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A microfibril assembly assay identifies different mechanisms of dominance underlying Marfan syndrome, stiff skin syndrome and acromelic dysplasias.

Jensen SA, Iqbal S, Bulsiewicz A, Handford PA - Hum. Mol. Genet. (2015)

Bottom Line: The majority of mutations affecting the human fibrillin-1 gene, FBN1, result in Marfan syndrome (MFS), a common connective tissue disorder characterised by tall stature, ocular and cardiovascular defects.Despite their apparent phenotypic differences, dysregulation of transforming growth factor β (TGFβ) is a common factor in all of these disorders.We show that substitutions in fibrillin-1 domains TB4 and TB5 that cause SSS and the acromelic dysplasias do not prevent fibrillin-1 from being secreted or assembled into microfibrils, whereas MFS-associated substitutions in these domains result in a loss of recombinant protein in the culture medium and no association with microfibrils.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Oxford, South Parks Rd, Oxford OX1 3QU, UK sacha.jensen@bioch.ox.ac.uk.

No MeSH data available.


Related in: MedlinePlus

Model of the mechanisms leading to different dominantly inherited fibrillinopathies associated with fibrillin-1 domains TB4 and TB5. (A) Wild-type fibrillin-1 is secreted from cells and assembled to produce a microfibril network that influences cell signalling through integrins and cell-surface HSPGs. Normal matrix deposition of growth factors (green and yellow), and growth factor activation, is mediated by interactions between cells and microfibrils (curved arrows). (B) When one of the alleles carries a pathogenic mutation that results in intracellular retention of the mutant protein, fewer functional microfibrils are produced in all tissues. Altered tissue dynamics, and possibly a reduction in growth factor binding sites in the matrix, lead to a general increase in growth factor activation and the development of MFS, but with microfibrils maintaining normal interactions with integrins and cell-surface HSPGs. (C) Alternatively, the mutant proteins could be secreted and assembled into structurally normal microfibrils, with defective interactions between microfibrils and integrins and/or cell-surface HSPGs being affected by altered binding site presentation. The resulting alterations in signalling lead to a secondary upregulation in the production of extracellular matrix. In these cases, the resulting phenotype is limited to tissues expressing the cell-surface proteins most sensitive to the altered binding sites on the microfibril, as seen in SSS and the acromelic dysplasias.
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DDV181F5: Model of the mechanisms leading to different dominantly inherited fibrillinopathies associated with fibrillin-1 domains TB4 and TB5. (A) Wild-type fibrillin-1 is secreted from cells and assembled to produce a microfibril network that influences cell signalling through integrins and cell-surface HSPGs. Normal matrix deposition of growth factors (green and yellow), and growth factor activation, is mediated by interactions between cells and microfibrils (curved arrows). (B) When one of the alleles carries a pathogenic mutation that results in intracellular retention of the mutant protein, fewer functional microfibrils are produced in all tissues. Altered tissue dynamics, and possibly a reduction in growth factor binding sites in the matrix, lead to a general increase in growth factor activation and the development of MFS, but with microfibrils maintaining normal interactions with integrins and cell-surface HSPGs. (C) Alternatively, the mutant proteins could be secreted and assembled into structurally normal microfibrils, with defective interactions between microfibrils and integrins and/or cell-surface HSPGs being affected by altered binding site presentation. The resulting alterations in signalling lead to a secondary upregulation in the production of extracellular matrix. In these cases, the resulting phenotype is limited to tissues expressing the cell-surface proteins most sensitive to the altered binding sites on the microfibril, as seen in SSS and the acromelic dysplasias.

Mentions: The role of TGFβ in the pathogenesis of MFS was initially demonstrated by Neptune et al. (49) in a mouse model, and current data from both patient samples and animal models suggest that the fibrosis observed in SSS and the acromelic dysplasias is also due to TGFβ dysregulation (15,50,51). The mechanisms leading to TGFβ dysregulation, and how they result in the different phenotypes observed in these syndromes, remains unclear. Our data show that MFS-associated substitutions in domains TB4 and TB5 reduce the concentration of fibrillin-1 found in the medium of cultured cells, with a corresponding lack of mutant protein being incorporated into microfibrils. This is consistent with a dominant mechanism in which a functional haploinsufficiency of fibrillin-1 or loss of microfibrils is at the heart of the pathogenesis of MFS, as shown by mouse models and human deletion mutants (52,53). Based on the ability of the non-MFS TB domain mutants presented here to secrete and be incorporated into microfibrils, we propose that the phenotypes of SSS and the acromelic dysplasias are not caused by a primary defect in microfibril assembly. Instead, these mutants are more likely to alter the cell–matrix interactions involving microfibrils (Fig. 5) leading to secondary effects on the extracellular matrix, such as fibrosis. The types of alterations would depend on the repertoire of cell-surface proteins in particular tissues and would explain the more limited range of tissues affected in SSS and the acromelic dysplasias compared with MFS. We therefore propose that the aberrant TGFβ signalling observed in each of the fibrillin-1-associated diseases is due to different causes. In MFS, we suggest that it is due to a loss of structural integrity in the fibrillin matrix, which affects TGFβ activation through a change in the mechanical properties of tissues. In contrast, the primary defect in SSS and the acromelic dysplasias is more likely to be defective cell-surface interactions with microfibrils rather than a defect in microfibril assembly or stability. The TGFβ dysregulation, fibrosis and dermal accumulation of microfibril aggregates observed in SSS and the acromelic dysplasias would then be a secondary response to an altered signalling program initiated by changes in the surface interactions with microfibrils. Further molecular and cellular studies will be required to investigate differences in cellular signalling in the different disorders.Figure 5.


A microfibril assembly assay identifies different mechanisms of dominance underlying Marfan syndrome, stiff skin syndrome and acromelic dysplasias.

Jensen SA, Iqbal S, Bulsiewicz A, Handford PA - Hum. Mol. Genet. (2015)

Model of the mechanisms leading to different dominantly inherited fibrillinopathies associated with fibrillin-1 domains TB4 and TB5. (A) Wild-type fibrillin-1 is secreted from cells and assembled to produce a microfibril network that influences cell signalling through integrins and cell-surface HSPGs. Normal matrix deposition of growth factors (green and yellow), and growth factor activation, is mediated by interactions between cells and microfibrils (curved arrows). (B) When one of the alleles carries a pathogenic mutation that results in intracellular retention of the mutant protein, fewer functional microfibrils are produced in all tissues. Altered tissue dynamics, and possibly a reduction in growth factor binding sites in the matrix, lead to a general increase in growth factor activation and the development of MFS, but with microfibrils maintaining normal interactions with integrins and cell-surface HSPGs. (C) Alternatively, the mutant proteins could be secreted and assembled into structurally normal microfibrils, with defective interactions between microfibrils and integrins and/or cell-surface HSPGs being affected by altered binding site presentation. The resulting alterations in signalling lead to a secondary upregulation in the production of extracellular matrix. In these cases, the resulting phenotype is limited to tissues expressing the cell-surface proteins most sensitive to the altered binding sites on the microfibril, as seen in SSS and the acromelic dysplasias.
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Related In: Results  -  Collection

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DDV181F5: Model of the mechanisms leading to different dominantly inherited fibrillinopathies associated with fibrillin-1 domains TB4 and TB5. (A) Wild-type fibrillin-1 is secreted from cells and assembled to produce a microfibril network that influences cell signalling through integrins and cell-surface HSPGs. Normal matrix deposition of growth factors (green and yellow), and growth factor activation, is mediated by interactions between cells and microfibrils (curved arrows). (B) When one of the alleles carries a pathogenic mutation that results in intracellular retention of the mutant protein, fewer functional microfibrils are produced in all tissues. Altered tissue dynamics, and possibly a reduction in growth factor binding sites in the matrix, lead to a general increase in growth factor activation and the development of MFS, but with microfibrils maintaining normal interactions with integrins and cell-surface HSPGs. (C) Alternatively, the mutant proteins could be secreted and assembled into structurally normal microfibrils, with defective interactions between microfibrils and integrins and/or cell-surface HSPGs being affected by altered binding site presentation. The resulting alterations in signalling lead to a secondary upregulation in the production of extracellular matrix. In these cases, the resulting phenotype is limited to tissues expressing the cell-surface proteins most sensitive to the altered binding sites on the microfibril, as seen in SSS and the acromelic dysplasias.
Mentions: The role of TGFβ in the pathogenesis of MFS was initially demonstrated by Neptune et al. (49) in a mouse model, and current data from both patient samples and animal models suggest that the fibrosis observed in SSS and the acromelic dysplasias is also due to TGFβ dysregulation (15,50,51). The mechanisms leading to TGFβ dysregulation, and how they result in the different phenotypes observed in these syndromes, remains unclear. Our data show that MFS-associated substitutions in domains TB4 and TB5 reduce the concentration of fibrillin-1 found in the medium of cultured cells, with a corresponding lack of mutant protein being incorporated into microfibrils. This is consistent with a dominant mechanism in which a functional haploinsufficiency of fibrillin-1 or loss of microfibrils is at the heart of the pathogenesis of MFS, as shown by mouse models and human deletion mutants (52,53). Based on the ability of the non-MFS TB domain mutants presented here to secrete and be incorporated into microfibrils, we propose that the phenotypes of SSS and the acromelic dysplasias are not caused by a primary defect in microfibril assembly. Instead, these mutants are more likely to alter the cell–matrix interactions involving microfibrils (Fig. 5) leading to secondary effects on the extracellular matrix, such as fibrosis. The types of alterations would depend on the repertoire of cell-surface proteins in particular tissues and would explain the more limited range of tissues affected in SSS and the acromelic dysplasias compared with MFS. We therefore propose that the aberrant TGFβ signalling observed in each of the fibrillin-1-associated diseases is due to different causes. In MFS, we suggest that it is due to a loss of structural integrity in the fibrillin matrix, which affects TGFβ activation through a change in the mechanical properties of tissues. In contrast, the primary defect in SSS and the acromelic dysplasias is more likely to be defective cell-surface interactions with microfibrils rather than a defect in microfibril assembly or stability. The TGFβ dysregulation, fibrosis and dermal accumulation of microfibril aggregates observed in SSS and the acromelic dysplasias would then be a secondary response to an altered signalling program initiated by changes in the surface interactions with microfibrils. Further molecular and cellular studies will be required to investigate differences in cellular signalling in the different disorders.Figure 5.

Bottom Line: The majority of mutations affecting the human fibrillin-1 gene, FBN1, result in Marfan syndrome (MFS), a common connective tissue disorder characterised by tall stature, ocular and cardiovascular defects.Despite their apparent phenotypic differences, dysregulation of transforming growth factor β (TGFβ) is a common factor in all of these disorders.We show that substitutions in fibrillin-1 domains TB4 and TB5 that cause SSS and the acromelic dysplasias do not prevent fibrillin-1 from being secreted or assembled into microfibrils, whereas MFS-associated substitutions in these domains result in a loss of recombinant protein in the culture medium and no association with microfibrils.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Oxford, South Parks Rd, Oxford OX1 3QU, UK sacha.jensen@bioch.ox.ac.uk.

No MeSH data available.


Related in: MedlinePlus