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Devil facial tumour disease (DFTD) is an aggressive non-viral transmissible parasitic cancer among Tasmanian devils.〔〔〔〔 The first official case of DFTD was described in 1996 in Australia. In the subsequent decade the disease ravaged Tasmania's wild devils, with estimates of decline ranging from 20% to as much as 50% of the devil population, across over 65% of the state.〔DPIWE. 2005. (Devil Facial Tumour Disease – Update June 2005 )〕〔DPIWE. 2005. (Tasmanian Devil Facial Tumour Disease, Disease Management Strategy )〕 Affected high-density populations suffer up to 100% mortality in 12–18 months.〔DPIWE. (Disease Affecting Tasmanian Devils )〕 The disease has mainly been concentrated in Tasmania's eastern half. Visible signs of DFTD begin with lesions and lumps around the mouth. These develop into cancerous tumours that may spread from the face to the entire body. Devils usually die within six months from organ failure, secondary infection, or metabolic starvation as the tumours interfere with feeding. DFTD affects males and females equally.〔Lachish S, McCallum H, Jones M. (2009)〕 At present the population has dwindled by 70% since 1996. As of 2010, 80% of population is infected. The most plausible route of transmission is through biting, particularly when canine teeth come into direct contact with the diseased cells.〔Hawkins CE, McCallum H, Mooney N, Jones M, Holdsworth M. (2009). Sarcophilus harrisii. In: IUCN red list of threatened species. Version 2009.1 Six females have been found with a partial immunity. Breeding in captivity has begun in an attempt to save the population.〔 == Characteristics == Tasmanian devil cells have 14 chromosomes, while the oldest-known strain of the tumour cell contains thirteen chromosomes, nine of which are recognizable and four of which are mutated “marker” chromosomes.〔 More recently evolved strains have an additional mutant marker chromosome, for a total of fourteen chromosomes.〔〔 Researchers identified the cancer as a neuroendocrine tumour, and found identical chromosomal rearrangements in all the cancer cells.〔 The karyotype anomalies of DFTD cells are similar to those of cancer cells from canine transmissible venereal tumour (CTVT), a cancer of dogs that is transmitted by physical contact.〔 Among the different alterations present in the tumour genome can be found several single base mutations or shorts insertions and deletions (indels) like deletions in the chromosomes 1, 2 and 3 or trisomy in 5p. Some of the mutated or deleted genes in DFTD are RET, FANCD2, MAST3 and BTNL9-like gene. The theory that cancer cells themselves could be an infective agent (the Allograft Theory〔) was first supported by the researchers A-M. Pearse, K. Swift, and colleagues. In 2006, Pearse and Swift analyzed DFTD cells from several devils in different locations, and determined that all of the DFTD cells were not only genetically identical to each other, but also genetically distinct from their hosts, and from all known Tasmanian devils. Thus the cancer must have originated in a single individual and spread, rather than arising separately within each animal.〔〔(Tasmanian devils felled by rare cancer ), ''New Scientist'', 1 February 2006.〕〔Gramling, Carolyn: (Poor Devils: Critters' fights transmit cancer ), ''Science News'', 4 February 2006.〕 Twenty one different subtypes have been identified by analyzing the genomes (mitochondrial and nuclear) of 104 tumours from different Tasmanian devils.〔 Later researchers witnessed a previously-uninfected devil develop tumours from lesions caused by an infected devil’s bites, confirming that the disease is spread by allograft, and that the normal methods of transmission include biting, scratching, and aggressive sexual activity between individuals.〔 During biting, infection can spread from the bitten devil to the biter. Since June 2005, three females have been found that are partially resistant to DFTD.〔 Further research from the University of Sydney has shown that the infectious facial cancer may be able to spread because of low diversity in devil immune genes (MHC class I and II). The same genes are also found in the tumours, so the devil's immune system does not recognize the tumour cells as foreign.〔Leung, Chee Chee: (Extinction warning for Tassie devils ), ''The Age'', 4 October 2007.〕 There are at least four, and most likely more, strains of the cancer, showing that it is evolving, and may become more virulent.〔http://northerntasmania.yourguide.com.au/news/local/news/environmental-issues/disease-setback-for-tasmanian-devil-tumour-evolving-with-nine-strains-now-identified/1227014.aspx〕 Increased levels of tetraploidy have been shown to exist in the oldest strain of DFTD as of 2014, which correlates with the devils involved being subjected to a DFTD removal programme. Because ploidy slows the tumour growth rate, Ujvari et al. posit that the DFTD removal programme selected for slower-growing tumours, and that disease eradication programmes may result in the evolution of DFTD. The strains may also complicate attempts to develop a vaccine, and the mutation of the cancer may mean that it could spread to other related species, like the quoll. In a paper published in the January 2010 issue of ''Science'', an international team of researchers announced that devil facial tumour disease likely originated in the Schwann cells of a single devil.〔 Schwann cells are found in the peripheral nervous system, and produce myelin and other proteins essential for the functions of nerve cells in the peripheral nervous system.〔〔 The researchers sampled 25 tumours and found that the tumours were genetically identical.〔 Using deep sequencing technology, they then profiled the tumours' transcriptome, the set of genes that are active in tumours. The transcriptomes closely matched those of Schwann cells, revealing high activity in many of the genes coding for myelin basic protein production.〔 Several specific markers were identified by the team, including the MBP and PRX genes, which may enable veterinarians to more easily distinguish DFTD from other types of cancer, and may eventually help identify a genetic pathway that can be targeted to treat it.〔 Due to the decreased life expectancy of the devils due to DFTD, they have begun breeding at younger ages in the wild, with reports that many only live to participate in one breeding cycle.〔Owen and Pemberton, p. 6.〕 A study has suggested that Tasmanian devils have changed their breeding habits in response to the disease. Females previously started breeding at the age of two, then annually for about three more years until dying normally. Now they commonly breed at the age of one, and die of tumours shortly thereafter. It is speculated that the disease is spread by devils biting each other during the mating season.〔 Social interactions have been seen spreading DFTD in a local area.〔http://www.environment.gov.au/biodiversity/threatened/publications/recovery/pubs/draft-for-comment-tasmanian-devil.pdf〕 It is one of three known contagious cancers. In 2015, a variant form of the cancer was described which was tetraploid, not diploid like the main form of the cancer. The tetraploid form has been linked to lower mortality rates.〔http://www.abc.net.au/worldtoday/content/2015/s4304667.htm〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Devil facial tumour disease」の詳細全文を読む スポンサード リンク
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