Distinct expression and function of alternatively spliced Tbx5 isoforms in cell growth and differentiation.
Mutations in the T-box transcription factor Tbx5 cause Holt-Oram syndrome, an autosomal dominant disease characterized by a wide spectrum of cardiac and upper limb defects with variable expressivity. Tbx5 haploinsufficiency has been suggested to be the underlying mechanism, and experimental models are consistent with a dosage-sensitive requirement for Tbx5 in heart development. Here, we report that Tbx5 levels are regulated through alternative splicing that generates, in addition to the known 518-amino-acid protein, a C-terminal truncated isoform. This shorter isoform retains the capacity to bind DNA, but its interaction with Tbx5 collaborators such as GATA-4 is altered. In vivo, the two spliced isoforms are oppositely regulated in a temporal and growth factor-dependent manner and are present in distinct DNA-binding complexes. The expression of the long isoform correlates with growth stimulation, and its reexpression in postnatal transgenic mouse hearts promotes hypertrophy. Conversely, the upregulation of the short but not the long isoform in C2C12 myoblasts leads to growth arrest and cell death. The results provide novel insight into posttranscriptional Tbx5 regulation and point to an important role not only in cell differentiation but also in cell proliferation and organ growth. The data may help analyze genotype-phenotype relations in patients with Holt-Oram syndrome.
Georges
Romain
R
Research Unit in Cardiac Growth and Differentiation, Institut de Recherches Cliniques de Montréal, Montréal, QC, Canada.
Nemer
Georges
G
Morin
Martin
M
Lefebvre
Chantal
C
Nemer
Mona
M
D017398:Alternative Splicing; D000818:Animals; D002454:Cell Differentiation; D049109:Cell Proliferation; D050991:GATA4 Transcription Factor / Q000737:chemistry; D005786:Gene Expression Regulation; D005838:Genotype; D051379:Mice; D008822:Mice, Transgenic; D008954:Models, Biological; D041681:NIH 3T3 Cells; D010641:Phenotype; D020033:Protein Isoforms; D020133:Reverse Transcriptase Polymerase Chain Reaction; D020825:T-Box Domain Proteins / Q000737:chemistry*
D050991:GATA4 Transcription Factor; C484171:Gata4 protein, mouse; D020033:Protein Isoforms; D020825:T-Box Domain Proteins; C103172:T-box transcription factor 5