Mutations in human TBX5, a member of the T-box transcription factor gene fa
mily, cause congenital cardiac septation defects and isomerism in autosomal
dominant Holt-Oram syndrome. To determine the cellular function of TBX5 in
cardiogenesis, we overexpressed wild-type and mutant human TBX5 isoforms i
n vitro and in vivo. TBX5 inhibited cell proliferation of D17 canine osteos
arcoma cells and MEQC quail cardiomyocyte-like cells in vitro. Mutagenesis
of the 5' end of the T-box but not the 3' end of the T-box abolished this e
ffect. Overexpression of TBX5 in embryonic chick hearts showed that TBX5 in
hibits myocardial growth and trabeculation. TBX5 effects in vivo were aboli
shed by Gly80Arg missense mutation of the 5' end of the T-box. PCNA analysi
s in transgenic chick hearts revealed that TBX5 overexpression does suppres
s embryonic cardiomyocyte proliferation in vivo. Inhibitory effects of TBX5
on cardiomyocyte proliferation include a noncell autonomous process in vit
ro and in vivo. TBX5 inhibited proliferation of both nontransgenic cells co
cultured with transgenic cells in vitro and nontransgenic cardiomyocytes in
transgenic chick hearts with mosaic expression of TBX5 in vivo. Immunohist
ochemical studies of human embryonic tissues, including hearts, also demons
trated that TBX5 expression is inversely related to cellular proliferation.
We propose that TBX5 can act as a cellular arrest signal during vertebrate
cardiogenesis and thereby participate in modulation of cardiac growth and
development. (C) 2001 Academic Press.