来自哈佛医学院（HMS）和麻省总医院（HMS）的研究人员在乳腺癌、肝癌和其他种类的癌症中发现了一种具有潜在促癌作用（cancer-promoting）的基因。他们发现这种YAP基因可以转化人乳腺上皮细胞，为理解一种首先在果蝇中发现的细胞生长新调控通路，是怎样在人类癌症扮演重要角色的问题打开了大门。该研究工作发表《PNAS》的网络版上。
“我们在乳腺癌细胞中筛选与肿瘤发生相关的DNA。确定这些潜在导致癌症发生的新基因，对研究正常细胞转化成为癌细胞的新途径来说至关重要。”该文章的高级作者，来自HMS和MGH的教授Daniel Haber博士说。该研究由Haber博士实验室和HMS细胞生物学系Joan Brugge教授领导的实验室共同完成。
通过在BRCA1/p53基因缺失小鼠模型中进行的乳腺肿瘤的微阵列分析，Haber博士领导的研究小组发现在小鼠乳腺肿瘤中扩增的DNA区域仅仅包含一种已知基因——YAP。
“在一些人类肿瘤细胞中也发现了相似的DNA扩增区域，但这些区域通常包含了其他一些促进细胞生存的已知基因。”
Haber说，“因此，关于YAP基因是否在这些癌症中起作用的问题在很大程度上受到了忽视。我们发现排除这些扩增的DNA区域中的其它基因，我们能够专注于YAP作为候选研究对象的工作中。”
YAP基因在文献中首先是在果蝇实验中引起人们的注意，果蝇中的YAP基因，又称为Yorkie (Yki)，作用是促进细胞分裂和细胞生存，并受其它几个基因调控，如：Hippo (Hpo), Salvador (Sav), Warts (Wts), 和Mats。这些调控途径上游基因中任何一个发生变异或者Yki基因的过量表达将会引起果蝇眼睛或者翅膀细胞的过量生长。这种促进细胞分裂和细胞生存的作用的偶联是唯一的——其它能促进细胞分裂的基因如Myc，却会促进细胞的死亡。
“在癌症模型中用汽车来比喻癌症形成，Myc基因就像用汽油驱使汽车前进一样，驱动细胞分裂，但同时又能对细胞分裂轻微‘刹车’，所以当细胞分裂中出现错误的时候，细胞分裂之‘车’能够很快的停下来——通过清除细胞或者细胞死亡。”来自HMS细胞生物学系的研究伙伴，该文章的第一作者Michael Overholtzer博士说，“而Yki基因的活化，就像汽油驱动汽车前进的同时使刹车闸失效。这种作用对于癌症细胞生长来说求之不得。因此，这些发现于果蝇的基因：Yki(YAP), Hpo, Sav, Wts, 和 Mats，虽然很多是首先在果蝇中被发现的，却可能是控制着人类癌症的新途径的发生。
对YAP基因功能的早期研究结果并不支持它是一种人类致癌基因的说法，这是由于它的过量表达事实上促进细胞死亡而不是促进细胞存活。然而，由于在小鼠乳腺瘤中发现了YAP基因的扩增，Overholtzer和他的同事们决定在Brugge实验室开发的3D乳腺培养模型中进行YAP功能的研究。
利用这种模型，他们在三维的蛋白矩阵而不是二维矩阵上培养细胞，使得培养的乳腺细胞的生长具有人类乳腺相似的架构。
利用三维培养，研究者们能够证明YAP基因的过量表达戏剧性的改变了细胞侵入蛋白矩阵的行为。这种入侵行为正常情况下都与强烈促癌基因相关。研究者进一步表明，在3D培养和其它的分析实验中，YAP的过量表达既能激活细胞的生长又能抑制细胞的死亡。
此外，在实验室的研究中，YAP的过量表达能够将非癌的乳腺细胞转化成为癌细胞，这是通过YAP表达的细胞在软琼脂上生长——一种衡量癌症潜能的实验证明。与Overholtzer和他的同事的工作进行的同时，冷泉港的Scott Lowe博士的实验室证明了在一种小鼠模型中，YAP的过量表达可以导致肝癌的发生。(Cancer Cell, July 2006)。这表明YAP基因确实是一种新致癌基因。
“我们下一步想要知道的是YAP基因在人类细胞中是怎样受Hpo-Sav-Wts通路所控制的”，Overholtzer说。“虽然我们在小鼠乳腺瘤中发现了YAP基因的扩增，但实际上YAP扩增子（amplicon）在人类癌症中普遍存在，例如肺癌、胰腺癌、卵巢癌和其他癌症中。因此YAP基因可能在许多癌症的发生中起着重要的作用。
英文原文：
Hunt for DNA Amplified in Cancers Uncovers Important Target Gene
Researchers have discovered a new cancer-promoting role for a gene potentially involved in breast, liver, and other kinds of cancers. Their discovery that the gene YAP can transform mammary epithelial cells opens the door to understanding how a novel cell growth controlling pathway first discovered in fruit flies might be important in human cancers.
“We screened the DNA from breast cancer cells for amplifications that are associated with tumor development. The identification of these new potential cancer-causing genes is critical to uncovering novel pathways that drive the conversion of a normal cell to a cancerous one.” says senior author Daniel Haber, MD, PhD, the Laurel Schwartz professor of medicine at Harvard Medical School (HMS) and Massachusetts General Hospital (MGH) and director of the MGH Cancer Center. This research was conducted jointly by Haber’s lab and the lab of Joan Brugge, PhD, professor and chair of the Department of Cell Biology at HMS and is published in the Aug. 8 online early edition of the Proceedings of the National Academy of Sciences and will appear in the Aug. 15 print edition.
Through microarray analysis of a mammary tumor in a BRCA1/p53 deficient mouse model, Haber’s group discovered an amplified region of DNA in the mouse breast tumor that contained only one known gene, called YAP.
“A similar region of DNA is also amplified in some human tumors, but this amplified region often contains other genes that are known to promote cell survival,” says Haber, who worked with co-authors Jianmin Zhang, PhD, and Gromoslaw Smolen, PhD, both research fellows at MGH. “Thus, whether the YAP gene could play a role in these cancers had been largely ignored. The amplified region we discovered excluded these other genes, which allowed us to focus on YAP as a new candidate.”
The YAP gene has an interesting literature associated with it that comes from the fruit fly Drosophila melanogaster. The Drosophila version of the YAP gene, called Yorkie (Yki), functions to promote both cell division and cell survival and is controlled by several other genes called Hippo (Hpo), Salvador (Sav), Warts (Wts), and Mats. The mutation of any of these upstream genes or the over expression of Yki causes dramatic overgrowth of cells in the Drosophila eye or wing. This coupling of cell division and cell survival is unique – other genes that promote cell division, for example, Myc, also sensitize a cell to death.
“To use the car analogy that is often applied to cancer models, activation of Myc is like stepping on the gas to activate cell division but also lightly tapping on the brakes at the same time, so that should anything go wrong during division, the car can very quickly be stopped, or the cell can be removed by cell death,” says first author Michael Overholtzer, PhD, research fellow in cell biology at HMS. “Yki activation, on the other hand, is like stepping on the gas and disabling the brakes at the same time. Such an activity would be thought to be coveted by cancer cells. Therefore these genes, Yki (YAP), Hpo, Sav, Wts, and Mats, most of which were first discovered in the fruit fly, represent a relatively new and exciting pathway that might control human cancers.”
Earlier studies on YAP function in human cells did not support the notion that YAP might be a cancer causing gene because its over expression actually promoted cell death rather than cell survival (like Yki in Drosophila). Nevertheless, due to the amplification of YAP in a mouse breast tumor, Overholtzer and colleagues decided to examine the functions of YAP in a 3D mammary culture model developed in Brugge’s lab.
In this model, they grew cells in a 3-dimensional protein matrix rather than in 2-dimensions on plastic, which allows mammary cells to adopt architecture in culture that is similar to what occurs in the human breast. They had previously uncovered the effects of other genes using this model that would be missed in more conventional 2D models.
Using these 3D cultures, the authors were able to show that the over expression of YAP caused a dramatic change in cell behavior associated with invasion into the protein matrix. This type of invasive activity is normally associated with strong acting cancer-promoting genes. The authors were further able to show, in 3D cultures and other assays that YAP over expression both activated cell growth and inhibited cell death, just as one might have predicted from the studies of Yki in Drosophila.
Moreover, YAP over expression was able to turn their non-cancerous mammary cells into cancer-like cells in the lab, as evidenced by the ability of YAP expressing cells to grow in soft agar, an assay that measures cancerous potential. Parallel to Overholtzer and colleague’s work, the lab of Scott Lowe, PhD, of Cold Spring Harbor, also showed that YAP over expression could contribute to the development of liver tumors in a mouse model (Cancer Cell, July 2006). Thus, it appears that YAP is indeed a newly identified cancer-causing gene.
“What we would like to understand next is how YAP is controlled by the Hpo-Sav-Wts pathway in human cells”, says Overholtzer. “Also, although we found the YAP amplification in a mouse breast tumor, in human cancers this amplicon is actually much more common in other types such as lung, pancreatic, ovarian, and others. Thus it is possible that YAP plays an important role in the development of many different types of cancer.”
