Monthly Archives: June 2014

Remembering Ciro de Quadros and the Eradication of Polio in Latin America

Ciro de Quadros, who passed away at his home in Washington, D.C. on May 28, 2014, was a present-day hero, who ought to be much better known. In brief, de Quadros, a Brazilian epidemiologist, single-handedly initiated and then led efforts in the 1980s to eradicate polio from the Latin America continent.

The incidence of polio in Latin America had already been significantly diminished by the early 1960s via the introduction of Sabin’s vaccine. However, de Quadros insisted that epidemics would remain possible on the continent until mass immunization of the population might be achieved. He was particularly concerned with reaching unimmunized children who lived in the remotest areas. So, starting in 1985, in his role as an executive of the Pan American Health Organization (PAHO; a subsidiary of the United Nation’s WHO), de Quadros sent teams of health workers to 15 countries; some of which contained the most isolated and war-torn regions of Latin America.

El Salvador and Guatemala were particularly unstable at the time. So, in order to administer vaccinations in those nations, de Quadros first negotiated 24-hour cease-fire agreements between rebel and government forces. These so called “tranquility days” enabled health care workers to enter combat zones and carry out immunizations in relative safety.

In Peru, de Quadros was unable to secure cooperation from the Shining Path guerillas that operated there. So, he had his teams work around the areas controlled by the guerillas, and come back to those areas when the battle lines shifted elsewhere.

In 1994, the success of de Quadros’ immunization program led the PAHO to declare that polio had been officially eradicated from Latin American. The last reported case of polio on the continent occurred in 1991, in Pichanaki, Peru.

Interestingly, de Quadros had to fight an uphill battle to obtain support for his immunization efforts; even with the WHO, which preferred to use its limited resources to sustain primary health care. But, de Quadros forcefully maintained that vaccination is the starting point for effective primary health care, especially for children. And, while he was mainly concerned with polio, his vaccination teams also were prepared to immunize against measles, diphtheria, pertussis, tetanus, and tuberculosis.

Earlier in his career, in the 1970s, de Quadros was recruited by Donald A. Hendreson, then director of the WHO’s global smallpox eradication program, to help organize smallpox eradication in Ethiopia. In a phone interview with a NY Times reporter after de Quadros’ passing, Henderson recalled de Quadros’s steadfastness during Ethiopia’s civil war, when a half-dozen of his teams were kidnapped by rebels and one of his United Nations helicopters was commandeered along with its pilot. De Quedros helped negotiate the release of the health teams and the pilot, all of whom returned to their vaccination duties. Henderson noted that the helicopter pilot, who had vaccine aboard when he was hijacked, actually vaccinated the rebels who held him captive.

On May 2, 2014, less than one month before his death, de Quadros was honored by the PAHO/WHO as a Public Health Hero of the Americas. The award was presented to de Quadros by PAHO/WHO director Carissa F. Etienne, during an international vaccine symposium celebrating the 20th anniversary of the Sabin Vaccine Institute, where De Quadros was serving as Executive Vice President and Director of Vaccine Advocacy and Education. Etienne stated, “We at PAHO believe that no single person has done more to extend the benefits of immunization to people throughout the Americas.”

Dr. Carissa F. Etienne, Dr. Donald A. Henderson, Dr. Ciro de Quadros, Dr. Anthony Fauci, and Dr. Jon K. Andrus
Dr. Carissa F. Etienne, Dr. Donald A. Henderson, Dr. Ciro de Quadros, Dr. Anthony Fauci, and Dr. Jon K. Andrus

Etienne went on to say, “His (de Quadros’) leadership and vision were essential to our region’s becoming the first in the world to eradicate polio, a success story that inspired the global polio eradication campaign.” Also, de Quadros is regarded as a leader in the development of the surveillance and containment strategies that facilitated the eradication of smallpox worldwide, and he also directed measles eradication efforts in the Americas.

 

 

 

Advertisements

The Politics of Science: Vignettes Featuring Nobel Laureate Harold Varmus during his Tenure as Director of the NIH

During his extraordinary career, Nobel laureate Harold Varmus practiced science and served science with distinction. The vignettes that follow are, for the most part, about Varmus’ service to science during his tenure (1993-1999) as director of the U.S. National Institutes of Health. But first, we begin with a brief account of Varmus’ most significant scientific accomplishment.

In 1976, Varmus and collaborator Michael Bishop reported that retrovirus oncogenes (cancer-causing genes) are versions of genes that actually are present in the genomes of normal cells (1). Indeed, retroviruses acquired their oncogenes by “capturing” them from the genomes of their host cells. Perhaps the most singularly important conclusion to be drawn from Varmus’ and Bishop’s finding is that since retroviral oncogenes are versions of genes that are actually part of a normal cell’s genetic makeup, mutations in those particular cellular genes, or the inappropriate expression of those genes, might lead to cancer.

Varmus’ and Bishop’s findings led to a mushrooming of discoveries in cell signaling, cell growth, and cell differentiation (see Aside 1). Moreover, their discoveries are increasingly relevant clinically, as recounted below in the main text.

[Aside 1: The v-src gene of Rous sarcoma virus was the first retroviral oncogene that Varmus and Bishop showed is a version of a cellular gene. Next, in 1978, Raymond Erikson and coworkers isolated the Src protein. Then, Erikson’s research group and that of Bishop and Varmus independently discovered that Src has protein kinase activity. Protein kinases add phosphate groups to a specific target protein, generally triggering its activity. (Actually, Src phosphorylates itself, thus regulating its own activity.)

At the time that Erikson isolated Src, all protein kinases were believed to add phosphate to serine and threonine residues on their target proteins. Then, in 1980, Tony Hunter and Bart Sefton discovered that Src adds phosphotes to tyrosine residues. Thus, Src was the first known protein tyrosine kinase.

Stanley Cohen then discovered that the epidermal growth factor (EGF) receptor too is a protein tyrosine kinase, underscoring the role of tyrosine kinases in the control of normal cell proliferation, while also affirming the notion that inappropriate phosphorylation of a cellular protein can lead to cancer.

These discoveries led to a burst of research activity in cell signaling, and to the discovery of additional tyrosine and serine/threonine protein kinases, many of which act in mitogenic signaling pathways. What’ more many of the cellular genes encoding these proteins were initially discovered as retroviral oncogenes. For details on these points, see Virology: Molecular Biology and Pathogenesis.]

[Aside 2: An earlier posting on the blog, Renato Dulbecco and the Beginnings of Quantitative Animal Virology, noted that Renato Dulbecco shared the 1975 Nobel Prize for Physiology or Medicine, in recognition of his opening up the study of transformation by the DNA tumor viruses (i.e., the polyomaviruses, papillomaviruses, and adenoviruses). How then did analysis of transformation by the oncogenic retroviruses (i.e., the RNA tumor viruses) complement analysis of the DNA tumor viruses?

As suggested above; studies of the oncogenic retroviruses led to the identification of cellular signaling pathways that positively govern cell replication (i.e., that trigger cell growth). In contrast, studies of the DNA tumor viruses led to insights into cellular processes that negatively regulate cellular replication; in particular, processes affected by the key cellular tumor suppressor protein, p53, which activates apoptosis in cells that attempt to divide without having appropriately passed cell cycle checkpoints. The DNA tumor viruses affect transformation by inactivating tumor suppressor proteins. See Virology: Molecular Biology and Pathogenesis for details.]

[Aside 3: Varmus tells us that early in his career, in the late 1960s, he looked for places and people that might offer research training in the tumor viruses. “However, when I wrote to the already famous virologist Renato Dulbecco, at the Salk Institute in La Jolla, just North of San Diego, for a postdoctoral position, I was rebuffed by not one but two letters from his secretary (2).” See Aside 2.]

In the days before Varmus and Bishop published their findings, many cancer researchers actually were reluctant to believe that cancer has an underlying genetic basis. This was partly because it was not yet possible to clone and sequence genes, and there were no other apparent methods by which to identify putative cancer-related genes. [The means by which Varmus and Bishop made their breakthrough discovery are recounted in the Appendix, below.] In recognition of their discovery, Varmus and Bishop were awarded the 1989 Nobel Prize for Physiology or Medicine.

Varmus looked back on all aspects of his career in his 2009 autobiography, The Art and Politics of Science (2). [Unless otherwise noted, all of the quotations that follow are from Varmus’ 2009 memoir.] Here, are his remarks on the clinical significance of his and Bishop’s Nobel Prize-winning findings:

“In recent years, after our prize was awarded, mutant proto-oncogenes and the proteins they encode have become critical tools for the classification of cancers and promising targets for drugs and antibodies—treatments that have, in some cases, proven to be effective for a significant and growing number of cancers, including leukemias and lymphomas, lung, gastrointestinal, and kidney cancers: and cancers of the breast.”

In 1993, Varmus was named by President Bill Clinton to serve as Director of the U.S. National Institutes of Health; a position he held through 1999. As such, he was the only Nobel laureate to ever serve as the NIH director and he was also the first NIH director to also run an active laboratory. What’s more, during his tenure as director, he managed to nearly double the NIH’s research budget.

nih clinical center NIH Clinical Center

One of Varmus’ major responsibilities as the NIH director, and also one of his most contentious ones, was to apportion research dollars among the individual NIH institutes and programs. Why was there contention? As might be expected, the directors of the individual institutes actively advocate for their shares of the NIH budget. But, a further source of contention was Congress, in which the most ardent NIH supporters were generally motivated by their interest in a particular disease or program. What’s more, public advocacy groups likewise championed their own favored disease. Consequently, as Varmus explains:

“Apart from the difficulties of predicting where and how discoveries will arise, the priority-setting process can be ugly—for instance, when advocates refuse to recognize, or to care, that funds for their disease must come from funds being spent elsewhere, including funds used for a disease important to another group of advocates.”

Here is one such instance that Varmus notes:

“One of my first exposures to this problem occurred soon after I arrived at the NIH, when I received a call from my own former congresswoman, Nancy Pelosi, asking me to add $50 million to the budget for AIDS research. As the representative from one of the districts most heavily affected by the epidemic, her wishes were understandable. Since she was a member of the House Appropriations Committee for the NIH, she was in a position to try to increase funds for AIDS research when the subcommittee was debating the size of the NIH budget, without taking the money from some other research program. But, in the period of spending caps, she had presumably been unsuccessful in negotiations with her fellow committee members and was now trying to fulfill a promise to her constituents by asking me to shift funds from some other budget categories into the OAR (Office of AIDS Research) account. I declined as politely as I could.”

Varmus notes that it can difficult to refuse such requests (demands?) when they come from powerful people; especially so when the come from the President. For instance, President Bill Clinton “requested” that $10 million more be spent on spinal cord research; this coming after he spent an afternoon with recently paralyzed actor, Christopher Reeve.

“But the President’s wishes are always obeyed. When the next accounting was made of disease-specific spending at the neurology institute (formerly known as the National Institute of Neurological Diseases and Stroke, or NINDS), the funds for spinal cord research were accordingly higher, and funds for other purposes were lower.”

Here is an additional example, this time involving the vice-president:

“But Vice-President Al Gore posed a potentially serious dilemma for the NIH late in 1997 when he proposed that the National Cancer Institute (NCI) should receive a much larger share than the other institutes in the record-breaking $1 billion budget increase that the president was going to request for the NIH for fiscal year 1999. Possibly as a result of promises made to cancer research advocates, possibly because of personal concerns about cancer (his sister died of lung cancer at an early age), possibly because cancer research was popular politically, Gore asked that the cancer institute’s budget grow at twice the rate accorded the others.”

Varmus continues:

“I was very unhappy about this. The differential rates of growth were not in accord with clearly defined medical needs or with carefully considered scientific opportunities. No major changes in disease rates or outcomes and no sudden developments in cancer research made the needs for the NCI any greater than those for brain disorders, metabolic diseases, or infections. By any measure, the NCI was already the largest institute by a considerable margin, and Gore’s plan would further accentuate the differences. And, of course, there would be strong negative reaction from the supporters of the other institutes when the plan was announced. But, he was the vice-president, and conceivably the next president, so the idea of arguing with him on this issue was not appealing.”

However, Varmus had an ally in Donna Shalala, the Secretary of Health and Human Services (HHS), who supported his position. And, with her help, Varmus was able to take the issue directly to Gore:

“… we were able to reach a rapprochement when I pointed out that many institutes did cancer research, not simply the NCI, and he was very pleased to learn this. That gave us an opening for a compromise: we would ensure a relatively large increase for cancer research, but it would be spread among all the institutes that could be said to do cancer research.”

[Aside 4: Varmus was featured in an earlier posting on the blog that recounted how, in 1986; he resolved the dispute between Luc Montagnier and Robert Gallo over the right to name the AIDS virus (3). It’s been said that Varmus developed diplomatic skills while resolving the naming dispute that served him well as Director of the NIH. The following comment from Varmus shows his subtle diplomacy when interacting with the directors of other government agencies that he competed against:

“Often the best way to support the NIH and science in general was to make a magnanimous gesture toward the other agencies, emphasizing their importance in an increasingly interdisciplinary world of science and hoping the gesture would be reciprocated. This strategy was appreciated by my colleagues in other disciplines, helped to dispel jealousies about our fiscal success, and is remembered as a hallmark of my time at the NIH.”]

Irrespective of any political considerations, the setting of research priorities is an inherently difficult process. The following quotation points up the often conflicting scientific and public health considerations that Varmus took under consideration when determining research priorities. And, bearing in mind his recounting of Nancy Pelosi’s request for additional funding for AIDS, these remarks also demonstrate that he was hardly insensitive to the AIDS issue:

“For much of my time at the NIH, I was castigated by advocates for research on heart disease because the NIH was spending about as much on AIDS research as on studies of heart disease, even though there were about twenty times more deaths from heart disease than from AIDS in the United States each year. The arguments tended to ignore other important facts: that AIDS was a new and expanding disease, that it is infectious, that it is devastating large parts of the world, or that age-adjusted death rates from heart disease have fallen by two-thirds in the past 50 years.”

Elsewhere Varmus notes: “Of course, very different impressions can be produced by the use of different criteria—the number of people living with a condition, the number who die from it each year, the age adjusted death rate, the number of healthy individuals at risk, the number diagnosed each year, the annual medical expenditures, the annual cost to society, or the degree of pain and suffering. These are legitimate aspects of the nation’s burden of disease, but they are crude tools for deciding how to spend research dollars appropriately.”

Another difficulty that Varmus had to contend with was that laypeople, both in Congress and in public advocacy groups, often did not appreciate that science usually works best when scientists are free to investigate the particular issues that most intrigue them. And, when biomedical scientists follow their own inclinations, they often focus on basic or fundamental questions that may seem to have no apparent clinical relevance. Yet, and importantly, the knowledge gained from untargeted basic research may have a more positive affect on the understanding and treatment of a particular disease than all of the clinical research specifically targeted at that disease. [Indeed, the Nobel Prize-winning research of Varmus and Bishop is a good example of that very point.]

Speaking to that notion, Varmus said the following in a June 2009 interview with Catherine Clabby in American Scientist:

“Look at what pride people take now in advances made in diabetes and cancer research and infectious disease research. Almost all of it is based on recombinant DNA technology, genomics and protein chemistry. These are methods that grew out of basic science that was funded for years and years in a non-categorical way.”

Still and all, while basic research often may lead to significant clinical advances, Varmus acknowledges that the NIH still must have programs that are targeted at public health concerns:

“One of the potential strengths of the NIH is its ability to encourage scientists throughout the country to pay greater attention to underserved and deserving problems, even when the opportunities may not be obvious. Simply by encouraging attention to such problems—autism, rare neurological diseases, imaging methods, emerging infections, or bioengineering, to mention a few areas promoted during my tenure—new ideas may emerge to create those opportunities.”

But, Varmus adds:

“In this regard, the NIH must walk a narrow line: to respond responsibly to public health needs and yet to provide the freedom for investigators to exercise their imaginations as freely as possible.”

[Aside 5: An earlier posting on the blog, Jonas Salk and Albert Sabin: One of the Great Rivalries of Medical Science, described how the National Foundation for Infantile Paralysis financed the crusade against polio in the pre-NIH days of the 1950s. But, the Foundation’s efforts went beyond merely raising money for research. It also attempted to provide direction to the research, which often placed it at odds with its grantees. That was so because the principal goal of Harry Weaver, the Foundation’s director of research, was to bring a vaccine to the public. In contrast, most of the Foundation’s grantees were more interested in investigating basic virological issues, such as poliovirus transmission, replication, and dissemination.]

Research involving human embryonic stem cells was a particularly contentious issue that Varmus dealt with as NIH director. Stem cell research “attracted controversy mainly because the cells are obtained from human embryos, linking stem cell research to historical battles over abortion and over the legal and moral status of the human embryo and fetus.”

Yet Varmus took up the cause for stem cell research because “embryonic stem cells were likely to have the potential to develop into many specific tissue types…if so they could be used to repair damaged tissues or to treat chronic degenerative diseases of the brain or spinal cord, endocrine organs (such as pancreatic islets), muscles, joints, or other tissues.”

In 1993 Varmus assembled the Human Embryo Research Panel, tasked to advise him on what types of stem cell research might be suitable for federal funding. Not surprisingly, an immediate hullabaloo followed the panel’s recommendation that in vitro fertilization might be used to create embryos, from which stem cells could then be derived. Varmus remarked on the reaction to the panel’s recommendation as follows:

“Although well received by scientists who were watching its work, the panel’s report ignited a storm of government opposition; even within the liberal Clinton administration…the White House was in shock from the Democratic Party’s loss of control of both congressional chambers in the midterm elections held a month earlier. Democrats across the nation, especially those at the highest ranks of the Clinton administration, were concerned about a shift in the electorate toward the conservative policies of Newt Gingrich and his Republican revolutionaries, and already anxious about the presidential election of 1996…I remember getting a call from Leon Panetta, then the White House chief of staff, telling me that I was supposed to repudiate some of the panel’s recommendations, in particular any that might permit the use of federal funds to create embryos for research purposes. I refused to reject the recommendations of my panel summarily. I was not fired, as the tone of Panetta’s call had threatened.”

Although Varmus wasn’t fired for his independence, the Clinton White House quickly issued an executive order forbidding the use of federal funds to create human embryos for research. Varmus attributed the political pushback to the undue influence (“on the conduct of science in a diverse society”) of a few conservative religious groups. Varmus went on to say:

“Few arguments can seem as insulting to medical scientists as the claim that we are ethically irresponsible when we toil to extract stem cells from donated early human embryos, which would otherwise be destroyed, and use them for beneficial, potentially lifesaving purposes.”

Varmus lamented the fact that President George W. Bush limited federal funding for stem cell research during his administration. Nevertheless, stem cell research was being done even during the Bush presidency, although it was supported by the private sector and by several states (California, New York, Massachusetts, Wisconsin and others). Yet because potential stem cell investigators would need to obtain funding from less well-endowed non-federal sources to do this research, it is likely that many were discouraged from entering the field.

Varmus also fought a difficult and frustrating battle to secure federal funding for needle-exchange programs. By way of background, intravenous drug abusers were accounting for one-quarter of all new HIV infections in the United States. And, while other industrialized nations had needle-exchange programs that were successful at reducing the number of new HIV infections, many powerful individuals in the United States, including General Barry McCaffrey, head of the Clinton White House Office on Drug Control, regarded efforts to make drug use safer to be the equivalent of condoning drug use.

In 1998, HHS Secretary Donna Shalala, using evidence compiled by Varmus, advised President Bill Clinton that needle exchange programs were proven to be effective at preventing HIV transmission and, moreover, they did not increase drug use. Nevertheless, the President did not lift the ban on federal support for needle exchange programs.

By coincidence, the day that Clinton announced his decision not to lift the ban, Varmus and his wife were having dinner with Rahm Emanuel, who, at that time, was a domestic policy advisor in the Clinton White House. Interestingly, the liberal Emanuel was not sympathetic to lifting the ban. Instead, he believed that doing so would open the Democratic Party to charges that it was soft on drugs. At any rate, not lifting the ban did not enable the Democrats to regain either house of Congress. Varmus adds:

“The only satisfaction we received was the later admission by Bill Clinton, speaking at an international AIDS conference in Spain, less than two years after he left the White House, that his failure to lift the ban on funding needle exchange was wrong and one of the worst decisions he made during his presidency.”

Another of the good fights that Varmus fought on behalf of science was to establish new approaches to publishing scientific papers. His purpose was to enhance access to the scientific literature by taking advantage of new opportunities being offered by the internet and by new computational tools. His efforts resulted in two important new ways in which scientific research is published, stored, and retrieved; specifically, public digital libraries and “open access” publishing.

Varmus credits Stanford biologist Pat Brown with pushing him, in 1998, to think about improving access to the scientific literature by making the most of the internet. Brown had earlier worked with Varmus and Bishop on retroviral integration in the 1980s.

Varmus was still the NIH director when he helped to launch PubMed Central; the NIH’s full-text public digital library for the biomedical sciences, and the first of its kind. [In contrast to PubMed Central, PubMed is the NIH’s on-line archive of titles, authors, and abstracts. Access to full text articles was possible via PubMed, but only if one had a personal subscription to the particular journal, or had access via their institution.] But, many scientists and journal publishers were initially opposed to PubMed Central. Consequently, in 2000, after Varmus had left the NIH, he and Brown, together with Mike Eisen, a computational biologist at Berkeley (who had worked as a post-doc with Brown), took more vigorous steps to promote it:

“Pat, Mike, and I wrote a short declaration of purpose—we called it a pledge, publishers called it a boycott—in which we said that one year hence, the signatories would no longer submit articles, provide reviewing or editing services, or purchase individual subscriptions to journals that had not agreed to deposit their articles with PubMed Central.”

Thirty thousand scientists worldwide signed the pledge, but most didn’t. One reason for the lack of wider support was that leading scientists typically strove to have their papers published in the most prestigious journals, and most of those journals had not bought in to the open access idea. Journal publishers were opposed because their revenues from subscriptions would be undermined by the open access model.

Since most journal publishers were not willing to participate in PubMed Central, Varmus, Brown, and Eisen decided to found open access journals themselves. They began by creating two outstanding journals, PLoS (for Public Library of Science) Biology and PLoS Medicine. Their business model was to use author’s fees to cover publication costs, usually paid from research grants. [Incidentally, no favorably reviewed paper would be turned away for inability to pay the fee. All of the PLoS journals can be seen by anyone, anytime, at http://www.plos.org.]

The public access situation began to change dramatically in 2007 when a coalition of leading scientists, open access publishers (including PLoS), and concerned members of Congress advocated for a policy that would require all scientific papers reporting NIH-funded research to be deposited in PubMed Central. This cause came to fruition when President George W. Bush signed the 2008 appropriations bill, which included a clause making the NIH public access policy the law of the land.

So, wrapping things up, considering that the NIH director’s job can grind one down with its “incessant and inevitable conflicts,” why did Varmus put up with it? His answer is he enjoyed it:

“Above all, there was the pride, excitement, and (at times) historical significance of being the leader of the largest funding agency for medical research in the world. The position represents medical science and the good things it does for the country, if not the world. I felt this when working within the administration, when speaking to members of Congress, when talking to reporters, and when addressing the public at commencement exercises, and elsewhere.”

Varmus left the NIH to become President of the Memorial Sloan-Kettering Cancer Center in New York City; a position he held from 2000 until 2010. He then returned to the NIH, where he serves as director of the National Cancer Institute.

In the Epilogue to his memoir (2), Varmus refers to the work of scientists, and its potential benefit to society, as follows:

“Scientists may work and compete as individuals, but the competitive efforts are ultimately directed to the construction of a common edifice, knowledge of the natural world. There are few other fields in which such fierce independence serves the public good in such a transparently shared fashion”

But he adds: “…our knowledge does not improve the societies in which we live unless other kinds of actions, both political and pragmatic, are taken.”

References:

1. Stehelin D, Varmus HE, Bishop JM, Vogt PK., 1976. DNA related to the transforming gene(s) of avian sarcoma viruses is present in normal avian DNA. Nature 260:170-173.

2. Varmus, H. 2009. The Art and Politics of Science, (W. W. Norton & Company)

3. How the Human Immunodeficiency Deficiency Virus (HIV) Got Its Name, on the blog

Appendix:

Varmus and Bishop turned to nucleic acid hybridization to test their hypothesis that v-src might be a version of a cellular gene. They could not use the complete Rous sarcoma virus genome as a probe for the putative cellular src gene, because the complete virus genome might have detected endogenous retrovirus sequences within the cellular genome, rather than a cellular src gene per se. So, they needed to generate a more specific probe.

In the days before recombinant DNA procedures, Varmus and Bishop cleverly generated their specific src probe by making use of a transformation-defective mutant of Rous sarcoma virus, isolated earlier by Peter Vogt. The important feature of this mutant virus was that its src gene was deleted.

Varmus and Bishop generated their src-specific probe by first using reverse transcriptase to make a radioactively labeled, single-stranded DNA copy of the entire standard Rous sarcoma virus genome, which contains the src gene. This cDNA was then fragmented and annealed to an excess of RNA genomes of the src deletion mutant. The only DNA fragments that did not anneal were those containing only src sequences. These single-stranded DNA fragments could be separated from the annealed product and used as the src nucleic acid hybridization probe.

Using their cDNA probe, Varmus and Bishop were able to demonstrate the presence of src not only in the genomes of normal chicken cells, but also in the genomes of many other vertebrates as well, including humans (reference 1). These experimental findings led to the remarkable conclusions that the cellular src gene was present early in vertebrate evolution and that it has remained conserved to this day.

More experiments of this kind demonstrated that other highly oncogenic retroviruses contain other oncogenes of their own, which likewise have their counterparts in normal cell genomes. Indeed, each of the known retroviral oncogenes corresponds to a gene present in a normal cellular genome, and each of these retroviral oncogenes appears to be derived from a cellular genome.

But, why did Varmus and Bishop suspect that v-src might have originated as a cellular gene? In part it was because Steve Martin had earlier isolated a mutant of Rous sarcoma virus that was temperature-sensitive for transformation, but not for replication. Why would a virus carry a gene that it did need for it to replicate?