Evidence of horizontal transmission of the cancer-associated Steamer retrotransposon among ecological cohort bivalve species

Abstract

Bivalve specimens from legacy frozen tissue collections, and others freshly obtained, were surveyed for the presence of the Steamer long terminal repeat (LTR)-retrotransposon associated with disseminated hemic neoplasia of the soft-shelled clam Mya areneria. Of 22 species investigated using primers for the pol region, only Atlantic M. arenaria, Atlantic and North Sea razor clams Ensis directus, and Baltic clams Macoma balthica from the North Sea were found to possess copies of Steamer in their genomes. Notably, close relatives like Mya truncata and Siliqua patula did not exhibit evidence of Steamer. Amplified Steamer sequences were uniformly identical in all M. areneria specimens, and were highly variable across specimens of E. directus. Variation in the latter included nucleotide polymorphisms among and within individuals as well as length variation in 2 specimens corresponding to the deletion of a predicted stable hairpin structure. Results implicate Atlantic razor clams as the proximal source for horizontal transmission of Steamer among ecologically similar yet markedly distantly related bivalves. The consequences of cross-species transmission of the Steamer retrotransposon are unknown, and the finding of Steamer in 3 bivalve species suggests that further spread is possible. KEY WORDS: Disseminated neoplasia · Hemic neoplasia · Retrotransposon · Steamer · Bivalvia · CancerResale or republication not permitted without written consent of the publisher Dis Aquat Org 124: [165][166][167][168] 2017 pression of the Steamer retrotransposon. Steamer is a long terminal repeat (LTR)-retrotrans poson in the Mag family of Ty3/Gypsy elements, complete with a single open reading frame encoding gag and pol genes. Retrotransposons are present in genomic DNA and replicate through reverse transcription of an mRNA transcript into DNA, followed by integration into a new location within the genomic DNA of a cell. As they are notably lacking a retroviral env gene, they are not expected to be able to transfer from cell to cell, as retroviruses do. Similar retrotransposons have been identified in the genomes of 3 other bivalve species known to have transmissible neoplasias, one of which seems to cross the species boundary (Metzger et al. 2016).To date, the Steamer retrotransposon itself is un ique to M. arenaria. Here we examined legacy bio diversity collections and freshly acquired bivalve tissues for the presence of a Steamer signature in their genomes in a manner that might (1) better elucidate whether disseminated neoplasias are ecologically or phylogenetically determined and (2) shed light on the potential for horizontal spread and genomic modification by Steamerlike retrotransposons in the marine environment. MATERIALS AND METHODSBivalve tissue samples were obtained from the American Museum of Natural History's Ambrose Monell Cryo Collection (AMCC), and included 22 species from the North Atlantic (see Table S1 in the Supplement at www. int-res. com/ articles/ suppl/ d124 p165 _ supp. pdf). Fresh specimens of steam...

Horizontal transfer of retrotransposons between bivalves and other aquatic species of multiple phyla

“…SLE sequences were present in 11 families and nine orders. A previous study of bivalves from the AMCC using highly specific primers (16) identified SLEs nearly identical to Steamer in two bivalves (Baltic clam, Limecola balthica, and Atlantic razor clam, Ensis directus), and we confirmed those findings using the degenerate primers.…”

“…The finding of Steamer in both Pacific and European samples of M. arenaria suggests that the first transmission into the M. arenaria genome occurred at least 800 y ago, as M. arenaria is believed to have been brought to Europe around the 1300s (33). The Steamer sequences from the M. arenaria samples in this study and in 10 more studies published recently (16) are all nearly identical, again suggesting recent entry into the genome or a recent expansion of a single lineage within the species. The SLEs in cockles appear to be recently acquired as well.…”

“…Clams, and Baltic Clams. A recent report used highly specific primers to investigate the presence of sequence fragments of Steamer in the AMCC (16). Consistent with this study, we amplified and sequenced a fragment of the pol gene that is nearly identical to the Steamer elements from soft-shell clams (M. arenaria) in multiple Atlantic razor clams (E. directus) and in a single sample of Baltic clam (L. balthica) ( Fig.…”

“…E4232 | www.pnas.org/cgi/doi/10.1073/pnas. 1717227115 Metzger et al (16), and here we have expanded that study with broad and systematic strategies which show that there has been widespread HTT of SLEs throughout bivalve evolution as well as across phyla throughout the marine environment. Phylogenetic incongruence and a patchy distribution across phylogeny provide evidence to support claims of HTT, but it has been argued that phylogenetic analysis can be misleading in some cases (7,29).…”

Living infectious agents with the same organic wall assembly of Precambrian early-life fossils discovered in Canine Transmissible Venereal Tumour and human cancer: Giant viruses or living protocells? Evaluating the effects of an anti-cancer vaccine in stray dogs, while challenging the mysteries around the RNA world

“…The strongest evidence that this form of clams' neoplasia is transmissible comes from studies of unfiltered hemolymph that induce cancer in healthy individuals (14)(15). Subsequent studies were consistent with a retrovirus aetiology where the examination of the hemolymphatic transcriptome identified the presence of retroviral protease, RT and integrase (15)(16)(17). Surprisingly, the highest RT levels was detected in cultured hemocytes compared to cell-free extracts (18).…”