The tumor suppressor gene TP53 , encoding p53, is expressed as several transcripts. The fully spliced p53 (FSp53) transcript encodes the canonical p53 protein. The alternatively spliced p53I2 transcript retains intron 2 and encodes Δ40p53 (or ΔNp53), an isoform lacking first 39 N-terminal residues corresponding to the main transactivation domain. We demonstrate the formation of G-quadruplex structures (G4) in a GC-rich region of intron 3 that modulates the splicing of intron 2. First, we show the formation of G4 in synthetic RNAs encompassing intron 3 sequences by ultraviolet melting, thermal difference spectra and circular dichroism spectroscopy. These observations are confirmed by detection of G4-induced reverse transcriptase elongation stops in synthetic RNA of intron 3. In this region, p53 pre-messenger RNA (mRNA) contains a succession of short exons (exons 2 and 3) and introns (introns 2 and 4) covering a total of 333 bp. Site-directed mutagenesis of G-tracts putatively involved in G4 formation decreased by ∼30% the excision of intron 2 in a green fluorescent protein-reporter splicing assay. Moreover, treatment of lymphoblastoid cells with 360A, a synthetic ligand that binds to single-strand G4 structures, increases the formation of FSp53 mRNA and decreases p53I2 mRNA expression. These results indicate that G4 structures in intron 3 regulate the splicing of intron 2, leading to differential expression of transcripts encoding distinct p53 isoforms.

The TP53 tumour-suppressor gene is expressed as several protein isoforms generated by different mechanisms, including use of alternative promoters, splicing sites and translational initiation sites, that are conserved through evolution and within the TP53 homologues, TP63 and TP73. Although first described in the eighties, the importance of p53 isoforms in regulating the suppressive functions of p53 has only become evident in the last 10 years, by analogy with observations that p63 and p73 isoforms appeared indispensable to fully understand the biological functions of TP63 and TP73. This review summarizes recent advances in the field of ‘p53 isoforms’, including new data on p63 and p73 isoforms. Details of the alternative mechanisms that produce p53 isoforms and cis- and trans-regulators identified are provided. The main focus is on their biological functions (apoptosis, cell cycle, aging and so on) in cellular and animal models, including mouse, zebrafish and Drosophila. Finally, the deregulation of p53 isoform expression in human cancers is reviewed. Based on these latest results, several developments are expected in the future: the identification of drugs modulating p53 isoform expression; the generation of animal models and the evaluation of the use of p53 isoform as biomarkers in human cancers.

Ribosomal RNA (rRNA) undergoes post-transcriptional modification of over 200 nucleotides, predominantly 2′-O-methylation (2′-O-Me). 2′-O-Methylation protects RNA from hydrolysis and modifies RNA strand flexibility but does not contribute to Watson-Crick base pairing. The contribution of 2′-O-Me to the translational capacity of ribosomes has been established. Yet, how 2′-O-Me participates in ribosome biogenesis and ribosome functioning remains unclear. The development of 2′-O-Me quantitative mapping methods has contributed to the demonstration that these modifications are not constitutive but rather provide heterogeneity to the ribosomal population. Moreover, recent advances in ribosome structure analysis and in vitro translation assays have proven, for the first time, that 2′-O-Me contributes to regulating protein synthesis. This review highlights the recent data exploring the impact of 2′-O-Me on ribosome structure and function, and the emerging idea that the rRNA epitranscriptome is involved in translational control.