Aziz Sancar -Nobel Laureate

Aziz Sancar -Nobel Laureate in Chemistry

Submitted by Prof. Dr. Ibrahim B. Syed

Aziz Sancar (Sancar – pronounced “SAN-jar”) is a Turkish American biochemist and molecular biologist specializing in DNA repair, cell cycle checkpoints, and circadian clock. In 2015, he was awarded the Nobel Prize in Chemistry along with Thomas Lindahl and Paul L. Mordich for their mechanistic studies of DNA repair. He has made contributions on photolyase and nucleotide excision repair in bacteria that have changed his field.

Sancar is currently the Sarah Graham Kenan Professor of Biochemistry and Biophysics at the University of North Carolina School of Medicine and a member of the UNC Lineberger Comprehensive Cancer Center. He is the co-founder of the Aziz & Gwen Sancar Foundation, which is a non-profit organization to promote Turkish culture and to support Turkish students in the United States. [2]

Life

Aziz Sancar (Sancar – pronounced “SAN-jar”) was born 8 September 1946, in Savur in southeast Turkey in a lower middle class family. Sancar was born the seventh of eight children. His parents had no education but considered education important for their children. Sancar studied hard in school and played hard on the soccer field. During his senior year of high school, Sancar, who played goalkeeper, was invited to attend tryouts for Turkey’s national under-18 soccer team. He spent his youth hoping to be the next Turgay Seren, the former goalkeeper for the Istanbul-based team whose heroic saves during a 1951 game against West Germany earned him the nickname “Panther of Berlin.” “He was my hero,” Sancar said.

In his senior year of high school he realized, “I was not tall enough to be a goalkeeper,” and decided on a career in medicine or chemistry instead.

Sancar excelled in many scientific disciplines in high school, and, after graduating, he narrowed his career choices to chemistry or medicine. He scored high enough on his university entrance examinations to attend the school of his choice in Turkey, and he entered Istanbul Medical School (Istanbul, Turkey) in 1963.

In many ways, he says, leaving his village for the teeming city of Istanbul was like moving to a foreign country. It was a difficult transition.

After taking a biochemistry class during his second year of the six-year program and becoming highly interested in the concepts learned, Sancar decided to become a research biochemist. When he discussed his desire to pursue a Ph.D. with his biochemistry professor, however, Sancar was advised to practice medicine, at least for a little while. In the opinion of his professor, “anyone who gets a medical degree and gets all this training should practice for a couple of years before going into the basic sciences,” says Sancar. He finished at the top of his class of 625 students. Even though he had already made up his mind, he followed the advice and spent two rewarding years as a rural physician near his hometown of Savur.

“Those were probably the happiest years of my life,” said Sancar, who felt beloved by his patients and gratified by the work. Many of those he treated had never seen a doctor and were grateful for any help.

“A shot of antibiotics could save a kid’s life,” he said. But it bothered him that he didn’t understand, at a molecular level, why an antibiotic worked against one type of infection and not another.

For Sancar, practicing medicine felt like being a highly trained technician. “You are presented with a set of symptoms, and you prescribe certain tests. You make a certain diagnosis, and you use certain prescriptions for treatment,” he said.

A practicing physician does not necessarily advance human understanding, Sancar said. For him, practicing medicine wasn’t enough. He wanted to know how it worked.

After concluding his medical practice in 1971, Sancar hoped to continue his biochemistry training in the United States. [3]

His interest was piqued by one phenomenon in particular: when bacteria are exposed to deadly doses of UV radiation, they can suddenly recover if they are illuminated with visible blue light. Sancar was curious about this almost magical effect; how did it function chemically?

In Sancar’s case, he had become interested in the phenomenon of photore-activation, whereby DNA damage caused by UV light can be repaired by longer-wavelength blue light. This reaction is mediated by the enzyme photolyase, which was identified years earlier by Claud Rupert at Johns Hopkins University (Baltimore, MD). At the time, in 1973, Rupert was teaching at the University of Texas at Dallas, and Sancar joined his laboratory and the university’s molecular biology program. [3]

Background

DNA repair – providing chemical stability for life

From one cell to another, from one generation to the next. The genetic information that governs how human beings are shaped has flowed through our bodies for hundreds of thousands of years. It is constantly subjected to assaults from the environment, yet it remains surprisingly intact.

The foundation of who you are was created when 23 chromosomes from a sperm combined with 23 chromosomes from an egg. Together, they formed the original version of your genome, your genetic material.

All the genetic information required to create you was present in that fusion. If someone had pulled out the DNA molecules from this first cell and laid them in a row, it would have been two meters long.

When the fertilized egg subsequently divided, the DNA molecules were copied and the daughter cell also obtained a full set of chromosomes. After that, the cells divided again; two became four, four became eight. After the first week you consisted of 128 cells, each one with its own set of genetic material. The total length of your DNA began to approach 300 meters.

Today – many, many billions of cell divisions later – your DNA could stretch all the way to the sun and back, around 250 times. Even though your genetic material has been copied so many times, the most recent copy is remarkably similar to the original that was once created in the fertilized egg. This is where life’s molecules display their greatness, because from a chemical perspective this ought to be impossible.

All chemical processes are prone to random errors. Additionally, your DNA is subjected on a daily basis to damaging radiation and reactive molecules. In fact, you ought to have been a chemical chaos long before you even developed into a fetus. Your DNA is monitored by a swarm of proteins. Our DNA remains astonishingly intact, year after year, due to a host of molecular repair mechanisms: a swarm of proteins that monitor the genes. They continually proof-read the genome and repair any damage that has occurred. The Nobel Prize in Chemistry 2015 is awarded to Tomas Lindahl, Paul Modrich and Aziz Sancar for having mapped these fundamental processes at the molecular level. Their systematic work has made a decisive contribution to the understanding of how the living cell functions, as well as providing knowledge about the molecular causes of several hereditary diseases and about mechanisms behind both cancer development and aging. The three Nobel laureates have, independently of each other, mapped several processes for DNA repair that are relevant to humans.

Life exists – so DNA must be repairable.

“How stable is DNA, really?”, in the 1960s, the scientific community believed that the DNA molecule – the foundation of all life – was extremely resilient; anything else was simply out of the question. Evolution does require mutations, but only a limited number per generation. If genetic information were too unstable no multi-cellular organisms would exist. [4]

Mentors and heroes

So, Sancar went back to school, this time at the University of Texas at Dallas. There, DNA pioneer Claud S. Rupert taught and ran a lab that Sancar regarded as the center of the universe for the study of a certain kind of DNA repair, in which bacteria recover from deadly doses of ultraviolet radiation if they are exposed to a blue light.

Rupert was an ideal advisor for Sancar. “He understood my capabilities and limitations,” says Sancar. “He encouraged me, gave me advice, and pointed me in the right direction. But, most importantly, he gave me the freedom to develop my own ideas and test them. As both a scientist and a gentleman, he has been the most influential person in my career.”

When Sancar joined Rupert’s group, the major question regarding photolyase was the nature of its chromophore, a question Sancar became obsessed with answering. “I told a fellow graduate student that I was willing to give my right arm to identify the chromophore, and I meant it!” says Sancar. Before taking such drastic steps, however, he tried an experimental approach to the problem. “About the time I started my research, recombinant DNA was born, and I realized I could use this technology to overproduce photolyase and identify the chromophore,” he says. “All I had to do was clone the gene into a multicopy plasmid. However, to do that, I first needed an E. coli mutant lacking photolyase.” [1]

Sancar was working in Rupert’s lab. It was there in 1976 that Sancar made his first major contribution to the field: He cloned the gene for photolyase, the enzyme that repairs the UV-damaged DNA in bacteria, though at the time, he didn’t understand how it worked.

Besides his tutelage under Rupert, Sancar had launched another partnership while in Dallas, with a fellow molecular biology doctoral student named Gwendolyn Boles, with whom he sometimes competed for late-night access to lab equipment.

“I just thought he was really interesting,” Gwen Sancar said. “He had a different sort of outlook from a lot of American men I had run into. And he was very accepting of a woman who was dedicated to a career.”

On the seventh day, he didn’t rest. He just didn’t work as hard. [3]

A Scientist, a Technician, a Wordsmith

Sancar devised a conceptually simple method to isolate photolyase-deficient mutants, which involved damaging bacterial cells with germinated UV light and then restoring them with normal light. He notes that the method was simpler in concept than in execution, because it did not work on the first, second, or third try. Sancar persisted, and, 11 months after his first attempt, he managed to isolate a photolyase-deficient phr ^– mutant strain. He considers that experiment the one that truly made him a scientist. “It reinforced my conviction that I had the ability to gather disparate facts from several fields to create a hypothesis, enough technical ability to carry out the experiments, and the perseverance to continue in the face of adversity,” he says.

In the spring of 1976, using that time’s blunt tools for molecular biology, he succeeded in cloning the gene for the enzyme that repairs UV-damaged DNA, photolyase, and also in getting bacteria to over-produce the enzyme. This work became a doctoral dissertation, but people were hardly impressed. “I believe it was the first gene to be cloned east of the Rockies,” he says wryly. “At least that’s what I tell my students to impress them.” After a four-month return to Turkey to perform compulsory military service, Second Lieutenant Sancar returned to Texas to finish characterizing the cloned photolyase gene. He had hoped to purify the protein as well, but Rupert told him he had done enough to write his Ph.D. dissertation and graduate. He received his doctorate at the University of Texas, Dallas, in 1977.

Although graduating was fairly simple, moving on proved difficult. Though Sancar believes he was the first person east of the Rockies to clone a gene, the accomplishment didn’t result in immediate offers of postdoctoral positions. Sancar had hoped to continue studying DNA repair, but all three laboratories he applied to rejected him. Sancar’s fiancée, fellow graduate student Gwendolyn Boles, had secured a position in New York. “Fortunately, I learned that Dean Rupp at Yale was interested in cloning repair genes, so I contacted him,” says Sancar. Although Rupp did not have a postdoctoral position available, he had a technician vacancy, and Sancar was hired, nominally, as a technician in 1977. At Yale’s School of Medicine that was one of several at the university doing DNA repair research. Aziz Sancar is married to Gwen Boles Sancar and they were married in 1978.

Like Rupert, Rupp proved a valuable mentor who further contributed to Sancar’s growth as a researcher. As in Dallas, Sancar had managed to land in the middle of the action. “Besides Rupp, Yale had other pioneers of DNA repair such as Paul Howard-Flanders, who helped discover excision repair and recombinational repair, Frank Hutchinson, and Charles Radding,” says Sancar. “Yale was one of, if not the, center for DNA repair.” Here he started the work that would eventually result in the Nobel Prize in Chemistry.

Aziz Sancar investigated how cells repair UV damage. By then it was clear that bacteria have two systems for repairing UV damage: in addition to light-dependent photolyase, a second system that functions in the dark had been discovered. Aziz Sancar’s new colleagues

at Yale had studied this dark system since the mid-1960s, using three UV-sensitive strains of bacteria that carried three different genetic mutations:

uvrA, uvrB and uvrC. [4]

Feeding off this exciting environment, Sancar identified and cloned several E. coli repair genes within two years, including the uvrA, uvrB, and uvrC genes involved in excision repair. Armed with his newly cloned genes, Sancar purified the three Uvr proteins and reconstructed the mechanism of excision repair.

As in his previous studies of photolyase, Sancar began investigating the molecular machinery of the dark system. Within a few years he had managed to identify, isolate and characterise the enzymes coded by the genes uvrA, uvrB and uvrC.

To Sancar’s surprise, the complex did not just nick the DNA near the damage, which was a popular working model at the time, but he showed that these enzymes can identify a UV-damage, then making two incisions in the DNA strand, one on each side of the damaged part. A fragment of 12-13 nucleotides, including the injury, is then removed.

Sancar termed the enzyme for this activity excision nuclease, or excinuclease. With Rupp’s help, Sancar also invented a method for identifying plasmid-encoded proteins through bacterial cells called maxicells. These maxicells were critical to his success in purifying the Uvr proteins. Within days of publishing his paper on maxicells in the Journal of Bacteriology in 1979, Sancar received more than 100 letters requesting his new cells, and he joyfully plastered these letters all over Rupp’s office. To this day, Sancar’s maxicell paper remains his most cited. Besides advancing science, these two studies secured the terms “excinuclease” and “maxicell” as entries in the Oxford Dictionary of Biochemistry and Molecular Biology.

In summary, UV radiation can make two thymines bind to each other incorrectly. In ground-breaking in vitro experiments, he showed that the enzyme exinuclease finds the damage and cuts the DNA strand. Twelve nucleotides are removed. DNA polymerase fills in the resulting gap. DNA ligase seals the DNA strand. Now the injury has been dealt with.

Similar mechanisms for UV damage repair in humans and bacteria Aziz Sancar’s ability to generate knowledge about the molecular details of the process changed the entire research field. He published his findings in 1983 [4]

Return to Photolyase

In 1982, Sancar received an offer to join the faculty at the University of North Carolina (Chapel Hill, NC) as an Associate Professor of Biochemistry. By that time, his mentor, Rupert, had left research to become the Dean of Arts and Sciences of the University of Texas at Dallas, and his departure allowed Sancar to resume his work on photolyase. Sancar joined the University of North Carolina, and, together with Boles and other collaborators, he identified the photolyase’s long sought-after chromophore—both of them, in fact. “I was expecting one, and I found two,” Sancar says. “One is FADH^–, and the other is a pterin.” Sancar developed a model for the reaction mechanism of photolyase repair but had difficulty proving his scheme because he could not experimentally capture the proposed radical intermediates. “I worked with ultrafast spectroscopists in three different continents,” he says. “Wherever there was an ultrafast lab in the world, I found it.”

Sancar continued studying other DNA repair pathways. Having answered some key questions about excision repair in E. coli, Sancar turned his attention to excision repair in humans. Using a strategy that took nearly five years to work out, Sancar showed in 1992 that humans excise thymine dimers by the same mechanism as E. coli. “This finding was especially significant since, unlike all other repair mechanisms, the genes for excision repair are not conserved between E. coli and humans, indicating this is a convergent mechanism,” he says. With the help of an in vitro system developed by his postdoctoral fellow, Christopher Selby, Sancar also managed to uncover the molecular mechanism behind the phenomenon of transcription-coupled repair, whereby transcribed DNA is repaired at a faster rate than nontranscribed DNA. “I consider this paper one of the most aesthetically pleasing ones of my career,” says Sancar. “It employed both classic and modern methods, the data are unambiguous and of high quality, and every experiment worked as predicted by the hypothesis.”

In addition, he helped to demonstrate that a human equivalent to photolyase helps us set the circadian clock. “While trying to prove cryptochrome was a photoreceptor, we ended up proving it was an essential component of the circadian clock itself.” [1]

Honors

Currently Sarah Graham Kenan Professor of Biochemistry at the University of North Carolina School of Medicine (Chapel Hill, NC), Sancar has employed a strategy of hard work, perseverance, and technical simplicity in his science. His honors include the Presidential Young Investigator Award from the National Science Foundation (1984) and the highest awards from the American Society for Photobiology (1990) and the Turkish Scientific Research Council (1995). Sancar, the first Turkish-American member of the National Academy of Sciences, as well as its first University of Texas at Dallas alumnus, was elected in 2005. [2]

Sancar received numerous honors and awards for his achievements: 1995, Associate Fellow, Third World Academy of Sciences. 1995, Scientific and Technological Research Council of Turkey, Basic Science Award. 2004, Fellow of the American Academy of Arts and Sciences. 2006, Member of the Turkish Academy of Sciences. 2007, Vehbi Koç Award from the Koç Foundation of Turkey. 2015, The Vallee Award in Biomedical Science, American Society for Biochemistry and Molecular Biology

2015, The Nobel Prize in Chemistry [5]

1969 MD, Summa Cum Laude (1st in class of 625). 1995 NIH MERIT Award. 2005 National Academy of Sciences, USA. 2009 Univ. of Texas at Dallas Distinguished Alumni Award. 2014 Distinguished Visiting Professor – Academia Sinica. 2016 TWESCO International Turkish Academy – Gold Medal at UN. 2016 ASBMB American Society for Biochemistry & Molecular Biology – Bert and Natalie Vallee Award. 2016 O. Max Gardner Award – highest honor by UNC Board of Governors. 2016 Carnegie Corporation’s Immigrant of the Year. 2016 North Carolina Award – the highest civilian honor given by the state. 2016 National Academy of Medicine. [6]

References

1. https//www.pnas.org/content/102/45/16125.full?ijkey=433d53fbe2d224a1e2d446ffb81eb6a8faa06a03&keytype2=tf_ipsecsha

A Scientist, a Technician, a Wordsmith

Return to Photolyase

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