cancers Review p53/p73 Protein Network in Colorectal Cancer and Other Human Malignancies Andela¯ Horvat †, Ana Tadijan † , Ignacija Vlaši´c † and Neda Slade * Laboratory for Protein Dynamics, Division of Molecular Medicine, Ruder¯ Boškovi´cInstitute, Bijeniˇcka54, 10000 Zagreb, Croatia; [email protected] (A.H.); [email protected] (A.T.); [email protected] (I.V.) * Correspondence: [email protected] † Contributed equally. Simple Summary: The p53 family of proteins comprises p53, p63, and p73, which share high structural and functional similarity. The two distinct promoters of each locus, the alternative splicing, and the alternative translation initiation sites enable the generation of numerous isoforms with different protein-interacting domains and distinct activities. The co-expressed p53/p73 isoforms have significant but distinct roles in carcinogenesis. Their activity is frequently impaired in human tumors including colorectal carcinoma due to dysregulated expression and a dominant-negative effect accomplished by some isoforms and p53 mutants. The interactions between isoforms are particularly important to understand the onset of tumor formation, progression, and therapeutic response. The understanding of the p53/p73 network can contribute to the development of new targeted therapies. Abstract: The p53 tumor suppressor protein is crucial for cell growth control and the maintenance of genomic stability. Later discovered, p63 and p73 share structural and functional similarity with p53. To understand the p53 pathways more profoundly, all family members should be considered. Citation: Horvat, A.; Tadijan, A.; Each family member possesses two promoters and alternative translation initiation sites, and they Vlaši´c,I.; Slade, N. p53/p73 Protein undergo alternative splicing, generating multiple isoforms. The resulting isoforms have important Network in Colorectal Cancer and Other Human Malignancies. Cancers roles in carcinogenesis, while their expression is dysregulated in several human tumors including 2021, 13, 2885. https://doi.org/ colorectal carcinoma, which makes them potential targets in cancer treatment. Their activities arise, 10.3390/cancers13122885 at least in part, from the ability to form tetramers that bind to specific DNA sequences and activate the transcription of target genes. In this review, we summarize the current understanding of the Academic Editors: Maja T. Tomicic biological activities and regulation of the p53/p73 isoforms, highlighting their role in colorectal and Thomas Efferth tumorigenesis. The analysis of the expression patterns of the p53/p73 isoforms in human cancers provides an important step in the improvement of cancer therapy. Furthermore, the interactions Received: 13 May 2021 among the p53 family members which could modulate normal functions of the canonical p53 in Accepted: 3 June 2021 tumor tissue are described. Lastly, we emphasize the importance of clinical studies to assess the Published: 9 June 2021 significance of combining the deregulation of different members of the p53 family to define the outcome of the disease. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: p53 isoforms; p73 isoforms; colorectal cancer; p53 family; isoform crosstalk published maps and institutional affil- iations. 1. Introduction p53 has a central role in tumorigenesis; it regulates cellular response to stress signals or Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. oncogenic cellular stimuli by inducing transient or permanent cell-cycle arrest, DNA repair, This article is an open access article apoptosis, or senescence [1]. The inactivation of the p53 tumor suppressor is the single most distributed under the terms and common genetic defect in human cancer. The mutations of TP53 have been found in nearly conditions of the Creative Commons all tumor types and are estimated to contribute to more than 50% of all cancers. Attribution (CC BY) license (https:// In the late 1990s, two novel family members TP73 [2] and TP63 [3] were described. Both creativecommons.org/licenses/by/ encode several proteins whose structures and functions are similar to those of p53 but not 4.0/). identical, each displaying peculiar functional features revealed by mouse model studies. Cancers 2021, 13, 2885. https://doi.org/10.3390/cancers13122885 https://www.mdpi.com/journal/cancers Cancers 2021, 13, x FOR PEER REVIEW 2 of 43 In the late 1990s, two novel family members TP73 [2] and TP63 [3] were described. Both Cancers 2021, 13, 2885 encode several proteins whose structures and functions are similar to those of p53 but2 ofnot 42 identical, each displaying peculiar functional features revealed by mouse model studies. All members of the p53 family are evolutionarily conserved and have a very high structural similarity within three main functional domains: transactivation domain All members of the p53 family are evolutionarily conserved and have a very high (TAD), DNA-binding domain (DBD), and oligomerization/tetramerization domain structural similarity within three main functional domains: transactivation domain (TAD), (OD/TD) (Figures 1 and 2). All family members share significant amino-acid homology in DNA-binding domain (DBD), and oligomerization/tetramerization domain (OD/TD) DBD (over 60%), which enables them to bind the p53-responsive elements (p53RE) and (Figures1 and2). All family members share significant amino-acid homology in DBD (over transactivate a large number of the same target genes. Accordingly, p63, p73, and p53 60%), which enables them to bind the p53-responsive elements (p53RE) and transactivate proteins form a family of transcription factors. At the sequence level, p63 and p73 share a large number of the same target genes. Accordingly, p63, p73, and p53 proteins form a higher homology to each other than to p53 [4]. However, despite many overlapping roles, family of transcription factors. At the sequence level, p63 and p73 share higher homology each member has its unique identity and functions. to each other than to p53 [4]. However, despite many overlapping roles, each member has The family complexity has been enriched by the transcription from alternative pro- its unique identity and functions. moters,The alternative family complexity splicing, hasand been diverse enriched translation by the initiation transcription sites from[4,5]. alternativeConsequently, pro- severalmoters, protein alternative isoforms splicing, with anddistin diversect N- and translation C- termini initiation are encoded. sites [ 4,5]. Consequently, severalThe protein roles isoformsof each particular with distinct p53 N- family and C- member termini arewere encoded. determined by transgenic knockoutThe roles mice. of While each particularthe p53 knockout p53 family mice member show high were susceptibility determined byto transgenicspontaneous knock- and inducedout mice. carcinogenesis, While the p53 thereby knockout defining mice p53 show as an high important susceptibility tumor suppressor, to spontaneous total andp63 andinduced p73 knockout carcinogenesis, mouse thereby models defining lack a cancer p53 as ph anenotype. important Instead, tumor p63- suppressor, and p73-null total mice p63 exhibitand p73 various knockout developmental mouse models deficiencies lack a cancer revealing phenotype. their Instead, crucial p63-functions and p73-null in epithelial mice andexhibit central various nervous developmental system formation, deficiencies respecti revealingvely [4]. their Although crucial p63 functions and p73 in are epithelial rarely mutatedand central in cancer, nervous TP63 system is situated formation, within respectively the locus frequently [4]. Although amplified p63 and in squamous p73 are rarely cell carcinomamutated in [6]. cancer, TheirTP63 roleis in situated tumorigenesis within the is locusdefined frequently by several amplified isoforms in squamouswith opposed cell functions,carcinoma in [6 ].the Their same role cellular in tumorigenesis context [7]. isHowever, defined bytheir several tumor isoforms suppressive with potential opposed wasfunctions, not fully in the elucidated same cellular until contextthe establishment [7]. However, of the their isoform-specific tumor suppressive knockout potential mice. was In thisnot fullyreview, elucidated we summarize until the the establishment structural and of thefunctional isoform-specific properties knockout of different mice. p53 In and this p73review, isoforms we summarize and their theroles structural in tumor and formatio functionaln with properties the emphasis of different on colorectal p53 and cancer p73 (CRC).isoforms and their roles in tumor formation with the emphasis on colorectal cancer (CRC). Figure 1. TheThe TP53TP53 genegene architecture architecture and generation of the p53 isoforms. (A) The scheme of the TP53 gene structure. The human TP53 gene consists of 11 exons andand twotwo alternativealternative exonsexons 99ββ andand 99γγ.. The exons are shown as boxes of different colors and noncodingnoncoding sequences in black.black. Introns 2 (i2) and 4 (i4) are shown as boxes with a striped pattern. The TP53 gene can be transcribed from two different promoters, the canonical promoter P1 upstream of exonexon 11 (giving(giving riserise toto thethe longlong p53/p53/DΔ40p5340p53 isoforms) and the alternative P2 located in intron 4 (giving rise to the short Δ133/Δ160p53 isoforms). isoforms) and the alternative P2 located in intron 4 (giving rise to the short D133/D160p53 isoforms). The TP53 mRNAs can be alternatively