Dysfunctional p53 deletion mutants in cell lines derived from Hodgkin's lymphoma.

Classical Hodgkin's lymphoma (cHL) is a distinct malignancy of the immune system. Despite the progress made in the understanding of the pathology of cHL, the transforming events remain to be elucidated. It has been proposed that mutations in the TP53 gene in biopsy material as well as cell lines derived from cHL are rare and therefore not notably involved in the pathogenesis of the malignant H&RS cells. Re-evaluating the expression in cHL-derived cell lines, we found that in 3/6 of these cell lines, TP53 transcripts are characterized by deletions within exon 4 (L428 cells) and nearly a complete loss of exons 10 - 11 (L1236) or exons 8 - 11 (HDLM-2), respectively. These changes were found in otherwise rarely mutated regions of TP53. Cell lines L1236 and HDLM-2 harbour fusions with alu-repeats in their TP53 mRNA 3'-ends, resulting in the carboxyterminal truncation and loss of the transcriptional activity of p53. Transcriptional inactivity was also found for p53 in L428 cells. This study characterizes mutations in TP53 transcripts within cHL cell lines with associated functional defects in the resulting p53 proteins and therefore reintroduces the concept that mutations of TP53 might be involved in the pathogenesis of Hodgkin's lymphoma.

p21/CDKN1A mediates negative regulation of transcription by p53.

The tumor suppressor p53 regulates transcription positively and negatively, depending on the target gene. Whereas p53 induces transcription through direct interaction with promoter DNA, the mechanism of p53-mediated transcriptional repression is less well understood. Early reports described the alleviation of p53-mediated repression by inhibitors of apoptosis, suggesting that negative regulation of transcription might occur only in conjunction with programmed cell death. More recently, it has been proposed that certain genes, such as survivin, are repressed by direct association of p53 with their promoters, followed by recruitment of a repressor complex. We show here that p53-mediated negative regulation of transcription could occur independently of apoptosis. In contrast, the amino-terminal transactivation domain of p53 was required for negative regulation of transcription. Similarly, the p53 homologue p73 diminished the expression of survivin and stathmin, depending on its transactivation domain. Mutation of the putative p53 binding site within the survivin promoter did not impair its repression. These observations raised the hypothesis that activation of an effector gene might be required for repression by p53. Strikingly, when the p53-inducible p21/CDKN1A gene was deleted, p53 no longer repressed any one among 11 genes that it down-regulates otherwise. Most of these genes were also repressed by ectopic p21 in the absence of p53. Overexpressed c-Myc reduced the transcription of p21/CDKN1A and impaired p53-mediated repression but did not abolish repression by ectopic p21. Taken together, these results strongly suggest that increased expression of p21/CDKN1A is necessary and sufficient for the negative regulation of gene expression by p53.