An invertebrate mdm homolog interacts with p53 and is differentially expressed together with p53 and ras in neoplastic Mytilus trossulus haemocytes

The mussel Mytilus trossulus can develop a neoplasia of the haemolymph, which occurs with high frequency (up to 40%) in nature. Associated with this disease are pro-apoptotic tumor-suppressor protein p53 isoforms, which are highly conserved between molluscs and vertebrates. The vertebrate wildtype p53 protein is maintained at low levels by the MDM2 protein in non-stressed cells to prevent undesired apoptosis. Identification of a putative invertebrate MDM-like homolog suggests early evolution of this mechanism of p53 regulation. The M. trossulus MDM homolog consists of a conserved NH2-terminal p53 binding domain, an acidic domain with highly conserved phosphorylation sites, and a highly conserved C-terminal RING-finger Zn-binding domain. Although BLAST queries predict this homologue to be more similar to vertebrate MDM2 than to MDM4, phylogenetic analysis suggests that it may be an ancestral form to both vertebrate MDM genes. Using yeast-two-hybrid assays and pull-down assays, we show that this molluscan MDM is able to bind to its p53 counterpart. We also show that MDM expression levels are directly correlated with p53 expression levels in healthy and in neoplastic haemocytes, but not with other p53 isoforms or with the proto-oncogene RAS. The combination of expression levels of five gene transcripts (p53, mdm, ras, ∆Np63/73, and TAp63/73) is significantly correlated with late-stage haemic neoplasia in M. trossulus.