Leukemia cell specific protein of the bivalve mollusc Mya arenaria

The Ethyl Acetate Extract of the Marine Edible Gastropod Haliotis tuberculata coccinea: a Potential Source of Bioactive Compounds

The phylum Mollusca represents one of the largest groups of marine invertebrates. Nowadays, molluscan shellfish belonging to the classes Bivalvia and Gastropoda are of commercial interest for fisheries and aquaculture. Although bioactive properties of bivalve molluscs have been widely investigated and several dietary supplements have been brought to the market, the bioactive potentialities of marine gastropods are poorly documented. The present study investigated the bioactive properties of tissue extracts derived from Haliotis tuberculata coccinea, or "European abalone," an edible abalone species distributed in the Mediterranean Sea and the northeast Atlantic Ocean. A bioactive organic compound-rich extract was obtained using ethyl acetate as extracting solvent. It showed antimicrobial activity towards the methicillin-resistant Staphylococcus epidermidis strain RP62A, the emerging multi-drug-resistant Stenotrophomonas maltophilia D71 and Staphylococcus aureus ATCC 6538P, being the most sensitive strain. It also showed anthelmintic activity, evaluated through the toxicity against the target model helminth Caenorhabditis elegans. In addition, the ethyl acetate extract demonstrated a selective cytotoxic activity on the cancer cell lines A375, MBA-MD 231, HeLa, and MCF7, at the concentration of 250 µg/mL. The fatty acid composition of the bioactive extract was also investigated through FAME analysis. The fatty acid profile showed 45% of saturated fatty acids (SAFA), 22% of monounsaturated fatty acids (MUFA), and 33% of polyunsaturated fatty acids (PUFA). The presence of some biologically important secondary metabolites in the extract was also analysed, revealing the presence of alkaloids, terpenes, and flavonoids.

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.

Identification of relevant cancer related-genes in the flat oyster Ostrea edulis affected by disseminated neoplasia

Disseminated neoplasia (DN), an oyster disease resembling leukaemia, has been reported in a number of species of marine bivalve molluscs. The disease is characterised by a proliferation of abnormal circulating cells of unknown origin resulting in the invasion of tissues and organs, frequently with a fatal end of the affected individuals. To obtain a more comprehensive view of bivalve cancer processes, suppressive subtracted hybridisation (SSH) and quantitative RT-PCR (q-PCR) approaches were combined to investigate changes in the transcriptome of Ostrea edulis haemolymph cells associated to DN. Two SSH libraries were constructed and 587 expressed sequence tags (ESTs) were sequenced, obtaining 329 ESTs which showed expression changes in neoplastic process. Transcription expression analyses (q-PCR) were done for a total of 24 genes that could be relevant in neoplastic process, including genes with role in the regulation of cell cycle, apoptosis or chromosomal defects. Most of those genes had not been reported in association with cancer in non-vertebrate organisms. The over-expression and under-expression of some of those genes in DN-affected oysters was in agreement with observations in vertebrate cancer. The results herein reported contribute to cancer understanding in bivalve molluscs.

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.

Disseminated neoplasia in cockles Cerastoderma edule: ultrastructural characterisation and effects on haemolymph cell parameters

Disseminated neoplasia (DN) has been detected in cockles from various beds in Galicia (NW Spain). A study was performed to characterise cockle neoplastic cell ultrastructure and to evaluate the effect of this disease at different severity stages on various haemolymph cell parameters. Examination of cockle neoplastic cells with transmission electron microscopy (TEM) showed round shapes and a lack of pseudopods, a high nucleus:cytoplasm diameter ratio, Golgi complexes, abundant mitochondria, ribosomes, and numerous endoplasmic reticulum tubes and electron-lucent vesicles. Various haemolymph cell parameters (cell mortality, non-specific esterase and lysosome biovolume, reactive oxygen intermediates [ROI] production, phagocytosis ability, intracellular Ca2+ and actin levels) were compared between DN severity categories by flow cytometry; haemocyte mortality, non-specific esterase activities and lysosome biovolume were found to be higher with increasing DN severity. The phagocytic ability of neoplastic cells was sharply reduced with regard to haemocytes. The cytoplasmic-free Ca2+ level was higher and actin content lower in haemolymph cells of diseased cockles compared to unaffected ones. A significant increase in ROI production was detected in later stages of disease progression.

Mass culture and characterization of tumor cells from a naturally occurring invertebrate cancer model: applications for human and animal disease and environmental health

On the northeastern coast of the United States and Canada, Mya arenaria, the soft shell clam, develops a diffuse, hemopoetic tumor (a fatal leukemia-like cancer) resulting from inactivation of p53-like family member proteins.These malignant cells provide a model for an unrelated set of human cancer cells that are also characterized by mortalin-based cytoplasmic sequestration of wild-type p53 tumor suppressor protein (mortalin is the mitochondrial Hsp70 protein). Here we describe methods for mass culture and long-term storage of tumor cells from this cancer. These are the first successful efforts at maintaining malignant cells from any marine invertebrate in vitro. Following passage (subculture), these cultures undergo transition from primary cultures to non-immortalized cell lines that continue to proliferate and do not re-differentiate the normal hemocyte phenotype. We also characterize normal clam hemocytes and the pathology of cancerous clam hemocytes in vitro and in vivo using light and electron microscopy, cyto- and immunocytochemistry, molecular biology, and a phagocytosis assay. Our protocols provide biomedical and environmental researchers with ready access to this naturally occurring cancer model. We discuss the clam cancer model regarding (a) human health and disease; (b) animal health, disease, and aquaculture; (c) environmental health monitoring; and (d) future research directions.

Immunophenotyping of Mya arenaria neoplastic hemocytes using propidium iodide and a specific monoclonal antibody by flow cytometry

Disseminated neoplasia (DN) is a disorder referred to as hemic neoplasia (HN) in the soft-shell clam Mya arenaria. Traditionally, diagnosis is performed by hematocytology or histology. The intensity of the disease is generally given as the percentage of transformed neoplastic cells out of total number of hemocytes. Flow cytometry techniques have found a field of application in diagnosis of HN with analysis of ploidy. Hemocytes of the soft-shell clams with HN display tetraploid DNA content, as shown by propidium iodide staining. This feature makes difficult HN diagnosis in the soft-shell clam, especially for early stages of the condition, since the percentage of normal circulating cells undergoing mitosis, which also are tetraploid, remains unknown in molluscs. Use of specific monoclonal antibodies in a flow cytometry assay was foreseen as a way to overcome the difficulty. The purpose of this study was to develop a double staining protocol using propidium iodide for hemocyte cycle analysis and the MAb 1E10 for staining of HN cells. Our results showed a correlation between tetraploid and MAb 1E10-stained hemocytes in a single clam with moderate HN. This protocol offers some potential for further investigation of this cell disorder. However, a validation step will be necessary to confirm our preliminary results.

Is there a causal association between genotoxicity and the imposex effect?

There is a growing body of evidence that indicates common environmental pollutants are capable of disrupting reproductive and developmental processes by interfering with the actions of endogenous hormones. Many reports of endocrine disruption describe changes in the normal development of organs and tissues that are consistent with genetic damage, and recent studies confirm that many chemicals classified to have hormone-modulating effects also possess carcinogenic and mutagenic potential. To date, however, there have been no conclusive examples linking genetic damage with perturbation of endocrine function and adverse effects in vivo. Here, we provide the first evidence of DNA damage associated with the development of imposex (the masculinization of female gastropods considered to be the result of alterations to endocrine-mediated pathways) in the dog-whelk Nucella lapillus. Animals (n = 257) that displayed various stages of tributyltin (TBT) -induced imposex were collected from sites in southwest England, and their imposex status was determined by physical examination. Linear regression analysis revealed a very strong relationship (correlation coefficient of 0.935, p < 0.0001) between the degree of imposex and the extent of DNA damage (micronucleus formation) in hemocytes. Moreover, histological examination of a larger number of dog-whelks collected from sites throughout Europe confirmed the presence of hyperplastic growths, primarily on the vas deferens and penis in both TBT-exposed male snails and in females that exhibited imposex. A strong association was found between TBT body burden and the prevalence of abnormal growths, thereby providing compelling evidence to support the hypothesis that environmental chemicals that affect reproductive processes do so partly through DNA damage pathways.

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.

Monoclonal antibodies to European flat oyster Ostrea edulis hemocytes: characterization and tissue distribution of granulocytes in adult and developing animals

Monoclonal antibodies specific for hemocyte sub-populations of the European flat oyster, Ostrea edulis, were prepared using separated granulocytes and hyalinocytes as antigen sources. Six monoclonal antibodies specific for hemocytes were selected. Five of them showed a specificity for more than one hemocyte type and one hybridoma produced a monoclonal antibody reacting specifically with granulocytes. At the ultrastructural level, this monoclonal antibody demonstrated epitopes principally in dense cytoplasmic granules of granulocytes. Western blotting analysis indicated that a peptide of 50kDa was recognized by this antibody. It was therefore used to investigate granulocyte distribution and ontogenesis in European flat oysters using immunohistochemistry. Granulocytes were mostly observed in connective tissues in different organs. Their distribution pattern in digestive gland, mantle, gills and gonad may indicate different functional status. Moreover, the absence of granulocytes in early larval stages can partly correspond to an immature immune system in oyster larvae.

Expression of homologues for p53 and p73 in the softshell clam (Mya arenaria), a naturally-occurring model for human cancer

Homologues for human p53 (Hsp53) and p73 (Hsp73) genes were cloned and expression patterns for their corresponding proteins analysed in tissues from normal and leukemic softshell clams (Mya arenaria). These are the first structural and functional data for p53 and p73 cDNAs and gene products in a naturally occurring, non-mammalian disease model. Core sequence of the predicted clam p53 (Map53) and p73 (Map73) proteins is virtually identical and includes the following highly conserved regions: the transcriptional activation domain (TAD), MDM2 binding site, ATM phosphorylation site, proline rich domain, DNA binding domains (DBDs) II-V, nuclear import and export signals and the tetramerization domain. The core sequence is a structural mosaic of the corresponding human proteins, with the TAD and DBDs resembling Hsp53 and Hsp73, respectively. This suggests that Map53 and Map73 proteins may function similarly to human proteins. Clam proteins have either a short (Map53) or long (Map73) C-terminal extension. These features suggest that Map53 and Map73 may be alternate splice variants of a p63/p73-like ancestral gene. Map73 is significantly upregulated in hemocytes and adductor muscle from leukemic clams. In leukemic hemocytes, both proteins are absent from the nucleus and sequestered in the cytoplasm. This observation suggests that a non-mutational p53/p73-dependent mechanism may be involved in the clam disease. Further studies of these gene products in clams may reveal p53/p73-related molecular mechanisms that are held in common with Burkitt's lymphoma or other human cancers.

Detection of mutant p53 in clam leukemia cells

Leukemia in the soft-shell clam, Mya arenaria, is characterized by tumor cells which are detected initially in the hemolymph. This disease is much more common in clams inhabiting polluted waters, suggesting an environmental component to its pathogenesis. In this study, leukemia cells were identified using a murine monoclonal antibody, 1E10, which recognizes a leukemia-specific protein expressed by tumor cells. Mutant p53 protein was detected using a murine monoclonal antibody (PAb 240) which reacts with mutant p53. Using immunofluorescence, the reactivity of clam cells to the 1E10 antibody was evaluated along with mutant p53 protein reactivity. Reverse transcriptase-polymerase chain reactions followed by sequence analyses were utilized to examine clams with hemocytes reacting with the p53 antibody for possible p53 gene mutations. Mutant p53 protein was expressed by tumor cells from five animals with advanced disease (in which greater than 90% of cells reacted with 1E10). A C-->G transversion was detected at the end of exon 6 from two of the five animals that reacted with both the mutant p53 antibody and 1E10. This substitution changes the amino acid of this codon from proline to alanine. Overall, our results suggest that environmentally induced alterations in p53 can contribute to the pathogenesis of leukemia in soft-shell clams inhabiting polluted water and/or sediment.

Specific reactivity of leukemia cells to polyclonal anti-PCB antibodies

Bivalve molluscs such as the soft shell clam (Mya arenaria) develop leukemias in the hemolymph which are fatal. The prevalence of leukemia in Mya was evaluated using a murine monoclonal antibody which recognizes a leukemia-specific protein expressed by tumor cells. The reactivity with a polyclonal antibody to polychlorinated biphenyls (PCBs) of both normal circulating cells and tumor cells was also determined. Both leukemia prevalence and PCB reactivity were ascertained by flow cytometry. Analytical chemistry was used to quantitate the amount of Aroclor per tumor cell population and compared directly to flow cytometric results. Our results show that the prevalence of leukemia consistently exceeds 60% when clams are retrieved from New Bedford Harbor, a site heavily contaminated with PCBs. Both normal circulating cells and tumor cells are extremely reactive with the PCB antibody. When clams from two other sites were compared with clams from New Bedford Harbor, both disease prevalence and cell reactivity to the PCB antibody were reduced. Our experiments are the first which use the flow cytometer to demonstrate PCBs in cell populations of marine invertebrates. Our results further demonstrate that the presence of polychlorinated biphenyls in vivo is directly correlated with environmentally linked leukemia.

Immunostimulation of tiger shrimp (Penaeus monodon) hemocytes for generation of microbicidal substances: analysis of reactive oxygen species

NBT staining was used to determine the presence of superoxide anions (O2-) produced by tiger shrimp (Penaeus monodon) hemocytes attached to a coverslip. When cells were treated with beta-glucan, blue granules were observed in 41% of studied hemocyte cytoplasm. For zymosan-treated, PMA-treated, and control cells, the percentages of hemocytes showing similar blue granules were 31, 9, and 5%, respectively. A comparison of stimulative effects on 15 hemocyte suspensions, each collected from a single tiger shrimp, showed that beta-glucan had the strongest effect on intracellular O2- generation, followed by zymosan and PMA (2.5, 2, and 1.3 times greater than the O2- generated by the control group, respectively). After oxidizing phenol red to measure the amounts of hydrogen peroxide (H2O2) produced by the hemocytes, we found that beta-glucan had the strongest stimulative effect (12.2 nmol/mg protein), followed by zymosan and PMA (7.2 and 2.6 nmol/mg, respectively). However, a luminol-enhanced chemiluminescence analysis of hypochlorites (OCl-) produced by the experimental hemocytes showed that neither zymosan nor beta-glucan had a stimulative effect on OCl- production. However, following PMA stimulation, hemocyte chemiluminescence was detected although only at 1.7 mV. Using H2O2 as substrate and guaiacol as an electron acceptor, the enzyme activity of crude enzyme extract derived from broken hemocytes was analyzed; enzyme activity similar to that of human myeloperoxidase (MPO) (0.104 U/mg protein) was observed. The data showed that only PMA had any stimulative effect on MPO-like enzyme activity (2.23 times that of the control group); zymosan and beta-glucan did not have any observable effects on this specific enzyme activity. This is the first documented demonstration of a respiratory burst in shrimp hemocytes.

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.

Field and laboratory comparisons of mortality in normal and neoplastic Mya arenaria

The results of a 6-month mark and recapture experiment involving approximately 900 adult Mya arenaria demonstrated that under natural conditions, significantly higher (P much less than .001, chi 2 test) mortality occurred among animals with neoplasia than those diagnosed as normal. Using a blood screening technique, the clams were diagnosed and placed in one of three diagnostic groups based on the severity of the disease (the percentage neoplastic cells per total number of blood cells): Nonneoplastic (NN), 0%; low severity neoplastic (LSN), less than 50%; and high severity neoplastic (HSN), greater than 50%. Fifty-one percent of those clams initially diagnosed as HSN died by the end of the test period as compared to 8% of the LSN clams and only 3% of the normals. Both progression and remission of the disease were also evident. Approximately 10% of the clams in the NN and LSN groups progressed to a LSN or HSN condition, whereas 16% of those clams initially identified as LSN, and that were recovered alive, underwent complete remission during the test period. Comparison of the field results with those of an 18-week laboratory study suggests that studies of mortality done under laboratory conditions may not provide useful data for the interpretation of the quantitative effects of a disease process, such as molluscan neoplasia, on the natural population of the animal studied.

Evolutionary immunobiology

Identifying the evolutionary origin of inducible, specific immune recognition represents a major objective in developmental immunology. In order to address this issue from an overall phylogenetic perspective, major studies of cellular and humoral immune function are being undertaken using lower vertebrate and invertebrate models. Here, C. Reinisch and G. Litman discuss the application of new technologies, particularly molecular genetic approaches, that is providing important new insights into the genetic mechanisms that have influenced the evolutionary diversification of immunological function.