藍色情懷

科學家證明刪除 1 個基因 使哺乳類動物再生
1 gene lost = 1 limb regained? Scientists demonstrate mammalian regeneration through single gene deletion

March 15, 2010

一次超過十年以上的追尋，在一次偶然的觀察中達到了一個里程碑：確認出一個可能調控「哺乳類動物再生」的基因。缺少這一個基因，稱為 p21，賦予老鼠某種早已被認為在演化中喪失的治癒潛能。而這種治癒潛能被扁蟲、海綿以及某些蠑螈（salamander）物種所保留。

A quest that began over a decade ago with a chance observation has reached a milestone: the identification of a gene that may regulate regeneration in mammals. The absence of this single gene, called p21, confers a healing potential in mice long thought to have been lost through evolution and reserved for creatures like flatworms, sponges, and some species of salamander.

在一項今日發表在 PNAS 的報告中，來自 The Wistar Institute 的研究者證明，缺乏 p21 基因的老鼠獲得再生喪失或損傷組織的能力。

In a report published today in the Proceedings of the National Academy of Sciences, researchers from The Wistar Institute demonstrate that mice that lack the p21 gene gain the ability to regenerate lost or damaged tissue.

不像典型的哺乳類動物，那透過疤的形成來治癒（healing）傷口，這些老鼠則從形成胚芽（blastema）開始，那是一種與快速細胞生長以及去分化（de-differentiation）相關的構造，一如在兩棲類中所見。根據 Wistar 研究者，p21 的喪失導致這些老鼠的細胞表現更像胚胎幹細胞而非成年哺乳類動物的細胞，而他們的發現提供了牢靠證據，使「組織再生」與「細胞分裂的控制」相連。

Unlike typical mammals, which heal wounds by forming a scar, these mice begin by forming a blastema, a structure associated with rapid cell growth and de-differentiation as seen in amphibians. According to the Wistar researchers, the loss of p21 causes the cells of these mice to behave more like embryonic stem cells than adult mammalian cells, and their findings provide solid evidence to link tissue regeneration to the control of cell division.

"十分類似一隻失去肢體的蠑螈（newt），這些老鼠將以健康的組織取代缺失或受損的組織，那缺乏任何結疤的徵象，" 計畫領導科學家 Ellen Heber-Katz, Ph.D.，Wistar 的分子與細胞腫瘤發生計畫教授。"雖然我們才剛開始了解這些發現的反響，也許，有朝一日我們將能透過暫時性地使 p21 基因失去活性加速人類的康復。"

"Much like a newt that has lost a limb, these mice will replace missing or damaged tissue with healthy tissue that lacks any sign of scarring," said the project's lead scientist Ellen Heber-Katz, Ph.D., a professor in Wistar's Molecular and Cellular Oncogenesis program. "While we are just beginning to understand the repercussions of these findings, perhaps, one day we'll be able to accelerate healing in humans by temporarily inactivating the p21 gene."

Heber-Katz 以及她的同事使用一種 p21 基因剔除鼠來協助解決一個首次在 1996 年於她實驗室裡的另一種老鼠品種中所遇到的謎。MRL 鼠，那已在某種自體免疫實驗中經過試驗，牠們的耳朵被穿刺幾個洞以創造出一種常用的、終身的身分證明標記。幾週之後，研究者發現這些耳洞在沒有留下任何痕跡的情況下消失。雖然這些實驗已一無所有，不過它留下一個新問題給研究者：MRL 鼠是進入哺乳類動物再生的一扇窗嗎？

Heber-Katz and her colleagues used a p21 knockout mouse to help solve a mystery first encountered in 1996 regarding another mouse strain in her laboratory. MRL mice, which were being tested in an autoimmunity experiment, had holes pierced in their ears to create a commonly used life-long identification marker. A few weeks later, investigators discovered that the earholes had closed without a trace. While the experiment was ruined, it left the researchers with a new question: Was the MRL mouse a window into mammalian regeneration?

這項發現使 Heber-Katz 實驗室開始二種並行的路線。與 Drexel 大學的遺傳學家 Elizabeth Blankenhorn, Ph.D. 以及 Washington 大學的遺傳學家 James Cheverud, Ph.D. 合作，這間實驗室開始聚焦在測繪將 MRL 鼠變成治療者的關鍵基因。於此同時，在 Wistar 繼續進行的細胞研究揭露，MRL 細胞的表現與那些來自培養中「非治癒者」老鼠品種的細胞非常不一樣。Khamilia Bedebaeva, M.D., Ph.D. 在車諾比（Chernobyl）反應爐輻射災變後開始研究遺傳效應，立即注意到這些細胞非常的「非典型（atypical）」，在細胞週期特徵與 DNA 損傷中展現出深刻的差異。這導致 Andrew Snyder, Ph.D. 探索 DNA 損傷路徑及其對於細胞週期控制的影響。

The discovery set the Heber-Katz laboratory off on two parallel paths. Working with geneticists Elizabeth Blankenhorn, Ph.D., at Drexel University, and James Cheverud, Ph.D., at Washington University, the laboratory focused on mapping the critical genes that turn MRL mice into healers. Meanwhile, cellular studies ongoing at Wistar revealed that MRL cells behaved very differently than cells from "non-healer" mouse strains in culture. Khamilia Bedebaeva, M.D., Ph.D., having studied genetic effects following the Chernobyl reactor radiation accident, noticed immediately that these cells were atypical, showing profound differences in cell cycle characteristics and DNA damage. This led Andrew Snyder, Ph.D., to explore the DNA damage pathway and its effects on cell cycle control.

Snyder 發現 p21，一種細胞週期調控者，在來自 MRL 鼠耳的細胞中一直不活化。P21 表現密切地受到腫瘤抑制者 p53 控制，另一種細胞分裂的調控者與許多癌症類型的一種已知因子。這項最終實驗證明，一隻缺乏 p21 的老鼠會表露出一種類似在 MRL 老鼠中所見到的再生反應。而這「就是那個光」。如其所示， p21 剔除鼠已經被創造出來，很容易取得，而且廣泛用在許多研究中。沒有被注意到的是，這些老鼠能治癒牠們的耳朵。

Snyder found that p21, a cell cycle regulator, was consistently inactive in cells from the MRL mouse ear. P21 expression is tightly controlled by the tumor suppressor p53, another regulator of cell division and a known factor in many forms of cancer. The ultimate experiment was to show that a mouse lacking p21 would demonstrate a regenerative response similar to that seen in the MRL mouse. And this indeed was the case. As it turned out, p21 knockout mice had already been created, were readily available, and widely used in many studies. What had not been noted was that these mice could heal their ears.

"在一般細胞中，p21 作用如同煞車，在 DNA 損傷事件中阻止細胞週期進行，阻止細胞分裂以及潛在地變得像癌症（cancerous），" Heber-Katz 表示。"在這些沒有 p21 的老鼠中，我們確實在 DNA 損傷中看見預期中的增加，不過出人意外的是，沒有癌症增加的報告出現。"

"In normal cells, p21 acts like a brake to block cell cycle progression in the event of DNA damage, preventing the cells from dividing and potentially becoming cancerous," Heber-Katz said. "In these mice without p21, we do see the expected increase in DNA damage, but surprisingly no increase in cancer has been reported."

事實上，這些研究者在 MRL 鼠中看見細胞自毀（apoptosis）的增加 -- 亦稱為已程式化的細胞死亡 -- 這種細胞自毀機制通常當 DNA 已經損傷時開啟。根據 Heber-Katz 表示，這恰好是那種可在自然再生生物中看見的表現。

In fact, the researchers saw an increase in apoptosis in MRL mice - also known as programmed cell death - the cell's self-destruct mechanism that is often switched on when DNA has been damaged. According to Heber-Katz, this is exactly the sort of behavior seen in naturally regenerative creatures.

"「高度再生的細胞以及細胞自毀」增加的結合效應，也許讓這些生物體的細胞迅速分裂而不會失去控制進而變得像癌症，" Heber-Katz 說。"事實上，這類似在哺乳類動物胚胎中所見，在此 p21 也在 DNA 損傷後出現不活化。p21 的下調（down regulation）促進哺乳類動物細胞的誘導式多能性狀態，強調了幹細胞、組織再生以及細胞週期間的相互關係。"

"The combined effects of an increase in highly regenerative cells and apoptosis may allow the cells of these organisms to divide rapidly without going out of control and becoming cancerous," Heber-Katz said. "In fact, it is similar to what is seen in mammalian embryos, where p21 also happens to be inactive after DNA damage. The down regulation of p21 promotes the induced pluripotent state in mammalian cells, highlighting a correlation between stem cells, tissue regeneration, and the cell cycle."

※ 相關報導：

＊ Lack of p21 expression links cell cycle control and appendage regeneration in mice


Khamilia Bedelbaeva, Andrew Snyder, Dmitri Gourevitch,
Lise Clark, Xiang-Ming Zhang, John Leferovich,
James M. Cheverud, Paul Lieberman, and Ellen Heber-Katz
PNAS, Published online before print March 15, 2010,
doi: 10.1073/pnas.1000830107