PATHOLOGY AND DIAGNOSIS
The work of Loh et al. (2006a, 2006b) and Pearse and Swift (2006) set the scene for the characterisation of DFTD. Using transcriptome analysis, Murchison et al. (2010) exposed the Schwann cell lineage of this unique transmissible cancer.
3.1 Gross pathology
Tumours are often located around the face or mouth, forming multicentric, well-circumscribed, firm soft tissue masses usually with flattened, centrally ulcerated and exudative surfaces.
Ulcerative lesions are usually round to ovoid, pink-red, variably sized, raised to vegetative and more pronounced in mucocutaneous tissue. Subcutaneous lesions are variably sized tumours of solid texture and irregular boundaries. As lesions progress they ulcerate further and large rapidly growing vegetative masses protrude from the skin surface. They aggressively efface the surrounding tissue and grow around more solid features, grossly distorting and replacing these tissues. The cut surface is firm, pale and slightly translucent with visible fibrous septa, sometimes with a necrotic core. Necrosis and inflammation of adjacent tissues is common, with crusting and scabs commonly at the periphery of ulcerated lesions. The tumour readily metastasises with no organ predilection. Metastatic lesions are commonly well-circumscribed, irregular, tan-pink, solid masses that start focally and expand to coalesce in multicentric invasion to replace organ tissue. Central necrosis of lesions is common, with fragmentation of cut surfaces and marked friability, which is a feature, particularly in exposed lesions.
3.2 Histopathology
The histological characteristics of DFT cells are consistent with an undifferentiated neoplasm. It presents as multinodular masses of densely compact cells arranged in bundles, chords or streams, with a high nuclear to cytoplasmic ratio, indistinct cell borders and contained within a pseudo-capsule and associated with rich blood supply.
Central necrosis and a marked lack of inflammatory cell infiltrates are common (Loh et al. 2006a). Morphological variants of the tumour are common, depending on the geographical distribution of the sampled animals and the time since arrival in a given population (unpublished).3.3 Immunohistochemistry
The histologic diagnosis and differentiation of DFT1 and DFT2 is aided by the utilisation of marker antibodies. The specific marker periaxin (PRX- Schwann cell marker) and the initial characterising matrix of S-100 (peripheral nerve sheath marker), peripheral myelin protein 22 (myelin marker) with variable specificity of primary and metastatic lesions to vimentin, neuron specific enolase (NSE), chromogranin A (CGA) and synaptophysin, can be used as a diagnostic protocol for DFT1 (Loh et al. 2006b; Trovar et al. 2011). DFT2 is not positive to PRX allowing differentiation between the two types of DFT. A comprehensive investigation of immunohistochemical stains was undertaken by Hayes (2019) on five cytogenetic strains of DFTD1 revealing that DFT1 expresses markers commonly associated with stem cells and functional selfrenewal (i.e. SOX2 (SRY-box 2), POU5F1 (POU Class 5 Homebox 1), NES (nestin), NGFR (Nerve Growth Factor Receptor), CD44 Molecule, PROM1 (Prominin 1) and EPCAM (Epithelial Cell Adhesion Molecule)). The work indicates from a functional perspective that DFT1 emerged from a neural crest stem cell origin giving it pluripotency and characteristics of a progenitor like cell with Schwann cell and melanocyte lineages and possessing self-renewal characteristics. This finding indicates a possible mechanism for DFT1 transmissibility as a progenitor-like state contributes to MHC down regulation, a characteristic of DFT1 cells.
3.4 Cytogenetics
The cytogenetic analysis of DFT1 was pivotal to its initial identification and subsequently to understanding the evolution of the cancer in affected populations. Pearse and Swift (2006) described the complex chromosomal rearrangements that were a diagnostic marker for the disease and showed that the tumour had not arisen from somatic change within an individual Tasmanian devil.
The marker was common to all of the initial tumours and the possession of a pericentric change in one host’s chromosome 5, which was absent in the same animal’s tumour’s karyotype, confirmed that the tumours were not from that animal. Continued surveillance of the karyotypes over the subsequent 5 yr showed that the tumour was evolving in different geographical areas of Tas., raising questions as to whether this evolution would affect the pathogenicity of the tumour, given that gene rearrangements would possibly affect the expression of the genes themselves (Pearse et al. 2012). Changes in pathogenicity would change the progression and possible management of the disease in free-ranging Tasmanian devil populations.Cytogenetic analysis of DFT2 demonstrated it is distinctly different from the initial clone, providing a useful tool for differentiation. This tumour carries a Y chromosome (Pye et al. 2016b).
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