Different TBX5 interactions in heart and limb defined by Holt-Oram syndrome mutations

Abstract

To better understand the role of TBX5, a T-box containing transcription factor in forelimb and heart development, we have studied the clinical features of Holt-Oram syndrome caused by 10 different TBX5 mutations. Defects predicted to create null alleles caused substantial abnormalities both in limb and heart. In contrast, missense mutations produced distinct phenotypes: Gly80Arg caused significant cardiac malformations but only minor skeletal abnormalities; and Arg237Gln and Arg237Trp caused extensive upper limb malformations but less significant cardiac abnormalities. Amino acids altered by missense mutations were located on the three-dimensional structure of a related T-box transcription factor, Xbra, bound to DNA. Residue 80 is highly conserved within T-box sequences that interact with the major groove of target DNA; residue 237 is located in the T-box domain that selectively binds to the minor groove of DNA. These structural data, taken together with the predominant cardiac or skeletal phenotype produced by each missense mutation, suggest that organ-specific gene activation by TBX5 is predicated on biophysical interactions with different target DNA sequences.

Figure 1

Fourteen mutations in TBX5 that cause Holt–Oram syndrome. (A) TBX5 genomic structure (our unpublished data and refs. and 11) is shown with approximate intron sizes. Exons 1–9 are shown with vertical bars. Exons 1a, 1b, or 1c are alternatively spliced as the first exon of the TBX5 cDNA. Alternative splicing of the 3′ region of the gene accounts for the variable addition of exon 9. Arrows indicate the translocation t(5;12)(q15;q24) found in the family IIa proband [designated t(5;12)], which disrupts TBX5 in intron 1a and the location of intron 2 acceptor site (AS) mutations Int2ASC₋₂A and Int2ASG₊₁C. Acceptor site residues were numbered from the splice site with the conserved G residue designated as +1. (B) Schematic representation of TBX5 cDNA (GenBank accession nos. U80987 and U89353) illustrating the largest of the alternatively spliced transcripts. Untranslated sequence (dark shading), exons 1–9 (numbered boxes), and locations of amino acids 1 and 517 are indicated. Codons (gray shading) that encode the T-box DNA binding domain [residues 56–238 (exons 3–7)] were defined by homology to other T-box gene family members. TBX5 mutations that are predicted to truncate TBX5 are shown above; missense mutations are indicated below (bold, italics). Mutations are designated by the name and number of the first substituted amino acid residue: Δ, deletion; Ins, insertion. FSter indicates frameshift mutations with resultant premature stop codons; ter indicates nonsense mutation. Mutations Glu69ter, Arg237Gln, InsAsn198FSter, Arg279ter, and InsSer387FSter have been described (10, 11). Mutation Ser196ter, reported here, was also previously observed in an unrelated family (11).

Figure 2

A chromosome 5;12 translocation causes severe skeletal and composite cardiac malformations. (A) The affected child in family IIa has left arm phocomelia, right radial hypoplasia, and right thumb aplasia. (B) Cardiac angiography demonstrated a common atrium and a complete atrioventricular canal defect. Contrast dye injected from a catheter passed across the atrioventricular septal defect from the right to left ventricle reveals a marked deformity (arrowheads, characteristic of this malformation. (C and D) FISH analysis of metaphase chromosomes from the affected child in family IIa demonstrates a breakpoint in the intron following exon 1a of TBX5. Pink signal indicates hybridization of TBX5.4 cDNA (encoding protein sequence) to a normal and derivative chromosome 12 in C. Hybridization of a genomic clone (JE98) containing exon 1a (D) produced pink signal in a normal chromosome 12 and the derivative chromosome 5.

Figure 3

(Upper) Three-dimensional structure of the X. laevis Xbra T-box bound to a 24-nt DNA duplex indicating the location of residues Gly-80 (red) and Arg-237 (yellow) mutated in the missense mutations Gly80Arg, Arg237Gln, and Arg237Trp. Note that the T-box binds its target DNA (teal and blue) as a homodimer. (Lower) Human TBX5 and X. laevis Xbra T-box sequences are aligned. The Gly80Arg (R) and Arg237Gln (Q) missense mutations are located in the T-box sequence. A vertical line indicates identical sequences in Xbra and TBX5, two dots indicate functionally related residues, and one dot denotes nonconserved residues.