerve and the muscles innervated by its superior division, the levator palpebrae superioris and superior rectus, as well as the medial rectus muscle innervated by its inferior division. The oculomotor nerve also aberrantly innervated the lateral rectus muscle, normally innervated by the abducens nerve. The findings were consistent with axon guidance defects. In addition, affected individuals in some of the families showed developmental delay or learning disabilities, and some had mild dysgenesis of the corpus callosum. In vitro studies showed patchy mutant heterodimer incorporation into microtubules. Further studies showed decreased polymerization rates and increased depolymerization rates, as well as altered interaction with kinesins. The mutation was not found in over 1,700 control chromosomes.
In affected members of 3 unrelated pedigrees with congenital fibrosis of extraocular muscles-3A (CFEOM3A; 600638), Tischfield et al. (2010) identified a heterozygous 904G-A transition in exon 4 of the TUBB3 gene, resulting in an ala302-to-thr (A302T) substitution in a region proposed to mediate lateral interactions between tubulin heterodimers, which assemble to form microtubules. Two affected individuals in 1 of the families showed learning disabilities and hypoplasia of the corpus callosum. In vitro functional expression studies showed that the A302T mutant incorporated into microtubules, increased the stability of microtubules, and diminished overall microtubule dynamics. The mutation was not found in over 1,700 control chromosomes.
Notably, a mutation affecting the same residue, A302V (602661.0008), was identified as causative of complex cortical dysplasia and other brain malformations (614039) (Poirier et al., 2010).
In a mother and 2 sons with congenital fibrosis of extraocular muscles-3A (CFEOM3A; 600638), Tischfield et al. (2010) identified a heterozygous 1249G-C transversion in the TUBB3 gene, resulting in an asp417-to-his (D417H) substitution in a region on the external surface of microtubules that mediate interactions with motor proteins and other proteins. Other clinical features included facial weakness, wrist and finger contractures, peripheral axonal neuropathy in 2, and developmental delay in 1. Brain imaging was not performed. In vitro functional expression studies in yeast showed that the mutation resulted in altered kinesin microtubule interactions. The mutation was not found in over 1,700 control chromosomes. Another mutation in this residue was also identified (D417N; 602661.0004).
In affected members of 4 unrelated pedigrees with congenital fibrosis of extraocular muscles-3A (CFEOM3A; 600638), Tischfield et al. (2010) identified a heterozygous 1249G-A transition in exon 4 of the TUBB3 gene, resulting in an asp417-to-asn (D417N) substitution in a region on the external surface of microtubules that mediate interactions with motor proteins and other proteins. Affected members of 2 families had isolated CFEOM, but affected members in the other 2 families also developed axonal peripheral neuropathy in the second to third decade. One Belgian family, previously reported by Abeloos et al. (1990), had 9 affected individuals, 4 of whom also had developmental delay or learning disabilities. Five mutation carriers in this family had peripheral neuropathy in the absence of CFEOM. In vitro functional expression studies in yeast showed that the mutation resulted in altered kinesin microtubule interactions. The mutation was not found in over 1,700 control chromosomes. Another mutation in this residue was also identified (D417H; 602661.0003).
In 6 unrelated individuals with sporadic congenital fibrosis of extraocular muscles-3A (CFEOM3A; 600638), Tischfield et al. (2010) identified a heterozygous 1228G-A transition in exon 4 of the TUBB3 gene, resulting in a glu410-to-lys (E410K) substitution in a region on the external surface of microtubules that mediate interactions with motor proteins and other proteins. All of the mutations appeared to be de novo. In addition to CFEOM, all individuals had developmental delay and facial weakness. Some had agenesis of the corpus callosum on brain MRI. One also had peripheral neuropathy with wrist and finger contractures. In vitro functional expression studies and yeast studies showed that the mutation resulted in decreased polymerization rates and increased depolymerization rates, as well as altered kinesin microtubule interactions. The mutation was not found in over 1,700 control chromosomes.
In a patient with complex cortical dysplasia and other brain malformations-1 (CDCBM1; 614039), Poirier et al. (2010) identified a de novo heterozygous c.533C-T transition in exon 4 of the TUBB3 gene, resulting in a thr178-to-met (T178M) substitution in the N-terminal domain. The mutation was not detected in 360 Caucasian controls. Protein modeling showed that the mutation localized to the surface of the alpha-beta tubulin heterodimer. Structural data suggested that the mutation may influence GTP binding. In vitro functional expression assays showed that the mutant protein was translated and could form tubulin heterodimers that were properly incorporated into microtubules, although the microtubules were less stable than wildtype. The patient had microcephaly, virtually no psychomotor development, axial hypotonia, spastic tetraplegia, refractory epilepsy, cortical gyral disorganization, hypoplasia of the cerebellum and brainstem, agenesis of the corpus callosum, and fusion of the caudate and putamen.
In affected members of 2 unrelated families with complex cortical dysplasia and other brain malformations-1 (CDCBM1; 614039), Poirier et al. (2010) identified a heterozygous c.613G-A transition in exon 4 of the TUBB3 gene, resulting in a glu205-to-lys (E205K) substitution in the intermediate domain. The mutation occurred de novo in 1 family and was inherited in an autosomal dominant pattern in the other family. The mutation was not detected in 360 Caucasian controls. In vitro functional expression studies showed that the mutant protein did not form tubulin heterodimers and had a subtle impairment in its ability to incorporate into the cytoskeleton. Studies of patient fibroblasts showed no major alterations in the microtubule network, but the microtubules were less stable than wildtype. The patients had moderate to severe mental retardation, polymicrogyria, dysplastic cerebellum, hypoplasia of the brainstem, and fusion of the caudate and putamen.
In 4 affected members of a family with complex cortical dysplasia and other brain malformations-1 (CDCBM1; 614039), Poirier et al. (2010) identified a heterozygous c.905C-T transition in exon 4 of the TUBB3 gene, resulting in an ala302-to-val (A302V) substitution in the intermediate domain. The mutation was not detected in 360 Caucasian controls. In vitro functional expression studies showed that the mutant protein did not form tubulin heterodimers. The mother had mild mental retardation, gyral disorganization, and dysplasia of the cerebellar vermis. One of her daughters was homozygous for the mutation and had a more severe phenotype with axial hypotonia, gyral disorganization, dysplastic cerebellar vermis, hypoplastic brainstem, thin corpus callosum, and fusion of the caudate and putamen.
Notably, a mutation affecting the same residue, A302T (602661.0002), had been identified as causative of CFEOM3A (600638) (Tischfield et al., 2010).
In a father and son with complex cortical dysplasia and other brain malformations-1 (CDCBM1; 614039), Poirier et al. (2010) identified a heterozygous c.967A-G transition in exon 4 of the TUBB3 gene, resulting in a met323-to-val (M323V) substitution in intermediate domain. Protein modeling showed that the mutation localized to the surface of the alpha-beta tubulin heterodimer. In vitro functional expression assays showed that the mutant protein was translated, but generated diminished levels of tubulin heterodimers. The patients had mental retardation, gyral disorganization, dysplastic cerebellum, hypoplastic brainstem, thin corpus callosum, and mild fusion of the basal ganglia.
In a 27-week-old fetus (individual 17) with complex cortical dysplasia with other brain malformations-1 (CDCBM1; 614039) manifest as microlissencephaly, Fallet-Bianco et al. (2014) identified a heterozygous c.1162A-G transition in the TUBB3 gene, resulting in a met388-to-val (M388V) substitution. Functional studies of the variant and studies of patient cells were not performed.
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