Contagious Cancer Found in North American Catfish
Contagious Cancer Found in North American Catfish
北美鲶鱼中发现传染性癌症
The idea that cancer could behave like a contagious disease may seem absurd; in humans, cancer cannot be transmitted from one person to another. Yet nature is still full of surprises. In exceptional cases, cancer cells themselves can pass from one individual to another and continue to multiply in a new host. This rare phenomenon has been detected in animals such as dogs, Tasmanian devils, and some species of mollusks, and it seems we can add one more to that exclusive club: catfish.
癌症像传染病一样传播，这种想法似乎很荒谬；在人类中，癌症无法在人与人之间传播。然而，大自然总是充满惊喜。在极少数情况下，癌细胞本身可以从一个个体转移到另一个个体，并在新宿主体内继续增殖。这种罕见现象此前已在狗、塔斯马尼亚恶魔（袋獾）以及某些软体动物身上被发现，现在看来，我们可以在这个独特的名单中再增加一位成员：鲶鱼。
An international team of researchers has identified a transmissible melanoma in the brown catfish (Ameiurus nebulosus)—the first documented case in fish and also the first recorded in a freshwater ecosystem. Their findings were reported this month in the journal Nature.
一个国际研究团队在褐美鲶（Ameiurus nebulosus）身上发现了一种可传播的黑色素瘤——这是鱼类中首个有记录的病例，也是淡水生态系统中的首例。他们的研究结果于本月发表在《自然》杂志上。
An Impossible Pattern
不可能的模式
The story began in 2012, when fishermen and biologists detected an unusually high number of catfish with black lesions in Lake Memphremagog, a body of water straddling Vermont and Quebec. Previous studies revealed that these spots were malignant melanomas, and that between 23 percent and 37 percent of the specimens examined had these tumors—an unusually high incidence for this species.
故事始于2012年，当时渔民和生物学家在横跨佛蒙特州和魁北克省的门弗雷梅戈格湖（Lake Memphremagog）中，发现大量鲶鱼身上长有黑色病变。此前的研究表明，这些斑点是恶性黑色素瘤，在受检样本中，有23%到37%的鲶鱼患有此类肿瘤——对于该物种而言，这一发病率高得异常。
“That was surprising,” said Julie Dragon, a researcher at the University of Vermont and co-lead author of the study. “We wanted to know how a bottom-dwelling fish was getting a cancer we associate with exposure to too much sunlight.”
“这令人惊讶，”佛蒙特大学研究员、该研究的共同主要作者朱莉·德拉贡（Julie Dragon）说，“我们想知道，一种底栖鱼类是如何患上我们通常认为与过度日晒有关的癌症的。”
Initially, it was thought that the cause might be some toxic substance or a pathogen linked to pollution. One suspicion was that the flooding caused by Tropical Storm Irene in 2011 had degraded water quality by washing environmental pollutants into the lake. There were also concerns about the health of the lake, which supplies drinking water to more than 175,000 people. However, initial genetic analyses began to point in an unexpected direction.
起初，人们认为病因可能与某种有毒物质或与污染有关的病原体有关。一种怀疑是，2011年热带风暴“艾琳”引发的洪水将环境污染物冲入湖中，导致水质恶化。人们还担心该湖的健康状况，因为它为超过17.5万人提供饮用水。然而，初步的基因分析开始指向一个意想不到的方向。
It was then that the researchers wondered whether the tumors were descended from a single original cancer that had learned to spread among individual fish, rather than arising independently in each fish.
就在那时，研究人员开始怀疑这些肿瘤是否源自最初的单一癌症，这种癌症学会了在个体鱼类之间传播，而不是在每条鱼身上独立产生。
To test this unusual hypothesis, the team sequenced the complete genomes of tumors and healthy tissues from affected fish, and compared the data with that from healthy specimens from different populations.
为了验证这一不同寻常的假设，研究团队对受影响鱼类的肿瘤和健康组织进行了全基因组测序，并将这些数据与来自不同种群的健康样本数据进行了比对。
The results indicated that the tumor cells from different fish were much more closely related to one another than to the animals in which they developed. In other words, the tumors shared their own genetic identity, distinct from that of their hosts. Furthermore, hundreds of thousands of genetic variants appeared repeatedly in the tumors but were absent from the healthy tissues of the same fish.
结果显示，不同鱼类体内的肿瘤细胞彼此之间的亲缘关系，远比它们与宿主鱼类之间的关系更为密切。换句话说，这些肿瘤拥有共同的遗传特征，且与宿主本身的基因截然不同。此外，数十万种基因变异在肿瘤中反复出现，但在同一条鱼的健康组织中却不存在。
Such a pattern would be virtually impossible if each melanoma had arisen independently. By way of comparison, the researchers analyzed hundreds of human melanomas and found that nearly all of their mutations were unique to each patient. In contrast, most of the mutations present in the tumors of these fish were shared by numerous individuals—a hallmark of a transmissible clonal cancer.
如果每例黑色素瘤都是独立产生的，这种模式几乎是不可能的。作为对比，研究人员分析了数百例人类黑色素瘤，发现几乎所有的突变对于每位患者来说都是独一无二的。相比之下，这些鱼类肿瘤中存在的大多数突变是由多个个体共享的——这是可传播克隆性癌症的标志。
Another possibility was that a virus or some microorganism was causing the melanomas. But after examining the genetic material in the samples, no viruses or bacteria specifically associated with the tumors were found. Although they cannot rule out the possibility that a pathogen may have played a role in the cancer’s origin or facilitated its spread, the evidence suggests that the transmissible agent is the cancer cells themselves.
另一种可能性是病毒或某种微生物导致了黑色素瘤。但在检查了样本中的遗传物质后，并未发现与肿瘤明确相关的病毒或细菌。尽管他们不能排除病原体可能在癌症起源中发挥作用或促进其传播的可能性，但证据表明，传播媒介正是癌细胞本身。
Unanswered Questions
未解之谜
It is still unclear how this cancer is transmitted among catfish. The authors propose several possibilities. During reproduction, catfish congregate in confined spaces and maintain close physical contact, which could ease the exchange of cells. It is also possible that tumor cells survive temporarily in the water or in sediments and enter other fish through the skin, gills, or mouth. Another hypothesis is that hormonal changes associated with reproduction alter the immune response and promote the establishment of cancer.
目前尚不清楚这种癌症是如何在鲶鱼之间传播的。作者提出了几种可能性。在繁殖期间，鲶鱼会聚集在狭小的空间内并保持密切的身体接触，这可能促进了细胞的交换。肿瘤细胞也有可能在水或沉积物中暂时存活，并通过皮肤、鳃或口腔进入其他鱼类体内。另一种假设是，与繁殖相关的荷尔蒙变化改变了免疫反应，从而促进了癌症的形成。
“It seems to only happen in larger fish that are of spawning age,” said biologist Mark Henderson, a co-author of the study, regarding the absence of cancer in younger fish. “Maybe some part of spawning behavior leads to the spreading of the cancer between animals.”
“这种情况似乎只发生在达到产卵年龄的大型鱼类身上，”该研究的共同作者、生物学家马克·亨德森（Mark Henderson）在谈到幼鱼中未发现癌症时说，“也许产卵行为的某些部分导致了癌症在动物间的传播。”
In a release from the University of Vermont, the researchers noted that the tumor cells contained elevated levels of arsenic and that they observed a correlation between the geographic distribution of the cancer and areas with high natural concentrations of this element. They emphasize, however, that this association does not prove that arsenic is the cause of the cancer.
在佛蒙特大学发布的一份新闻稿中，研究人员指出，肿瘤细胞中含有较高水平的砷，并且他们观察到癌症的地理分布与该元素天然高浓度区域之间存在相关性。然而，他们强调，这种关联并不能证明砷就是癌症的病因。
Despite the concerns, the scientists also note that the phenomenon could open up new avenues of research into cancer development, including in humans. “By studying how cancers survive and spread outside their original host, we can learn a great deal about what keeps cancers contained—and what happens when those boundaries break down,” said Dragon. Meanwhile, Henderson had advice for anglers.
尽管存在担忧，科学家们也指出，这一现象可能为癌症发展的研究开辟新途径，包括人类癌症的研究。“通过研究癌症如何在原始宿主之外生存和传播，我们可以深入了解是什么限制了癌症的扩散，以及当这些边界崩溃时会发生什么，”德拉贡说。与此同时，亨德森给垂钓者提了一个建议。
“Personally,” he said, “I wouldn’t eat the ones with tumors.”
“就我个人而言，”他说，“我不会吃那些长了肿瘤的鱼。”