Internal translation initiation generates novel WT1 protein isoforms with distinct biological properties

The Wilms' tumor 1 gene, WT1, is homozygously mutated in a subset of Wilms' tumors. Heterozygous mutations in WT1 give rise to congenital anomalies. During embryogenesis, WT1 is expressed mainly in the kidneys, uterus, and testes. Alternative splicing of the WT1 mRNA results in synthesis of four main WT1 protein isoforms with molecular masses of 52-54 kDa. In addition, translation initiation at a CUG upstream of the initiator AUG generates four larger WT1 proteins of 60-62 kDa. We describe here the existence of novel WT1 isoforms and demonstrate that they are derived from translation initiation at the second in-frame AUG of the WT1 mRNA. These N-terminally truncated WT1 proteins of 36-38 kDa can be detected in several cell lines, mouse testes, and Wilms' tumor specimens. They can bind to DNA and direct transcription from reporter constructs. The shorter WT1 protein lacking the two splice inserts has a greater transcription activation potential than the corresponding main WT1 protein isoform but shows no transcription repression potential. Overexpression of full-length or N-terminally truncated WT1 efficiently induces apoptosis. These data show that additional WT1 isoforms with distinct transcription-regulatory properties exist, which further increases the complexity of WT1 expression and activity.