Samuel Frederick Mock Hart

Bivalve transmissible neoplasias (BTNs) are transmissible cancers that have arisen in multiple bivalve species, including Eastern soft-shell clam ( Mya arenaria ) populations on the East Coast of North America. Two sublineages of a single clone (termed MarBTN) currently circulate at low enzootic levels (1 to 5%) in New England, the United States, and Prince Edward Island, Canada, but MarBTN has not previously been observed in soft-shell clams on the West Coast. In 2022, we collected soft-shell clams in Puget Sound, Washington, and unexpectedly found MarBTN in two sites. Prevalence increased in subsequent years, surpassing 75% at both sites in 2024, while remaining undetectable in other populations, suggesting the early stages of a severe disease outbreak. Based on the presence of somatic transposon insertion sites found only in the USA-sublineage, the cancer was likely recently transplanted from New England. We also found that soft-shell clams in multiple Puget Sound locations are hybridizing populations of M. arenaria and Mya japonica . M. japonica may have decreased susceptibility to this cancer, though further sampling will be needed to confirm this hypothesis. We further developed a sensitive environmental DNA (eDNA) assay, targeting somatic mutations in the MarBTN mitogenome. Using this assay, we surveyed 51 sites throughout Puget Sound, detecting cancer at high levels at sites where MarBTN-positive clams were observed and in the surrounding area. These results identify a severe outbreak of transmissible cancer in a hybridizing population, due to transfer of disease from another ocean, and demonstrate the utility of eDNA methods to track BTN through the environment.

Despite dozens of tools to identify mutational signatures in cancer samples, there is not an established metric for quantifying whether signature exposures differ significantly between two heterogeneous groups of samples. We demonstrate that a signature-agnostic metric - the aggregate mutation spectrum distance permutation method (AMSD) - can rigorously determine whether mutational exposures differ between groups, a hypothesis that is not directly addressed by signature analysis. First, we reanalyze a study of carcinogen exposure in mice, determining that eleven of twenty tested carcinogens produce significant mutation spectrum shifts. Only three of these carcinogens were previously reported to induce distinct mutational signatures, suggesting that many carcinogens perturb mutagenesis by altering the composition of endogenous signatures. Next, we interrogate whether patient ancestry has a measurable impact on human tumor mutation spectra, finding significant ancestry-associated differences across ten cancer types. Some have been previously reported, such as elevated SBS4 in African lung adenocarcinomas, while some have not to our knowledge been reported, such as elevated SBS17a/b in European esophageal carcinomas. These examples suggest that AMSD is a robust tool for detecting differences among groups of tumors or other mutated samples, complementing descriptive signature deconvolution and enabling the discovery of environmental and genetic influences on mutagenesis.

Oncogenesis involves widespread genetic and epigenetic alterations, yet the full spectrum of somatic variation genome-wide remains unresolved. We generated a near-telomere-to-telomere (T2T) diploid assembly of a donor paired with deep short- and long-read sequencing of their melanoma. This revealed that 16% of somatic variants occur in sequences absent from GRCh38, with satellite repeats acting as hotspots for UV-induced damage due to sequence-intrinsic mutability and inefficient repair. Centromere kinetochore domains emerged as focal sites of structural, genetic, and epigenetic variation, leading to remodeling of centromere kinetochore binding domains during tumor evolution. Single-molecule telomere reconstructions uncovered cycles of attrition, deletion, and telomerase-mediated extension that shape cancer telomeres. Finally, diploid chromatin maps exposed that copy number alterations and epimutations, rather than point mutations, predominate in rewiring cancer regulatory programs. These findings define the full landscape of a cancer’s somatic variation and their functional impact, establishing a blueprint for T2T studies of mosaicism.

Despite dozens of tools to identify mutational signatures in cancer samples, there is not an established metric for quantifying whether signature exposures differ significantly between two heterogeneous groups of samples. We demonstrate that a signature-agnostic metric - the aggregate mutation spectrum distance permutation method (AMSD) - can rigorously determine whether mutational exposures differ between groups, a hypothesis that is not directly addressed by signature analysis. First, we reanalyze a study of carcinogen exposure in mice, determining that eleven of twenty tested carcinogens produce significant mutation spectrum shifts. Only three of these carcinogens were previously reported to induce distinct mutational signatures, suggesting that many carcinogens perturb mutagenesis by altering the composition of endogenous signatures. Next, we interrogate whether patient ancestry has a measurable impact on human tumor mutation spectra, finding significant ancestry-associated differences across ten cancer types. Some have been previously reported, such as elevated SBS4 in African lung adenocarcinomas, while some have not to our knowledge been reported, such as elevated SBS17a/b in European esophageal carcinomas. These examples suggest that AMSD is a robust tool for detecting differences among groups of tumors or other mutated samples, complementing descriptive signature deconvolution and enabling the discovery of environmental and genetic influences on mutagenesis.

Bivalve transmissible neoplasias (BTNs) are leukemia-like cancers found in at least 10 bivalve species, in which the cancer cells themselves transfer from one individual to another, spreading as an unusual form of infectious disease. Before the infectious etiology was known, there were reports of lethality and outbreaks of cancer in the soft-shell clam ( Mya arenaria ) on the east coast of North America. Using sensitive and specific qPCR assays, we followed the progression of BTN in naturally-infected soft-shell clams from Maine, USA. We observed variable outcomes, with about half of clams (9/21) progressing to high levels of cancer and death, about half exhibiting long-term non-progression (11/21), and a single animal showing regression of cancer. We also observe a significant decrease in survival in animals that progress to >10% cancer in their hemolymph, while we see no effect on survival in clams with BTN that are long-term non-progressors. As most bivalves do not physically contact each other, and BTN cells can survive in seawater, it has been proposed that BTN is spread through release of cancer cells into the water. We used qPCR to detect BTN-specific DNA in environmental DNA (eDNA) in the tanks of animals throughout this experiment. We show that cancer cell release can be detected in tank water of most clams with >24% cancer in their hemolymph, but not below this level. Cancer cell release is variable and occurs in bursts, but above 24% detection in eDNA correlates with progression of cancer in the hemolymph. This study demonstrates both the lethality of BTN and the presence of a block to the progression of BTN in a large portion of clams in a population with enzootic disease. This also further supports the hypothesis that BTN cells transmit through seawater and provides insights into the mechanisms of the transmission dynamics.

Transmissible cancers are unique instances in which cancer cells escape their original host and spread through a population as a clonal lineage, documented in Tasmanian devils, dogs, and ten bivalve species. For a cancer to repeatedly transmit to new hosts, these lineages must evade strong barriers to transmission, notably the metastasis-like physical transfer to a new host body and rejection by that host’s immune system. We quantified gene expression in a transmissible cancer lineage that has spread through the soft-shell clam (Mya arenaria) population to investigate potential drivers of its success as a transmissible cancer lineage, observing extensive differential expression of genes and gene pathways. We observed upregulation of genes involved with genotoxic stress response, ribosome biogenesis and RNA processing, and downregulation of genes involved in tumor suppression, cell adhesion, and immune response. We also observe evidence that widespread genome instability affects the cancer transcriptome via gene fusions, copy number variation, and transposable element insertions. Finally, we incubated cancer cells in seawater, the presumed host-to-host transmission vector, and observed conserved responses to halt metabolism, avoid apoptosis and survive the low-nutrient environment. Interestingly, many of these responses are also present in healthy clam cells, suggesting that bivalve hemocytes may have inherent seawater survival responses that may partially explain why transmissible cancers are so common in bivalves. Overall, this study reveals multiple mechanisms this lineage may have evolved to successfully spread through the soft-shell clam population as a contagious cancer, utilizing pathways known to be conserved in human cancers as well as pathways unique to long-lived transmissible cancers.

Bivalve transmissible neoplasia (BTN) is one of three known types of naturally transmissible cancer— cancers in which the whole cancer cells move from individual to individual, spreading through natural populations. BTN is a lethal leukemia-like cancer that has been observed throughout soft-shell clam (Mya arenaria) populations on the east coast of North America, with two distinct sublineages circulating at low enzootic levels in New England, USA, and Prince Edward Island, Canada. Major cancer outbreaks likely due toMya arenariaBTN (MarBTN) were reported in 1980s and the 2000s and the disease has been observed since the 1970s, but it has not been observed in populations of this clam species on the US west coast. In 2022, we collected soft-shell clams from several sites in Puget Sound, Washington, USA, and unexpectedly found high prevalence of BTN in two sites (Triangle Cove on Camano Island and near Stanwood in South Skagit Bay). Prevalence of BTN increased in subsequent years, reaching >75% in both sites in 2024, while it was not observed in other sites, suggesting the early stages of a severe disease outbreak following recent introduction. We observed that these cancer cells contain several somatic transposing insertion sites found only in the USA-sublineage of MarBTN, showing that it likely was recently transplanted from New England to this location. We then developed a sensitive environmental DNA (eDNA) assay, using qPCR to target somatic mutations in the MarBTN mitogenome, and showed that MarBTN can be detected in seawater at Triangle Cove, as well as several kilometers outside of the cove. We then used this assay to survey 50 sites throughout Puget Sound, confirming that the disease can be detected at high levels at Triangle Cove and South Skagit Bay, and showing that it extends beyond these known sites. However, while normal soft-shell clam mtDNA was widely detected, MarBTN was undetectable throughout most of Puget Sound and currently remains limited to the South Skagit Bay area and north Port Susan. These results identify a previously unknown severe outbreak of a transmissible cancer due to long-distance transplantation of disease from another ocean, and they demonstrate the utility of eDNA methods to track the spread of BTN through the environment.

Transmissible cancers are unique instances in which cancer cells escape their original host and spread through a population as a clonal lineage, documented in Tasmanian Devils, dogs, and ten bivalve species. For a cancer to repeatedly transmit to new hosts, these lineages must evade strong barriers to transmission, notably the metastasis-like physical transfer to a new host body and rejection by that host’s immune system. We quantified gene expression in a transmissible cancer lineage that has spread through the soft-shell clam ( Mya arenaria ) population to investigate potential drivers of its success as a transmissible cancer lineage, observing extensive differential expression of genes and gene pathways. We observed upregulation of genes involved with genotoxic stress response, ribosome biogenesis and RNA processing, and downregulation of genes involved in tumor suppression, cell adhesion, and immune response. We also observe evidence that widespread genome instability affects the cancer transcriptome via gene fusions, copy number variation, and transposable element insertions. Finally, we incubated cancer cells in seawater, the presumed host-to-host transmission vector, and observed conserved responses to halt metabolism, avoid apoptosis and survive the low-nutrient environment. Interestingly, many of these responses are also present in healthy clam cells, suggesting that bivalve hemocytes may have inherent seawater survival responses that may partially explain why transmissible cancers are so common in bivalves. Overall, this study reveals multiple mechanisms this lineage may have evolved to successfully spread through the soft-shell clam population as a contagious cancer, utilizing pathways known to be conserved in human cancers as well as pathways unique to long-lived transmissible cancers.

Transmissible cancers are infectious parasitic clones of malignant cells that metastasize to new hosts, living past the death of the founder animal in which the cancer initiated. Several lineages of transmissible cancer have recently been identified in bivalves, including one that has spread through the soft-shell clam ( Mya arenaria ) population along the east coast of North America. To investigate the evolutionary history of this transmissible cancer lineage, we assembled a highly contiguous 1.2 Gb soft-shell clam reference genome and characterized somatic mutations from cancer sequences. We show that all cancer cases observed descend from a single founder and cluster into two geographically distinct sub-lineages. We discover a previously unreported clock-like mutational signature that predicts the cancer lineage to be 344 to 877 years old, indicating that it spread undetected long before it was first observed in the 1970s. We observe high mutation density, widespread copy number gain, structural rearrangement, loss of heterozygosity, variable telomere lengths, mitochondrial genome expansion, and transposable element activity, all indicative of an unstable cancer genome. Our study reveals the ability for an invertebrate cancer lineage to survive for centuries while its genome continues to structurally mutate, likely contributing to the ability of this lineage to adapt as a parasitic cancer. SUMMARY: The genome of a contagious cancer in clams reveals structural instability of multiple types throughout the ∼500 years since its origin.