p53 Superfamily proteins in marine bivalve cancer and stress biology.

The human p53 tumour suppressor protein is inactivated in many cancers and is also a major player in apoptotic responses to cellular stress. The p53 protein and the two other members of this protein family (p63, p73) are encoded by distinct genes and their functions have been extensively documented for humans and some other vertebrates. The structure and relative expression levels for members of the p53 superfamily have also been reported for most major invertebrate taxa. The functions of homologous proteins have been investigated for only a few invertebrates (specifically, p53 in flies, nematodes and recently a sea anemone). These studies of classical model organisms all suggest that the gene family originally evolved to mediate apoptosis of damaged germ cells or to protect germ cells from genotoxic stress. Here, we have correlated data from a number of molluscan and other invertebrate sequencing projects to provide a framework for understanding p53 signalling pathways in marine bivalve cancer and stress biology. These data suggest that (a) the two identified p53 and p63/73-like proteins in soft shell clam (Mya arenaria), blue mussel (Mytilus edulis) and Northern European squid (Loligo forbesi) have identical core sequences and may be splice variants of a single gene, while some molluscs and most other invertebrates have two or more distinct genes expressing different p53 family members; (b) transcriptional activation domains (TADs) in bivalve p53 and p63/73-like protein sequences are 67-69% conserved with human p53, while those in ecdysozoan, cnidarian, placozoan and choanozoan eukaryotes are ≤33% conserved; (c) the Mdm2 binding site in the transcriptional activation domain is 100% conserved in all sequenced bivalve p53 proteins (e.g. Mya, Mytilus, Crassostrea and Spisula) but is not present in other non-deuterostome invertebrates; (d) an Mdm2 homologue has been cloned for Mytilus trossulus; (e) homologues for both human p53 upstream regulatory and transcriptional target genes exist in molluscan genomes (missing are ARF, CIP1 and BH3 only proteins) and (f) p53 is demonstrably involved in bivalve haemocyte and germinoma cancers. We usually do not know enough about the molecular biology of marine invertebrates to address molecular mechanisms that characterize particular diseases. Understanding the molecular basis of naturally occurring diseases in marine bivalves is a virtually unexplored aspect of toxicoproteomics and genomics and related drug discovery. Additionally, increases in coastal development and concomitant increases in aquatic pollutants have driven interest in developing models appropriate for evaluating potential hazardous compounds or conditions found in the aquatic environment. Data reviewed in this study are coupled with recent developments in our understanding the molecular biology of the marine bivalve p53 superfamily. Taken together, they suggest that both structurally and functionally, bivalve p53 family proteins are the most highly conserved members of this gene superfamily so far identified outside of higher vertebrates and invertebrate chordates. Marine bivalves provide some of the most relevant and best understood models currently available for experimental studies by biomedical and marine environmental researchers.