DISTINCT EXPRESSION AND FUNCTION OF ALTERNATIVELY SPLICED TBX5 ISOFORMS IN CELL GROWTH AND DIFFERENTIATION

Research Unit in Cardiac Growth and Differentiation, Institut de recherches cliniques de Montréal (IRCM), Montréal QC CANADA; Programme de biologie moléculaire, Université de Montréal, CANADA; Department of Biochemistry, University of Ottawa CANADA

^* To whom correspondence should be addressed. Email: mona.nemer{at}ircm.qc.ca.

	Abstract

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 as 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. Expression of the long isoform correlates with growth stimulation and its reexpression in postnatal transgenic mice hearts promotes hypertrophy. Conversely, upregulation of the short but not long isoform in C2C12 myoblasts leads to growth arrest and cell death. The results provide novel insight into post-transcriptional 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 Holt Oram patients.