Isolation of a viral agent causing hematopoietic neoplasia in the soft-shell clam, Mya arenaria

Survival and Detection of Bivalve Transmissible Neoplasia from the Soft-Shell Clam Mya arenaria (MarBTN) in Seawater

Many pathogens can cause cancer, but cancer itself does not normally act as an infectious agent. However, transmissible cancers have been found in a few cases in nature: in Tasmanian devils, dogs, and several bivalve species. The transmissible cancers in dogs and devils are known to spread through direct physical contact, but the exact route of transmission of bivalve transmissible neoplasia (BTN) has not yet been confirmed. It has been hypothesized that cancer cells from bivalves could be released by diseased animals and spread through the water column to infect/engraft into other animals. To test the feasibility of this proposed mechanism of transmission, we tested the ability of BTN cells from the soft-shell clam (Mya arenaria BTN, or MarBTN) to survive in artificial seawater. We found that MarBTN cells are highly sensitive to salinity, with acute toxicity at salinity levels lower than those found in the native marine environment. BTN cells also survive longer at lower temperatures, with 50% of cells surviving greater than 12 days in seawater at 10 °C, and more than 19 days at 4 °C. With one clam donor, living cells were observed for more than eight weeks at 4 °C. We also used qPCR of environmental DNA (eDNA) to detect the presence of MarBTN-specific DNA in the environment. We observed release of MarBTN-specific DNA into the water of laboratory aquaria containing highly MarBTN-diseased clams, and we detected MarBTN-specific DNA in seawater samples collected from MarBTN-endemic areas in Maine, although the copy numbers detected in environmental samples were much lower than those found in aquaria. Overall, these data show that MarBTN cells can survive well in seawater, and they are released into the water by diseased animals. These findings support the hypothesis that BTN is spread from animal-to-animal by free cells through seawater.

Bacterial death and TRADD-N domains help define novel apoptosis and immunity mechanisms shared by prokaryotes and metazoans

Several homologous domains are shared by eukaryotic immunity and programmed cell-death systems and poorly understood bacterial proteins. Recent studies show these to be components of a network of highly regulated systems connecting apoptotic processes to counter-invader immunity, in prokaryotes with a multicellular habit. However, the provenance of key adaptor domains, namely those of the Death-like and TRADD-N superfamilies, a quintessential feature of metazoan apoptotic systems, remained murky. Here, we use sensitive sequence analysis and comparative genomics methods to identify unambiguous bacterial homologs of the Death-like and TRADD-N superfamilies. We show the former to have arisen as part of a radiation of effector-associated α-helical adaptor domains that likely mediate homotypic interactions bringing together diverse effector and signaling domains in predicted bacterial apoptosis- and counter-invader systems. Similarly, we show that the TRADD-N domain defines a key, widespread signaling bridge that links effector deployment to invader-sensing in multicellular bacterial and metazoan counter-invader systems. TRADD-N domains are expanded in aggregating marine invertebrates and point to distinctive diversifying immune strategies probably directed both at RNA and retroviruses and cellular pathogens that might infect such communities. These TRADD-N and Death-like domains helped identify several new bacterial and metazoan counter-invader systems featuring underappreciated, common functional principles: the use of intracellular invader-sensing lectin-like (NPCBM and FGS), transcription elongation GreA/B-C, glycosyltransferase-4 family, inactive NTPase (serving as nucleic acid receptors), and invader-sensing GTPase switch domains. Finally, these findings point to the possibility of multicellular bacteria-stem metazoan symbiosis in the emergence of the immune/apoptotic systems of the latter.

Implementation of various approaches to study the prevalence, incidence and progression of disseminated neoplasia in mussel stocks

Marine mussel production is of substantial economic interest in numerous coastal areas worldwide, making crucial the study of pathologies that affect them. Disseminated neoplasia (DN) has recently been suggested to be linked to blue mussel, Mytilus edulis, mortality outbreaks observed in France since 2014, although the evidence remains indirect. In order to improve DN detection and monitoring, we compared the sensitivity of four diagnostic tools, namely haemocytology, histology, flow cytometry, and genetics. Haemocytological examination gave the best results in sensitivity and had the advantage of being non-invasive, allowing disease progression to be followed in affected mussels. Using this approach, we showed that DN progression is usually slow, and we provide evidence of remission events. We observed a high diversity of forms and mitotic features of neoplastic cells located in the vesicular connective tissue but rarely in the haemolymph. Circulating cells occur as four main types but are homogenous in morphology and DNA content within a single individual. Polyploidy proved very high, from 8 N to 18 N. Genetic analysis of haemolymph DNA showed that a Mytilus trossulus genetic signal was associated with almost all the DN cases here diagnosed by haemocytological examination, regardless of the DN type. This result corroborates DN is a transmissible cancer that first originated in a M. trossulus host and subsequently crossed into M. edulis. No pre-neoplastic conditions were detectable. The prevalence of the disease was quite low, which, together with the low morbidity observed in the lab, suggest DN is unlikely to be the direct cause of mortality outbreaks in France.

Researching new diseases: assumptions and trajectories

New diseases in humans and animals have been the subject of considerable research as well as policy development and popular attention. Researchers commonly proceed on the basis of plausible assumptions about mechanisms, pathways, and dangers but seldom question the assumptions themselves. Studies in the history and sociology of science show that research trajectories are conditioned by social, political, and economic arrangements. The assumptions underlying research into three new diseases—devil facial tumor disease in Tasmanian devils, AIDS in humans, and leukemia in soft-shell clams—are examined, and dominant and alternative research programs compared. In each case, most research has assumed the disease is spread through “natural processes”, while research about possible human influences has been left undone.

Nuclear morphometry and ploidy of normal and neoplastic haemocytes in mussels

Haemic neoplasia (HN) in bivalves has been reported in association with mass mortality events in various species of molluscs. The aim of this work was to quantify the nuclear morphometry and DNA content of neoplastic cells of mussels Mytilus galloprovincialis affected by HN using nuclear densitometry in Feulgen-stained preparations. The results were also compared with a population of normal mussel haemocytes. We captured 256 images of 3 different neoplasia stages and 120 images of normal haemocytes; thus, a total of 120,166 nuclei were analysed. We extracted 21 morphological parameters from normal and neoplastic nuclei. Eighteen of these parameters were different (P<0.05). Among those (expressed in pixel units—inter-pixel distance of 0.45 micrometres—as: normal vs. neoplastic) nuclear area (117.1±94.1 vs. 423.1±226.9), perimeter (44.9±14.0 vs. 79.0±21.3) and (IOD) integrated optical density (13.47±34.5 vs. 177.1±150.8) were relevant features to discriminate between normal and neoplastic cells. Those differences allowed identifying two distinctive populations of neoplastic nuclei, occasionally in the same individuals at a given phase of the disease. Moreover, neoplastic haemocytes in less extended lesions showed a ploidy value of 6.2 n along with the presence of a second population of circulating cells with a DNA content of 10.7n. In samples with moderate disease only one peak at 7n was observed. Finally, in more severe conditions, a further ploidy peak of 7.8n was recorded, accompanied by a shallow but broad peak of 31n. This latter extreme value is thought to be due to the presence of giant multinucleated cells where individual nuclei overlap in space and cannot be discerned individually. Computer-based imaging allowed the direct visualization of the cell populations and simultaneous collection of ploidy data as well as morphological features of nuclei.

A Preliminary Molecular and Phylogenetic Analysis of the Genome of a Novel Endogenous Retrovirus in the Sea Slug Elysia chlorotica

An endogenous retrovirus that is present in the sea slug Elysia chlorotica is expressed in all individuals at the end of the annual life cycle. But the precise role of the virus, if any, in slug senescence or death is unknown. We have determined the genomic sequence of the virus and performed a phylogenetic analysis of the data. The 6060-base pair genome of the virus possesses a reverse transcriptase-domain-containing protein that shows similarity to retrotransposon sequences found in Aplysia californica and Strongylocentrotus purpuratus. However, nucleotide BLAST analysis of the whole genome resulted in hits to only a few portions of the genome, indicating that the Elysia chlorotica retrovirus is novel, has not been previously sequenced, and does not have great genetic similarity to other known viral species. When more invertebrate retroviral genomes are examined, a more precise phylogenetic placement of the Elysia chlorotica retrovirus can be determined.

Mass mortality in bivalves and the intricate case of the Pacific oyster, Crassostrea gigas

Massive mortality outbreaks in cultured bivalves have been reported worldwide and they have been associated with infection by a range of viral and bacterial pathogens. Due to their economic and social impact, these episodes constitute a particularly sensitive issue in Pacific oyster (Crassostrea gigas) production. Since 2008, mortality outbreaks affecting C. gigas have increased in terms of intensity and geographic distribution. Epidemiologic surveys have lead to the incrimination of pathogens, specifically OsHV-1 and bacteria of the Vibrio genus, in particular Vibrio aestuarianus. Pathogen diversity may partially account for the variability in the outcome of infections. Host factors (age, reproductive status...) including their genetic background that has an impact on host susceptibility toward infection, also play a role herein. Finally, environmental factors have significant effects on the pathogens themselves, on the host and on the host-pathogen interaction. Further knowledge on pathogen diversity, classification, and spread, may contribute toward a better understanding of this issue and potential ways to mitigate the impact of these outbreaks.

Horizontal transmission of clonal cancer cells causes leukemia in soft-shell clams

Outbreaks of fatal leukemia-like cancers of marine bivalves throughout the world have led to massive population loss. The cause of the disease is unknown. We recently identified a retrotransposon, Steamer, that is highly expressed and amplified to high copy number in neoplastic cells of soft-shell clams (Mya arenaria). Through analysis of Steamer integration sites, mitochondrial DNA single-nucleotide polymorphisms (SNPs), and polymorphic microsatellite alleles, we show that the genotypes of neoplastic cells do not match those of the host animal. Instead, neoplastic cells from dispersed locations in New York, Maine, and Prince Edward Island (PEI), Canada, all have nearly identical genotypes that differ from those of the host. These results indicate that the cancer is spreading between animals in the marine environment as a clonal transmissible cell derived from a single original clam. Our findings suggest that horizontal transmission of cancer cells is more widespread in nature than previously supposed.

Activation of transcription and retrotransposition of a novel retroelement, Steamer, in neoplastic hemocytes of the mollusk Mya arenaria

Bivalve mollusks of the North Atlantic, most prominently the soft shell clam Mya arenaria, are afflicted with an epidemic transmissible disease of the circulatory system closely resembling leukemia. The disease is characterized by a dramatic expansion of blast-like cells in the hemolymph with high mitotic index. Examination of hemolymph of diseased clams revealed high levels of reverse transcriptase activity, the hallmark of retroviruses and retroelements. By deep sequencing of RNAs from hemolymph, we identified transcripts of a novel retroelement, here named Steamer. The DNA of the element is marked by long terminal repeats and encodes a single large protein with similarity to mammalian retroviral Gag-Pol proteins. Steamer mRNA levels were specifically elevated in diseased hemocytes, and high expression was correlated with disease status. DNA copy number per genome was present at enormously high levels in diseased hemocytes, indicative of extensive reverse transcription and retrotransposition. Steamer activation in M. arenaria is an example of a catastrophic induction of genetic instability that may initiate or advance the course of leukemia.

Biomarkers and transcription levels of cancer-related genes in cockles Cerastoderma edule from Galicia (NW Spain) with disseminated neoplasia

Disseminated neoplasia (DN) is a pathological condition reported for several species of marine bivalves throughout the world, but its aetiology has not yet been satisfactorily explained. It has been suggested that chemical contamination could be a factor contributing to neoplasia. The aim of the present study was to compare cell and tissue biomarkers and the transcription level of cancer-related genes in cockles (Cerastoderma edule) affected by DN with those of healthy cockles in relation to chemical contaminant burdens. For this, cockles were collected from a natural bed in Cambados (Ria de Arousa, Galicia) in May 2009. The prevalence of DN was 12.36% and 3 degrees of DN severity were distinguished. No significant differences in metal accumulation, non-specific inflammatory responses and parasites were observed between healthy and DN-affected cockles. Lysosomal membrane stability was significantly reduced in cockles affected by DN, which indicates a poorer health condition. Very low frequencies of micronuclei were recorded and no significant differences were detected between DN severity groups. Haemolymph analyses showed a higher frequency of mitotic figures and binucleated cells in cockles affected by moderate and heavy DN than in healthy ones. Neoplastic animals showed significantly higher transcription levels of p53 and ras than healthy cockles and mutational alterations in ras gene sequence were detected. Low concentrations of metals, polycyclic aromatic hydrocarbons, polychlorinated biphenyls and phthalate esters were measured in cockles from Cambados. In conclusion, cockles affected by DN suffer a general stress situation and have altered patterns of cancer-related gene transcription. Further studies are in progress to elucidate mechanisms of carcinogenesis in this species.

Complete genome sequence of acute viral necrosis virus associated with massive mortality outbreaks in the Chinese scallop, Chlamys farreri

BACKGROUND

Acute viral necrosis virus (AVNV) is the causative agent of a serious disease resulting in high mortality in cultured Chinese scallops, Chlamys farreri. We have sequenced and analyzed the complete genome of AVNV.

RESULTS

The AVNV genome is a linear, double-stranded DNA molecule of 210,993 bp with a nucleotide composition of 38.5% G + C. A total of 123 open reading frames were predicted to encode functional proteins, ranging from 41 to 1,878 amino acid residues. The DNA sequence of AVNV is 97% identical to that of ostreid herpesvirus 1 (OsHV-1), and the amino acid sequences of the encoded proteins of these two viruses are 94-100% identical. The genomic organization of AVNV is similar to that of OsHV-1, and consists of two unique regions (170.4 kb and 3.4 kb, respectively), each flanked by two inverted repeats (7.6 kb and 10.2 kb, respectively), with a third unique region (1.5 kb) situated between the two internal repeats.

CONCLUSIONS

Our results indicate that AVNV is a variant of OsHV-1. The AVNV genome sequence provides information useful for understanding the evolution and divergence of OsHV-1 in marine molluscs.

Novel ssDNA virus recovered from estuarine Mollusc (Amphibola crenata) whose replication associated protein (Rep) shares similarities with Rep-like sequences of bacterial origin

Over the past couple of years highly diverse novel ssDNA viruses have been discovered. Here, we present the first ssDNA virus, Gastropod-associated circular ssDNA virus (GaCSV), recovered from a mollusc Amphibola crenata Martyn 1784, which is a deposit feeder that grazes micro-organisms and organic detritus on the surface of tidal mudflats. The GaCSV (2351 nt) genome contains two large bidirectionally transcribed ORFs. The smaller ORF (874 nt) has similarities to viral replication-associated protein (Rep) sequences of some bacteria and circoviruses, whereas the larger ORF (955 nt) does not relate to any sequences in public databases and we presume it potentially encodes the capsid protein. Phylogenetic analysis shows that the GaCSV Rep clusters with Rep-like sequences of bacterial origin, highlighting the role of ssDNA viruses in horizontal gene transfer. The occurrence of previously unknown viruses in organisms associated with human pollution is a relatively unexplored field.

Selective initiation and transmission of disseminated neoplasia in the soft shell clam Mya arenaria dependent on natural disease prevalence and animal size

Disseminated neoplasia, a diffuse tumor of the hemolymph system, is one of the six most destructive diseases among bivalve mollusk populations, characterized by the development of abnormal, rounded blood cells that actively proliferate. Though the specific etiology of disseminated neoplasia in Mya arenaria remains undetermined, the involvement of viral pathogens and/or environmental pollutants has been suggested and considered. The current study used 5-bromodeoxyuridine (BrDU) known to induce the murine leukemia virus and filtered neoplastic hemolymph to initiate disseminated neoplasia in clams from different populations and size classes respectively. M. arenaria from three locations of different natural neoplasia occurrences were divided into a control and three experimental treatments and injected with 200 μl of sterile filtered seawater or 50-200 μg/ml BrDU respectively. In a concurrent experiment, animals from different size classes were injected with 2.5% total blood volume of 0.2 μm filtered blood from a fully neoplastic animal. Animals were biopsied weekly and cell neoplasia development was counted and scored as 0-25, 26-50, 51-75 and 76-100% neoplastic hemocytes (stages 1-4) in 50 μl samples. BrDU injection demonstrated that neoplasia development in M. arenaria was dose dependent on BrDU concentration. In addition, natural disease prevalence at the source location determined initiation of neoplasia induction, with animals from the area of the highest natural disease occurrence displaying fastest neoplasia development (p=0.0037). This could imply that depending on the natural disease occurrence, a potential infectious agent may remain dormant in normal (stage 1) individuals in higher concentrations until activated, i.e. through chemical injection or potentially stress. The size experiment demonstrated that only M. arenaria between 40 and 80 mm developed 26-100% neoplastic hemocytes when injected with filtered neoplastic hemolymph, indicating that individuals smaller than 20mm or larger than 80 mm were not or no longer susceptible to disease development. So far neoplasia studies have not considered natural disease prevalence or size involvement in neoplasia development and our results indicate that these should be future considerations in neoplasia examinations.

Haemocytic leukemia in Prince Edward Island (PEI) soft shell clam (Mya arenaria): spatial distribution in agriculturally impacted estuaries

Intensive farming of potatoes in Prince Edward Island (PEI) relies on the repeated and widespread application of fertilizers and pesticides. In PEI the main potato farming areas are in close proximity and drain directly to estuaries. Runoff from high agricultural activity watersheds could impact benthic organism health in the depositional zone of downstream estuaries. The estuarine filter feeder Mya arenaria (soft-shell clam) could be particularly vulnerable to both particle-adsorbed and water soluble contaminants. M. arenaria is susceptible to haemocytic leukemia. In May 2009, we established that heavily proliferated leukemia (HPL) prevalence was generally higher in PEI estuaries located downstream of high intensity potato farming (Dunk and Wilmot estuaries) watersheds than in estuaries downstream of lower intensity areas. Using Mab-1E10 based immunocytochemistry we observed that leukemic haemocytes from the Dunk and Wilmot estuaries were 1E10 negative whereas those from the Ox/Sheep estuary (low potato farming intensity) were 1E10 positive. The expression of genes in the p53 tumour suppressor pathway enabled us to differentiate groups of leukemic and normal M. arenaria, validating our diagnoses. In October 2009, we confirmed that HPL prevalence was elevated in the Dunk and Wilmot estuaries compared to reference (Souris River). Moreover, leukemia prevalence declined with distance from the river mouths along transects through the Dunk and Wilmot estuaries. The pesticides ß-endosulfan and α-endosulfan were detected in surface sediments from the Dunk and Wilmot estuaries, but not in sediments from either the Souris River or several other lower intensity potato farming watersheds. Our study provides evidence of an association between intensity of potato farming and prevalence of clam leukemia at downstream estuaries in PEI.

Lack of detection of a putative retrovirus associated with haemic neoplasia in the soft shell clam Mya arenaria

Haemic neoplasia (HN) is a leukemia-like disease that affects at least 20 species of marine bivalves including soft shell clam, Mya arenaria. Since the disease was discovered in 1969, the etiology remains unknown. A retroviral etiology has been suggested based on the detection of reverse transcriptase activity and electron microscopic observation of retroviral-like particles using negative staining. To date, however no virus isolate and no retroviral sequence from HN has been obtained. Moreover, transmission of the disease by cell-free filtrate from affected clams has not been reproduced. In the current study, we reinvestigated the association of HN with a putative retrovirus. Sucrose gradient centrifugation followed by assessment of reverse transcriptase activity, electrophoretic analysis of protein and RNA, and electron microscopic examinations of fractions corresponding to retroviral density were employed. Detection of retroviral pol sequences using degenerate RT-PCR approaches was also attempted. Our results showed visible bands at the expected density of retrovirus in HN-positive and HN-negative clam tissues and both with reverse transcriptase activity. Electron microscopy, RNA analysis, protein analysis, and PCR systems targeting the pol gene of retroviruses did not however provide clear evidence supporting presence of a retrovirus. We point out that the retrovirus etiology of HN of Mya arenaria proposed some 25 years ago should be reconsidered in the absence of a virus isolate or virus sequences.

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.

Evidence of retroviral etiology for disseminated neoplasia in cockles (Cerastoderma edule)

Epizootiologic outbreaks of disseminated neoplasia have been reported in association with massive mortalities of various bivalve species. In cockles, Cerastoderma edule, this pathological condition was described in Ireland and France. Since 1997, different populations affected by this pathology have been detected in Galicia (NW Spain). Transmission electron microscopy allowed the visualization of virus-like particles in neoplastic cells, resembling a retrovirus-like agent. To confirm this hypothesis, we used a commercial kit for detection and quantification of reverse transcriptase (RT) activity, based on the use of bromo-deoxyuridine triphosphate (BrdUTP) and a BrdU binding antibody conjugated to alkaline phosphatase. In addition, we developed a product-enhanced RT assay using RNA of hepatitis A virus as a template. These two assays showed positive RT activity in 90.9 and 81.8% of samples, respectively, from cockles displaying disseminated neoplasia as determined by light microscopy. These results strongly support the hypothesis of retroviral etiology for this pathological condition.

Disseminated neoplasia in blue mussels, Mytilus galloprovincialis, from the Black Sea, Romania

Disseminated neoplasia, also called leukemia or hemic neoplasia, has been detected in 15 species of marine bivalve mollusks worldwide. The disease is characterized by the presence of single anaplastic cells with enlarged nuclei and sometimes frequent mitosis, in hemolymph vessels and sinuses. The neoplastic cells gradually replace normal hemocytes leading to the increased mortality of animals. The neoplasia reaches epizootic prevalences in blue mussels, Mytilus trossulus, in some areas, whereas prevalences in Mytilus edulis are generally very low. Mytilus galloprovincialis was suggested to be resistant to the disease although very low prevalences were documented from Spain in the Atlantic Ocean and Italy in the Mediterranean Sea. A case of disseminated neoplasia was discovered in M. galloprovincialis from among 200 specimens studied from the coast of the Romanian Black Sea. Histological preparation revealed the presence of large anaplastic cells with lobed nuclei. This observation extends the geographic range of marine bivalve mollusks with disseminated neoplasia to include the Black Sea.

Viral infections of aquatic animals with special reference to Asian aquaculture

Worldwide, the number of communicable diseases of animals raised in aquaculture continue to increase. Viral infections of cultivated shellfish, crustacea, and finfish have been frequently recognized in the past few years. In the Asian regions, penaeid shrimp and several teleost fish underwent epizootics associated with heavy losses in aquaculture. Baculoviruses are particularly harmful to shrimp and prawns. Herpes-, irido-, reo-, or rhabdovirus-like agents can cause outbreaks in fish farms. Viral diseases are important limiting factors in the expansion of aquaculture. However, studies on viral infections of aquatic animals have been focused primarily on economically important farmed fish. Therfore, certain viral diseases of teleost fish are relatively well understood. In contrast, our knowledge of viral infections of farmed aquatic invertebrates is still very spare. Although a great number of viruses have been detected in farmed molluscs and crustaceans, the pathogenicity and epizootiology of most of the agents is not known.

Multiple protein differences distinguish clam leukemia cells from normal hemocytes: evidence for the involvement of p53 homologues

In coastal locations, marine invertebrates, primarily molluscs, develop fatal leukemias in their blood or hemolymph. In the clam Mya arenaria, non-adhesive, mitotic, spherical leukemia cells replace adhesive, motile, normal hemocytes as leukemia progresses. End-stage leukemia cells express a unique antigen, IE10, while normal cells express the 2A4 marker. The goals of this work were to further differentiate the normal and leukemia specific antigens relative to protein structure, determine if other protein distinctions exist, and examine p53 gene family expression in both cell types. Recognized by the monoclonal antibody 2A4, normal cells express a 185-kDa glycoprotein that may have multiple forms. Detected by the monoclonal antibody 1E10, leukemic cells express a very hydrophobic 252-kDa glycoprotein that is likely to be a transmembrane protein with spectrin/dystrophin-like characteristics. After normalization to the major cytoskeletal protein actin, sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals major distinguishing protein and glycoprotein differences between the two cell types. Most obvious is the near-absence of tubulin in the non-mitotic normal hemocytes. We have also characterized the expression of p53 gene family members in normal and end-stage leukemia cells, finding shifts in expression of the p53 gene homologues p73 and p97 coincident with leukemia-specific protein synthesis.

Polyploidy in a natural population of mussel, Mytilus trossulus

We have analyzed natural polyploidy in a population of Mytilus trossulus from Vancouver Island (British Columbia, Canada) by means of cytogenetic techniques. Results obtained are the first reporting on this type of numerical chromosome aberrations in mussels.

Key words: mussel, Mytilus trossulus, polyploidy, chromosome.

Environmental contaminants and the prevalence of hemic neoplasia (leukemia) in the common mussel (Mytilus edulis complex) from Puget Sound, Washington, U.S.A

The relationship between hemic neoplasia, a blood cell disorder in bivalve molluscs, and chemical contaminants was evaluated in the common mussel (Mytilus edulis complex). Hemic neoplasia (HN) is endemic to mussel populations in Puget Sound. The prevalence of hemic neoplasia ranged from 0 to 30% in mussels from nine sites in Puget Sound, Washington. Organic chemical contamination in sediment from these sites range from 0.1 to 64.0 ppm of polycyclic aromatic hydrocarbons (PAHs) and 0.07 to 0.50 ppm chlorinated hydrocarbons. No relationship between the body burden of environmental contaminants and the prevalence of HN in mussels was identified. To evaluate the short-term ability of chemical contaminants to induce HN in mussels, mussels, from a site where mussels were previously determined to be HN free, were fed microencapsulated PAHs (composed of a mixture of phenanthrene, flouranthene, and benzo[a]pyrene) or PCBs (Aroclor 1254) and the prevalence of HN was assessed after 30 days of exposure. Although an apparent increase in HN prevalence (20 to 30%) was observed in all treatments groups except the untreated controls, no significant difference in the prevalence of HN was observed between the control group of mussels fed corn oil (vehicle) and mussels fed either PAHs or PCBs in corn oil. A long-term (180-day) exposure study was conducted to evaluate the influence of PAHs or PCBs in modulating the prevalence of HN in a mussel population already exhibiting a moderate HN prevalence. Mussels, from a site where mussels were previously determined to exhibit a background prevalence of HN, fed microencapsulated PAHs, PCBs, and corn oil (vehicle) over a long time period (180 days), revealed an apparent increased prevalence of HN (30 to 40%) above the low levels (20%) initially present. However, no significant difference in the prevalence of HN was observed between the control group of mussels fed corn oil (vehicle) and mussels fed either PAHs or PCBs in corn oil. Although chemical contaminants have been proposed as a modulating factor in the development and promotion of HN in bivalve molluscs from environmentally stressed and degraded habitats, we find no evidence that chemical contaminants induce or promote the development of HN in the mussel M. edulis complex.

Retroviral diversity and distribution in vertebrates

We used the PCR to screen for the presence of endogenous retroviruses within the genomes of 18 vertebrate orders across eight classes, concentrating on reptilian, amphibian, and piscine hosts. Thirty novel retroviral sequences were isolated and characterized by sequencing approximately 1 kb of their encoded protease and reverse transcriptase genes. Isolation of novel viruses from so many disparate hosts suggests that retroviruses are likely to be ubiquitous within all but the most basal vertebrate classes and, furthermore, gives a good indication of the overall retroviral diversity within vertebrates. Phylogenetic analysis demonstrated that viruses clustering with (but not necessarily closely related to) the spumaviruses and murine leukemia viruses are widespread and abundant in vertebrate genomes. In contrast, we were unable to identify any viruses from hosts outside of mammals and birds which grouped with the other five currently recognized retroviral genera: the lentiviruses, human T-cell leukemia-related viruses, avian leukemia virus-related retroviruses, type D retroviruses, and mammalian type B retroviruses. There was also some indication that viruses isolated from individual vertebrate classes tended to cluster together in phylogenetic reconstructions. This implies that the horizontal transmission of at least some retroviruses, between some vertebrate classes, occurs relatively infrequently. It is likely that many of the retroviral sequences described here are distinct enough from those of previously characterized viruses to represent novel retroviral genera.

Experimental field studies with Mya arenaria (Bivalvia) on the induction and effect of hematopoietic neoplasia

A field experiment was conducted from 1991 to 1992 to examine induction and impact of hematopoietic neoplasia on the marine bivalve Mya arenaria in southeastern Massachusetts. Clams were collected from Little Buttermilk Bay and separated into three size classes (20-29, 30-39, and 40-49 mm shell length) in the laboratory. These sizes span the range of adults found in the population. A random subsample of these clams was taken to estimate disease prevalence at the start of the experiment, and this was found to be </=10% in all size classes. Remaining clams were assigned randomly to two groups: Control and Treated. "Controls" were injected with filtered seawater, while "Treated" clams were injected with hemocytes extracted from diseased individuals. Injection of diseased hemocytes was performed to increase disease prevalence in the Treated group. Clams were returned to New Bedford Harbor, a more contaminated field location, where hematopoietic neoplasia is more prevalent, in January 1991, and characteristics of both groups were monitored for 555 days. Among Controls, probability of survival was size-dependent, with higher survival rates in larger clams. Treated clams had a lower probability of survival than Controls, and the magnitude of treatment effect increased with size class. The impact on survival was evident after 89 days, but it was first shown to be statistically significant after 189 days. Among Controls, probability of disease was strongly season-dependent, increasing in the large size class from 0.19 in spring to 0.50 in summer. During summer, Treated clams had a higher probability of being diseased than Controls. Among survivors, no significant sublethal effects due to treatment were detected in the field experiment. Experimental manipulation of disease prevalence may be a useful tool in future studies. In addition to results pertaining to disease, this study obtained long-term growth information, by size class, on somatic and reproductive tissue and shell size.

Human endogenous retrovirus type I-related viruses have an apparently widespread distribution within vertebrates

Retroviruses from lower vertebrate hosts have been poorly characterized to date. Few sequences have been isolated, and those which have been reported are all highly divergent when compared to the retroviruses known to be harbored by mammals and birds. Here we show that retroviruses with significant homology to the human endogenous retrovirus type I (HERV-I) are present within the genomes of fish, reptiles, birds, and mammals and that they may well be widespread within many vertebrates. Phylogenetic analysis of nucleotide sequences strongly supported the inclusion of viruses from each of these vertebrate classes into one monophyletic group. This analysis also demonstrated that the HERV-I-related viruses are more closely related to retroviruses belonging to the murine leukemia virus genus than to members of the other retroviral genera. The presence of HERV-I-related retroviruses in so many disparate vertebrate hosts suggests that other endogenous human retroviruses may also have a much wider distribution than is currently appreciated.

Implications of oil pollution in production of disease in marine organisms

Contamination of marine waters by petroleum, whether as a consequence of acute or chronic events, constitutes an additional source of stress for marine organisms — one often reflected in pathological changes. Morphological, behavioural, physiological or biochemical abnormalities may result from exposure to petroleum or its component chemicals. Among the morphological changes that have been associated, at least circumstantially, with petroleum contamination of marine habitats are (for fish) fin erosion, fin ray deformation, ovarian histopathology, olfactory lesions, degeneration of ventricular myocardium and cytogenetic anomalies; and (for invertebrates) tissue hyperplasia, gill and gut epithelial necrosis, gonadal tumours and kidney tubule occlusion. Literature on petroleum-associated diseases and abnormalities of fish and shellfish can best be summarized in four major categories: that dealing with integumental lesions offish; that concerned with various other kinds ofhistopathologies, usually experimentally induced; that emphasizing neoplasms of molluscs, and that related to genetic and morphological abnormalities in eggs and larvae, as a consequence of exposure to petroleum or its components. A. general conclusion is that petroleum, in sufficient concentration, can be toxic to marine animals, and toxicity may be expressed as morphological changes. . Some of the effects of petroleum, such as the enhancement of latent viral infections in clams and immunosuppression in fish, are quite probably associated with increased stress. The development of neoplasia and hyperplasia in petroleum-exposed marine animals deserves further examination, as does the entire matter of pollution-related integumental lesions.

A novel adhesion protein expressed by ciliated epithelium, hemocytes, and leukemia cells in soft-shell clams

The ontogeny of circulating hemocytes and tumor cells in mollusks has been approached using monoclonal antibodies to normal cells. A monoclonal antibody, previously shown to identify an adhesion related protein (p130), has been used to define the reactivity of cells in tissues from normal soft-shell clams (Mya arenaria) and soft-shell clams with leukemia. Using immunoperoxidase technology, we have determined that hemocytes, connective tissue cells, and a subset of leukemia cells that are adherent share a cross-reactive epitope with cilia.

Serum-cell interactions in transmission of sarcoma in the soft shell clam, Mya arenaria L

1. Serum proteins from sarcomatous soft shell clams, Mya arenaria L., enhanced transmission of sarcoma. 2. Sarcoma cells were isolated and administered to the recipients at the same cell density in different sarcoma-protein-free diluents: seawater, serum from normal clams, heat-treated sarcoma serum or protease-digested sarcoma serum. 3. Transmission in these groups was significantly slower than in the group where cells were administered in intact sarcoma serum, demonstrating that the tumor promoting factors in the serum were heat-sensitive proteins. 4. Normal hemocytes administered in sarcoma serum caused mortality but not sarcoma transmission, suggesting the presence of cytotoxic factors in sarcoma serum.

Different proteins in the hemolymph sera from sarcomatous and healthy soft shell clams, Mya arenaria L

1. Serum proteins showed quantitative and qualitative differences between sarcomatous and healthy soft shell clams, Mya arenaria L. 2. Total protein concentration was significantly higher in the serum of sarcomatous clams than of healthy clams. 3. According to SDS-PAGE, more serum proteins with more variability distinguished sarcomatous clams from healthy ones. 4. Sarcomatous clams had unique serum proteins of M(r)23,000, 45,000 and 54,000. Healthy clams had unique serum proteins of M(r) 24,000, 103,000 and 105,000. 5. During disease progression, sarcoma-specific proteins appeared while normal proteins disappeared. 6. We propose that some sarcoma-associated proteins may have tumor promoting and/or cytotoxic functions and that some normal proteins which disappear during disease progression may be involved in the humoral defense system.

Genotoxic, carcinogenic, and teratogenic hazards in the marine environment, with special reference to the Mediterranean Sea

Genotoxic, carcinogenic, and teratogenic hazards arising out of pollution in the marine environment are discussed in this article, with special reference to the situation in the Mediterranean area. A number of chemical compounds or complex mixtures relevant to marine pollution, either natural or of anthropogenic origin, are tentatively listed, along with protective factors which may play a counteracting role in the same environment. Harmful substances tend to undergo interactions and transformations in seawater, sediments, and marine biota, due to physical, chemical, microbial, or light-mediated mechanisms. Bioaccumulation phenomena in marine organisms may result from food-chain biomagnification processes or from concentration of pollutants by filter feeders. A variety of sources can account for marine pollution by genotoxic, carcinogenic, and teratogenic compounds, but there is a relative paucity of analytical data concerning the Mediterranean. Metabolic transformations of xenobiotics occur in all marine organisms, the biochemical mechanisms in fish being comparable to those which have been extensively investigated in mammals. Induction of metabolic pathways, and especially of the mixed-function oxygenase system, represents the earliest warning signal of exposure to pollutants. Occurrence of neoplastic diseases is documented by experimental and field studies in marine vertebrates as well as in invertebrates. The association with local pollution phenomena has been recognized in several studies, but other etiopathogenetic factors may be also involved, and in some cases tumors have been reported to be unrelated to chemical pollution. Genotoxic agents have been detected by means of suitable techniques in seawater, sediments, and marine organisms. Several studies have investigated the presence of carcinogen-DNA adducts, DNA damage and repair processes, and cytogenetic alterations, such as chromosomal aberrations, sister-chromatid exchanges, and micronuclei, in tissues of marine organisms. However, monitoring of these end-points under field conditions encounters some limitations and problems. Even more fragmentary is the information on teratogenic effects in marine organisms, although interesting test systems have been set up. On the whole, a quite extensive database on all these toxicological issues is already available in the literature, but further studies are warranted for an adequate assessment of genotoxic, carcinogenic, and teratogenic hazards, and possibly counteracting factors in the marine environment, and specifically in the Mediterranean Sea.

Alternate pathogenesis of systemic neoplasia in the bivalve mollusc Mytilus

The proliferative disease systemic neoplasia, also termed hemic neoplasia or disseminated sarcoma, was studied in four Puget Sound, Washington populations of the bay mussel (Mytilus sp.). Using flow cytometric measurement of DAPI-stained cells withdrawn from the hemolymph, DNA content frequency histograms were generated for 73 individuals affected by the disease. The cells manifesting systemic neoplasia were found to exist as either of two separate types, characterized by G0G1 phase nuclear DNA contents of either approximately 4.9 x haploid (pentaploid form) or approximately 3.8 x haploid (tetraploid form). The two disease forms were found to coexist in all four mussel populations sampled, with overall relative prevalences of 66% pentaploid form, 29% tetraploid form, and 5% exhibiting both disease forms simultaneously. These findings represent the first unequivocal demonstration of multiple cell types in a bivalve neoplasia. The two forms appear to represent separate pathogenetic processes rather than sequential stages of a single pathogenesis. Two cell cycling parameters associated with proliferative activity were employed to compare the alternate forms: (i) the percentage of cells assigned to the DNA Synthesis (S) phase of the neoplastic cell cycle, and (ii) the proportion of neoplastic cell mitotic figures in hemocytological preparations. Mean values for both parameters were significantly higher for mussels with the tetraploid form of the disease, suggesting a higher rate of proliferation relative to the pentaploid form. Qualitatively, cells of the tetraploid form contained slightly lower nuclear and cytoplasmic volumes compared to those of the pentaploid form. An observed wide variation in neoplastic cell nuclear size within either disease form may reflect the distribution of cells in the G0G1, S, and G2M phases of the cell cycle. Potential etiologic relationships between the two forms are discussed.

Human cancer and the food chain: an alternative etiologic perspective

Oncogenic viruses are among the known or presumed initiating agents of human cancer. Although evidence suggests that DNA and RNA oncoviruses may be acquired through multiple routes, our attention focuses chiefly on the ingestion pathway. We have two reasons for this. One is the possibility that viral as well as nonviral oncogenic amino acid sequences might be acquired at the top of the food chain. The other is that the food chain-infection hypothesis may reconcile several biological, ecological, and epidemiological phenomena. Transfection experiments suggest that the concept of infection may have to be broadened to embrace the cellular precursors of oncogenic viruses. Accumulating circumstantial evidence from viral oncology and molecular biology provides a basis for the belief that oncogenic viruses and their cellular precursors might be transmitted from animals to humans through the ingestion pathway. The possibility that such transmission may give rise to some human cancers must now be considered. The ingestion and genomic integration of food-associated DNA sequences may directly account for the increased risk of human cancer associated with an elevated intake of animal fat and protein. This paper addresses the role of infective oncogenic agents as the initiators, rather than the promoters, of cancer.

Leukemia cell specific protein of the bivalve mollusc Mya arenaria

Soft shell clams, Mya arenaria, develop leukemias in the hemolymph which are fatal. Tissue sections and hemolymph samples from normal and tumor-bearing clams were tested with an anti-leukemic cell specific monoclonal antibody (Mab) "IEII." Evaluation of leukemic cells and normal hemocytes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analyses showed that Mab IEII bound to a large protein of approximately 200 kDa from the tumor cell, but not from the normal cell preparation.

Activation of mammalian carcinogens to bacterial mutagens by microsomal enzymes from a pelecypod mollusk, Mercenaria mercenaria

Several kinds of neoplastic diseases have been described in mollusks collected from the field. The etiology of these lesions is unknown; however, the involvement of chemical carcinogens has been suggested. Experimental models for chemically-induced neoplasia in bivalves have yet to be developed. We have obtained data which suggest that aromatic amines may be more appropriate candidates than polycyclic aromatic hydrocarbons for putative molluscan carcinogens. Digestive gland enzymes from a bivalve mollusk, Mercenaria mercenaria, were found to be able to convert aromatic amines to frameshift mutagens as detected by the Ames Salmonella tester strains. Extensive mutagenesis was obtained with 2-aminoanthracene, 2-aminofluorene, 2-acetylaminofluorene and 4-amino-trans-stilbene which are all mammalian procarcinogens. Mutagenic activation of benzo[a]pyrene and 3-methylcholanthrene was minimal. Low levels of aromatic amine-activating enzyme(s) were found in all tissues examined, but activity was greatest in the digestive gland. Enzymatic activity was heat-labile, NADPH-dependent, and apparently not inducible by polychlorinated biphenyls (Aroclor 1254).

Unique antigens on neoplastic cells of the soft shell clam Mya arenaria

Soft shell clams (Mya arenaria) are commonly found in coastal waters of both the eastern and western United States. These invertebrates, which have an open circulatory system, may develop neoplasms of the haemolymph which ultimately kill the host. In this study we have 1) recorded the prevalence of hematopoietic neoplasms (HN) in Mya arenaria within a 50 mile radius of Woods Hole, Massachusetts and 2) utilized cells from one HN bearing clam to generate a series of monoclonal antibodies. Our data show that determinants are expressed on HN cells which are not detected on normal clam haemocytes, suggesting separate ontogenetic pathways of cell differentiation.