Alternative titles; symbols
HGNC Approved Gene Symbol: SMAD6
Cytogenetic location: 15q22.31 Genomic coordinates (GRCh38) : 15:66,702,236-66,782,849 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 15q22.31 | {Craniosynostosis 7, susceptibility to} | 617439 | Autosomal dominant | 3 |
| {Radioulnar synostosis, nonsyndromic} | 179300 | Autosomal dominant | 3 | |
| Aortic valve disease 2 | 614823 | Autosomal dominant | 3 |
MAD-related proteins, named for Drosophila 'mothers against decapentaplegic,' act as second messengers distal to the transforming growth factor-beta family of receptors (e.g., TGFBR1; 190181).
Using a differential display approach in cultured endothelial cells subjected to multiple soluble and biomechanical stimuli, Topper et al. (1997) isolated a human endothelial cell cDNA encoding MADH6, which they called SMAD6. The predicted 235-amino acid MADH6 protein exhibits significant sequence similarity to other human MAD-related proteins, including the presence of a C-terminal MH2 domain; however, it lacks the conserved N-terminal MH1 domain, and certain conserved C-terminal serine residues that are involved in receptor-mediated phosphorylation. In situ hybridization and immunohistochemical studies on human tissues showed that MADH6 is expressed predominantly in vascular endothelium.
Riggins et al. (1996) reported an expressed sequence tag containing the C-terminal sequence of MADH6. By PCR screening of a somatic cell hybrid panel and YACs, they mapped the MADH6 gene to 15q21-q22.
Topper et al. (1997) demonstrated that MADH6 and MADH7 (602932) can form complexes in endothelial cells. MADH6 was induced in cultured vascular endothelium by fluid mechanical forces, and it modulated endothelial gene expression in response to both humoral and biomechanical stimuli in vitro.
Galvin et al. (2000) explored the role of an inhibitory SMAD in vivo by targeted mutation of Madh6, which encodes the Smad6 protein, in mice. Targeted insertion of a LacZ reporter demonstrated that Smad6 expression is largely restricted to the heart and blood vessels, and that Madh6 mutants have multiple cardiovascular abnormalities. Hyperplasia of the cardiac valves and outflow tract septation defects indicated a function of Smad6 in the regulation of endocardial cushion transformation. The role of Smad6 in the homeostasis of the adult cardiovascular system was indicated by the development of aortic ossification and elevated blood pressure in viable mutants. These defects highlighted the importance of SMAD6 in the tissue-specific modulation of TGFB superfamily signaling pathways in vivo.
Aortic Valve Disease 2
In 2 patients with bicuspid aortic valve (BAV) and aortic valve stenosis (AOVD2; 614823), 1 of whom also had aortic coarctation, Tan et al. (2012) identified heterozygosity for missense mutations in the SMAD6 gene (602931.0001, 602931.0002). Neither mutation was found in more than 1,800 controls, and both mutants had significantly lower activity than wildtype SMAD6.
By targeted resequencing of the SMAD6 gene in 473 unrelated patients with nonsyndromic thoracic aortic aneurysm (AAT) who were negative for mutation in known AAT genes, Luyckx et al. (2019) identified 6 probands with likely pathogenic variants (see, e.g., 602931.0006 and 602931.0007), all of whom also had bicuspid aortic valve.
In a 42-year-old Korean man with a severely calcified BAV and aneurysm of the ascending aorta, Park et al. (2019) screened 20 BAV-associated genes and identified heterozygosity for a 6-bp in-frame duplication in the SMAD6 gene (602931.0008).
Craniosynostosis 7
In 13 probands with nonsyndromic craniosynostosis affecting the sagittal and/or metopic sutures (CRS7; 617439), Timberlake et al. (2016) identified heterozygosity for 13 different variants in the SMAD6 gene (see, e.g., 602931.0003-602931.0005). Analysis of family members revealed 4 additional affected individuals. All 17 carried a SMAD6 mutation, but striking incomplete penetrance was observed in the families, as none of 10 parental mutation carriers were affected (overall penetrance, 57%). Genotyping for the 'C' risk allele of a candidate common variant located downstream of the BMP2 gene (rs1884302; 112261.0002) demonstrated that all 14 individuals with a SMAD6 mutation and the risk allele had craniosynostosis, whereas only 3 (19%) of 16 individuals with a SMAD6 mutation and no rs1884302 risk allele were affected. In addition, none of the 18 family members who carried only the rs1884302 risk allele, including 2 homozygotes, had craniosynostosis. Segregation analysis in a parametric 2-locus linkage model yielded a 7.37 maximum lod score. Of the 13 SMAD6 variants, 8 were frameshift or premature termination mutations, supporting haploinsufficiency as the mechanism of the genetic contribution of SMAD6 to craniosynostosis. Timberlake et al. (2016) suggested a threshold-effect model in which quantitative increases in SMAD signaling, resulting from reduced inhibition due to SMAD6 haploinsufficiency as well as from increased BMP2 expression via the rs1884302 risk allele, cause accelerated closure of midline sutures.
Luyckx et al. (2019) screened for the presence of the BMP2 'risk allele' for craniosynostosis (rs1884302) in 22 probands with bicuspid aortic valve and thoracic aortic aneurysm who carried SMAD6 variants, and detected the BMP2 risk allele in 6 probands, none of whom exhibited craniosynostosis. Luyckx et al. (2019) stated that they could not support the digenic model proposed by Timberlake et al. (2016) and suggested that additional factors contribute to the phenotypic outcomes of patients with cardiovascular disease or craniosynostosis.
Radioulnar Synostosis
Yang et al. (2019) performed exome sequencing on 117 patients with sporadic radioulnar synostosis (RUS; 179300) and found significant enrichment for loss-of-function variants in the SMAD6 gene. Yang et al. (2019) identified 22 SMAD6 rare variants (with a minor allele frequency of less than 0.0001) that occurred in 22 nonsyndromic RUS patients. Logistic regression showed that SMAD6 loss-of-function variants were significantly associated with increased risk of nonsyndromic RUS (OR 430; 95% CI 237.5-780.1; p less than 0.000001). Segregation analysis was used to test whether these SMAD6 variants segregated with nonsyndromic RUS. Parental genomic DNA was available for 11 probands with SMAD6 rare variants. Six were inherited from the proband's unaffected mother and 1 was affected from the proband's unaffected father. Additional sequencing detected 5 rare variants, 2 from 8 sporadic nonsyndromic RUS patients and 3 from 10 nonsyndromic RUS families. Yang et al. (2019) identified a total of 27 rare variants (19 loss of function and 8 missense) on SMAD6 that occurred in 24 of 125 sporadic cases and 3 of 10 families with nonsyndromic RUS. Among the 19 loss-of-function variants, 14 (73.7%) occurred in exon 1 of SMAD6, and 16 were located in the N domain of the SMAD6 protein. Among the 8 rare missense variants of SMAD6, 3 were located in the MH2 domain and 5 are evenly distributed in the N domain section between amino acids 154 and 267. Yang et al. (2019) identified 30 nonsyndromic RUS patients (sporadic and familial) who harbored SMAD6 rare variants. Among them 25 were male and 5 were female, for a male-to-female ratio of 5:1. In addition, 17 were affected by bilateral RUS and 13 were affected by unilateral RUS (left 9, right 4).
Horiki et al. (2004) found that transgenic mice overexpressing Smad6 in chondrocytes showed postnatal dwarfism, with lower average body weight and lower average skeletal component length than wildtype. Analysis of cultured explants of metatarsal primordial cartilage of transgenic mice showed that excessive Smad6 blocked Smad signaling. Transgenic mice displayed osteopenia, but both osteoclast precursors and osteoclast-supporting activities of osteoblast/stromal cells appeared normal in bone marrow. Compared with wildtype mice, transgenic mice had much smaller skeleton, delayed hypertrophy of chondrocytes, and reduced population of hypertrophic chondrocytes, in spite of normal chondrocyte proliferation. Chondrocyte hypertrophy induced by Bmp2 (112261) was downregulated in cartilage explants of transgenic mice. Unlike Smad6, overexpression of Smurf1 (605568) in chondrocytes of transgenic mice did not significantly affect chondrocyte proliferation or hypertrophy induced by Bmp2. However, overexpression of Smurf1 in Smad6 transgenic mice enhanced the phenotype of Smad6 transgenic mice.
In a man with bicuspid aortic valve, aortic valve stenosis, and coarctation and calcification of the aorta (AOVD2; 614823), Tan et al. (2012) identified heterozygosity for a nonsynonymous variant in exon 4 of SMAD6, resulting in a cys484-to-phe (C484F) substitution at a highly conserved residue within the MH2 domain. The mutation was not found in 1,000 Caucasian controls of British ancestry, in 629 individuals in the 1000 Genomes Project database, or in 200 individuals in the Danish Exome Project database. Functional analysis demonstrated that the MH2 domain of the mutant protein inhibited BMP (see 112264) signaling less efficiently than the equivalent domain of wildtype SMAD6, with relative luciferase readings that were 24-fold higher than wildtype. Tan et al. (2012) suggested that C484F could be considered a null allele, since there was almost no inhibitory effect on BMPR1A (601299) activity in the assay.
Park et al. (2019) performed immunoblot analysis of transfected C2C12 cells and observed that the C484F mutant appeared to inhibit BMP signaling less efficiently than wildtype. The wildtype protein also inhibited luciferase activity, whereas the mutant could not. In addition, wildtype SMAD6 inhibited osteoblast differentiation, whereas the C484F mutant showed impaired inhibition, suggesting that the mutant protein has less efficacy in preventing tissue calcification.
In an infant with bicuspid aortic valve and moderate aortic stenosis (AOVD2; 614823), Tan et al. (2012) identified heterozygosity for a nonsynonymous variant in exon 4 of SMAD6, resulting in a pro415-to-leu (P415L) substitution at a highly conserved residue within the MH2 domain. The mutation was not found in 1,000 Caucasian controls of British ancestry, in 629 individuals in the 1000 Genomes Project database, or in 200 individuals in the Danish Exome Project database. Functional analysis demonstrated that the MH2 domain of the mutant protein inhibited BMP (see 112264) signaling less efficiently than the equivalent domain of wildtype SMAD6, with relative luciferase readings that were 7-fold higher than wildtype, consistent with a hypomorphic allele.
In 2 brothers with craniosynostosis (CRS7; 617439), Timberlake et al. (2016) identified heterozygosity for a 1-bp deletion in the SMAD6 gene (c.1034delG, NM_005585), causing a frameshift predicted to result in a premature termination codon (Arg345fsTer194) within the MH2 domain. The 1-year-old brother, who had sagittal craniosynostosis, also carried the 'C' risk allele of a common variant downstream of the BMP2 gene (rs1884302; 112261.0002). His 5-year-old brother, who had metopic craniosynostosis, carried only the SMAD6 1-bp deletion, which was inherited from his unaffected father; their unaffected mother carried only the rs1884302 risk allele, as did an unaffected sister. The brothers had no other abnormalities.
In a 5-year-old girl with metopic craniosynostosis (CRS7; 617439), Timberlake et al. (2016) identified heterozygosity for a c.667C-T transition (c.667C-T, NM_005585) in the SMAD6 gene, resulting in a gln223-to-ter (Q223X) substitution within the MH1 domain. The proband also carried the 'C' risk allele of a common variant downstream of the BMP2 gene (rs1884302; 112261.0002), which she inherited from her unaffected mother. Her unaffected father was heterozygous for the SMAD6 Q223X mutation, as was her unaffected sister; neither carried the rs1884302 risk allele. The proband, who underwent suture repair at age 5 months, also had speech delay, gross motor delay, and fine motor impairment, with persistent delays at age 5 years.
In 15-year-old twin brothers with metopic craniosynostosis (CRS7; 617439), Timberlake et al. (2016) identified heterozygosity for a c.1393C-T transition (c.1393C-T, NM_005585) in the SMAD6 gene, resulting in an arg465-to-cys (R465C) substitution within the MH2 domain. Both brothers also carried the 'C' risk allele of a common variant downstream of the BMP2 gene (rs1884302; 112261.0002), for which their unaffected mother was homozygous and an unaffected sister heterozygous; neither unaffected individual carried the SMAD6 R465C mutation, and DNA was unavailable from the father. The brothers, who both underwent suture repair at age 1 year, also exhibited global developmental delay with a sensory disorder, marked speech delay, and attention deficit hyperactivity disorder.
In a 37-year-old woman with bicuspid aortic valve and aneurysm of the ascending aorta (AOVD2; 614823), Luyckx et al. (2019) identified heterozygosity for a c.42G-A transition (chr15.66,995,638, GRCh37) in the SMAD6 gene, resulting in a trp14-to-ter (W14X) substitution. Her 17-year-old daughter was also heterozygous for W14X but was asymptomatic. The variant was not present in the gnomAD database.
In a 57-year-old man with bicuspid aortic valve, aneurysm of the ascending aorta, and coarctation of the aorta (AOVD2; 614823), Luyckx et al. (2019) identified heterozygosity for a 1-bp deletion (c.794del; chr15.66,996,390del, GRCh37), predicted to cause a frameshift resulting in a premature termination codon (His265ProfsTer274). His father, who died at age 80 due to rupture of a thoracic-abdominal diffuse aneurysm, was also heterozygous for the mutation. The father had known aortic tortuosity and arterial wall calcification, but no valvular abnormalities were reported. The variant was not present in the gnomAD database.
In a 42-year-old Korean man with a severely calcified bicuspid aortic valve and aneurysm of the ascending aorta (AOVD2; 614823), Park et al. (2019) identified heterozygosity for a 6-bp duplication (c.1168_173dup) in the SMAD6 gene, resulting in an in-frame duplication (Gly390_Ile391dup) within the MH2 domain. The mutation was not found in his unaffected mother or brother, or in the 1000 Genomes Project, gnomAD, or Korean Reference Genome databases; DNA was unavailable from his deceased father, who had no history of cardiovascular disease. Immunoblot analysis of transfected C2C12 cells indicated that the Gly390_Ile391dup mutant inhibited BMP signaling less efficiently than wildtype. The wildtype protein also inhibited luciferase activity, whereas the mutant could not. In addition, wildtype SMAD6 inhibited osteoblast differentiation, whereas the Gly390_Ile391dup mutant showed impaired inhibition, suggesting that the mutant protein has less efficacy in preventing tissue calcification.
In a father (M2692) and daughter (M2553) with unilateral radioulnar synostosis (RUS; 179300), Yang et al. (2019) identified a heterozygous C-to-G transversion (c.1050C-G, NM_005585) at nucleotide 1050 of the SMAD6 gene leading to a tyrosine-to-premature termination codon substitution at amino acid 350 (Y350X). This variant was found at a frequency of 0.00001268 in gnomAD.
In a male (M1180) with unilateral radioulnar synostosis (RUS; 179300), Yang et al. (2019) identified a heterozygous T-to-A transversion (c.613T-A, NM_005585) at nucleotide 613 of the SMAD6 gene leading to a cysteine-to-serine substitution at codon 205 (C205S). A cysteine is found at this position in all members of the SMAD family. This variant was absent from gnomAD.
In a father (M2661) with unilateral and son (M2660) with bilateral radioulnar synostosis (RUS; 179300), Yang et al. (2019) identified a heterozygous G-to-A transition (c.1412G-A, NM_005585) at nucleotide 1412 of the SMAD6 gene leading to a glycine-to-aspartic acid substitution at codon 471 (G471D). This variant was absent from gnomAD.
In 2 cousins (M2694 and M2466) with bilateral radioulnar synostosis (RUS; 179300), Yang et al. (2019) identified a heterozygous 10-bp duplication (c.1304_1313dup, NM_005585) in exon 1 of the SMAD6 gene, resulting in frameshift and premature termination of the protein (Phe349HisfsTer129). The variant was inherited in each cousin from the unaffected father and mother, respectively, who were sibs. This variant was absent from gnomAD.
In a female (M2111) with bilateral radioulnar synostosis (RUS; 179300), Yang et al. (2019) identified a de novo heterozygous 7-bp deletion (c.465_471del, NM_005585) in exon 1 of the SMAD6 gene, resulting in a frameshift and premature termination of the protein (Gly156ValfsTer23). This variant was absent from gnomAD.
Galvin, K. M., Donovan, M. J., Lynch, C. A., Meyer, R. I., Paul, R. J., Lorenz, J. N., Fairchild-Huntress, V., Dixon, K. L., Dunmore, J. H., Gimbrone, M. A., Jr., Falb, D., Huszar, D. A role for Smad6 in development and homeostasis of the cardiovascular system. Nature Genet. 24: 171-174, 2000. [PubMed: 10655064] [Full Text: https://doi.org/10.1038/72835]
Horiki, M., Imamura, T., Okamoto, M., Hayashi, M., Murai, J., Myoui, A., Ochi, T., Miyazono, K., Yoshikawa, H., Tsumaki, N. Smad6/Smurf1 overexpression in cartilage delays chondrocyte hypertrophy and causes dwarfism with osteopenia. J. Cell Biol. 165: 433-445, 2004. [PubMed: 15123739] [Full Text: https://doi.org/10.1083/jcb.200311015]
Luyckx, I., MacCarrick, G., Kempers, M., Meester, J., Geryl, C., Rombouts, O., Peeters, N., Claes, C., Boeckx, N., Sakalihasan, N., Jacquinet, A., Hoichen, A., and 10 others. Confirmation of the role of pathogenic SMAD6 variants in bicuspid aortic valve-related aortopathy. Europ. J. Hum. Genet. 27: 1044-1053, 2019. [PubMed: 30796334] [Full Text: https://doi.org/10.1038/s41431-019-0363-z]
Park, J. E., Park, J. S., Jang, S. Y., Park, S. H., Kim, J.-W., Ki, C.-S., Kim, D.-K. A novel SMAD6 variant in a patient with severely calcified bicuspid aortic valve and thoracic aortic aneurysm. Molec. Genet. Genomic Med. 7: e620, 2019. Note: Electronic Article. [PubMed: 30848080] [Full Text: https://doi.org/10.1002/mgg3.620]
Riggins, G. J., Thiagalingam, S., Rozenblum, E., Weinstein, C. L., Kern, S. E., Hamilton, S. R., Willson, J. K. V., Markowitz, S. D., Kinzler, K. W., Vogelstein, B. Mad-related genes in the human. Nature Genet. 13: 347-349, 1996. [PubMed: 8673135] [Full Text: https://doi.org/10.1038/ng0796-347]
Tan, H. L., Glen, E., Topf, A., Hall, D., O'Sullivan, J. J., Sneddon, L., Wren, C., Avery, P., Lewis, R. J., ten Dijke, P., Arthur, H. M., Goodship, J. A., Keavney, B. D. Nonsynonymous variants in the SMAD6 gene predispose to congenital cardiovascular malformation. Hum. Mutat. 33: 720-727, 2012. [PubMed: 22275001] [Full Text: https://doi.org/10.1002/humu.22030]
Timberlake, A. T., Choi, J., Zaidi, S., Lu, Q., Nelson-Williams, C., Brooks, E. D., Bilguvar, K., Tikhonova, I., Mane, S., Yang, J. F., Sawh-Martinez, R., Persing, S., and 12 others. Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles. eLife 5: e20125, 2016. Note: Electronic Article. [PubMed: 27606499] [Full Text: https://doi.org/10.7554/eLife.20125]
Topper, J. N., Cai, J., Qiu, Y., Anderson, K. R., Xu, Y.-Y., Deeds, J. D., Feeley, R., Gimeno, C. J., Woolf, E. A., Tayber, O., Mays, G. G., Sampson, B. A., Schoen, F. J., Gimbrone, M. A., Jr., Falb, D. Vascular MADs: two novel MAD-related genes selectively inducible by flow in human vascular endothelium. Proc. Nat. Acad. Sci. 94: 9314-9319, 1997. [PubMed: 9256479] [Full Text: https://doi.org/10.1073/pnas.94.17.9314]
Yang, Y., Zheng, Y., Li, W., Li, L., Tu, M., Zhao, L., Mei, H., Zhu, G., Zhu, Y. SMAD6 is frequently mutated in nonsyndromic radioulnar synostosis. Genet. Med. 21: 2577-2585, 2019. Note: Erratum: Genet. Med. 21: 2409 only, 2019. [PubMed: 31138930] [Full Text: https://doi.org/10.1038/s41436-019-0552-8]