superman

作者：Jacquelyn K. Beals, PhD  
出處：WebMD醫學新聞

　　May 24, 2010 — Venter研究中心的科學家們報告指出，創造出第一個「合成細胞(synthetic cell)」— 山羊黴漿菌(Mycoplasma capricolum)細胞，將絲狀黴漿菌(Mycoplasma mycoides)的實驗室組裝基因組移入並控制複製。雖然有些科學家指出，只有基因組是合成的，研發者解釋，接受細胞的細胞質已經在每次分裂中稀釋，因此，培養30代之後，有些子代除了合成來源的染色體，已經沒有原始細胞的蛋白質了。
　　
　　5月20日Science Express網站上，研究作者在科學期刊發表的文章中指出，這項成就源自我們過去15年的努力，建立一個只有必須基因的小細胞，這些努力包括，在1995年將生殖道黴漿菌(Mycoplasma genitalium)基因組定序，將一個細菌菌株的基因組轉植入另一個菌株的細胞中，在2008年，從化學合成的DNA片段組裝一個完整的細菌基因組。
　　
　　這項研究由資深作者、J. Craig Venter研究中心的發起人兼主席J. Craig Venter博士發表，他們整合轉植細菌基因組的專門技術和能力，建立得自DNA片段的基因組，因為生殖道黴漿菌生長緩慢，研究者選擇生長較快速的絲狀黴漿菌亞種capri，作為捐贈者菌株，將山羊黴漿菌亞種capricolum作為接受者。
　　
　　使用絲狀黴漿菌兩個實驗室菌株的定序資訊，謹慎地構成合成的捐贈者基因組，這兩個菌株的基因組在95個地點分開，所以，可校正有生物意義的差異，以符合那些在酵母菌細胞成功複製的基因組。其他19個地點顯示沒有生物意義，但是，合成的和天然的基因組之間有差異。
　　
　　複雜的組裝過程始於對長度超過1000個鹽基對(1080 bp)的DNA進行定序，合併產生10,080-bp (10-kb)的中間物，證明定序之後，這些單元被組合而形成100-kb的中間物，最後，1,077,947-bp的絲狀黴漿菌合成基因組。在酵母菌中組裝，合成基因組在酵母菌細胞中植入，最後轉植到山羊黴漿菌接受細胞。標記植入合成的染色體，在培養皿中展現，使研究者可以辨識細胞已經合併的合成絲狀黴漿菌基因組。
　　
　　【我們的實驗有百分之99失敗】
　　該研究小組已經對絲狀黴漿菌相當熟悉，那是他們多年來用於轉植實驗的一種細菌。Venter博士在NPR之Science Friday的訪問中表示，你可以想像，我們的實驗有百分之99都失敗，近十年來，解決這些複雜問題之後才獲得這一點，所以，我們希望可以從已經知道、至少和生活相容的某件事情著手開始實驗。
　　
　　不過，他們的研究遇到一些阻礙，挑戰之一是，單一bp的缺失打斷了染色體複製的重要基因，而無法成功轉植進入合成染色體。作者們表示，我們之前不知道這個突變，指出他們的成功是這些缺失造成數週失敗之後才達到。其他小技術問題包括，百萬個以上的核甘酸對之中只有1個漏失，但是在修補受影響的基因組段落時引起漫長的延遲。
　　
　　校正這些錯誤之後，合成的基因組成功地轉植進入接受細胞，產生預期表現型的細菌，且持續自我複製。作者們觀察指出，這顯示我們的合成基因組、轉植的絲狀黴漿菌細胞的特徵增加，意味著上面的DNA序列已足以準確證明一個有適當性質的活細胞。
　　
　　除了「首次證明根據電腦中設計的基因組序列產生細胞的原則」這項成功創舉之外，這項研究受到媒體、聯邦政府與生命倫理學家的注目。
　　
　　來自白宮的反應中，歐巴馬總統將「合成生物學」指定為生命倫理議題研究委員會的首要研究計畫，他在5月20日給委員會主席的信件中，指示他們考慮這項研究的環境、醫療與安全利益，也要考慮可能的風險，他們的報告與建議將在6個月內發表。
　　
　　Venter博士在10多年前即已經預期該研究之可能的倫理影響。Venter博士在全國公共廣播電台中表示，首次的生命倫理回顧發表於1999年，在我們進行第一次實驗之前，那也是我們要求進行的回顧。我們擔心的是，儘管有諸多討論— 這篇報告發表之前，網路上有超過100,000個相關的部落格 —許多人依舊是首次聽到這個研究，我們知道，對它勢必會有一些震驚與反應。
　　
　　【科學家們沒有可以創造生命的生物學知識】
　　8位合成生物學家於5月20日在科學期刊的意見專欄發表「合成細胞之後的生命」為題的看法，這些專家之一、賓州大學生命倫理中心主任Arthur Caplan博士將該研究視為顯示出某個物質可以被操作產生我們視為生命的東西，藉此，他們得以將持續數千年之生命本質的辯論畫下句點。
　　
　　就他所知，多數宗教的形而上觀點中，陷入疑義的是：生命可以從無生命的部份創造，縱令這些是源自一個細胞。Venter博士的成就似乎平息了「生命需要一個特定的力量或機能來存在」的爭論。
　　
　　瑞士巴塞爾聯邦理工大學化學與生物工程研究中心主任、生物科技與生物工程教授Martin Fussenegger博士在科學期刊中提出一個不同的看法，他指出，就這個速度[產生新的有機體]以及出現生命系統相關的新科技，引起一些不安。
　　
　　如果一個計畫好的合成基因組的物種可變為有用，Fussenegger博士預期，生產環境將有限制。Fussenegger博士表示，如果它是面對自然生態系，它將會受到競爭者的挑戰，將不足以面對競爭。不論它們的起源，合成產生的有機體依舊要遭受競爭和演化，這是無法避免的天然法則。
　　
　　波士頓大學工程學院BioDynamics中心共同主任、生物醫學工程系教授Jim Collins博士對此提出懷疑，他指出，這篇研究只能說在我們的有機體重組工程的能力中是重要進步；它並不是從頭開始製造一個新生命。
　　
　　Collins博士將「合成細胞」視為只有合成其中的DNA，而不是建立一個新的生命。Collins博士表示，坦白說，它的基因組是已經存在的有機體DNA的複製段落縫合在一起而成，其中只有一些扭曲，科學家還不知道足以創造生命的生物學知識。
　　
　　Science Express. 線上發表於2010年5月20日。
　　Nature. 線上發表於2010年5月20日。
　　Science Friday. 公開於2010年5月20日。


Did Scientists Create a Synthetic Cell? It Depends Who You Ask

By Jacquelyn K. Beals, PhD
Medscape Medical News

May 24, 2010 — Scientists at the Venter Institute have reported the creation of the first "synthetic cell" — Mycoplasma capricolum cells that received and are controlled by a laboratory-assembled genome of Mycoplasma mycoides. Although some scientists note that only the genome is synthetic, the developers explain that the recipient cell's cytoplasm is diluted with each division. Thus, after 30 generations in culture, some progeny not only have a chromosome of synthetic origin but also contain no proteins from the original cell.

The achievement "grew out of our efforts over the past 15 years to build a minimal cell that contains only essential genes," said authors of the study, posted by Science May 20 on its Science Express Web site. These efforts included sequencing the Mycoplasma genitalium genome in 1995, transplanting the genome from 1 bacterial strain into cells of another strain, and in 2008, assembling a full bacterial genome from chemically synthesized DNA fragments.

The present study by senior author J. Craig Venter, PhD, founder, chairman, and president of the J. Craig Venter Institutes, Rockville, Maryland, and San Diego, California, and his colleagues combined their ability to transplant a bacterial genome with the technological expertise to build that genome from DNA fragments. Because M genitalium grows slowly, the investigators chose the more rapidly growing M mycoides subspecies capri as the donor strain and M capricolum subspecies capricolum as the recipient.

The synthetic donor genome was carefully constructed using sequencing information from 2 laboratory strains of M mycoides. Genomes of the 2 strains diverged at 95 sites, so differences with biological significance were corrected to match those of the genome that had been cloned successfully in yeast cells. The 19 remaining sites appeared to have no biological significance but serve to differentiate between the synthetic and natural genomes.

The complex assembly process started with DNA sequences just over 1000 base pairs (1080 bp) long, which were combined to produce 10,080-bp (10-kb) intermediates. After sequences were verified, these units were assembled to form 100-kb intermediates, and finally the 1,077,947-bp synthetic genome of M mycoides. Assembly occurred in yeast, and the synthetic genome was cloned in yeast cells and finally transplanted into M capricolum recipient cells. Markers built into the synthetic chromosome and expressed in culture enabled researchers to identify cells that had incorporated the synthetic M mycoides genome.

"99 out of 100 of Our Experiments Haven't Worked"

The research group was already very familiar with M mycoides — a bacterium they had worked with in transplant experiments for several years. "If you can imagine, 99 out of 100 of our experiments haven't worked, and this has been almost a decade of complex problem solving to get to this point," said Dr. Venter in an interview on NPR's Science Friday. "So we wanted to start the experiments with something that we at least knew would be compatible with life."

Nevertheless, their work encountered obstacles. One challenge was a single-bp deletion that disrupted a gene essential for chromosomal replication and prevented successful transplantation of the synthetic chromosome. "We were previously unaware of this mutation," said the authors, noting that their "success was thwarted for many weeks" by the deletion. The glitch involved only 1 missing nucleotide pair out of more than a million but caused a lengthy delay to repair the affected section of the genome.

After the error was corrected, the synthetic genome was successfully transplanted into recipient cells, producing bacteria with expected phenotypes that continue to self-replicate. "The demonstration that our synthetic genome gives rise to transplants with the characteristics of M. mycoides cells implies that the DNA sequence upon which it is based is accurate enough to specify a living cell with the appropriate properties," observe the authors.

Apart from scoring a coup as the first "proof of principle for producing cells based upon genome sequences designed in the computer," the work has attracted widespread attention from the media, the federal government, and bioethicists.

In a response from the White House, President Obama designated "synthetic biology" as the first study project for his Commission for the Study of Bioethical Issues. His May 20 letter to the chair of the commission instructed them to consider the environmental, medical, and security benefits of this research, as well as its potential risks. Their report and recommendations are due within 6 months.

Dr. Venter anticipated the potential ethical implications of the research more than a decade ago. "The first bioethical review was published in 1999, before we did the first experiments, and that's a review that we had asked for," said Dr. Venter on National Public Radio. "We were concerned that despite all the discussion — there's over 100,000 blogs on the Internet before this publication — that still this would be the first time the majority of people heard about this research, and we figured there would be some of the usual shock and responses to it."

"Scientists Do Not Know Enough About Biology to Create Life"

Eight synthetic-biology experts aired their views on "Life After the Synthetic Cell" in a May 20 opinion piece in Nature. One of these experts, Arthur Caplan, PhD, director of the Center for Bioethics, University of Pennsylvania, Philadelphia, saw the study as a demonstration that "the material world can be manipulated to produce what we recognize as life. In doing so they bring to an end a debate about the nature of life that has lasted thousands of years," he writes.

To his mind, the metaphysical views of the major religions are "cast into doubt by the demonstration that life can be created from non-living parts, albeit those harvested from a cell. Venter's achievement would seem to extinguish the argument that life requires a special force or power to exist."

Martin Fussenegger, PhD, professor of biotechnology and bioengineering and director of the Institute for Chemical and Bioengineering, ETH Zurich, Basel, Switzerland, reflected a different perspective in Nature: "It is this speed [with which new organisms can be generated], and the appearance of a new technology associated with living systems, that trigger discomfort," he noted.

If a species with a programmed synthetic genome were to become useful, Dr. Fussenegger expects that it would be restricted to the production environment. "If it were ever to face a natural ecosystem, it would be challenged by rivals and would be unprepared for the competition." Despite their origin, synthetically generated organisms would still be subject to competition and to evolution, says Dr. Fussenegger, "a natural law that cannot be tricked."

And with casual skepticism, Jim Collins, PhD, professor, Department of Biomedical Engineering, and codirector of the Center for BioDynamics, College of Engineering, Boston University, Massachusetts, responded to the hype: "The work...is an important advance in our ability to re-engineer organisms; it does not represent the making of new life from scratch," he pointed out.

Dr. Collins considers the "synthetic cell" as synthetic only "in the sense that its DNA is synthesized, not in that a new life form has been created. Its genome is a stitched-together copy of the DNA of an organism that exists in nature, with a few small tweaks thrown in.... Frankly," said Dr. Collins, "scientists do not know enough about biology to create life."

Science Express. Published online May 20, 2010.

Nature. Published online May 20, 2010.

Science Friday. Aired May 21, 2010.